Conferencistas

Adsorption of atoms and organic molecules on metal surfaces typically leads to strong hybridization of the frontier molecular orbitals with the substrate electronic bands. This results in broad energy levels reflecting the ultrashort lifetime of excited states in tunneling experiments. A monolayer of MoS2 is a direct‐bandgap semiconductor. Here, we show that single‐layer MoS2 on Au(111) acts as an efficient decoupling layer for organic molecules and magnetic adatoms. Molecular resonances within the semiconducting band gap of MoS2 exhibit widths of only a few meV. This exquisite energy resolution allows to study vibrational excitations and their spatial variations within the individual molecules.   

Furthermore, we investigate single Fe atoms on MoS2/Au(111). We find that the decoupling efficiency strongly varies across the moiré pattern induced by the lattice mismatch at the interface. Fe atoms located on the minima of the moiré structure show sharp inelastic spin excitations. In contrast, Fe atoms on the moiré maxima exhibit a Kondo resonance. We track the evolution of magnetic excitations by investigating Fe atoms on different adsorption sites with respect to the moiré pattern and ascribe the gradual increase of Kondo correlations to the moiré‐modulated density of states. 

The variations in exchange coupling evoked by the moiré structure are also present for individual Mn atoms on MoS2 on Au(111). We explore the interatomic exchange coupling when two atoms are brought into sufficiently close distance. At a separation of a few lattice sites, the interaction is dominated by substrate‐mediated exchange, while nearest neighbor sites lead to direct exchange coupling and a molecular‐singlet ground state. The singlet‐triplet transition probed by tunneling electrons is again very sensitive to the relative adsorption of the dimer with respect to the moiré structure. We ascribe these variations to renormalization of the interatomic exchange imposed by the electronic moiré variations in the surrounding. 

In this talk, I will delve into two new topics in the field of two-dimensional (2D) materials physics: the impact of strong laser illumination on materials with an existing topological phase, and the interaction between electrons and chiral phonons. The first part of my talk will explore how intense laser illumination can be used to generate tunable spin-polarized photocurrents in the presence of spin-orbit interaction, and how it can cause a switch in chirality of the edge states in graphene when in the quantum Hall regime. In the second part, I will tackle the intriguing issue of electron-phonon interactions with chiral phonons. The theoretical prediction and subsequent observation of chiral phonons in 2D materials has added a fresh dimension to this persistent challenge. I will present an overview of this area, emphasizing our recent findings on the effects of electron-chiral phonon interactions in 2D materials. By using a non-perturbative solution, we demonstrate that electron-phonon interactions trigger inelastic Umklapp processes, leading to peculiar edge states.

I will present a systematic study of the formation of skyrmion crystals in triangular lattice models, including Kondo Lattice Hamiltonians relevant for metallic magnets and anisotropic Heisenberg models relevant for insulating magnets and quantum paramagnets. We will see that the skyrmion crystal lattice parameter of weakly-coupled metallic magnets is dictated by the Fermi wave-vector and that this property can naturally lead to a very large topological Hall effect. For the case of frustrated insulating quantum paramagnets, we will see that CP2 skyrmion crystals, like the one shown in the figure, can be induced by application of an external magnetic field. Unlike the regular CP1 skyrmions, which have opposite magnetic polarization in the core and the periphery, the dipolar field of the CP2 skyrmions vanishes away from the skyrmion core, where there is only a net quadrupolar field. 

Las propiedades físicas de los superconductores basados en Fe nos brindan la posibilidad de estudiar con gran sensibilidad cómo los defectos a escala atómica afectan las propiedades electrónicas de volumen, un tema que es de interés general en la ciencia de materiales. Adicionalmente, en el caso de superconductores este tema brinda información relevante para entender la conexión entre la naturaleza y concentración de defectos atómicos con la temperatura crítica hasta la que el material puede presentar resistencia nula. En la familia de superconductores basados en Fe, el compuesto FeSe ha capturado la atención de la comunidad científica debido a su sencilla estructura cristalina compuesta por un apilamiento de planos de Fe con átomos de Se levemente desplazados hacia arriba y abajo. A pesar de esta sencillez de la estructura, las propiedades electrónicas de los superconductores basados en Fe son fuertemente afectadas por deformaciones de la estructura cristalina introducidas mediante dopaje o aplicación de presión. En este coloquio presentaré resultados experimentales que nos permitieron dilucidar que los defectos atómicos del tipo mancuerna, visualizados directamente con microscopía túnel de barrido, alteran el espectro de niveles electrónicos de core en el volumen de las muestras. Mediante simulaciones de density functional theory mostramos el buen acuerdo cuantitativo entre la modificación de las nubes electrónicas de los átomos en el entorno de estos defectos y la forma espectral de los niveles electrónicos de core en estos compuestos. Adicionalmente, discutiré cómo en los defectos del tipo mancuerna se produce una modificación estructural que explicaría el dramático deterioro de la superconductividad cuando estos defectos proliferan.

In the past couple of years, machine learning has permeated many areas of physics and found numerous applications in condensed matter and chemistry. In particular, we have witnessed remarkable progress toward developing computational methods using neural networks as variational estimators. Variational representations of quantum states abound and have successfully been used to guess ground-state properties of quantum many-body systems. Some are based on partial physical insight (Jastrow, Gutzwiller projected, and fractional quantum Hall states, for instance), and others operate as a black box that may contain information about the underlying structure of entanglement and correlations (tensor networks, neural networks) and offer the advantage of a large set of variational parameters that can be efficiently optimized. However, using variational approaches to study excited states and, in particular, calculating the excitation spectrum, remains a challenge. 

In this talk, I present two variational methods to calculate the dynamical properties and spectral functions of quantum many-body systems in the frequency domain: The first one consists of encoding the Green's function of the problem in the form of a neural network. We introduce a natural gradient descent approach to solve linear systems of equations and use Monte Carlo to obtain the dynamical correlation function. The second approach is based on a Chebyshev expansion of the spectral function and a neural network representation for the wave functions. The Chebyshev moments are obtained by recursively applying the Hamiltonian and projecting on the space of variational states. I will present results for the one-dimensional and two-dimensional Heisenberg model on the square lattice and compare to those obtained by other methods in the literature. 

The past decade marked breakthrough discoveries of novel topological polarization structures in nanostructured ferroelectrics. These findings lie at the crossroad of the forefront of the physics of nanostructured materials, advanced topological concepts, and cutting-edge industrial nanotechnologies. The observed polarization structures have been interpreted in terms of the real-space vector field topology as multiple combinations of vortices, skyrmions, merons, and other topological formations. This topological approach was successfully used in the physics of dislocation, liquid crystals, and magnetism.

Ferroelectricity has been viewed as an electric dual of magnetism; however, this visible analogy misses ferroelectricity's cornerstone, the dominant role of the long-range electrostatic forces. In this talk, we present a generic topological foundation of ferroelectricity arising from its electrostatic essence and discuss from this viewpoint the recent trends in the field. We overview the existing topological approaches widely utilized for the description of the vector fields based on the homotopy groups in real space, usually employed in magnetism, and discuss the particular shortcomings of these approaches in application to ferroelectrics.  We establish that the physics of ferroelectrics can be constructed in the framework of topological hydrodynamics, an advanced formalism of the topology of an incompressible flow of vector fields. Based on the introduced topological identity of the electrostatics of ferroelectrics and hydrodynamics of the incompressible liquid, we overview the methods of topological hydrodynamics and set up a unified approach for the explanation of topological states in ferroelectrics. We give a comprehensive description of the observed polarization topological structures and show that their rich variety can be exhaustively described by the fundamental formations of topological hydrodynamics, vortices, and Hopfions. We highlight an inherent relation of topological matter in ferroelectrics to manifestations of topological hydrodynamics in the magnetic confinement fusion, magnetohydrodynamics of stellar and planetary interiors, and to the advanced topological aspects of gauge quantum field theories. We outline the prospects of implications of topological hydrodynamics to the physics of ferroelectrics and their diverse applications to the related emergent technologies.

El dióxido de carbono, CO2, y metano, CH4, son los principales gases responsables por el efecto invernadero en la atmosfera del planeta, produciendo un aumento de las temperaturas y el cambio climático. La reacción entre el metano y el dióxido de carbono para producir gas de síntesis (CO/H2), eliminando en un solo proceso dos gases responsables por el efecto invernadero, está atrayendo mucho interés debido a su naturaleza ecológica. Además, una alternativa es su conversión a metanol (CH3OH) −un combustible líquido con un alto valor en la industria química. En esta charla, analizamos los resultados recientes sobre catalizadores modelo de nanopartículas metálicas (Ni, Co, Pt) soportadas sobre óxido de cerio (CeO2) para ambas reacciones. El énfasis se pone aquí en estudios computacionales en combinación con experimentos que utilizan métodos in situ/operando y pruebas catalíticas, y se presta especial atención a los efectos de la carga de metal y las interacciones entre el metal y el soporte. Los resultados apuntan hacia una posible estrategia para producir catalizadores activos y estables que pueden emplearse para la activación y conversión de CH4 y CO2 en productos de alto valor añadido, acelerando el cambio hacia un futuro más limpio.

Los fonones acústicos suelen considerarse como una fuente primaria de efectos no deseados en optoelectrónica y tecnologías cuánticas basadas en sistemas en el estado sólido. En esta reunión, presentaré una serie de técnicas experimentales y nanodispositivos en los que los fonones acústicos constituyen un recurso central para descubrir fenómenos físicos fascinantes y novedosos.

Las ondas acústicas a escala nanométrica pueden utilizarse como plataforma de simulación para demostrar fenómenos difíciles o imposibles de acceder en fotónica y electrónica, como herramienta para controlar otras excitaciones del estado sólido, e incluso como una nueva herramienta en el diseño de nanodispositivos optoelectrónicos, tanto en el régimen clásico como cuántico. En esta presentación abordaré, entre otros resultados, la implementación de potenciales efectivos para nanovibraciones acústicas, la concepción de modos topológicos fonónicos y la demostración de resonadores optofonónicos tridimensionales basados en micropilares y antenas ópticas. También discutiré sobre el potencial de la nanoacústica para abrir nuevas direcciones en la ingeniería de nanodispositivos y en el estudio de nuevos fenómenos físicos en el estado sólido.

Despite PbTe being the archetypal thermoelectric material, still today some of the most exciting advances in the efficiency of this and related materials are being achieved by tuning their properties at the nanoscale. Its inherently low lattice thermal conductivity can be lowered to its fundamental limit by designing a structure capable of scattering phonons over a wide range of length scales. Intrinsic defects, such as vacancies or grain boundaries, can and do play the role of these scattering sites. Here we assess the effect of these defects by means of molecular dynamics simulations. For this we purposely parametrized a Buckingham potential that provides an excellent description of the thermal conductivity of this material over a wide temperature range. Our results show that intrinsic point defects and grain boundaries can reduce the lattice conductivity of PbTe down to a quarter of its bulk value. By studying the size dependence, we also show that typical defect concentrations and grain sizes realized in experiments normally correspond to the bulk lattice conductivity of pristine PbTe. We then address the issue of the survival of nanostructuring against thermal coarsening using a phase field model. Finally, we mention some recent results on grain boundary thermal resistance in Bi2Te3, the most widely used thermoelectric material at room temperature.