• The dynamical evolution of close-in binary systems formed by a super-Earth and its host star. Case of the Kepler-21 system

Autores: Santiago H. Luna, Hugo D. Navone, Mario Melita

Revista: Astronomy & Astrophysics 641, A109 (2020)

The aim of this work is to develop a formalism for the study of the secular evolution of a binary system which includes interaction due to the tides that each body imparts on the other. We also consider the influence of the J2-related secular terms on the orbital evolution and the torque, caused by the triaxiality, on the rotational evolution, both of which are associated only to one of the bodies. We apply these set of equations to the study of the orbital and rotational evolution of a binary system composed of a rocky planet and its host star in order to characterize the dynamical evolution at work, particularly near spin-orbit resonances.

We used the equations of motion that give the time evolution of the orbital elements and the spin rates of each body to study the dynamical evolution of the Kepler-21 system as an example of how the formalism that we have developed can be applied.

We obtained a set of equations of motion without singularities for vanishing eccentricities and inclinations. This set gives, on one hand, the time evolution of the orbital elements due to the tidal potentials generated by both members of the system as well as the oblateness of one of them. On the other hand, it gives the time evolution of the stellar spin rate due to the corresponding tidal torque and of the planet’s rotation angle due to both the tidal and triaxiality-induced torques. We found that for the parameters and the initial conditions explored here, the tidally and triaxiality-induced modifications of the tidal modes can be more significative than expected and that the time of tidal synchronization strongly depends on the values of the rheological parameters.

• Sixteen overlooked open clusters in the fourth Galactic quadrant. A combined analysis of UBVI photometry and Gaia DR2 with ASteCA

Autores: G. I. Perren, E. E. Giorgi, A. Moitinho, G. Carraro, M. S. Pera and R. A. Vázquez

Revista: Astronomy and Astrophysics 637, A95 (2020)

This paper has two main objectives: (1) To determine the intrinsic properties of 16 faint and mostly unstudied open clusters in the poorly known sector of the Galaxy at 270° −300° to probe the Milky Way structure in future investigations. (2) To address previously reported systematics in Gaia DR2 parallaxes by comparing the cluster distances derived from photometry with those derived from parallaxes.

Methods. Deep UBVI photometry of 16 open clusters was carried out. Observations were reduced and analyzed in an automatic way using the ASteCA package to obtain individual distances, reddening, masses, ages, and metallicities. Photometric distances were compared to those obtained from a Bayesian analysis of Gaia DR2 parallaxes.

Ten out of the sixteen clusters are true or highly probable open clusters. Two of them are quite young and follow the trace of the Carina Arm and the already detected warp. The remaining clusters are placed in the interarm zone between the Perseus and Carina Arms, as expected for older objects. We found that the cluster van den Berg-Hagen 85 is 7.5 × 109 yr old, which means that it is one of the oldest open clusters detected in our Galaxy so far. The relationship of these ten clusters with the Galaxy structure in the solar neighborhood is discussed. The comparison of distances from photometry and parallaxes data in turn reveals a variable level of disagreement.

Various zero-point corrections for Gaia DR2 parallax data recently reported were considered for a comparison between photometry- and parallax-based distances. The results tend to improve with some of these corrections. Photometric distance analysis suggests an average correction of ∼+0.026 mas (to be added to the parallaxes). The correction may have a more intricate dependence on distance, but addressing this level of detail will require a larger cluster sample.

• Alterations of the erythrocyte membrane by in vitro action of Trichinella spiralis muscle larvae

Autores: P. Ponce De León, M. Bellini, H. Castellini, B. Riquelme

Revista: Journal of Helminthology, 94, e17 (2020)

Resumen: The complex life cycle of Trichinella spiralis includes the migration of newborn larvae through the bloodstream to their encystment in muscle. The parasite establishes an intimate contact with the erythrocytes of the host both during the migration of the newborn larvae and when encysting, as this parasite causes intense vascularization in the muscle cell. The goal of this work was to study the effects of various concentrations of T. Spiralis muscle larvae (ML) on erythrocyte membranes. The treatment was performed by incubating human erythrocytes with equal volume of different concentrations of ML for 30 minutes, with controlled agitation (37°C). The control erythrocytes (with no contact with the larvae) were incubated in the same way with an equal volume of physiological solution. To evaluate the alterations to the erythrocytes by the action of the larvae and in the respective controls, an Erythrocyte Rheometer and a Digital Image Analysis technique were used. The results indicated that when the larval concentration was higher, the aggregation and erythrocyte membrane alterations were also higher. Also, the erythrocyte deformability index and the erythrocyte elasticity increased. The values of isolated cell coefficient varied from 0.51 in the treatment with 100 larvae/ml to 0.91 in the incubation with 1000 larvae/ml. This experiment shows that T. Spiralis muscle larvae affect significantly the red blood cell aggregation and the erythrocyte viscoelastic properties.

• Estudio preliminar de los efectos de las radiaciones Gamma sobre los glóbulos rojos humanos

Autores: E. Estrada, J. Borraz, H. Castellini, A. Acosta, L. Di Tullio, J. Borraz, A. Chinellato, I. Tack, M D’Arrigo, B. Riquelme, Mariel Elisa Galassi

Revista: Anales AFA 31 (2), 51 (2020)

Resumen: En este estudio se analizaron los parámetros viscoelásticos y de agregación en glóbulos rojos humanos sometidos a los procedimientos habituales de irradiación gamma con fines transfusionales. Las muestras fueron irradiadas a diferentes dosis a fin de determinar los posibles cambios hemorreológicos que pudieran afectar a la salud de los pacientes y su relación con las modificaciones bioquímicas observadas. Los resultados obtenidos muestran alteraciones en el tiempo de agregación, en la viscosidad superficial de membrana y en el tamaño de los agregados eritrocitarios en las muestras irradiadas, sugiriendo que el daño producido por la radiación ionizante afecta a las propiedades físicas de la membrana del glóbulo rojo en diferentes niveles.

• Linear and non-linear viscoelasticity of red blood cells using a new optical erythrocyte rheometer

Autores: B. Riquelme, H. Castellini, B. Albea

Revista: Latin America Optics and Photonics Conference, OSA Technical Digest (Optical Society of America, 2018), paper Th4A.41 (2018)

Resumen: Linear and non-linear viscoelasticity of red blood cells was characterized using a new Optical Erythrocyte Rheometer based on laser diffractometry technique. Optical and mechanical models were developed to obtain representative parameters for clinical application.

• Atomic RABBITT-like experiments assisted by a circularly polarized laser

Autores: D. I. R. Boll and O. A. Fojón

Revista: Journal of Physics: Conference Series 1412, 072026 (2020)

Resumen: We study theoretically the single ionization of atoms by the combined action of an attosecond pulse train with linear polarization and an assistant laser field with circular polarization. By means of a non-perturbative model, we obtain closed-form expressions for the angular distributions of photoelectrons. We show that such a configuration of light fields is similar to the usual polarization control technique where both the attosecond pulse train and the assistant laser field are linearly polarized with a controlled relative orientation.

• Absolute total cross sections of multiple electron processes for molecules of biological interest by fast proton impact

Autores: P N Terekhin, M A Quinto, J M Monti, O A Fojón and R D Rivarola

Revista: Journal of Physics: Conference Series 1412, 152064 (2020)

Resumen: We present the study of multiple electron processes produced by impact of proton beams on molecular targets from intermediate to high projectile impact energies. Transition probabilities and absolute cross sections were determined by using the Continuum Distorted Wave-Eikonal Initial State approximation. Results were compared with previous calculations and also with recent experimental data.

• Photoionization of water: influence of the initial state representations

Autores: L. Martini, D. I. R. Boll and O. A. Fojón

Revista: Journal of Physics: Conference Series 1412, 152090 (2020)

Resumen: We study theoretically the photoionization of water molecules by monochromatic linearly polar-ized photons. We elaborated a model in which the final state wavefunctions are given by Coulomb continuum w avefunctions with distorted charges and the bound states are represented by using different wavefunctions. We contrast our calculations for photon energies between 20 and 300 eV with more elaborated theories and experimental results. We achieve a very good agreement with experiments, especially at enough high energies where elaborated results are not available.

• Ionization of pyrimidine molecule induced by proton

Autores: M. A. Quinto, J. M. Monti, R. D. Rivarola

Revista: Journal of Physics: Conference Series 1412, 212002 (2020)

Resumen: In the present study will be investigated the ionization of isolated pyrimidine molecules induced by ions. The total and differential cross section have been computed with a quantum mechanical method within the continuum distorted wave-eikonal initial state (CDW-EIS) approximation. A comparison with the available data is presented.

• Simulation of single cell irradiation by proton beams using TILDA-V

Autores: M E Alcocer-Avila, M A Quinto, J M Monti, P F Weck, O A Fojón, R D Rivarola and C Champion

Revista: Journal of Physics: Conference Series 1412, 212004 (2020)

Resumen: The purpose of the present work is to emphasize the need for realistic modeling of the biological medium when studying the transport of protons in living matter at the very small scale. In this context, we have used the TILDA-V Monte Carlo track-structure code to simulate the irradiation of single cells with proton beams over the energy range of 10 keV – 100 MeV.

• Radiodosimetry endpoints obtained with the TILDA-V Monte Carlo tracking structure code

Autores: M A Quinto, M E Alcocer-Avila, J M Monti, P F Weck, O A Fojón, R D Rivarola and C Champion

Revista: Journal of Physics: Conference Series 1412, 212008 (2020)

Resumen: The present study aims at scrutinizing at the nanometer scale the radiodosimetry endpoints induced by ion beams in a realistic biological medium. In this context, self-consistent quantum mechanical modeling of the ionization and the electron capture processes are reported within the continuum distorted wave-eikonal initial state (CDW-EIS) framework for both isolated water molecules and DNA components impacted by ions.

• Electron emission induced by Helium ions on water molecule

Autores: M. A. Quinto, J. M. Monti, C. Champion, R. D. Rivarola

Revista: Journal of Physics: Conference Series 1412, 212009 (2020)

Resumen: This study focuses on the electron emission induced by He ions on water molecule. In this context, the ionization and the electron capture processes will be investigated using a quantum mechanical description within the continuum distorted wave-eikonal initial state (CDW-EIS) approximation.

• Electron emission from CH4 molecule induced by 5.5 MeV/u bare C ions

Autores: A. Mandal, C. Bagdia, M. Roychowdhury, J. M. Monti, R. D. Rivarola, L. C. Tribedi

Revista: Journal of Physics: Conference Series 1412, 142014 (2020)

Resumen: We present the experimental and theoretical study of electron emission from methane (CH4) in collisions with 5.5 MeV/u bare carbon projectiles. We have measured double differential cross section of electron emission and compared the data with the prior form of the continuum distorted wave - eikonal initial state (CDW-EIS) model calculation. The CDW-EIS calculations are in very good agreement with the data

• Delay control in laser-assisted photoionization of water molecules by attopulses

Autores: Lara Martini, DiegoI. R. Boll and Omar A. Fojón

Revista: Proceedings of SPIE - The International Society for Optical Engineering 11270, 112701J (2020)

Resumen: The knowledge of the ionization of water is essential in different fields such as Biology and Atomic Physics. Basic reactions involving this molecule are crucial to understand the interaction between radiation and the biological tissue because living cells are composed mostly by water. Therefore, we study theoretically the laser-assisted photoionization of water molecules by attopulses in the streaking regime by means of a Coulomb-Volkov model. We analyze reactions initiated by an extreme ultraviolet single attosecond pulse assisted by a near-infrared laser. The initial molecular wavefunctions are described by using the Moccia’s monocentric wavefunctions whereas the final state wavefunctions are given by the separable Coulomb-Volkov type wavefunctions. We obtain analytical expressions for the observables of interest. We calculate photoelectron spectra as a function of the delay between the attopulse and the assistant laser field for water molecules. Several polarization configurations of pulses and assistant laser are considered. Particularly, we focus on the conditions where asymmetries are generated in the observables and we examine those under which these asymmetries could be enhanced and/or diminished leading to a directional selectivity of the photoelectron emission. Consequently, we hope our work promotes progress on the control of the chemical reactivity of water as this could be useful in many domains of radiobiology and medical physics. Finally, we expect these studies contribute to the improvement of attopulses and assistant laser technologies as well as to the development of new polarization and delay control experiments.

• Auger electron emission in proton‐induced interactions in living matter: A TILDA‐V Monte Carlo tracking

Autores: Michele Arcangelo Quinto, Juan Manuel Monti, Mario Enrique Alcocer Avila, Philippe F. Weck, Omar Ariel Fojón, Roberto Daniel Rivarola, Christophe Champion

Revista: X-Ray Spectrometry 49(1), 95 (2020) - Special Issue: Proceedings of the 19th International Conference on the Physics of Highly Charged Ions (HCI 2018), Caparica, Lisboa, Portugal, 3‐7 September 2018

Resumen: The present work focuses on studying the contribution of the Auger electron emission in proton‐induced interactions in biological matter. The Monte Carlo track‐structure code, TILDA‐V, was then used for modeling the protons beams of 10 keV to 100 MeV in biological matter, namely, water vapor and hydrated DNA. The main ionizing processes are described by means of an extensive set of ab initio differential and total cross sections computed within a quantum‐mechanical CDW‐EIS approximation.

• The physics of irradiation of biological matter by ion beams

Autores: M. A. Quinto, J. M. Monti, C. A. Tachino, P. F. Weck, O. A. Fojón, C. Champion and R. D. Rivarola

Revista: Radiation Physics and Chemistry 167, 108337 (2020)

Resumen: In radiobiology, predicting the evolution of irradiated biological matter is nowadays an active field of research to identify DNA lesions or to adapt the radiotherapeutic protocols in radiation oncology. In this context, the numerical methods, based on Monte Carlo track-structure simulations, represent the most suitable and powerful tools for understanding the radiobiological damages induced by ionizing particles. In the present work, we report the theoretical differential and total cross sections, computed within the quantum mechanical continuum distorted wave-eikonal initial state (CDW-EIS) approach, for ion impact on water vapor and DNA nucleobases. These cross sections have been used to build up the input database for the homemade Monte Carlo track-structure TILDA-V. A comparison between the theoretical predictions and the available experimental data is presented. Micro-dosimetry results obtained with TILDA-V are also reported.

• Off-resonance-enhanced polarization control in two-color atomic ionization

Autores: D. I. R. Boll, L. Martini, O. A. Fojón, and A. Palacios

Revista: Physical Review A 101, 013428 (2020)

Resumen: We investigate a polarization control scheme in two-color two-photon atomic ionization. Experiments employing free-electron laser or high-order harmonic generation sources have shown light polarization dependencies in above threshold ionization of atoms assisted with an IR field. These were mostly performed at relatively high photoelectron energies where the soft-photon approximation remains valid. We use here a perturbation theory approach to show the larger degree of control that can be attained when the short wavelength photon energy is below the ionization threshold. This largely unexplored energy region offers unique possibilities to achieve polarization control of the reaction.

• Radiobiological effectiveness of iodouracil and the influence of atomic giant resonance

Autores: Anuvab Mandal, Madhusree Roy Chowdhury, Chandan Bagdia, Juan M. Monti, Roberto D. Rivarola, P. F. Weck, and Lokesh C. Tribedi

Revista: Physical Review A 102, 062811 (2020)

Resumen: Hadron therapy combined with nanotechnology has been proposed as an elegant alternative for cancer treatment. Internal amplification of electron emission causing radiobiological effectiveness in nanoinserted biomolecules is of prime importance and has been measured here for the iodouracil molecule. Our experiment involves the measurement of angle and energy resolved double differential cross section (DDCS) of electron emission from iodouraciil and uracil (and also water) in collisions with fast C6+ ions. The electron emission from iodouracil is substantially enhanced over that from uracil or water. The enhancement is much larger than the state-of-the-art model for Coulomb ionization based on the continuum distorted wave-eikonal initial state (CDW-EIS) approximation. The electron sensitizing factor (≈2.4) is in excellent agreement with the strand-breaking sensitizing factor (≈2.0) for metal nanoparticle embedded in a DNA. The enhancement is explained in terms of collective excitation of strongly correlated 4d electrons, known as atomic giant dipole resonance (GDR) in I atoms. The GDR contribution to the enhancement is derived, which is in excellent agreement with recent theoretical prediction, thereby providing conclusive experimental evidence of the crucial role of collective excitation in radio sensitization.

• Electron emission from CH4 molecules in collisions with fast bare C ions

Autores: Anuvab Mandal, Chandan Bagdia, Madhusree Roy Chowdhury, Shamik Bhattacharjee, Deepankar Misra, Juan M. Monti, Roberto D. Rivarola, and Lokesh C. Tribedi

Revista: Phys. Rev. A 101, 062708 (2020)

Resumen: We present the energy and angular distributions of electron emission from a CH4 molecule in collisions with fast bare C ions with energies 3.5 and 5.5 MeV/u. The absolute double differential cross sections (DDCS) are measured for the ejected electrons having energies from 11 eV to 330 eV for 3.5−MeV/u projectiles and from 5eV to 330 eV for 5.5 MeV/u bare C ions. The emission is measured in the angular range from 20∘ to 160∘. The forward-backward angular asymmetry, the single differential cross sections (SDCS), and the total cross section are deduced from the measured DDCS values. The energy and angular distributions of the DDCS and SDCS are compared with those calculated using a theoretical model based on the prior form of the continuum distorted wave–eikonal initial state (CDW-EIS) approximation. The dynamics of the process is considered within the CDW-EIS approximation, while the initial orbitals of the molecular target are represented using the complete neglect of the differential overlap approximation. The calculations are found to be in very good agreement with the measured cross sections. The angle dependence of the carbon K-LL Auger emission and the total Auger emission cross section are also derived for both projectile energies.

• Electron emission in ionization of adenine molecule induced by 5 MeV/u bare C ions

Autores: Shamik Bhattacharjee, Anuvab Mandal, Madhusree Roy Chowdhury, Chandan Bagdia, Juan M. Monti, Roberto D. Rivarola, Lokesh C. Tribedi

Revista: European Physical Journal D 74(8), 163 (2020)

Resumen: The double differential cross sections (DDCS) of electron emission in ionization of adenine molecule (C5H5N5) induced by 5 MeV/u bare C ions are measured using the electron spectroscopy technique. Emission of electrons from adenine is measured in the energy range from 20 eV to 460 eV and in the angular range from 20° to 160°. The single differential cross sections (SDCS) and total ionization cross section (TCS) are deduced from the e-DDCS. The experimental results are compared with the prior form of the continuum distorted wave- eikonal initial state (CDW-EIS) model prediction. Overall, a very good agreement between the theory and experiment is observed.

• Ionization of atoms and molecules using 200-keV protons and 5.5-MeV/u bare C ions: Energy-dependent collision dynamics

Autores: Madhusree Roy Chowdhury, A. Mandal, A. Bhogale, H. Bansal, C. Bagdia, S. Bhattacharjee, J. M. Monti, R. D. Rivarola, and Lokesh C. Tribedi

Revista: Phys. Rev. A 102, 012819 (2020)

Resumen: Using electron spectroscopy technique, we measure the absolute double-differential cross sections (DDCSs) of electrons emitted in collisions of 200-keV protons on He, CH4, and O2 and 5.5-MeV/u bare C ions colliding on O2. The emitted electrons are measured in the energy range from 1 to 400 eV for He and CH4 targets and up to 600 eV for O2 to include the K−LL Auger line of oxygen. The electrons are detected over different emission angles varying from 20∘ in the extreme forward direction to 160∘ in the backward direction. The single-differential cross section (SDCS) and total cross section are deduced from the measured DDCSs spectra for all the targets. The DDCS and SDCS are compared with the state-of-the-art continuum distorted wave–eikonal initial state (CDW-EIS) theoretical model. The CDW-EIS model provides excellent agreement with the oxygen data at MeV energy, whereas the deviation in the case of keV energy is substantial, in spite of having nearly the same perturbation strength. The forward-backward angular asymmetry shows a saturation effect in the case of keV energy protons but no such signature is observed for the high-energy collision. A systematic analysis reveals that the asymmetry at low electron energy is sensitive to the associated atomic or molecular structure and is in close agreement with the theory.

• Angularly resolved two-photon above-threshold ionization of helium

Autores: D. I. R. Boll, Omar Fojón, C. W. Mccurdy y A. Palacios

Revista: Physical Review A, 99, 023416 (2019)

Resumen: Angularly resolved two-photon single ionization yields of helium resulting after the interaction with an ultrashort XUV pulse are obtained by numerically solving the full dimension time-dependent Schrödinger equation. The angular distributions reveal the underlying dominant mechanism, which depends on the effective photon energy absorbed and the pulse parameters. We specifically explore the contributions of radial and angular electron correlation terms. A single active electron picture is a qualitatively valid approach for the lowest photon energies, even in the above-threshold ionization region. Nonetheless, angular correlation plays a detectable role in the low-energy region and a major role at higher energies when autoionizing states are populated. As the photon energy increases, sequential ionization-excitation dominates; therefore, the resulting probability distributions are explained as the result of two active uncorrelated electrons. This uncorrelated picture fails again for photon energies above ionization potential of the ion.

• Interferences in the photoionization of water molecules

Autores: Lara Martini, Diego I. R. Boll y Omar Fojón

Revista: Journal of Physics B - Atomic, Molecular and Optical Physics, 52, 105204 (2019)

Resumen: We study theoretically the photoionization of water molecules by monochromatic and linearly polarized radiation. The final state wavefunctions are given by coulomb continuum wavefunctions and the water molecule bound states are represented using linear combinations ofSlater-type orbitals located on the centers of the molecule. We obtain total and differential cross sections. We compare them with more elaborated theoretical results and experiments obtaining a very good agreement in particular at enough high energies where there is a lack of predictions.We put in evidence three-center interference effects not only the total cross sections, but also show that these effects may be detected in a direct way in the angular distributions of photoelectrons corresponding to randomly oriented molecules. In particular, we find that the interference effects under certain conditions may provoke a partial suppression of the emission of photoelectrons in the classical direction given by the polarization vector.

• Proton transport modeling in a realistic biological environment by using TILDA-V

Autores: M. E. Alcocer-Ávila, M. A. Quinto, J. M. Monti, R. D. Rivarola, C. Champion

Revista: Scientific Reports 9, 14030 (2019)

Resumen: Whether it is in radiobiology to identify DNA lesions or in medicine to adapt the radiotherapeutic protocols, a detailed understanding of the radiation-induced interactions in living matter is required. Monte Carlo track-structure codes have been successfully developed to describe these interactions and predict the radiation-induced energy deposits at the nanoscale level in the medium of interest. In this work, the quantum-mechanically based Monte Carlo track-structure code TILDA-V has been used to compute the slowing-down of protons in water and DNA. Stopping power and range are then reported and compared with existing data. Then, a first application of TILDA-V to cellular irradiations is also reported in order to highlight the absolute necessity of taking into account a realistic description of the cellular environment in microdosimetry.

• Neutral hydrogen versus proton-induced ionization in water vapor

Autores: M. Quinto, Juan Manuel Monti, C. Champion, Roberto Rivarola

Revista: Physical Review A 100, 042704 (2019)

Resumen: In the current work, we investigate the ionization of water molecules impacted by neutral hydrogen and proton beams. The continuum distorted wave-eikonal initial-state approximation is used to describe the reaction. Differential and total cross sections are compared with the available experimental data and a fine analysis of the influence of the projectile charge state on the ionization process is reported.

• Bare-carbon-ion-impact electron emission from adenine molecules: Differential and total cross-section measurements

S. Bhattacharjee, C Bagdia, M. R. Chowdhury, A. Mandal, Juan Manuel Monti, Roberto Rivarola y L. C. Tribedi

Revista: Physical Review A 100, 012703 (2019)

Resumen: Double-differential ionization cross sections (DDCS) for bare-carbon-ion-induced ionization of vapor-phase adenine molecules (C5H5N5) have been measured. The experiment has been performed using an electron spectroscopy technique. Electrons ejected from adenine were analyzed by a hemispherical electrostatic deflection analyzer over an energy range of 1-450eV for emission angles from 20 to 160. The single-differential cross section (SDCS) and total ionization cross section were also deduced. The experimental results have been compared with the continuum distorted wave-eikonal initial-state model calculation. We have observed a very good agreement between the theory and experiment. The angular distribution of the DDCS, SDCS and the asymmetry parameter for low-energy (Ee≤0.5 a.u.) electron display an oscillatory behavior which is in contrast to that observed in ion-atom collisions. A comparison is also made with available experimental cross-section results for uracil target colliding with the same velocity bare carbon ions and the scalability of ionization cross sections among these molecules is discussed.

• Roadmap on photonic, electronic and atomic collision physics: III. Heavy particles: With zero to relativistic speeds

Autores: F. Aumayr, K. Ueda, E. Sokell, S. Schippers, H. Sadeghpour, F. Merkt, T. F. Gallagher, F. B. Dunning, P. Scheier, O. Echt, T. Kirchner, S. Fritzsche, A. Surzhykov, X., Ma, Roberto Rivarola, Omar Fojón, L. Tribedi, E. Lamour, J. R. Crespo López-Urrutia, Y. Litvinov, V. Shabaev, H. Cederquist, H. Zettergren, M. Schleberger, R. A. Wilhelm, T. Azuma, P. Boduch, H. T. Schmidt, T. Stöhlker

Revista: Journal of Physics B: Atomic, Molecular and Optical Physics 52, 171003 (2019)

Resumen: We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. Roadmap III focusses on heavy particles: with zero to relativistic speeds. Modern theoretical and experimental approaches provide detailed insight into the wide range of many-body interactions involving projectiles and targets of varying complexity ranging from simple atoms, through molecules and clusters, complex biomolecules and nanoparticles to surfaces and crystals. These developments have been driven by technological progress and future developments will expand the horizon of the systems that can be studied. This Roadmap aims at looking back along the road, explaining the evolution of the field, and looking forward, collecting nineteen contributions from leading scientists in the field.

• Fully differential cross sections for ionization of H2+ by ion impact: Classical and quantum effects

Autores: Carmen Tachino, Roberto Rivarola

Revista: Journal of Physics B: Atomic, Molecular and Optical Physics 52, 065203 ( 2019)

Resumen: Single ionization of H2 + molecular ions by impact of bare ions is theoretically studied. Fully differential cross sections are calculated by employing the prior version of the continuum distorted wave-eikonal initial state model within an independent electron approximation. It is shown in this work that, under certain conditions there can exist a competition between classical and quantum mechanisms in the ionization reaction that gives rise to the suppression of characteristic classical structures, namely the binary and recoil peaks, from the ionization angular spectra. The existence of destructive quantum interferences due to the coherent emission of electrons from the neighbourhood of both molecular centres may be held as the responsible for the suppression of these classical signatures. Different behaviours are observed depending on whether the ground (gerade) or the first excited (ungerade) states of the H2 + target are analysed. Different values of the projectile nuclear charge and collision geometries are considered when calculating FDCS.

• Electron Transport Modeling in Biological Tissues: From Water to DNA

Autores: M. E. Alcocer-Ávila, M. A. Quinto, J. M. Monti, R. D. Rivarola, C. Champion

Revista: Springer Proceedings in Physics 230, 137 (2019)

Resumen: A proper and reliable description of charged particles interactions in the biological matter remains a critical aspect of radiation research. All the more so when new and better methods for treating cancer through the use of ionizing radiation are emerging on the horizon such as targeted alpha therapy. In this context, Monte Carlo track-structure codes are extremely useful tools to study radiation-induced effects at the atomic scale. In the present work, we review the latest version of CELLDOSE, a homemade Monte Carlo track-structure code devoted to electron dosimetry in biological matter. We report here some recent results concerning the stopping power and penetration range of electrons in water and DNA for impact energies ranging from 10 eV to 10 keV.

• Single- and multiple-electron processes in water molecules colliding with proton beams

Autores: P. N. Terekhin, M. A. Quinto, J. M. Monti, O. A. Fojón, R. D. Rivarola

Revista: Journal of Physics B: Atomic, Molecular and Optical Physics 51, 235201 (2018)

Resumen: Single- and multiple-electron removal processes (ionization, capture and transfer-ionization) from water molecules by the impact of protons have been studied. A prior version of the three-body continuum distorted wave-eikonal initial state (3B-CDW-EIS) approximation within the independent electron approximation is used to calculate transition probabilities as a function of the impact parameter and consequently pure and net absolute cross sections for the collisions under consideration. A unitarization procedure is employed to avoid possible overestimation of the 3B-CDW-EIS single-particle impact parameter probabilities at intermediate collision energies. Multiple-electron transitions are determined using a statistical multinomial distribution. A critical analysis of the validity of this type of distribution for describing pure single-electron processes is presented. The results are compared with other theoretical calculations and available experimental data at impact energies from 50 keV to 5 MeV. New physical insights into the reactions studied are introduced.

• Electron capture by swift ions from molecules of biological interest

Autores: M. A. Quinto, P. R. Montenegro, J. M. Monti, O. A. Fojón, R. D. Rivarola

Revista: Journal of Physics B: Atomic, Molecular and Optical Physics 51 (16), 165201 (2018)

Resumen: Electron capture from molecular targets impacted by swift ions, H+, He2+, Li3+ and C6+, is investigated in the framework of the quantum-mechanical continuum distorted wave-Eikonal initial state model. Biological molecules considered are nitrogen, methane, carbon monoxide, carbon dioxide and water. In particular, for water, the calculation of the corresponding cross sections plays a fundamental role for the determination of energy deposition in biological matter. A detailed analysis on the contributions coming from different molecular orbitals to total cross sections (TCS) are discriminated as well as those of capture to fundamental and excited projectile states. A good agreement with measurements is found for cases where experimental data exist. For other systems, the theoretical results here reported are useful for the prediction of the corresponding TCS.

• Electron capture to the continuum manifestation in fully differential cross sections for ion impact single ionization

Autores: M. F. Ciappina, O. A. Fojón, R. D. Rivarola

Revista: Journal of Physics B: Atomic, Molecular and Optical Physics 51(8), 085204 (2018)

Resumen: We present theoretical calculations of single ionization of He atoms by protons and multiply charged ions. The kinematical conditions are deliberately chosen in such a way that the ejected electron velocity matches the projectile impact velocity. The computed fully differential cross sections (FDCS) in the scattering plane using the continuum-distorted wave-eikonal initial state show a distinct peaked structure for a polar electron emission angle θ k = 0°. This element is absent when a first order theory is employed. Consequently, we can argue that this peak is a clear manifestation of a three-body effect, not observed before in FDCS. We discuss a possible interpretation of this new feature.

• Energy and angular distribution of electrons ejected from water by the impact of fast O8+ ion beams

Autores: S. Bhattacharjee, C. Bagdia, M. R. Chowdhury, J. M. Monti, R. D. Rivarola, L. C. Tribedi

Revista: European Physical Journal D 72 (1), 15 (2018)

Resumen: Double differential cross sections (DDCS) of electrons emitted from vapor water molecules (in vapor phase) by 2.0 MeV/u and 3.75 MeV/u bare oxygen ion impact have been measured by continuum electron spectroscopy technique. The ejected electrons were detected by an electrostatic hemispherical deflection analyzer over an energy range of 1–600 eV and emission angles from 20∘ to 160∘. The DDCS data has been compared with the continuum-distorted-wave-eikonal-initial state (CDW-EIS) approximation and a reasonable agreement was found with both version of the models i.e. post and prior version. By numerical integration of the DDCS data, the single differential cross section (SDCS) and total ionization cross section (TCS) were obtained. The obtained TCS results were compared with other available TCS results for water target within the same energy range. The total ionization cross sections values are seen to saturate as the projectile charge state (qp) increases, which is in contrast to the first-Born predicted qp 2 dependence. This is also in contrast to the prediction of the CDW-EIS models.

• Detection of Acoustic Plasmons in Hole-Doped Lanthanum and Bismuth Cuprate Superconductors Using Resonant Inelastic X-Ray Scattering

Autores: Abhishek Nag, M. Zhu, Matias Bejas, J. Li, H. C. Robarts, Hiroyuki Yamase, A. N. Petsch, D. Song, H. Eisaki, A. C. Walters, M. Garcia-Fernandez, Andres Greco, S. M. Hayden, Ke-Jin Zhou

Revista: Physical Review Letters 125, 257002 (2020)

Resumen: High Tc superconductors show a rich variety of phases associated with their charge degrees of freedom. Valence charges can give rise to charge ordering or acoustic plasmons in these layered cuprate superconductors. While charge ordering has been observed for both hole- and electron-doped cuprates, acoustic plasmons have only been found in electron-doped materials. Here, we use resonant inelastic X-ray scattering (RIXS) to observe the presence of acoustic plasmons in two families of hole-doped cuprate superconductors [La2-xSrxCuO4 (LSCO) and Bi2Sr1.6La0.4CuO6+d (Bi2201)], crucially completing the picture. Interestingly, in contrast to the quasi-static charge ordering which manifests at both Cu and O sites, the observed acoustic plasmons are predominantly associated with the O sites, revealing a unique dichotomy in the behaviour of valence charges in hole-doped cuprates.

• Interplay between spatial anisotropy and further exchange interactions in the triangular Heisenberg model

Autores: M. G. Gonzalez, E. A. Ghioldi, C. J. Gazza, L. O. Manuel, A. E. Trumper

Revista: Physical Review B 102, 224410 (2020)

Resumen: We investigate the interplay between spatial anisotropy and further exchange interactions in the spin-12 Heisenberg antiferromagnetic model on a triangular lattice. We use the Schwinger boson theory by including Gaussian fluctuations above the mean-field approach. The phase diagram exhibits a strong reduction of the long range collinear and incommensurate spirals regions with respect to the mean-field ones. This reduction is accompanied by the emergence of its short range order counterparts, leaving an ample room for 0-flux and nematic spin liquid regions. Remarkably, within the neighborhood of the spatially isotropic line, there is a range where the spirals are so fragile that only the commensurate 120∘ Néel ones survive. The good agreement with recent variational Monte Carlo predictions gives support to the rich phase diagram induced by spatial anisotropy.

• Close inspection of plasmon excitations in cuprate superconductors

Autores: Andrés Greco, Hiroyuki Yamase, Matías Bejas

Revista: Physical Review B 102, 024509 (2020)

Resumen: Recently resonant inelastic x-ray scattering experiments reported fine details of the charge excitations around the in-plane momentum q∥=(0,0) for various doping rates in electron-doped cuprates La2−xCexCuO4. We find that those new experimental data are well captured by acoustic-like plasmon excitations in a microscopic study of the layered t-J model with the long-range Coulomb interaction. The acoustic-like plasmon is not a usual plasmon typical to the two-dimensional system, but has a small gap proportional to the interlayer hopping tz.

• Ferromagnetic fluctuations in the Rashba-Hubbard model

Autores: Andrés Greco, Matías Bejas, Andreas P. Schnyder

Revista: Physical Review B 101, 174420 (2020)

Resumen: We study the occurrence and the origin of ferromagnetic fluctuations in the longitudinal spin susceptibility of the t-t′-Rashba-Hubbard model on the square lattice. The combined effect of the second-neighbor hopping t′ and the spin-orbit coupling leads to ferromagnetic fluctuations in a broad filling region. The spin-orbit coupling splits the energy bands, leading to two van Hove fillings, where the sheets of the Fermi surface change their topology. Between these two van Hove fillings the model shows ferromagnetic fluctuations. We find that these ferromagnetic fluctuations originate from interband contributions to the spin susceptibility. These interband contributions only arise if there is one holelike and one electronlike Fermi surface, which is the case for fillings in between the two van Hove fillings. We discuss implications for experimental systems and propose a test on how to identify these types of ferromagnetic fluctuations in experiments.

• Doping dependence of d-wave bond-charge excitations in electron-doped cuprates

Autores: H. Yamase, Matías Bejas y Andrés Greco

Revista: Physical Review B 99, 014513 (2019)

Resumen: Motivated by the recent experiments reporting the doping dependence of the short-range charge order (CO) in electron-doped cuprates, we study the resonant x-ray scattering spectrum from d-wave bond-charge fluctuations obtained in the two-dimensional t-J model. We find that (i) the CO is short-range, (ii) the CO peak is pronounced at low temperature, (iii) the peak intensity increases with decreasing carrier doping δ down to δ≈0.10 and is substantially suppressed below δ≈0.10 due to strong damping, and (iv) the momentum of the CO decreases monotonically down to δ≈0.10 and goes up below δ≈0.10. These results reasonably capture the major features of the experimental data, and the observed short-range CO can be consistently explained in terms of bond-charge fluctuations with an internal d-wave symmetry.

• Origin of high-energy charge excitations observed by resonant inelastic X-ray scattering in cuprate superconductors

Autores: Andrés Greco, Hiroyuki Yamase y Matías Bejas

Revista: Communications Physics 2, (2019)

Resumen: The recent development of x-ray scattering techniques revealed the charge-excitation spectrum in high-Tc cuprate superconductors. While the presence of a dispersive signal in the high-energy charge-excitation spectrum is well accepted in the electron-doped cuprates, its interpretation and universality are controversial. Since charge fluctuations are observed ubiquitously in cuprate superconductors, the understanding of its origin is a pivotal issue. Here, we employ the layered t − J model with the long-range Coulomb interaction and show that an acoustic-like plasmon mode with a gap at in-plane momentum (0, 0) captures the major features of the high-energy charge excitations. The high-energy charge excitations, therefore, should be a universal feature in cuprate superconductors and are expected also in the hole-doped cuprates. Acoustic-like plasmons in cuprates have not been recognized yet in experiments. We propose several experimental tests to distinguish different interpretations of the high-energy charge excitations.

• Influence of Rashba spin-orbit coupling on the 0-π transition and Kondo temperature in one-dimensional superconductors

Autores: I. J. Hamad, F. T. Lisandrini, C. J. Gazza, A. M. Lobos

Revista: Physical Review B 100, 235110 (2019)

Resumen: Using the framework of the density-matrix renormalization group (DMRG), we study a quantum dot coupled to a superconducting nanowire with strong Rashba spin-orbit coupling. Regarding the singlet-to-doublet "0-π" transition that takes place when the Kondo effect is overcome by the superconducting gap, we show that the Rashba coupling modifies the critical values at which the transition occurs, favoring the doublet phase. In addition, using a generalized Haldane's formula for the Kondo temperature TK, we show that it is lowered by the Rashba coupling. We benchmark our DMRG results, comparing them with previous numerical renormalization group results. The good agreement obtained opens the possibility of studying chains or clusters of impurities coupled to superconductors by means of DMRG.

• Microscopic model for magnetoelectric coupling through lattice distortions

Autores: D. C. Cabra, Ariel O. Dobry, Claudio J. Gazza, G. L. Rossini

Revista: Physical Review B 100, 161111 (2019)

Resumen: We propose a microscopic magnetoelectric model in which the coupling between spins and electric dipoles is mediated by lattice distortions. The magnetic sector is described by a spin S=1/2 Heisenberg model coupled directly to the lattice via a standard spin-Peierls term and indirectly to the electric dipole variables via the distortion of the surrounding electronic clouds. Electric dipoles are described by Ising variables for simplicity. We show that the effective magnetoelectric coupling which arises due to the interconnecting lattice deformations is quite efficient in one-dimensional arrays. More precisely, we show using bosonization and extensive density matrix renormalization group numerical simulations that increasing the magnetic field above the spin-Peierls gap, a massive polarization switch-off occurs due to the proliferation of soliton pairs. We also analyze the effect of an external electric field when the magnetic system is in a gapped (plateau) phase and show that the magnetization can be electrically switched between clearly distinct values. More general quasi-one-dimensional models and two-dimensional systems are also discussed.

• Large-S limit of the large-N theory for the triangular antiferromagnet

Autores: Shang.Shun Zhang, Esteban A. Ghioldi, Yoshitomo Kamiya, Luis O. Manuel, Adolfo E. Trumper, Cristian D. Batista

Revista: Physical Review B 100, 104431 (2019)

Resumen: Large-S and large-N theories (spin value S and spinor component number N) are complementary, and sometimes conflicting, approaches to quantum magnetism. While large-S spin-wave theory captures the correct semiclassical behavior, large-N theories, on the other hand, emphasize the quantumness of spin fluctuations. In order to evaluate the possibility of the non-trivial recovery of the semiclassical magnetic excitations within a large-N approach, we compute the large-S limit ofthe dynamic spin structure of the triangular lattice Heisenberg antiferromagnet within a Schwinger boson spin representation. We demonstrate that, onlyafter the incorporation of Gaussian (1=N) corrections to the saddle-point (N = 1) approximation, we are able to exactly reproduce the linear spin wave theory results in the large-S limit. The key observation is that the eect of 1=N corrections is to cancel out exactly the main contribution of the saddle-point solution; while the collective modes (magnons) consist of two spinon bound states arising from the poles of the RPA propagator. This result implies that it is essential to consider the interaction of the spinons with the emergent gauge fields and that the magnon dispersion relation should not be identied with that of the saddle-point spinons.

• Fully compensated Kondo effect for a two-channel spin S = 1 impurity

Autores: Germán G. Blesio, Luis O. Manuel, Armando A. Aligia, Pablo Roura-Bas

Revista: Physical Review B 100, 75434 (2019)

Resumen: We study the low-temperature properties of the generalized Anderson impurity model in which two localizedconfigurations, one with two doublets and the other with a triplet, are mixed by two degenerate conduction channels. By using the numerical renormalization group and the noncrossing approximation, we analyze the impurity entropy, its spectral density, and the equilibrium conductance for several values of the model parameters.Marked differences with respect to the conventional one-channel spin s = 1/2 Anderson model, which can betraced as hallmarks of an impurity spin S = 1, are found in the Kondo temperature, the width and position of thecharge-transfer peak, and the temperature dependence of the equilibrium conductance. Furthermore, we analyzethe rich effects of a single-ion magnetic anisotropy D on the Kondo behavior. In particular, as shown before,for large enough positive D the system behaves as a "non-Landau" Fermi liquid that cannot be adiabaticallyconnected to a noninteracting system turning off the interactions. For negative D the Kondo effect is stronglysuppressed. While the model is suitable for the description of a single Ni impurity embedded in an O-doped Auchain, it is a generic one for S = 1 and two channels and might be realized in other nanoscopic systems.

• Correlated Partial Disorder in a Weakly Frustrated Quantum Antiferromagnet

Autores: Matías G. Gonzalez, Franco T. Lisandrini, Germán G. Blesio, Adolfo E. Trumper, Claudio J. Gazza, Luis O. Manuel

Revista: Physical Review Letters 122, 017201 (2019)

Resumen: Partial disorder - the microscopic coexistence of long-range magnetic order and disorder - is a rare phenomenon that has been experimentally and theoretically reported in some Ising- or easy plane-spin systems, driven by entropic effects at finite temperatures. Here, we present an analytical and numerical analysis of the S=1/2 Heisenberg antiferromagnet on the 3×3-distorted triangular lattice, which shows that its quantum ground state has partial disorder in the weakly frustrated regime. This state has a 180° Néel ordered honeycomb subsystem coexisting with disordered spins at the hexagon center sites. These central spins are ferromagnetically aligned at short distances, as a consequence of a Casimir-like effect originated by the zero-point quantum fluctuations of the honeycomb lattice.

• Magnetic phase diagram of the infinite- U Hubbard model with nearest- and next-nearest-neighbor hoppings

Autores: Germán G. Blesio, Matías G. Gonzalez, Franco T. Lisandrini

Revista: Physical Review B 99, 174411 (2019)

Resumen: We study the infinite-U Hubbard model on ladders of two, four, and six legs with nearest- (t) and next-nearest- (t′) neighbor hoppings by means of the density-matrix renormalization group algorithm. In particular, we analyze the stability of the Nagaoka state for several values of t′ when we vary the electron density ρ from half filling to the low-density limit. We build the two-dimensional phase diagram, where the fully spin polarized and paramagnetic states prevail. We find that the inclusion of a nonfrustrating next-nearest-neighbor hopping stabilizes the fully spin polarized phase up until |t′/t|=0.5. Surprisingly, for this value of t′, the ground state is fully spin polarized for almost any electron density 1 ρ 0, connecting the Nagaoka state to itinerant ferromagnetism at low density. Also, we find that the previously found checkerboard insulator phase at t′=0 and ρ=0.75 is unstable against t′.

• Orbital-selective bad metals due to Hund's rule and orbital anisotropy: A finite-temperature slave-spin treatment of the two-band Hubbard model

Autores: A. Mezio, R. H. McKenzie

Revista: Physical Review B 100, 205134 (2019)

Resumen: We study the finiteerature properties of the half-filled two-band Hubbard model in the presence of Hund's rule coupling and orbital anisotropy. We use the mean-field treatment of the Z2 slave-spin theory with a finiteerature extension of the zeroerature gauge variable previously developed by Hassan and de' Medici [Phys. Rev. B 81, 035106 (2010)10.1103/PhysRevB.81.035106]. We consider the instability of the Fermi-liquid phases and how it is enhanced by the Hund's rule. We identify paramagnetic solutions that have zero quasiparticle weight with a bad metal, and the first-order transition temperature between the bad metal and the Fermi-liquid phase as a coherence temperature that signals the crossover to the bad metallic state. When orbital anisotropy is present, we found an intermediate transition to an orbital-selective bad metal (OSBM), where the narrow band becomes a bad metal while the wide band remains a renormalized Fermi liquid. The temperatures Tcoh and TOSBM at which the system transitions to the bad metal phases can be orders of magnitude less than the Fermi temperature associated with the noninteracting band. The parameter dependence of the temperature at which the OSBM is destroyed can be understood in terms of a ferromagnetic Kondo-Hubbard lattice model. In general, Hund's rule coupling enhances the bad metallic phases, reduces interorbital charge fluctuations, and increases spin fluctuations. The qualitative difference found in the ground state whether the Hund's rule is present or not, related to the degeneracy of the low-energy manifold, is also maintained for finite temperatures.

• Width of the charge-transfer peak in the SU( N ) impurity Anderson model and its relevance to nonequilibrium transport

Autores: J. Fernández, F. Lisandrini, P. Roura-Bas, C. Gazza, A. Aligia

Revista: Physical Review B 97, 45144 (2018)

Resumen: We calculate the width 2CT and intensity of the charge-transfer peak (the one lying at the on-site energy Ed ) in the impurity spectral density of states as a function of Ed in the SU(N) impurity Anderson model (IAM). We use the dynamical density-matrix renormalization group (DDMRG) and the noncrossing approximation (NCA) for N = 4 and a 1/N variational approximation in the general case. In particular, while for Ed , where is the resonant level half-width, CT = as expected in the noninteracting case, for −Ed N one has CT = N. In the N = 2 case, some effects of the variation of CT with Ed were observed in the conductance through a quantum dot connected asymmetrically to conducting leads at finite bias [J. Könemann et al., Phys. Rev. B 73, 033313 (2006)]. More dramatic effects are expected in similar experiments that can be carried out in systems of two quantum dots, carbon nanotubes or other, realizing the SU(4)IAM.

• Generalized One-Band Model Based on Zhang-Rice Singlets for Tetragonal CuO

Autores: I. J. Hamad, L. O. Manuel, A. A. Aligia

Revista: Physical Review Letters 120, 1770011 (2018)

Resumen: Tetragonal CuO (T-CuO) has attracted attention because of its structure similar to that of the cuprates. Ithas been recently proposed as a compound whose study can give an end to the long debate about the propermicroscopic modeling for cuprates. In this work, we rigorously derive an effective one-band generalizedt − J model for T-CuO, based on orthogonalized Zhang-Rice singlets, and make an estimative calculationof its parameters, based on previous ab initio calculations. By means of the self-consistent Bornapproximation, we then evaluate the spectral function and the quasiparticle dispersion for a single holedoped in antiferromagnetically ordered half filled T-CuO. Our predictions show very good agreement withangle-resolved photoemission spectra and with theoretical multiband results. We conclude that ageneralized t − J model remains the minimal Hamiltonian for a correct description of single-holedynamics in cuprates.

• Dynamical structure factor of the triangular antiferromagnet: Schwinger boson theory beyond mean field

Autores: E. A. Ghioldi, M. G. Gonzalez, Shang-Shun Zhang, Yoshitomo Kamiya, L. O. Manuel. A. E. Trumper, C. D. Batista

Revista: Physical Review B 98, 184403 (2018)

Resumen: We compute the zero-temperature dynamical structure factor S(q,ω) of the triangular lattice Heisenberg model (TLHM) using a Schwinger boson approach that includes the Gaussian fluctuations (1/N corrections) of the saddle-point solution. While the ground state of this model exhibits a well-known 120 -magnetic ordering, experimental observations have revealed a strong quantum character of the excitation spectrum. We conjecture that this phenomenon arises from the proximity of the ground state of the TLHM to the quantum melting point separating the magnetically ordered and spin-liquid states. Within this scenario, magnons are described as collective modes (two-spinon bound states) of a spinon condensate (Higgs phase) that spontaneously break the SU(2) symmetry of the TLHM. Crucial to our results is the proper account of this spontaneous symmetry breaking. The main qualitative difference relative to semiclassical treatments (1/S expansion) is the presence of a high-energy spinon continuum extending up to about three times the single-magnon bandwidth. In addition, the magnitude of the ordered moment (m=0.224) agrees very well with numerical results and the low-energy part of the single-magnon dispersion is in very good agreement with series expansions. Our results indicate that the Schwinger boson approach is an adequate starting point for describing the excitation spectrum of some magnetically ordered compounds that are near the quantum melting point separating this Higgs phase from the deconfined spin-liquid state.

• Topological quantum phase transition between Fermi liquid phases in an Anderson impurity model

Autores: G. G. Blesio, L. O. Manuel, P. Roura-Bas, A. A. Aligia

Revista: Physical Review B 98, 195435 (2018)

Resumen: We study a generalized Anderson model that mixes two localized configurations-one formed by two degenerate doublets and the other by a triplet with single-ion anisotropy DSz2-by means of two degenerate conduction channels. The model has been derived for a single Ni impurity embedded into an O-doped Au chain. Using the numerical renormalization group, we find a topological quantum phase transition, at a finite value Dc, between two regular Fermi liquid phases of high (low) conductance and topological number 2IL/π=0 (+1) for DDc), where IL is the well-known Luttinger integral. At finite temperature the two phases are separated by a non-Fermi liquid phase with fractional impurity entropy 12ln2 and other properties which are similar to those of the two-channel Kondo model.

• Competition between spin-induced charge instabilities in underdoped cuprates

Autores: R. Zeyher, A, Greco

Revista: Physical Review B 98, 224504 (2018)

Resumen: We study the static charge correlation function in a one-band model on a square lattice. The Hamiltonian consists of effective hoppings of the electrons between the lattice sites and the Heisenberg Hamiltonian. Approximating the irreducible charge correlation function by a single bubble yields the ladder approximation for the charge correlation function. In this approximation, one finds, in general, three charge instabilities - two of them are due to nesting, the third one is the flux phase instability. Since these instabilities cannot explain the experiments in hole-doped cuprates, we have included in the irreducible charge correlation function also Aslamasov-Larkin (AL) diagrams where charge fluctuations interact with products of spin fluctuations. We then find at high temperatures a nematic or d-wave Pomeranchuk instability with a very small momentum. Its transition temperature decreases roughly linearly with doping in the underdoped region and vanishes near optimal doping. Decreasing the temperature further, a secondary axial charge-density wave (CDW) instability appears with mainly d-wave symmetry and a wave vector somewhat larger than the distance between nearest-neighbor hot spots. At still lower temperatures, the diagonal flux phase instability emerges. A closer look shows that the AL diagrams enhance mainly axial and not diagonal charge fluctuations in our one-band model. This is the main reason why axial and not diagonal instabilities are the leading ones in agreement with experiment. The two instabilities due to nesting vanish already at very low temperatures and do not play any major role in the phase diagram. Remarkable is that the nematic and the axial CDW instabilities show a large reentrant behavior.

• Mechanism for Unconventional Superconductivity in the Hole-Doped Rashba-Hubbard Model

Autorres: A. Greco, A. P. Schnyder

Revista: Physical Review Letters 120, 177002 (2018)

Resumen: Motivated by the recent resurgence of interest in topological superconductivity, we study superconducting pairing instabilities of the hole-doped Rashba-Hubbard model on the square lattice with first- and second-neighbor hopping. Within the random phase approximation, we compute the spin-fluctuation-mediated pairing interactions as a function of filling. Rashba spin-orbit coupling splits the spin degeneracies of the bands, which leads to two van Hove singularities at two different fillings. We find that, for a doping region in between these two van Hove fillings, the spin fluctuations exhibit a strong ferromagnetic contribution. Because of these ferromagnetic fluctuations, there is a strong tendency towards spin-triplet f-wave pairing within this filling region, resulting in a topologically nontrivial phase. © 2018 American Physical Society.

• Transport properties of Rashba conducting strips coupled to magnetic moments with spiral order

Autor: J. A. Riera

Revista: Physica Review B 97, 035136 (2018)

Resumen: The magnetic and transport properties of a conducting layer with Rashba spin-orbit coupling (RSOC) magnetically coupled to a layer of localized magnetic moments are studied on strips of varying width. The localized moments are free to rotate and they acquire an order that results from the competition between the magnetic exchange energy and the kinetic energy of the conduction electrons. By minimizing the total Hamiltonian within the manifold of variational spiral orders of the magnetic moments, the phase diagram in the space of the interlayer exchange Jsd and the ratio of the Rashba coupling to the hopping integral λ/t was determined. Two main phases with longitudinal spiral order were found, one at large interlayer coupling Jsd with uniform order in the transversal direction, and the other at small Jsd showing a transversal staggered order. This staggered spiral order is unstable against an antiferromagnetic (AFM) one for large values of λ/t. In both spiral phases, the longitudinal spiral momentum departs from the expected linear dependence with the RSOC for large values of λ/t. Then, various transport properties, including the longitudinal Drude weight and the spin Hall conductivity, inside these two phases are computed in linear response, and their behavior is compared with the ones for the more well-studied cases of a fixed ferromagnetic (FM) and AFM localized magnetic orders.

• Resistivity humidity sensors based on hydrogenated amorphous carbon films

Autores: Javier Epeloa, Carlos E. Repetto, Bernardo J Gómez, Lucas Nachez y Ariel Dobry

Revista: Materials Research Express 6(2) 025604 (2019)

Resumen: We have studied the humidity dependence of the electrical properties in hydrogenated amorphous carbon (a-C:H) films. The films were prepared in two stages combining the techniques of physical deposition in vapor phase evaporation (PAVD) and plasma pulsed nitriding. They were deposited over printed circuit boards made of synthetic resin FR2, predesigned to measure DC and AC transport. The Raman spectrum showed a broad peak whose two components are characteristics of a-C:H. By treating the sample in N2-H2 plasma, the impedance became intrinsically sensitive to the relative humidity (RH) of the surrounding media. We observed that when RH increased, the electrical impedance (Z) of the films diminished. The complex impedance spectroscopy method was used to analyze the interaction between the water molecules and the a-C:H film. The main contribution to the humidity dependency of Z came from the resistivity component. The present work provides a fundamental support to develop humidity sensors based on the variation of the impedance of a-C:H films.

• La cuestión ambiental en la Revista de Enseñanza de la Física (2015-2019)

Autores: Hugo D. Navone, Lucas Niell, Manuel Bertoldi, Rodrigo Menchón, Andrea L. Fourty

Revista: Revista de Enseñanza de la Física 32, Nro. Extra (2020)

El tratamiento de la problemática ambiental es requerido en diversos documentos de política educativa, es demandado social-mente y, dada la gravedad de la situación actual del mundo, constituye un urgente desafío para la práctica de la enseñanza. En este trabajo, se presentan los resultados de un estudio de caso único basado en un análisis temático de contenido acerca de cómo se inscribe la cuestión ambiental en el campo de producción de conocimientos en enseñanza de la física. Se toma como representación discursiva de este campo al conjunto de artículos publicados en español en la Revista de Enseñanza de la Física durante el período 2015-2019. Los resultados obtenidos indican que existe un porcentaje relativamente bajo de documentos que abordan la cuestión ambiental, que en varios casos sólo se la menciona, y que su representación discursiva es débil. Sin embargo, también se releva la presencia de algunos artículos que plantean la problemática ambiental en términos de propuestas concretas y que realizan alguna apreciación crítica sobre la misma. De acuerdo con estos resultados, consideramos que es necesario promover y fortalecer un abordaje integral y crítico de esta temática en todas sus dimensiones, construyendo conocimientos y propuestas con particular énfasis en la práctica educativa.

• Reflexión crítica en torno a la inserción de las problemáticas de género en el diseño curricular del Profesorado de Educación Secundaria en Física de la provincia de Santa Fe

Revista: Revista de Enseñanza de la Física 31, Número Extra: Selección de trabajos presentados en REF (2019)

Autores: Lucas Niell, Lucía Koch, Manuel Bertoldi, Rodrigo Menchón, Andrea Fourty, Hugo D. Navone

Las problemáticas de género ingresan fuertemente en el campo educativo a partir de la sanción de la Ley Nacional N° 26.150/06 y su tratamiento es requerido en la formación docente de todas las disciplinas. Esta norma se inscribe en el contexto de las diversas demandas que los movimientos socio-sexuales vienen expresando sostenidamente desde hace muchos años en nuestro país. Teniendo en cuenta todo esto, en este trabajo de reflexión crítica presentamos los resultados de un estudio de caso único basado en un análisis temático de contenido acerca de cómo se inscriben las problemáticas de género en el Diseño Curricular Jurisdiccional del Profesorado de Educación Secundaria en Física de la Provincia de Santa Fe. Los resultados obtenidos indican que estas temáticas se concentran en una unidad curricular especial y que su presencia se diluye en el resto del documento, no estando explícitamente presentes en los espacios que constituyen el Campo de la Formación Específica. Finalmente, se concluye que el diseño analizado es un importante avance en el tratamiento curricular de las problemáticas de género, pero que su ausencia en las unidades del Campo de Formación Específica dificulta el tratamiento integral de las mismas.

• Análisis exploratorio y cualitativo acerca de la presencia de problemáticas de género en la Revista de Enseñanza de la Física en el período 2014-2018

Revista: Revista de Enseñanza de la Física 31, Número Extra: Selección de trabajos presentados en REF (2019)

Autores: Hugo D. Navone, Lucas Niell, Lucía Koch, Manuel Bertoldi, Rodrigo Menchón, Andrea Fourty

En este trabajo presentamos los resultados de un estudio de caso único basado en un análisis temático de contenido acerca de cómo se inscriben las problemáticas de género en el campo de producción de conocimientos en enseñanza de la Física. Se toma como representación de este campo a los documentos publicados en español en la Revista de Enseñanza de la Física durante el período 2014-2018. Los resultados obtenidos indican que existe un bajo porcentaje de presencia de las problemáticas de género en los documentos analizados y que la representación discursiva de las mismas es débil, estando generalmente basada en menciones a la temática. No obstante esto, se releva la presencia promisoria de algunos artículos que abordan el rol de las mujeres en ciencia. En base a estos resultados, teniendo en cuenta que las problemáticas de género ingresan fuertemente en el campo educativo a partir de la sanción de la Ley Nacional N° 26.150/06 y que su tratamiento es requerido en la formación docente de todas las disciplinas, consideramos que, si bien el campo de investigación educativa en enseñanza de la Física comienza a dar cuenta de estas demandas, es necesario promover el abordaje integral de estas temáticas y profundizaren la construcción de conocimientos en todas sus dimensiones.

• Why are we not teaching machine learning at high school? A proposal

Autores: I. Evangelista, G. Blesio, E. Benatti

Revista: 2018 World Engineering Education Forum - Global Engineering Deans Council (WEEF-GEDC) (2018).

Resumen: Although there are many online resources for youngsters to start learning about machine learning on their own, a majority of them require solid mathematical or programming background. In addition, there is a lack of material on how to effectively teach machine learning. This paper is motivated by a survey conducted among young students in a technical high-school to inquire on their interest in learning how computers learn. Given the fact that almost every high-school student has heard about artificial intelligence and is curious about it, this article proposes a way to give a friendly introduction to machine learning in the context of a short workshop. Through a series of problem-based activities, students are expected to understand the foundations of what does 'learning' mean for a computer. In addition, through analogies as well as toy and real problems, this short workshop will tackle students' preconceptions, give them an insight of what tools are important in order to deal with this popular topic and address the ethical issues that arise. Moreover, it is desirable that this stimulates students' interest for STEM degrees. Finally, the activities proposed can be easily adapted for an introductory engineering course.

• Estimate of the location of the neutron drip line for calcium isotopes from an exact Hamiltonian with continuum pair correlations

Autores: A. C. Dassie, R. M. Id Betan

Revista: Physical Review C 102(6), 064301 (2020)

Resumen: Background: The region of the neutron-rich calcium isotopes chain of the nuclei chart is, nowadays, of great activity. The experimental assessment of the limit of stability is of interest to confirm or improve microscopic theoretical models. Purpose: The goal of this work is to provide the drip line of the calcium isotopes from the exact solution of the pairing Hamiltonian which incorporates explicitly the correlations with the continuum spectrum of energy. Method: The modified Richardson equations, which include correlations with the continuum spectrum of energy modeled by the continuum single particle level density, is used to solve the many-body system. Three models are used, two isospin independent models with core Ca40 and Ca48, and one isospin dependent model. Results: One- and two-neutron separation energies and occupation probabilities for bound and continuum states are calculated from the solution of the Richardson equations. Conclusions: The one particle drip line is found at the nucleus Ca57, while the two neutron drip line is found at the nucleus Ca60 from the isospin independent model and at Ca66 from the isospin dependent one.

• White paper: from bound states to the continuum (Topical Review)

Autores: Calvin W Johnson, Kristina D Launey, Naftali Auerbach, Sonia Bacca, Bruce R Barrett, Carl R Brune, Mark A Caprio, Pierre Descouvemont, W H Dickhoff, Charlotte Elster, Patrick J Fasano, Kevin Fossez, Heiko Hergert, Morten Hjorth-Jensen, Linda Hlophe, Baishan Hu, Rodolfo M Id Betan, Andrea Idini, Sebastian König, Konstantinos Kravvaris, Dean Lee, Jin Lei, Alexis Mercenne, Rodrigo Navarro Perez, Witold Nazarewicz, Filomena M Nunes, Marek Płoszajczak, Jimmy Rotureau, Gautam Rupak, Andrey M Shirokov, Ian Thompson, James P Vary, Alexander Volya, Furong Xu, Remco G T. Zegers, Vladimir Zelevinsky and Xilin Zhang

Revista: Journal of Physics G: Nuclear and Particle Physics 47, 123001 (2020)

Resumen: This white paper reports on the discussions of the 2018 Facility for Rare Isotope Beams Theory Alliance (FRIB-TA) topical program ‘From bound states to the continuum: Connecting bound state calculations with scattering and reaction theory’. One of the biggest and most important frontiers in nuclear theory today is to construct better and stronger bridges between bound state calculations and calculations in the continuum, especially scattering and reaction theory, as well as teasing out the influence of the continuum on states near threshold. This is particularly challenging as many-body structure calculations typically use a bound state basis, while reaction calculations more commonly utilize few-body continuum approaches. The many-body bound state and few-body continuum methods use different language and emphasize different properties. To build better foundations for these bridges, we present an overview of several bound state and continuum methods and, where possible, point to current and possible future connections.

• Neutron-pair structure in the continuum spectrum of O26

Autores: S. Affranchino, R. M. Id Betan

Revista: Physical Review C 102(4), 044330 (2020)

Resumen: Background: The structure of O26 is currently being investigated on both theoretical and experimental fronts. It is well established that it is unbound and the resonance parameters are fairly well known. The theoretical analysis may involved two- and three-body interactions as well as correlations with the continuum spectrum of energy. Purpose: In order to properly assess the structure of the ground and excited states, it is imperative to include a large single-particle representation with the right asymptotic behavior. The purpose of this paper is to provide details of the single-particle continuum configurations of the ground and excited 0+ states. Method: We use a large complex energy single-particle basis, formed by resonances and complex energy scattering states, the so-called Berggren basis, and a separable interaction, which is convenient to solve in a large model space. Results: Three 0+ states were found in the complex energy plane. Changes in the resonant parameters, i.e., energy and width, were analyzed as a function of strength of the residual interaction. It is shown how a subtle difference in the interaction could change the unbound character of O26 into a Borromean nucleus. Conclusions: Only one of the two excited states can be considered as a candidate for a physical meaningful resonance. The calculated occupation probabilities are in agreement with other theoretical approaches although the calculated half-life is three orders of magnitude smaller than the experimental one.

• Gamow-shell-model description of Li isotopes and their mirror partners

Autores: X. Mao, J. Rotureau, W. Nazarewicz, N. Michel, R. M. Id Betan, and Y. Jaganathen

Revista: Physical Review C 102(2), 024309 (2020)

Resumen: Background: Weakly bound and unbound nuclei close to particle drip lines are laboratories of new nuclear structure physics at the extremes of neutron/proton excess. The comprehensive description of these systems requires an open quantum system framework that is capable of treating resonant and nonresonant many-body states on equal footing. Purpose: In this work, we develop the complex-energy configuration interaction approach to describe binding energies and spectra of selected 5≤A≤11 nuclei. Method: We employ the complex-energy Gamow shell model (GSM) assuming a rigid He4 core. The effective Hamiltonian, consisting of a core-nucleon Woods-Saxon potential and a simplified version of the Furutani-Horiuchi-Tamagaki interaction with the mass-dependent scaling, is optimized in the sp space. To diagonalize the Hamiltonian matrix, we employ the Davidson method and the Density Matrix Renormalization Group technique. Results: Our optimized GSM Hamiltonian offers a good reproduction of binding energies and spectra with the root-mean-square (rms) deviation from experiment of 160 keV. Since the model performs well when used to predict known excitations that have not been included in the fit, it can serve as a reliable tool to describe poorly known states. A case in point is our prediction for the pair of unbound mirror nuclei Li10-N10 in which a huge Thomas-Ehrman shift dramatically alters the pattern of low-energy excitations. Conclusion: The new model will enable comprehensive studies of structure and reactions aspects of light drip-line nuclei.

• Energy dependence of the optical potential of the weakly bound 9Be projectile on the 197Au target

Autores: F. Gollan, D. Abriola, A. Arazia. M. A.Cardona, E. de Barbará, D. Hojmana, R. M. Id Betan, G. V. Martí, A. J. Pacheco, D. Rodrigues, M. Togneri

Revista: Nuclear Physics A 1000, 121789 (2020)

Resumen: In this work we measured elastic and inelastic angular distributions of the weakly bound 9Be projectile on the 197Au target at several bombarding energies from 84% up to 140% of the Coulomb barrier. The elastic angular distributions were analyzed using a phenomenological Woods-Saxon potential and a double folding São Paulo potential and the energy dependence was extracted. Angular distributions from two inelastic peaks were compared with coupled channel calculations using reduced transition probabilities available in the literature. The energy dependence of the two interaction potential models show a similar trend in the region of the Coulomb barrier. Dispersion relation calculation demonstrates the presence of the breakup threshold anomaly proposed for weakly bound systems.

• An approximation to the Woods–Saxon potential based on a contact interaction

Autores: C. Romaniega, M. Gadella, R. M. Id Betan, L. M. Nieto

Revista: European Physical Journal Plus 135(4), 372 (2020)

Resumen: We study a non-relativistic particle subject to a three-dimensional spherical potential consisting of a finite well and a radial δ- δ′ contact interaction at the well edge. This contact potential is defined by appropriate matching conditions for the radial functions, thereby fixing a self-adjoint extension of the non-singular Hamiltonian. Since this model admits exact solutions for the wave function, we are able to characterize and calculate the number of bound states. We also extend some well-known properties of certain spherically symmetric potentials and describe the resonances, defined as unstable quantum states. Based on the Woods–Saxon potential, this configuration is implemented as a first approximation for a mean-field nuclear model. The results derived are tested with experimental and numerical data in the double magic nuclei 132Sn and 208Pb with an extra neutron.

• Algebraic Gorkov solution in finite systems for the separable pairing interaction

Autores: R. M. Id Betan, C. E. Repetto

Revista: Nuclear Physics A 994, 121676 (2020)

Resumen: An algebraic Quantum Field Theory formulation of separable pairing interaction for spherical finite systems is presented. The Lipkin formulation of the model Hamiltonian and model wave function is used. The Green function technique is applied to obtain the model energy through the spectral function. Closed equation for the many-body energy of the system is given and comparison with exact models are performed.

• The Gamow-state description of the decay energy spectrum of neutron-unbound 25O

Autores: R. M. Id Betan, R. de la Madrid

Revista: Nuclear Physics A 970, 398 (2018)

Resumen: We show the feasibility of calculating the decay energy spectrum of neutron emitting nuclei within the Gamow-state description of resonances by obtaining the decay energy spectrum of 25O. We model this nucleus as a valence neutron interacting with an 24O inert core, and we obtain the resulting resonant energies, widths and decay energy spectra for the ground and first excited states. We also discuss the similarities and differences between the decay energy spectrum of a Gamow state and the Breit?Wigner distribution with energy-dependent width.

• Shadow poles in coupled-channel problems calculated with the Berggren basis

Autores: R. M. Id Betan, A. T. Kruppa, T. Verste

Revista: Physical Review C 97, 024307 (2018)

Resumen: Background: In coupled-channels models the poles of the scattering S matrix are located on different Riemann sheets. Physical observables are affected mainly by poles closest to the physical region but sometimes shadow poles have considerable effect too. Purpose: The purpose of this paper is to show that in coupled-channels problems all poles of the S matrix can be located by an expansion in terms of a properly constructed complex-energy basis. Method: The Berggren basis is used for expanding the coupled-channels solutions. Results: The locations of the poles of the S matrix for the Cox potential, constructed for coupled-channels problems, were numerically calculated and compared with the exact ones. In a nuclear physics application the Jπ=3/2+ resonant poles of He5 were calculated in a phenomenological two-channel model. The properties of both the normal and shadow resonances agree with previous findings. Conclusions: We have shown that, with an appropriately chosen Berggren basis, all poles of the S matrix including the shadow poles can be determined. We have found that the shadow pole of He5 migrates between Riemann sheets if the coupling strength is varied.

• Mechanical properties and microstructural aspects of two high-manganese steels with twip/trip effects: A comparative study

Autores: M. Bordone, J. Perez-Ipiña, R. Bolmaro, A. Artigas, A. Monsalve

Revista: Metals 11(1), 1 (2021)

Resumen: This article is focused on the mechanical behavior and its relationship with the microstruc-tural changes observed in two high-manganese steels presenting twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP), namely Steel B and Steel C, respectively. Chemical compositions were similar in manganese, but carbon content of Steel B approximately doubles Steel C, which directly impacted on the stacking fault energy (SFE), microstructure and mechanical response of each alloy. Characterization of as-cast condition by optical microscope revealed a fully austenitic microstructure in Steel B and a mixed microstructure in Steel C consisting of austenite grains and thermal-induced (εt ) martensite platelets. Same phases were observed after the thermo-mechanical treatment and tensile tests, corroborated by means of X-Ray Diffraction (XRD), which confirms no phase transformation in Steel B and TRIP effect in Steel C, due to the strain-induced γFCC→εHCP transformation that results in an increase in the ε-martensite volume fraction. Higher values of ultimate tensile strength, yield stress, ductility and impact toughness were obtained for Steel B. Significant microstructural changes were revealed in tensile specimens as a consequence of the operating hardening mechanisms. Scanning Electron Microscopy (SEM) observations on the tensile and impact test specimens showed differences in fracture micro-mechanisms.

• Heat treatment effect on an AA6063 alloy

Autores: J. Alberto Muñoz, A. Komissarov, M. Avalos, R. E. Bolmaro

Revista: Materials Letters 277, 128338 (2020)

Resumen: The aging treatment effect on an AA6063 aluminum alloy was studied by characterizing the microstructure and mechanical properties. The aging impact increased the alloy strength from 100 MPa in the solubilization state to 200 MPa in the aged state. The aging treatment effect, in addition to the formation of precipitates, gave rise to a high number of geometrically necessary dislocations at (GND), and different concentrations of Si and Mg inside some grains. The early fracture of the aged material concerning the solubilization condition was demonstrated by the multiple formations of strain peaks and a high dislocation annihilation rate. The most significant strength contributions to the aged alloy came from precipitates, followed by the solid solution, dislocations, and grain size.

• Mechanical and microstructural behavior of a heterogeneous austenitic stainless steel processed by Equal Channel Angular Sheet Extrusion (ECASE)

Autores: Jairo Alberto Muñoz, Alexander Komissarov, Martina Avalos, Raúl E. Bolmaro

Revista: Materials Science and Engineering A 792, 139779 (2020)

Resumen: A sheet-shaped austenitic stainless steel (ASS) was processed by one ECASE pass. In this way, larger deformations were introduced close to the sheet edge vicinities, while the middle zone remained less affected by the deformation. The heterogeneous deformation produced larger amounts of deformation induced martensite (DIM) at the sheet edges, resulting in a heterogeneous structure along the sheet thickness. Tensile tests revealed that after the deformation process the material increased its yield strength more than 3 times from 200 MPa in its initial state to ~690 MPa after one ECASE pass. Tensile tests for the processed material in different zones of sheet thickness revealed the existence of two types of materials, one more resistant and less ductile (near the edges) and other more ductile, but less resistant (in the middle zone). Because of the heterogeneity, higher dislocation densities were found in the edges than in the middle of the sheet, giving rise to a plastic deformation gradient in which the region near the edge is capable of withstanding greater plastic deformations by generating a plastic instability as soon as the deformation starts. Through a rule of mixtures, it was found that the greatest strength contribution of the heterogeneous material came from the dislocations on the bcc martensite developed near the sheet edges.

• Microstructure, texture and interface integrity in sheets processed by Asymmetric Accumulative Roll-Bonding

Autores: Renan Pereira de Godoi, Danielle Cristina Camilo Magalhães, Martina Avalos, Raul Eduardo Bolmaro, Vitor Luiz Sordi, Andrea Madeira Kliauga

Revista: Materials Science and Engineering A 771, 138634 (2020)

Resumen: Accumulative Roll-Bonding (ARB) and Asymmetric Rolling (AR) techniques were combined to produce ultrafine-grained aluminum sheets with the mechanical characteristics of a Severe Plastic Deformation (SPD) process. Temperature and number of bonding cycles were varied to promote grain refinement, texture randomization and high-quality sheet bonding. Finite element simulation for a single pass was performed to clarify the strain distribution differences between symmetric and asymmetric roll -bonding. The microstructure and crystallographic texture were measured by Electron Backscatter Diffraction (EBSD) and X-ray diffraction. Hardness and tensile tests characterized strain distribution and bonding efficiency. A fine grain structure with a mean grain size of 1.0 μm was achieved at 350 °C, whereas a coarser grain structure was obtained at 400 °C. The grain size and shape distribution were linked to enhancing the mechanical strength in a transversal direction. During repeated bonding cycles at both temperatures, extra shear in the interfacial region yielded favorable homogeneous strain distribution and a weak shear texture across the sheet. Rotated-cube orientation was the strongest component in both processing temperatures. To increase the interfacial strength, mainly on the last bond interface, an extra 50% reduction step was added. This improved the adhesion in the last bonding interface, and thus enhanced the ductility. These findings helped to provide a basis for determining the processing conditions for aluminum alloys.

• Thermal analysis techniques applied to study corrosion mechanisms of SEN used in the steel continuous casting process

Autores: Elena Brandaleze, M. Ávalos, Edgardo Benavidez

Revista: Journal of Thermal Analysis and Calorimetry (2020)

Resumen: Recent advances in the technology of black refractories for steel continuous casting promote the development of materials that operate at extreme conditions. The effects of different damage mechanisms are critical, and it is necessary to provide detailed information about the thermal behaviour of the nozzle–mould flux system, at steel continuous casting process condition. Mould fluxes present a chemical composition that includes a complex mix of oxides, fluorite (CaF2) and carbonaceous materials (coke, graphite, etc.). Their physical properties [viscosity (η) and surface tension (γ)] are strongly dependent on chemical composition and temperature evolution at process conditions. For this reason, a complete characterization and deep knowledge of the thermal behaviour of the mould flux is the key to understand in detail the interaction phenomena present in the system (insert nozzle–mould flux), in order to identify the corrosion mechanisms. In these circumstances, the use of thermal analysis techniques (DTA-TG and dilatometry) can provide relevant information about nozzles and mould fluxes such as phase transformations, mass changes and volumetric variations, which occur in the system of study. In this paper, these results are correlated with a microstructural study of two samples: S1 (as received) and S2 (post-mortem) applying different microscopy techniques, X-ray diffraction and thermal simulation of the nozzle corrosion by FactSage 7.2 in order to identify the wear mechanisms and to obtain relevant information for the refractory and steelmaking industry.

• Prediction of Generation of High- and Low-Angle Grain Boundaries (HAGB and LAGB) During Severe Plastic Deformation

Autores: J. A. Muñoz, R. E. Bolmaro, A. M. Jorge, A. Zhilyaev, J. M. Cabrera

Revista: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 51(9), 4674 (2020)

Resumen: It is well known that a balance between the generation of low-angle and high-angle grain boundaries (LABG and HAGB) is achieved in materials undergoing a severe plastic deformation (SPD) process. It is also observed that most annealed materials evolve from a substantial fraction of LAGB at the early deformation steps towards a steady state in which an equilibrium between LAGB and HAGB fractions is attained. In the present work, such a balance is analyzed for different alloys, i.e., Cu, Al, and Fe based. A mathematical expression is proposed for the amount of LAGB and HAGB as a function of the strain impaired which in turn can be used to predict the number of passes (especially for ECAP) or the amount of total deformation for any SPD process necessary to attain a steady microstructure. The model seeks to serve as a first approximation of the mechanical and microstructural properties of ultrafine grain metal materials.

• Effects of laser shock processing and shot peening on 253 MA austenitic stainless steel and their consequences on fatigue properties

Autores: L. Spadaro, S. Hereñú, R. Strubbia, G. Gómez Rosas, R. Bolmaro, C. Rubio González

Revista: Optics and Laser Technology 122, 105892 (2020)

Resumen: A comparative study of the influence of laser shock peening without coating (LSPwC) and shot peening (SP) treatments on the low cycle fatigue life of 253 MA austenitic stainless steel was investigated. LSPwC treatments were performed with 2500 pulse/cm2 and 5000 pulse/cm2. To evaluate the effects of each treatment, residual stress, roughness, hardness, and microstructure were analyzed. It is of particular interest the correlation of the fatigue performance with the dislocation microstructure evolution. The results reveal that all the surface treatments increase the surface hardness, induce compressive residual stresses (CRS) and a high density of mechanical twins and dislocations. The stability of the workhardened near surface microstructure results in the improvement of the fatigue life of peened 253 MA. Moreover, the best improvement of fatigue life reached for SP is mainly due to the highest number of twins and the more superficial CRS.

• Protrusions formed during primary recrystallization of cross-rolled coarse-grained niobium

Autores: T. Brekailo, D. R. Almeida, Martina Ávalos, Raúl E. Bolmaro y H. R. Sandim

Revista: International Journal of Refractory Metals and Hard Materials 85, 105080 (2019)

Resumen: The growth of recrystallization nuclei towards the deformed matrix generally does not proceed isotropically. Local disturbances in the migration along grain boundaries lead to the formation of protrusions. This important aspect in recrystallization is rarely explored in the literature, especially in the sense of understanding which microstructural mechanisms explain its formation. The aim of this work is to study the formation of protrusions during the recrystallization of a niobium oligocrystal. A coarse-grained niobium plate was cut out from the longitudinal section of an electron-beam cast ingot. Five coarse grains (A, B, C, D and E) were randomly selected for this study to evaluate likely orientation effects on protrusion formation. The plate was cold rolled with several passes up to 50% thickness reduction. The sheet was then cut into two parts. The former was straight rolled until 70% thickness reduction while the latter was rotated 90° around the normal direction with respect to the initial rolling direction. Important orientation and rolling mode effects were noticed regarding their recrystallization behavior. Protrusions were more abundant in the cross-rolled specimens. It was not possible to identify and associate significant differences regarding either CSL-type special boundaries or highly-misoriented boundaries ahead of the boundaries showing protrusions.

• Investigation of cast and annealed Ti25Nb10Zr alloy as material for orthopedic devices

Autores: R. Bolmaro, A. C. Parau, V. Pruna, M. A: Surmeneva, L. R. Constantin, M. Avalos, C. M. Cotrut, R. Tutuianu, M. Braic, D. V. Cojocaru, I. Dan, S. Croitoru, R. A. Surmenev, A. Vladescu

Revista: Journal of Materials Research and Technology 8(4), 3399 (2019)

Resumen: In the present work, we report the preparation of a novel titanium-based alloy, namely Ti25Nb10Zr, by cold crucible levitation melting technique. The cast alloy consists of a complex microstructure with large Beta phase grains (54%, 50-150 μm) with a regularly connected net of Alpha′ (orthorhombic, 46%) phase running along boundaries and across the grains and keeping a regular misorientation with respect to the Beta phase. An intermeshed 51% Alpha and 49% Beta phases with lamellar microstructure were found by annealing. The electrochemical tests showed that both alloys were affected by the corrosion process. A good corrosion resistance in SBF at 37 °C was found for the cast form. The cast alloy is more resistant when immersed into solutions with pH2 and pH7, while the annealed one is resistant in pH5 solution. Surface potential of both alloys is negative, with the annealing process leading to a slight decrease of that property. Collectively, the biological results indicate a more favorable viability on cast form as compared to annealed one, suggesting that the cast alloy is promising for biomedical applications.

• Equal channel angular sheet extrusion (ECASE) as a precursor of heterogeneity in an AA6063-T6 alloy

Autores: J. A. Muñoz, O. F. Higuera, V. Tartalini, P. Risso, M. Ávalos, R. E. Bolmaro

Revista: International Journal of Advanced Manufacturing Technology 102 (9), 3459 (2019)

Resumen: We study the deformation inducing heterogeneity in an aluminum alloy 6063-T6 in the form of a sheet processed at room temperature by equal channel angular sheet extrusion (ECASE) up to a maximum equivalent strain of 1.86 following route C. The through thickness strain distribution showed higher strains in the edge vicinities than in the sheet core. The texture was heterogeneous between the edges and the sheet core with a strong cube component in the initial deformation stages, and a rolling texture with the S component in the sheet edges. Different microstructural characteristics, like grain size, average misorientation, and fraction of high angle grain boundaries (HAGB), decreased by increasing the deformation. The geometrically necessary dislocation (GND) calculations corroborated the existence of a heterogeneous microstructure along the sheet thickness, giving rise to gradients of plastic deformation which allowed to obtain a good strength-ductility relationship. It was demonstrated that ECASE process was a good alternative to produce heterogeneous microstructures. The material heterogeneity was found not to be randomly distributed across the sheet thickness but rather showing higher dislocation concentration and bigger grain size reductions in the edge’s vicinities than in its middle zone.

• Inducing heterogeneity in an austenitic stainless steel by equal channel angular sheet extrusion

Autores: J. A. Muñoz, O. F. Cobos, R. M'Doihoma, M Ávalos, R. E. Bolmaro

Revista: Journal of Materials Research and Technology 8 (2), 2473 (2019)

Resumen: An austenitic stainless steel was processed at room temperature by equal channel angular sheet extrusion (ECASE). The microstructure evolution was heterogeneous across the sheet thickness with a higher martensitic transformation in the zones closer to the edges than in the sheet core due to the deformation-induced martensite (DIM) phenomena. The material heterogeneity induced by one ECASE pass gave a good strength-ductility relationship.

• Analysis of the micro and substructural evolution during severe plastic deformation of ARMCO iron and consequences in mechanical properties

Autores: Jairo Alberto Muñoz, Oscar Fabián Higuera, José Antonio Benito, Djamel Bradai, Tarek Khelfa, Raúl E. Bolmaro, Alberto Moreira Jorge y José María Cabrera

Revista: Materials Science and Engineering A-Structural Materials Properties Microstructure and Processing 740, 108 (2019)

Resumen: ARMCO iron processed by ECAP up to a maximum equivalent strain of sixteen ECAP passes following route Bc was investigated by Electron Back Scattering Diffraction (EBSD), Transmission Electron Microscopy (TEM), TEM-EBSD and X-ray Diffraction Line Profile Analysis (XRDLPA). Different values of grain size were obtained with each technique. The lower values were obtained by XRDLPA whereas, the higher values were achieved by TEM-EBSD. Dislocation densities were also calculated by different techniques obtaining values of statistically stored and geometrically necessary dislocations as a function of the deformation. All differences in values allowed a better understanding of the nature of microstructures of the materials processed by severe plastic deformation (SPD). Mechanical properties showed a considerable increment in the material strength due to the progressive reduction of the grain size because of the Continuous Dynamic Recrystallization (CDRX) mechanism and the increment in the dislocation density.

• An ultra-fast annealing treatment by electropulsing during pure copper wire drawing

Autores: H. A. G. Rojas, A. J. S. Egea, S. Hameed, R. Bolmaro

Revista: Metals 9 (12), 1253 (2019)

Resumen: The influence of electropulses in situ on the drawing process of copper wires was investigated, with the aim of avoiding time consuming annealing thermal treatments. It was found that with the application of electropulses, tensile and drawing forces were reduced and the plasticity of the wire was improved. Meanwhile, compared with conventional drawing, hardness decreased. With the electropulsing treatment, no differences were found in the material’s resistivity. The microstructure exhibited similar grain size despite the joule effect, although some changes were found in the low and high angle grain boundaries. Compressive residual stresses were found on the surface during the assisted process; consequently, these specimens were expected to have a better performance under fatigue fracture. Therefore, we have found the first evidence of the advantages of this hybrid technique for pure copper wire drawing and, ultimately, to replace the conventional drawing annealing process with a more time effective electropulse-assisted drawing process.

• EBSD analysis of orientation gradients developed near grain boundaries

Autores: N. S. De Vincentis, A. Roatta, R. E. Bolmaro, J. W. Signorelli

Revista: Materials Research 22(1), e20180412 (2019)

Resumen: The local misorientation and orientation gradient development near grain boundaries (GBs) are analyzed in a deep drawing quality steel sheet (AKDQ) subjected to interrupted tensile tests in a notched sample. The microstructure is studied using electron backscatter diffraction (EBSD) with subgrain-level spatial resolution. The evolution of misorientation accumulation for particular GBs was traced in grains located in different zones inside the notch, identifying the effective area of influence of GBs inside the neighboring grains. A local study was performed, and the evolution in misorientation development near GB was investigated. The results show a low correlation between GB width and sharpness of the orientation gradient with the mesoscopic strain, but instead orientation gradients between GB zones and the interior of the grains were observed with increasing strain. The increase in severity observed in some GBs can be related to dislocation pile up development, which reduces the permeability of a boundary to dislocation transmission.

• Study of recovery and first recrystallisation kinetics in CGO Fe3%Si steels using misorientation-derived parameters

Autores: F. Cruz-Gandarilla, R. E. Bolmaro, H. F. Mendoza-León, A. M. Salcedo-Garrido, J. G. Cabañas-Moreno

Revista: Journal of Microscopy 275 (3), 133 (2019)

Resumen: Many metallurgical processes produce characteristic dislocation accumulation, with heterogeneous spatial and orientation distributions and further development of microstructures after heat treatment. Recovery and recrystallisation behaviours are direct consequences of those uneven dislocation distributions. The Electron BackScatter Diffraction (EBSD) technique can be used for the characterisation of such microstructural features, including: Density of Geometrically Necessary Dislocations (GND), Kernel Average Misorientations (KAM), Grain Orientation Spread (GOS), Grain Average Misorientation (GAM), Grain Reference Orientation Deviation (GROD – Angle) and GOS/D, where D is an assumed characteristic grain length. Production of Fe3%Si alloys with a Goss texture, essential step in the manufacture of electrical transformers, requires several different processing stages, including the one called primary recrystallisation, a key process preceding abnormal grain growth. The structure of grains and different microstructural aspects of the recrystallisation stage will provide the conditions for development of the Goss orientation during abnormal grain growth. In the present work we use GOS, GAM, GROD, GOS/D, GND and KAM, calculated from EBSD scans performed on cold rolled Fe3%Si alloys subject to increasing heat treatment times, to characterise the kinetics of recovery and primary recrystallisation in an Fe3%Si alloy. Difficulties in the interpretation of these results may arise from the interactive competition between various microstructural features. Hardness measurements were also performed in order to validate recovery and recrystallisation evolution by classical methods. It was found that the global GOS (i.e. including grains of all orientations) shows changes which can be related to those observed in the hardness for high annealing temperatures but it is not sensitive to microstructure evolution occurring at low temperatures. Meanwhile, GND undergoes changes at all annealing temperatures and, remarkably, it responds to the recovery that GOS cannot detect at low temperatures. The GAM parameter seems to follow better the microhardness results. When grains belonging to different texture components are analysed, gamma fibre grains are the first to recrystallise and alpha fibre grains the last. Lay Description: Many metallurgical processes produce characteristic dislocation accumulation, with heterogeneous spatial and orientation distributions. Further development of such microstructures occurs with subsequent heat treatments. Recovery and recrystallisation behaviours are directly affected by consequences of those uneven dislocation distributions. The Electron Back Scatter Diffraction (EBSD) technique can be used for the characterisation of such microstructural features using different magnitudes that describe locally or globally misorientations between various locations in the material. In search of the best parameters [among them: Density of Geometrically Necessary Dislocations (GND), Kernel Average Misorientations (KAM), Grain Orientation Spread (GOS), Grain Average Misorientation (GAM), Grain Reference Orientation Deviation (GROD – Angle) and GOS/D, where D is an assumed characteristic grain length], we characterised the kinetics of the recovery during the 1st recrystallisation in an Fe3%Si alloy. It was found that the global GOS (i.e. including grains of all orientations) shows changes that can be related to the advance of recrystallisation on the other hand, the GND (KAM, GAM etc.) parameter seems to better follow the progress of recovery phenomenon. When grains belonging to different texture components are analysed, gamma fibre grains are the first to recrystallise and alpha fibre grains the last.

• Heterogeneity of strain path, texture and microstructure evolution of AA6063-T6 processed by Equal Channel Angular Sheet Extrusion (ECASE)

Autores: Jairo Alberto Muñoz, Martina Avalos, Raúl E.Bolmaro

Revista: Journal of Alloys and Compounds 768, 349 (2018)

Resumen: An Aluminum alloy 6063T6 was processed at room temperature by Equal Channel Angular Sheet Extrusion (ECASE). Microstructure evolution was investigated by Electron Back Scattering Diffraction (EBSD), and X-ray diffraction analysis. The texture evolution was heterogeneous and represented by a mix of rolling and shearing components through the sheet thickness. Ncorr™ calculations showed higher shear strains close to the edges than in the sheet core, in agreement with the through thickness expected variation. The Geometrically Necessary Dislocation (GND) calculations corroborated the existence of a heterogeneous microstructure through the sheet thickness.

• Texture, microstructure, and surface mechanical properties of AZ31 magnesium alloys processed by ECASD

Autores: E. Mendoza Fandiño, R. E., Bolmaro, P Risso, P Fernández Morales, V. Tartalini, M. Ávalos

Revista: Advanced Engineering Materials 20(4), 1700228 (2018)

Resumen: The current work presents the results on Mg AZ31B alloy sheets subject to four passes using Equal Channel Angular Sheet Drawing (ECASD) at various temperatures (25, 100, and 200°C). Textures are determined by laboratory X-ray diffraction and EBSD. EBSD allows the evaluation of the evolution of crystal sizes in function of the distance to the surface and the presence of twinning. Twinning is evident by the analysis of the textures, which show mainly two components, one due to the spin induced by the shearing of ECASD and the other one as a direct product of twinning. Micro-hardness, by Knoop tests on the lateral face of the sheets, are performed, allowing the determination of the influence of SPD on the hardness from surface to surface, going through the center of the sheet. Almost 50% increase on hardening, with respect to the non-processed material, is obtained near to the surface after four and six passes. The effectiveness as a hardening technique declined after the first four passes.

• Thermal stability of ARMCO iron processed by ECAP

Autores:Jairo Alberto Muñoz, Oscar Fabián Higuera, Ana Hernández Expósito, Ahmed Boulaajaj, Raúl E. Bolmaro, Florina D. Dumitru, Pablo Rodriguez Calvillo, Alberto Moreira Jorge Jr, José María Cabrera

Revista: International Journal of Advanced Manufacturing Technology 98(9-12), 2917 (2018)

Resumen: The thermal stability of ARMCO iron processed by equal-channel angular pressing (ECAP) up to a true strain of 16 was investigated by differential scanning calorimetry (DSC). Particularly, the analysis was focused on deriving the recrystallization behavior (onset, peak, and offset temperatures) at three different heating rates (10, 20, and 40 °C/min). Additionally, the stored and activation energies were calculated. As well, the microstructure and the tensile response were evaluated at different numbers of passes before and after annealing heat treatment to assess any significant grain growth and its influence on the material ductility. The different energy contributions (dislocations, grain boundaries, and vacancies) were calculated, being verified that the main contribution came from vacancies.

• Severe plastic deformation by equal channel angular pressing and rolling: The influence of the deformation path on strain distribution

Autores: A. M. Kliauga, V. L. Sordi, N. S. De Vincentis, R. E. Bolmaro, N. Schell, H. -G. Brokmeier

Revista: Advanced Engineering Materials 20 (4), 1700055 (2018)

Resumen: The present work compares two deformation techniques, rolling and Equal Channel Angular pressing (ECAP), and the response offered by three different materials that differ in Stacking Fault Energy (SFE): AA1010 Al, commercially pure Cu, and an austenitic stainless steel. The objective of this investigation is to study the effect of each deformation mode on tensile behavior, deformation mechanism, texture, and microstructure and to establish the influence of the stacking fault energy on said effects. The results show that the different strain paths of ECAP and rolling do not affect the strength, but rolling leads to an accentuated texture and thus to elastic and plastic anisotropy. This finding has practical relevance for micro manufacturing techniques. Furthermore, it is observed that lower SFE results in smaller domain size and higher dislocation density, which are microstructural details related to strength and to the work hardening capacity. Finally, both techniques are able to produce a high amount of high angle grain boundaries, a feature that characterizes refined microstructures. These processes operate at different strain rates; thus, in low SFE materials, a more effective grain fragmentation by deformation-induced twins is observed after the ECAP process.

• Dynamic recrystallization during warm accumulative asymmetric roll bonding (Aarb) of the aa1050 aluminium

Autores: A. M . Kliauga, R. P. de Godoi, V. L. Sordi, R. E. Bolmaro

Revista: Materials Science Forum 941 MSF, 1342 (2018) 10th International Conference on Processing and Manufacturing of Advanced Materials, Paris, France.

Resumen: In this work asymmetric accumulative roll bonding (AARB) was applied to a AA1050 aluminum up to ten cycles at 350 and 400oC. The texture was measured by x ray diffraction and EBSD. Hardness and tensile tests characterized the strain distribution and bonding efficiency. At 350oC the microstructural refinement was stabilized after four cycles and mean grain sizes of one micron and a saturation yield strength of 160 MPa was achieved. At 400oC grain growth took place yielding a bimodal microstructure with mean grain size of 9 microns. During repeated bonding cycles recovery and dynamic recrystallization were observed and extra shear in the interfacial region yielded a fairly well homogeneous strain distribution and weak shear texture across the sheet for both temperatures. The strongest component in both cases was the rotated cube orientation. The last bonding surface was the weakest bond but adding an extra 50% reduction step to the process increased the interfacial strength considerably.

• Normal and off-normal incidence dissociative dynamics of O2( v, J) on ultrathin Cu films grown on Ru(0001)

Autores: J. G. Fallaque, M. Ramos, H. F. Busnengo, F. Martín, C. Díaz

Revista: Physical Chemistry Chemical Physics (2020)

Resumen: The dissociative adsorption of molecular oxygen on metal surfaces has long been controversial, mostly due to the spin-triplet nature of its ground state, to possible non-adiabatic effects, such as an abrupt charge transfer from the metal to the molecule, or even to the role played by the surface electronic state. Here, we have studied the dissociative adsorption of O2 on CuML/Ru(0001) at normal and off-normal incidence, from thermal to super-thermal energies, using quasi-classical dynamics, in the framework of the generalized Langevin oscillator model, and density functional theory based on a multidimensional potential energy surface. Our simulations reveal a rather intriguing behavior of dissociative adsorption probabilities, which exhibit normal energy scaling at incidence energies below the reaction barriers and total energy scaling above, irrespective of the reaction channel, either direct dissociation, trapping dissociation, or molecular adsorption. We directly compare our results with existing scanning tunneling spectroscopy and microscopy measurements. From this comparison, we infer that the observed experimental behavior at thermal energies may be due to ligand and strain effects, as already found for super-thermal incidence energies.

• Non-Thermalized Precursor-Mediated Dissociative Chemisorption at High Catalysis Temperatures.

Autores: R. Moiraghi, A. Lozano, E. Peterson, A. L. Utz, W. Dong, H. F. Busnengo

Revista: The Journal of Physical Chemistry Letters 11(6), 2211 (2020)

Quasi-classical trajectory calculations and vibrational-state-selected beam-surface measurements of CH$_4$ chemisorption on Ir(111) reveal a non-thermal, hot-molecule mechanism for C-H bond activation. Low-energy vibrationally excited molecules trap in the physisorption well, and react before vibrational and translational energies accommodate with the surface. Reaction probability is strongly surface-temperature dependent and arises from the pivotal role of Ir atom thermal motion. In reactive trajectories, the mean outward Ir atom displacement largely exceeds that of the transition state geometry obtained through a full geometry optimization. The study also highlights a new way for (temporary) surface defects to impact high-temperature heterogeneous catalytic reactivity. Instead of reactants diffusing to and competing for geometrically localized lower barrier sites; transient, thermally activated surface atom displacements deliver low-barrier surface reaction geometries to the physisorbed reactants.

• Site-Specific Product Selectivity of Stepped Pt Surfaces for Methane Dehydrogenation

Autores: M. E. Torio and H. F. Busnengo

Revista: Journal of Physical Chemistry C 124 (36), 19649 (2020)

Resumen: Through density functional theory (DFT) calculations, we show that, in contrast to terrace sites of Pt(211) and Pt(111), on steplike sites of Pt(211) and Pt(110)-(2 × 1), methylene is more stable than methyl. The resulting site-dependent product selectivity for methane dissociation allows us to conciliate between theory and recent reflection absorption infrared spectroscopy (RAIRS) experiments, which might open a door to new applications of vicinal surfaces of Pt in heterogeneous catalysis and as templates for nanoscale patterning of self-assembled monolayers.

• Enhancing Hydrogen Evolution Activity of Au(111) in Alkaline Media through Molecular Engineering of a 2D Polymer

Autores: P. Alexa, J. M. Lombardi, P. Abufager, H. F. Busnengo, D. Grumelli, V. Vyas, Frederik Haase, B. Lotsch, R. Gutzler, K. Kern

Revista: Angewandte Chemie (International Ed. in English) 59(22), 8411 (2020)

Resumen: The electrochemical splitting of water holds promise for the storage of energy produced intermittently by renewable energy sources. The evolution of hydrogen currently relies on the use of platinum as a catalyst—which is scarce and expensive—and ongoing research is focused towards finding cheaper alternatives. In this context, 2D polymers grown as single layers on surfaces have emerged as porous materials with tunable chemical and electronic structures that can be used for improving the catalytic activity of metal surfaces. Here, we use designed organic molecules to fabricate covalent 2D architectures by an Ullmann‐type coupling reaction on Au(111). The polymer‐patterned gold electrode exhibits a hydrogen evolution reaction activity up to three times higher than that of bare gold. Through rational design of the polymer on the molecular level we engineered hydrogen evolution activity by an approach that can be easily extended to other electrocatalytic reactions.

• Molecular Dynamics Study of Molecular and Dissociative Adsorption Using System-Specific Force Fields Based on Ab Initio Calculations: CO/Cu(110) and CH 4/ Pt(110)

Autores: Giulia Seminara, Iván Peludhero, W. Dong, Alejandra Martínez, H. Fabio Busnengo

Revista: Topics in Catalysis 62 (12-16), 1044 (2019)

Resumen: We present results of quasi-classical trajectory (QCT) calculations of the molecular and dissociative sticking probability of CO on Cu(110), and of CH 4 on Pt(110)-(2 × 1) respectively. Our QCT calculations make use of reactive force fields (RFF) specifically developed for the systems/processes of interest by fitting of a large set of Density Functional Theory (DFT) total energy data. Through these two selected examples of great importance for a deep understanding of relevant chemical reactions on metal surfaces, we illustrate the potentiality of QCT calculations based on accurate RFFs built on ab initio data.

• Site Selective Detection of Methane Dissociation on Stepped Pt Surfaces

Autores: A. Gutiérrez-González, M. E. Torio, H. F. Busnengo, R. D. Beck

Revista: Topics in Catalysis 62 (12-16), 859 (2019)

Resumen: We report a combined experimental and theoretical study comparing methane dissociation on three different platinum surfaces Pt(111), Pt(211), and Pt(110)-(1 × 2). Reflection absorption infrared spectroscopy (RAIRS) was used to detect chemisorbed methyl species formed by dissociative chemisorption of CH4 on specific surface sites and to measure surface-site-specific sticking coefficients of CH4 on the terrace, step, and ridge sites as function of incident translational energy. Methane dissociation is observed to be direct on all sites and diffusion of the chemisorbed methyl species is absent for surface temperature below 150 K. The experimental data are compared with the results of density functional (DFT) calculations that give minimum energy barriers for CH4 chemisorption that properly account for the experimental relative site-specific reactivities. Also in agreement with experiments, DFT results predict a negligible effect of co-adsorbed H and CH3 species on the vibrational frequency of a methyl group chemisorbed on terrace and step sites of Pt(211). However, the origin of the red-shift of the RAIRS peak of CH3 chemisorbed on terrace sites compared with that on step sites of Pt(211) remains elusive and still demands further investigation.

• Energy Dissipation Effects on the Adsorption Dynamics of N 2 on W(100)

Autores: A. Peña-Torres, H. F. Busnengo, J. I. Juaristi, P. Larregaray, C. Crespos

Revista: Journal of Physical Chemistry C 123 (5), 2900 (2019)

Resumen: Adsorption dynamics of N 2 on the W(100) surface is studied by means of quasi-classical trajectories making use of a six-dimensional potential energy surface obtained from density functional theory calculations. In our simulations, van der Waals (vdW) interactions are accounted for by using the vdW-DF2 functional. In view of the comparison with experiments, we show that this leads to a good description of the adsorption dynamics, providing a significant improvement with respect to semi-local exchange-correlation functionals used in the past. Particular emphasis is placed on the description of nonactivated pathways, leading to either dissociation or molecular adsorption. Dynamics calculations are performed within the generalized Langevin oscillator (GLO) model in order to simulate the energy exchange between molecule and surface atoms. Electron-hole (e-h) pair excitations are also implemented in the dynamics via the local density friction approximation (LDFA). Overall adsorption probability, including dissociative and nondissociative mechanisms, is enhanced when molecules can lose energy through surface phonons and electronic excitations. However, the energy exchange with phonons has a larger influence in the adsorption probability than e-h pair excitations. Nondissociative molecular adsorption only takes place when such energy dissipation channels are included in the simulations, underlying the importance of GLO and LDFA models in such theoretical studies.

• Dynamics of N2 sticking on W(100): The decisive role of van der Waals interactions

Autores: A. Peña-Torres, H. F. Busnengo, J. I Juaristi, P. Larregaray, C. Crespos

Revista: Physical Chemistry Chemical Physics 20 (29) 19326 (2018)

Resumen: The reactive dynamics of N2 on W(100) has been investigated by means of quasi-classical trajectory calculations using an interpolated six-dimensional potential energy surface (PES) based on density functional theory energies obtained employing the vdW-DF2 functional. The dynamics are compared to those obtained using the PW91 functional and to experimental data. The results show that the new PES provides a significant improvement in the description of the reactivity in this system. We show that the long standing problem that constituted the large qualitative disagreement between the simulations performed with the PW91-PES and the experiments was due to the presence of energy barriers in the entrance channel that disappear when vdW forces are accounted for.

• Thickness-Dependent Reactivity of O2 on Cu Layers Grown on Ru(0001) Surfaces

Autores: M. Ramos, C. Díaz, A. E. Martínez, F. Martín, H. F. Busnengo

Revista: Journal of Physical Chemistry C 122(27), 15529 (2018)

Resumen: The reaction of diatomic molecules on bimetallic surfaces, formed by one to few monolayers of a metal adsorbed on a different metal substrate, is relevant to understand the role of surface strain and substrate chemical binding in catalysis, which is interesting for industrial applications, challenges existing state-of-the-art theoretical methods, due to the additional complexity associated with having a molecule with triplet spin multiplicity. Here, we have studied the interaction of O2 with CuxML/Ru(0001) (x being the number of Cu monolayers), for which experimental data are available, by means of multidimensional classical dynamics simulations based on first-principles potential energy surfaces. Our results show, on the one hand, that the inclusion of the surface temperature on the simulations is essential to reproduce the experimental observations, and therefore, to analyze the physical mechanisms behind these observations, and, on the other hand, that electronic effects due to the binding between the two metallic species are only relevant for one Cu monolayer, whereas strain is responsible for the observed reactivity in O2 interacting with Cu(x≥2)ML/Ru(0001).

• On-surface transmetalation of metalloporphyrins

Autores: Diana Hötger, Paula Abufager, Claudius Morchutt, Patrick Alexa, Doris Grumelli, Jan Dreiser, Sebastian Stepanow, Pietro Gambardella, H. Fabio Busnengo, Markus Etzkorn, Rico Gutzler and Klaus Kernac

Revista: Nanoscale 10(45), 21116 (2018)

Resumen: Increasing the complexity of 2D metal-organic networks has led to the fabrication of structures with interesting magnetic and catalytic properties. However, increasing complexity by providing different coordination environments for different metal types imposes limitations on their synthesis if the controlled placement of one metal type into one coordination environment is desired. Whereas metal insertion into free-base porphyrins at the vacuum/solid interface has been thoroughly studied, providing detailed insight into the mechanisms at play, the chemical interaction of a metal atom with a metallated porphyrin is rarely investigated. Herein, the breadth of metalation reactions is augmented towards the metal exchange of a metalloporphyrin through the deliberate addition of atomic metal centers. The cation of Fe(ii)-tetraphenylporphyrins can be replaced by Co in a redox transmetalation-like reaction on a Au(111) surface. Likewise, Cu can be replaced by Co. The reverse reaction does not occur, i.e. Fe does not replace Co in the porphyrin. This non-reversible exchange is investigated in detail by X-ray absorption spectroscopy complemented by scanning tunneling microscopy. Density functional theory illuminates possible reaction pathways and leads to the conclusion that the transmetalation proceeds through the adsorption of initially metallic (neutral) Co onto the porphyrin and the expulsion of Fe towards the surface accompanied by Co insertion. Our findings have important implications for the fabrication of porphyrin layers on surfaces when subject to the additional deposition of metals. Mixed-metal porphyrin layers can be fabricated by design in a solvent-free process, but conversely care must be taken that the transmetalation does not proceed as an undesired side reaction.

• Energy dissipation to tungsten surfaces upon hot-atom and Eley-Rideal recombination of H2

Autores: O. Galparsoro, H. F. Busnengo, A. E. Martinez, J. I. Juaristi, M. Alducin, P. Larregaray

Revista: Physical Chemistry Chemical Physics 20 (33) 21334 (2018)

Resumen: Adiabatic and nonadiabatic quasi-classical molecular dynamics simulations are performed to investigate the role of electron-hole pair excitations in hot-atom and Eley-Rideal H2 recombination mechanisms on H-covered W(100). The influence of the surface structure is analyzed by comparing with previous results for W(110). In the two surfaces, hot-atom abstraction cross sections are drastically reduced due to the efficient energy exchange with electronic excitations at low incident energies and low coverage, while the effect on Eley-Rideal reactivity is negligible. As the coverage increases, the projectile energy is more efficiently dissipated into the other adsorbates. Consequently, the effect of electronic excitations is reduced. As a result, the reactivity and final energy distributions of the formed H2 molecules are similar for both abstraction mechanisms.

• Sol-gel synthesis and characterization of 0.5Pb(Zr0.52Ti0.48)O3-0.5Pb(Fe0.5Nb0.5)O3 thin films

Autores: Lucía Imhoff, Sebastián Barolín, Nora Pellegri y Marcelo Stachiotti

Revista: Ferroelectrics 545, 1 (2019)

Resumen: A simple route for the fabrication of (1-x)Pb(Zr0.52Ti0.48)O3-xPb(Fe0.5Nb0.5)O3 [(1-x)PZT-xPFN] thin films is presented. The PZT and PFN precursor solutions were obtained by a modified sol-gel process based on the use of acetoin as chelating agent. Both precursors were mixed to obtain a particular composition: 0.5PZT-0.5PFN. The precursor solution was deposited onto platinized silicon substrates by spin coating. Two different thermal treatments were compared: conventional furnace annealing (CFA) and rapid thermal annealing (RTA). Structural and morphological characterizations made by DRX and AFM indicated that RTA favors the perovskite phase formation. Dielectric measurements showed negligible frequency dispersion and low dissipation factor. It was finally shown that the films display ferroelectric and ferromagnetic behavior at room-temperature.

• Proton strings and rings in atypical nucleation of ferroelectricity in ice

Autores: J. Lasave, S. Koval, A. Laio, E. Tosatti

Revista: Proceedings of the National Academy of Sciences of the United States of America, 118(1), e2018837118 (2021)

Resumen: Ordinary ice has a proton-disordered phase which is kinetically metastable, unable to reach, spontaneously, the ferroelectric (FE) ground state at low temperature where a residual Pauling entropy persists. Upon light doping with KOH at low temperature, the transition to FE ice takes place, but its microscopic mechanism still needs clarification. We introduce a lattice model based on dipolar interactions plus a competing, frustrating term that enforces the ice rule (IR). In the absence of IR-breaking defects, standard Monte Carlo (MC) simulation leaves this ice model stuck in a state of disordered proton ring configurations with the correct Pauling entropy. A replica exchange accelerated MC sampling strategy succeeds, without open path moves, interfaces, or off-lattice configurations, in equilibrating this defect-free ice, reaching its lowtemperature FE order through a well-defined first-order phase transition. When proton vacancies mimicking the KOH impurities are planted into the IR-conserving lattice, they enable standard MC simulation to work, revealing the kinetics of evolution of ice from proton disorder to partial FE order below the transition temperature. Replacing ordinary nucleation, each impurity opens up a proton ring generating a linear string, an actual FE hydrogen bond wire that expands with time. Reminiscent of those described for spin ice, these impurity-induced strings are proposed to exist in doped water ice too, where IRs are even stronger. The emerging mechanism yields a dependence of the long-time FE order fraction upon dopant concentration, and upon quenching temperature, that compares favorably with that known in real-life KOH doped ice.

• Multiferroic properties of three-layer Aurivillius compound Bi4TiFeNbO12: A first-principles and experimental study

Autores: C. Lavado, A. F. Rébola, R. Machado, M. G. Stachiotti

Revista: Solid State Communications 320, 114028 (2020)

Resumen: We present a first-principles and experimental study of the structural, ferroelectric, and magnetic properties of the potentially multiferroic three-layer Aurivillius compound Bi4TiFeNbO12. This system can be realized by inserting a BiFeO3 formula unit into the two-layer Aurivilius Bi3TiNbO9 matrix. The calculations are performed using the PBEsol exchange–correlation functional within the DFT+U framework. First we search for potential cation site preference by comparing the relative stability of different Fe, Nb and Ti arrangements. We find a preference for the Fe3＋ ions to occupy the inner site within the pseudoperovskite block. This configuration exhibits a band gap of 1.2 eV (UFe=4eV) and ferroelectric and magnetic orders. A value of 66 μC/cm2 is obtained for the spontaneous polarization, which is similar to the one obtained for Bi4Ti3O12 (BIT). The magnetic ground state of this system is characterized by a strong antiferromagnetic coupling between the Fe3＋ ions located in the central layer. By mapping to a Heisenberg model, the superexchange antiferromagnetic coupling between nearest-neighbor Fe3＋cations is estimated to be J=53 meV. Finally, we synthesize Bi3.25La0.75TiFeNbO12 ceramics by the solid-state reaction method. Their structural, electric and magnetic properties are confronted with the theoretical predictions.

• Topology of the polarization field in PbTiO3 nanoparticles of different shapes by atomic-level simulations

Autores: F. Di Rino, M. Sepliarsky, M. G. Stachiotti

Revista: Journal of Applied Physics 127(14), 144101 (2020)

Resumen: An atomistic model approach parameterized from first-principles calculations is used to investigate size and shape effects on the polarization field in isolated stress-free PbTiO3 nanoparticles. The study was carried out by molecular dynamics simulations in free-standing nanodots of cylindrical, spherical, and ellipsoidal shapes. We show that in cylinders with diameter equal to height, the size-induced transformation from the vortex to the flux-closure domain configuration causes an anomaly in the behavior of the toroidal moment and the volume of the system. During this transformation, the vortex core evolves into domain walls while the resulting structure is stabilized due to the non-homogeneous distribution of polarization and strain inside the domains. A similar behavior is observed in elongated cylinders, spheres, and spheroids. The increment in the diameter/height relation of the nanoparticles gives rise to a succession of topological transformations that include multi-vortex configurations, ferroelectric bubble states, and multi-domain patterns. While the transformation path for flat cylinders is similar to the one previously obtained for cuboids, the thinner edge region of the spheroids prevents the stabilization of one- and two-bubble states. Despite this last difference, our results indicate that the polarization pattern of a nanoparticle depends more on its aspect ratio than on its shape.

• Site occupancy effects of Mg impurities in BaTiO3

Autores: Rodrigo Machado, Ariel Di Loreto, Agustín Frattini, Marcelo Sepliarsky, Marcelo Stachiotti

Revista: Journal of Alloys and Compounds 809, 151847 (2019)

Resumen: A first-principles based atomistic model is developed to investigate intrinsic effects of Mg incorporation into A- and B-sites of BaTiO3. We find that the replacement of Ba by Mg at A-site positions generates local electric dipoles due to Mg off-centering along [001] directions, which increase the Curie temperature and decrease the cell volume. The inverse dependence is observed for B-site doped compositions, where the defect dipoles decrease the Curie temperature and expand the volume. Temperature-composition phase diagrams are constructed for both site locations and the effect of the two types of defects on the switching process is investigated. The theoretical predictions are used to shed light on experimental results of Mg-doped ceramics manufactured to induce a given occupation site. The comparison indicates that the incorporation of Mg into the B-site is thermodynamically favorable whereas the properties observed for the A-site ceramics cannot be explained from the intrinsic effects described by the model.

• Ab initio study of the structure, isotope effects, and vibrational properties in KDP crystals

Autores: R. Menchón, G. Colizzi, C. Johnston, F. Torresi, J. Lasave, S. Koval, J. Kohanoff, R. Migoni

Revista: Physical Review B 98, 104108 (2018)

Resumen: The lattice dynamics of potassium dihydrogen phosphate (KDP) and its deuterated analog DKDP was studied via first-principles DFT calculations. A thorough assessment of the quality of a wide range of functionals supplemented with the approximate inclusion of quantum nuclear effects indicated that the nonlocal van der Waals functional vdW-DF [M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)PRLTAO0031-900710.1103/PhysRevLett.92.246401; J. Klimeš et al., Phys. Rev. B 83, 195131 (2011)PRBMDO1098-012110.1103/PhysRevB.83.195131] produces the best agreement with structural data for both compounds. This enabled the calculation of full phonon dispersions in the ferroelectric phase, and hence the phonon density of states and specific heat, in very good agreement with experimental data. Phonon bands and especially modes at the Γ point of the Brillouin zone were classified according to their vibrational pattern. This allowed for the assignment of stretching and bending modes of the hydrogen bonds. Internal modes involving the phosphate units were identified at lower frequencies, while the lowest-lying modes were those involving the K+ ion. These assignments were used to interpret infrared and Raman spectra along the c axis and in the perpendicular plane. Phonon modes calculated at the Γ point showed two types of instabilities. One was a normal mode polarized along the c axis of the crystal, while the other corresponded to a twofold-degenerate mode polarized in the perpendicular plane. The former gives rise to a spontaneous polarization in the ferroelectric phase at low temperatures by coupling to an optical K+-H2PO4- stretching mode, consistently with a significant off-diagonal Born effective charge on the hydrogen atoms. A mode describing the opposite rotation of neighboring PO4 tetrahedra was also found to couple strongly to the ferroelectric mode, as this modulates the O-O distance, which determines the barrier for proton transfer. The present study suggests that a minimal model to describe isotope effects in KDP should involve at least three fully coupled vibrational modes.

• Influence of A-site magnesium doping on structural and electrical properties of BaZr0.1Ti0.9O3 ceramics

Autores: A. Di Loreto, A. Frattini, R. Machado, M. G. Stachiotti

Revista: Journal of Materials Science: Materials in Electronics 29, 19783 (2018)

Resumen: The effects of the replacement of Ba2+ by off-center Mg2+ ions on the structural and electrical properties of BaZr0.1Ti0.9O3 ceramics were investigated. We show that the use of magnesium as A-site dopant favors the formation of the perovskite phase at a lower temperature and improves the densification of the ceramics. Combining XRD, SEM and electrical measurements, we determined that the solubility limit of Mg is ~ 2%. We show that Mg doping leads to a decrease in both the Curie temperature and remnant polarization of the ceramics. A 1% Mg content, however, enhances the room-temperature d33 piezoelectric coefficient due to the composition proximity to an impurity induced morphotropic phase boundary.

• Fe3+/Nb5+ co-doping effects on the properties of Aurivillius Bi4Ti3O12 ceramics

Autores: C. Lavado, M. G. Stachiotti

Revista: Journal of Alloys and Compounds 731, 914 (2018)

Resumen: Bi4Ti3-x(Nb0.5Fe0.5)xO12 (BTFNx) ceramics with 0 ≤ x ≤ 2 were synthesized to evaluate the effect of Fe3+/Nb5+ co-substitution into the B-site of Bi4Ti3O12 (BIT). We show that the XRD pattern for the compounds with x ≤ 1 is the characteristic one of a layered perovskite structure belonging to the n = 3 member of the Aurivillius family. Impurity peaks assigned to a pyrochlore phase are detected at higher concentrations. Raman measurements corroborated that Fe3+and Nb5+ ions are incorporated into the Ti sites of the Aurivillius compound. Frequency dependent dielectric studies at room-temperature displayed the reduction of both dielectric constant and loss tangent with substitution, while the electric-field-induced polarization switching behavior indicates ferroelectric character. It is observed that the ferroelectric transition temperature decreases with increasing Fe/Nb content. Magnetic measurements indicated no evidence of ferromagnetic ordering, but antiferromagnetic spin correlations between Fe3+ ions.

• Excellent pseudoelasticity of Al-rich Fe–33Mn–17Al–6Ni–0.15C (at%) shape memory single crystals obtained without an aging conditioning stage

Autores: J. M. Vallejos, M. F. Giordana, C. E. Sobrero, J. A. Malarria

Revista: Scripta Materialia 179, 25 (2020)

Resumen: The pseudoelastic behavior of a series of Fe–33Mn–17Al–6Ni–0.15C (at%) shape memory alloy single crystals was analysed with compression tests. The specimens, with different crystallographic orientations, showed almost 100% strain recovery after a full stress-induced martensitic transformation. The size and distribution of precipitates introduced in the alloy processing proved to be clearly suitable for achieving a completely reversible pseudoelastic transformation, without the need of additional aging in this Al-rich alloy. The transformation strain was analysed using the phenomenological theory of martensite transformation. This model roughly describes the pseudoelastic strain under compression as a function of the crystallographic orientation.

• Effect of the precipitates on the thermodynamics of the martensitic transformations in Ti-rich Ni-Ti-Co thin films

Autores: L. M. Isola, B. F. Malvasio, M. F. Giordana, J. Malarría

Revista: Journal of Alloys and Compounds 818, 152904 (2020)

Resumen: The wide-spread multiple transformation in Ti-rich Ni-Ti shape memory alloys produced by Ti2Ni nano-precipitates was investigated. For this purpose, a series of amorphous Ti-Ni-Co thin films with the same composition were subjected to different annealing treatments. The characteristic temperatures and heat of the transformations were measured by four-point probe resistivity and differential scanning calorimetry. A critical review of the thermodynamics of thermally-induced thermoelastic martensitic transformations is presented. The values of the frictional work and the stored elastic energy associated with the thermoelastic transformations were assessed from the transformation temperatures, and related with the transformation heats measured in the samples annealed at different temperatures. It was observed that the stored elastic energy decreases as the annealing temperature rises due to a change in the distribution of the Ti2Ni precipitates, in concordance with an increase of the transformation heat measured by the calorimeter. Nevertheless, the values of the frictional work were independent of the annealing temperature. The influence of the dissipation of the stored elastic energy on the transformation temperatures and thermal hysteresis is discussed. Finally, a better understanding of the wide-spread multiple transformation, as an effect of the local stresses introduced by Ti2Ni nano-precipitates in the matrix, is proposed.

• Growing Fe-Mn-Al-Ni single crystals by combining directional annealing and thermal cycling

Autores: J. M. Vallejos, J. A. Malarría

Revista: Journal of Materials Processing Technology 275, 116317 (2020)

Resumen: In pseudoelastic alloys, a fine-grained microstructure implies restrictions on the propagation and shrinkage of the martensitic phase. Thus, the pseudoelastic effect is adversely affected. For this reason, thermomechanical methods that accelerate grain growth are sought in these alloys. In this paper, we present a novel and extremely fast method to obtain single crystals of centimeter-order lengths. The process consists of combining two methods of abnormal crystal growth: directional annealing; and thermal cycling. This technique is applicable to pseudoelastic alloys that have a parent single-phase high-temperature region and a two-phase, low-temperature region. The crystallographic orientations measured in single crystals generated by this process are also shown. A full-strain recovery pseudoelastic behavior was observed in single crystalline specimens tested under compression.

• On the strain-induced martensitic transformation process of the commercial AISI 304 stainless steel during cyclic loading

Autores: F. Farias, I. Alvarez-Armas, A. F. Armas

Revista: International Journal of Fatigue 140, 105809 (2020)

Revista: Low-cycle fatigue tests were performed on metastable austenitic steel at two initial material states, the as-received, with a high crystal defect density, and the solution treated one, with fewer crystal defects. Cyclic curves have shown different hardening behavior depending on the initial material states. To understand these differences, the back and friction stresses were studied from the hysteresis loop analysis. The correlation of these results with the volume fraction of induced martensite and the observed microstructure revealed that the cyclic hardening behavior strongly depends on the existence of potential martensite nucleation sites.

• Effect of initial microstructure on surface relief and fatigue crack initiation in AISI 410 ferritic-martensitic steel

Autores: M. N. Batista, M. C. Marinelli, Iris Álvarez-Armas

Revista: Fatigue & Fracture of Engineering Materials & Structures 42, 61 (2019)

Resumen: This work is focused on the effect of the initial tempered‐lath microstructure on the surface relief and nucleation of microstructurally short fatigue cracksdeveloped during low‐cycle fatigue tests of the ferritic‐martensitic AISI 410steel. Transmission and scanning electron microscopy as well as electronback‐scattered diffraction were used to study the surface‐damage evolution insmooth, cylindrical, notched specimens. Results from the electron back‐scattered diffraction analysis broaden the understanding of the processes ofnucleation of microstructurally short fatigue cracks in the initial tempered‐lath microstructure of AISI 410 ferritic‐martensitic steels. Results prove that during fatigue, microcracks nucleate mainly at high‐angle boundaries represented byblock subunit interfaces formed in this tempered microstructure. Besides, theprogress of fatigue cycling causes the reorientation of the {112} systemsto a direction more favourable to slip, giving rise to the formation of extrusions within the blocks and consequently the formation of microcracks.

• Effects of Strain Rate on the TRIP–TWIP Transition of an Austenitic Fe-18Mn-2Si-2Al Steel

Autores: Marcio Raposo, Mauro Martín, María Florencia Giordana, Valeria Fuster y Jorge Malarría

Revista: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. 50 (9), 4058 (2019)

Resumen: A fully austenitic Fe-18Mn-2Si-2Al transformation-induced plasticity (TRIP) steel was tensile tested from quasi-static to low-dynamic regime at three different strain rates: 4.7 × 10−4, 1.3 × 10−1, and 8.3 × 100 s−1. Typical two-stage transformation mechanism, TRIP γ → ε → α′, was observed for samples tested at 4.7 × 10−4 s−1. At higher strain rates, the increase in temperature due to adiabatic plastic work shifts the stacking fault energy (SFE) towards a twinning-induced plasticity–SFE-range modifying the mechanical behavior of the alloy. This change on the deformation mechanism leads to a lower work hardening capacity and a higher elongation to rupture in samples tested at 1.3 × 10−1 and 8.3 × 100 s−1. In this context, the alloy maintains its energy absorption capability with a maximum reduction of 3.6 pct according to the Rm × A parameter. The Md temperature, experimentally determined in the present study, proved to be a useful tool for understanding the material’s behavior.

• The shape recovery conditions for Fe–Mn–Si alloys: An interplay between martensitic transformation and plasticity

Autores: A. Druker, P. Vermaut, J. Malarría

Revista: Materials Characterization 139, 319 (2018)

Resumen: This work analyses the improvement of the shape memory properties of Fe-Mn-Si based alloys, and expands upon and deepens recently published developments concerning this topic. We prepared TEM specimens from a Fe-15Mn-5Si-9Cr-5Ni sheet rolled at 800 °C and annealed at 650 °C–92% shape recovery – and from another sheet annealed at 1000 °C–45% shape recovery. Here, we analyse the development of microstructure during the γ ↔ ε martensitic transformation and correlate it with the shape memory behaviour. Among the observed microstructural features, the introduction of complex dislocation arrays due to plastic deformation appears to be critical for shape memory development, since they can arrest the backward movement of Shockley partial dislocations over the original path. Tailoring the microstructure in order to introduce a high stacking-fault density accompanied by an appropriate dislocation density remains a key issue in the design of these kinds of functional materials.

• Crystallographic orientation relationships in the α→γ′ martensitic transformation in an Fe-Mn-Al-Ni system

Autores: J. M. Vallejos, C. E. Sobrero, M. Ávalos, J. W. Signorelli, J. A. Malarría

Revista: Journal of Applied Crystallography 51, 990 (2018)

Resumen: The Fe43.5Mn34Al15Ni7.5 (at.%) alloy exhibits outstanding pseudoelastic behaviour over a wide range of temperatures. This alloy undergoes an unusual martensitic transformation from a disordered body-centred cubic (α) parent phase to a face-centred cubic γ′) product phase. In the present work, the orientations of the parent and product phases for quenched samples were analysed by electron backscatter diffraction. Bain, Kurdjumow-Sachs, Pitsch, Nishiyama-Wassermann and Greninger-Troiano orientation relationships between the parent and product phases were compared with experimental results. The Pitsch relationship appears to be the most suitable to describe the α→γ′ martensitic transformation. This result provides experimental support to the dislocation-based heterogeneous Bogers-Burgers type transformation mechanism. No indications of variant selection were detected in the thermally activated transformations. © 2018, Wiley-Blackwell. All rights reserved.

• Characterization of a Continuously Cooled Dual-Phase Steel Microstructure

Autores: F. Farías, M. Balbi, M. N. Batista, I. Álvarez-Armas

Revista: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49, 6010 (2018)

Resumen: Continuous-cooling transformation behavior of a DP steel was analyzed from dilation curves with cooling rates that range between 10 °C/s and 98 °C/s and data taken in 10 °C/s increments. For a precise understanding of the problem, several metallographic techniques were used in order to determine which phases and types of transformation are present, the grain structure and crystal defects generated for each cooling rate, among other characteristics. The local distribution of the main alloying elements was analyzed by wave dispersive spectroscopy. From the dilation curves, the relative amount of transformed phase was estimated, as well as the first derivatives as a function of both temperature and time to analyze the characteristics of the transformation and correlate these with a characteristic microstructure. To further understand these results, the mobility of suitable alloying elements such as Cr, Mn, Al, and P was evaluated. The analysis showed that at lower cooling rates, 10 °C/s to 20 °C/s, the transformation occurs at temperatures above 700 °C (at which the majority of alloying atoms have good mobility) in a relatively slow process producing polygonal ferrite. At cooling rates greater than 40 °C/s, the transformation occurs below 700 °C in a relatively short time, where massive transformation takes place. Finally, a cooling rate of 30 °C/s gives a mixed transformation, producing an appreciably smaller grain structure with a high density of crystal defects.

• Effect of holding time at an intercritical temperature on the microstructure and tensile properties of a ferrite-martensite dual phase steel

Autores: M. Balbi, I. Alvarez-Armas, A. Armas

Revista: Materials Science and Engineering A 733, 1 (2018)

Resumen: To analyze the influence of the initial microstructure in the production of a dual phase (DP) steel on the mechanical properties, a commercial DP steel was subjected to two heat treatments (HT): step quenching (SQ) and intermediate quenching (IQ). SQ samples were austenitized at 930 °C for 20 min, then annealed at 830 °C for 5, 15, 30, 60 and 120 min and finally water quenched. Shorter annealing times show allotriomorphic ferrite and martensite while at longer time polygonal ferrite is formed. Tensile tests showed that yield and tensile strength are higher than those of the commercial DP steel and besides, the uniform elongation is smaller. IQ samples were water quenched after austenitization and then annealed at 830 °C for the same holding times and water quenched again. The microstructure consists of martensite dispersed in a ferrite matrix. The remarkable results are that these samples had similar tensile properties to that of the as-received DP steel. Tensile properties were analyzed as a function of the characteristic microstructure and the assessment of the alloying elements partition during the different HTs. Evaluation of the Kolmogorov-Johnson-Mehl-Avrami theory indicates that the transformation in SQ and in IQ samples corresponds to a diffusional process.

• A miniaturized device for the measurement of sheet-metal formability using digital image correlation

Autores: M. E. Leonard, F. Ugo, M. Stout, J. W. Signorelli

Revista: Review of Scientific Instruments 89 (8) 085114 (2018)

Resumen: A laboratory-scale device to obtain forming-limit diagram data was designed to utilize the Marciniak and Kuczynski (MK) sample geometry. The design uses a high-resolution photographic camera, automatic trigger, and light-emitting diode (LED) lighting to record the time history of deformation calculated with the digital-image correlation technique. Because the testing device was miniaturized, it was possible to halt the forming experiments at intermediate strains and recrystallize the MK carrier blank. This permits large formability strains to be obtained without cracks developing at the carrier blank's central hole, an advantage over full-size specimens and conventional testing rates. A number of initial experiments were performed on a zinc alloy sheet (Zn-Cu-Ti) over the entire forming-limit range (-0.5 ≤ ϵ 2 /ϵ 1 ≤ 1), and the strain fields reduced employing the Bragard criterion to obtain limit strains. These results are compared favorably to previous data of this material obtained with a hemispherical, Nakazima, punch and a circle-grid pattern.

• A Geometric Version for the Description of the Dynamics of Different Models

Autor: Oscar P. Zandrón

Revista: Advanced Studies in Theoretical Physics 13 (4), 155 (2019)

Resumen: In many of our works, and over the years, the dynamics of different models were analyzed from a geometric point of view. In this work the advantages of the applications of this formalisms are studied comparatively for models that have diverse origins, from gravity and supergravity models, through models of condensed matter, to models of productive systems. The geometrical approach it is very useful tool for the study of the dynamics, and it is possible to apply it with the correct interpretation of the variables in game. In this paper we try to study the scope of this application.

• Shell Area-To-Volume Ratio in Ammonoids

Autores: H. Parent, M. Bejas, A. Greco

Revista: Paleontological Research 24(3), 216 (2020)

Resumen: The external area-to-volume ratio of the ammonite shell has been held to be related to morphology but never evaluated quantitatively. A dimensionless ratio, the Vogel number, was computed for large samples of Devonian to Cretaceous ammonites with a new method based on the ADA-model. The estimated ratios range from 2.4 to 3.4. The highest values are exhibited by uncoiled serpenticone ammonites, lowering in the sequence serpenticone-oxycone-spherocone. It is shown that the area-to-volume relationships are controlled by the involution (degree of overlapping) and the relative width of whorl section. The typical evolutionary trends serpenticone-spherocone and/or serpenticone-oxycone, broadly documented through the history of the Ammonoidea, could have been driven, at least in part, by the lowering of the area-to-volume ratio.

• Adaptación de un tubo de kundt para la determinación de la velocidad del sonido a distintas temperaturas

Autores: I. Novara, M. Matar, M. A. Parodi, A. Roatta, B. J. Gómez, C. E. Repetto

Revista: Anales de la Asociacion Fisica Argentina 30(2), 42 (2019)

Resumen: In this work we describe how we designed and built an acoustic system that allowed us to adapt a Kundt’s tube for the measurement of the speed of sound at different temperatures. The air column inside the tube was excited by a speaker at a frequency of 2 kHz. We changed the air column length by moving a piston throughout the tube, and we measured and recorded that length when we observed a resonance signal on the oscilloscope screen. This procedure was repeated at different temperatures, ranging from 19○ C to 115○ C. Then it was calculated the propagation of the speed of sound based on the temperature. The results were compared with those predicted by the classical model, which assumes that air is an ideal diatomic gas, and that the acoustic phenomenon is an adiabatic process. Finally, it was found a satisfactory agreement between the experimental values and those predicted by the classical model.

• Regulación de calidad de agua desde la perspectiva de la Ingeniería y la Economía Ambiental

Autores: Luis Herrera, Eduardo Luccini, Miguel Parodi, Maricel Matar y Javier Gómez Insausti

Revista: Energeia (Rosario) 16, (2019).

Resumen: Debido a su importancia para la vida humana y a la magnitud del despliegue necesario tanto para su disposición como para su cuidado, el control de la calidad del agua se ha transformado en las últimas décadas en unas de las áreas más prominentes de la ingeniería ambiental y de la economía ambiental. Una estrategia implementada por países que cuentan con una importante experiencia en este tipo de políticas ambientales consiste en recurrir a incentivos económicos, con el fin de sumar a los distintos agentes contaminadores al proceso mismo de reducción de la contaminación. En este trabajo analizamos los posibles resultados de este tipo de regulación, aplicada a calidad del agua en el curso de un río sobre el cual se producen vertidos de diversas fuentes, mediante la implementación de un modelo de control de la contaminación y a su correspondiente simulación computacional. Se plantea un problema de optimización sobre un modelo de simulación de mercado basado en agentes de descargas de efluentes (puntuales y no puntuales), desarrollado en Estados Unidos de América (USA) y que puede ser aplicable en muchos aspectos, con su correspondiente adaptación, a nuestro país.

• Análisis comparativo de la captura y almacenamiento de CO2 por especies vegetales de tres ecosistemas en Perú

Autores: Eduardo Luccini, Mario Flores, Karenth Ramírez, Harold Pérez, Luis Herrera, Miguel Parodi, Maricel Matar, Javier Gómez Insausti, Leonardo Barrea, Mirta Mechni, Grisel Calcagno.

Revista: Energeia (Rosario), 15, 13 (2018).

Resumen: La capacidad de captación y almacenamiento de CO2 atmosférico por diferentes especies vegetales es un parámetro crucial a determinar ante el presente escenario de cambio climático planetario. En este trabajo se cuantifica y compara la captura y el almacenamiento de CO2 por la flora nativa de tres regiones muy disímiles de Perú, de las cuales algunas comparten especímenes en común. Las regiones comprendidas son el humedal costero de Villa María, el Lago Titicaca en el Altiplano de Puno y la selva amazónica de Loreto. El muestreo abarca desde especies gramíneas hasta arbóreas. Se delimitaron áreas representativas ocupadas homogéneamente por cada especie donde se realizaron muestreos de campo. En la etapa de laboratorio, el contenido de carbono en la estructura vegetal se determinó prioritariamente por método de ?Walkley y Black? y la captura de dióxido de carbono se estimó mediante el ?factor de conversión de carbono a dióxido de carbono?. Los valores de cantidad de dióxido de carbono capturado abarcan desde 44 tCO2/ha en la especie gramínea hinea del humedal costero hasta 644 tCO2/ha en árboles grandes de la selva amazónica.

• A study of periodic potentials based on quadratic splines

Autores: M. Gadella, L. P. Lara

Revista: International Journal of Modern Physics C 29(8), 1850067 (2018)

Resumen: In this paper, we discuss a method based on a segmentary approximation of solutions of the Schrödinger equation by quadratic splines, for which the coefficients are determined by a variational method that does not require the resolution of complicated algebraic equations. The idea is the application of the method to one-dimensional periodic potentials. We include the determination of the eigenvalues up to a given level, and therefore an approximation to the lowest energy bands. We apply the method to concrete examples with interest in physics and discussed the numerical errors.

• Linear modelization of a real spring-mass system

Autores: M. Matar, M. A. Parodi, C. E. Repetto, A. Roatta

Revista: Revista Brasileira de Ensino de Fisica 40 (2), e2306 (2018)

Resumen: On university introductory courses, while studying the frequency of free oscillations of a vertical spring-mass system in the case of negligible dissipative forces and massless spring, such a frequency can be simply calculated as the square root of the relationship between the local gravity and the spring elongation. However, when an actual spring is used, the measured frequencies differ markedly below those predicted by the model. The analysis of the static response of the spring to the load reveals that it does not obey Hooke's law and prevents defining a single elastic constant kE in any load condition. By making a linear approximation of this response around the point of work and defining a dynamic constant kD, the resolution of the differential motion equation predicts a value of the frequency much closer to the measured one. A subsequent heuristic correction, which takes into account the mass of the spring, further decreases the relative discrepancy. Our analysis concludes that dissipative phenomena are insignificant compared to the elasticity and inertia of the system studied. This problem and its solution show the need to discuss in the classroom the modeling of this physical phenomenon.