Condensed Matter Physics Seminars

Antiferromagnetic (AFM) materials are promising for future spintronic applications owing their advantageous properties such as robustness against perturbations due to magnetic fields, absence of parastatic stray fields or ultrafast dynamics (up to THz ranges). In a wide variety of AFM materials, transition metal oxides (e.g. NiO or CoO) play an important role due to their unique properties. These properties are especially evident in systems of reduced dimensionality, as pointed out by a number of studies. However, in thin film growth, as necessary for devices, the quality is often disappointing, dictated by the defect density.

Here we demonstrate a route for preparing high quality ultrathin antiferromagnetic oxide films on a metallic substrate. Mixed nickel, iron and cobalt oxides have been grown on Ru(0001) by high temperature oxygen-assisted molecular beam epitaxy. The nucleation and growth process are observed in real time by means of Low Energy Electron Microscopy (LEEM), which enables to optimize of the growth parameters. A comprehensive characterization is performed combining LEEM and LEED for structural characterization and PEEM (PhotoEmission Electron Microscopy) with synchrotron radiation for chemical and magnetic analysis via X-ray Absorption Spectroscopy and X-ray Magnetic Linear Dichroism (XAS-PEEM and XMLD-PEEM, respectively).

Depending on the chosen stoichiometry and conditions, the growth leads to the formation of high quality 2D islands of different compositions. The high crystalline and morphological quality of prepared films result in optimized properties with respect to films grown by other methods, such as magnetic domains, larger by several orders of magnitude or Ńeel temperature, which can be adjusted at will.

In January 1987 Philip W Anderson (December 13, 1923 – March 29, 2020) published one of the most controversial and influential papers in the history of superconductivity[1]. Now cited over 6000 times, the paper introduced the Resonating Valence Bond theory, or RVB, of superconductivity as an explanation of the then very recently discovered 30K superconductivity in the doped quantum, antiferromagnet La_{2-x}Ba_{x}CuO_4. The core idea is that the strong antiferromagnetic exchange interaction combined with doping leads to a novel two-dimensional quantum spin liquid of the Cu spins, in which neighbouring spins form a liquid of fluctuating singlet pairs. Doped holes move through this novel quantum spin liquid as charged bosonic 'holons' which condense into a superfluid explaining high Tc superconductivity. While several aspects of the original idea have been now proved incorrect, the ideas in the theory were highly influential in developing new models of superconductivity in strongly correlated electron systems which do not involve electron-phonon interactions. In this talk I shall review the ideas in the RVB and its successors and try to show which aspects of the original idea remain valid and useful after over 30 years of research into cuprate high Tc superconductivity.

[1] PW Anderson, Science 235 1191 (1987)

Due to the growing demand for electricity, combined with increasing environmental awareness, renewable sources and alternative fuels have become increasingly popular in recent years. Renewable energy sources, although they make it possible to obtain energy in a non-polluting manner, face many limitations, especially in terms of the continuity of energy supply. The use of hydrogen as an energy source is not subject to such restrictions. We briefly present methods of generating energy from hydrogen and hydrogen production, with particular emphasis on photoelectrocatalytic decomposition of water. In the second part we focus on hematite (110) surface as a promising electrode for water-splitting process. We discuss its advantages, disadvantages and the approaches to improve its efficiency in hydrogen production.

First, I will revisit Landau's Fermi liquid (FL) theory for normal metals, and thereby outline the properties of the non-Fermi liquid (NFL) metals (also called "strange" metals) which cannot be described within the Landau framework, due to the destruction of the Landau quasiparticles. In particular, I will focus on critical Fermi surface states, where there is a well-defined Fermi surface, but no quasiparticles, as a result of strong interactions between the Fermi surface and some emergent massless boson(s). I will deal with examples involving quantum phase transitions where finite-density fermions interact with an Ising-nematic order parameter, or U(1) gauge field(s). I will outline a framework to extract the low-energy physics of such systems in a controlled approximation, using the tool of dimensional regularization.

Finally, I will focus on Sachdev-Ye-Kitaev (SYK) models, which are toy-models of exactly soluble NFLs. I will consider a model with SYK fermions coupled to non-interacting lead fermions, which can be realized in a graphene flake connected to external leads. This system exhibits a quantum phase transition from an NFL phase to an FL one as a function of the ratio of the numbers of the two kinds of fermions. I will discuss the characteristics of the system after a sudden quench to the NFL, where a thermal state is reached rapidly via collapse-revival oscillations of the quasiparticle residue of the lead fermions. In contrast, the quench to the FL, across the NFL-FL transition, leads to multiple pre-thermal regimes and much slower thermalization.

In this talk, I will present a device that operates as a robust charge, spin and heat transistor. The device consists of a quantum spin Hall insulator in proximity contact with a ferromagnetic insulator and a superconductor. After a quick introduction to the quantum spin Hall insulator, I will discuss the phenomenon on which the operation of the device is based on, spin pumping. I will show that besides spin currents, heat currents are also pumped out of the device as well as charge currents.

Subsequently, I will show that the device supports two robust operation regimes. In one regime, the pumped charge, spin and heat are quantized and related to each other due to a topological winding number of the reflection coefficient. In the second regime, a Majorana zero mode switches off the pumping of currents owing to the topologically protected perfect Andreev reflection. We show that the interplay of these two topological effects can be utilized so that the device operates as a robust charge, spin and heat transistor.

The study of nonequilibrium phenomena in correlated materials has recently become one of the most active and exciting branches of condensed matter physics. This is largely due to advances in light sources and time-­resolved spectroscopies on the ultra­ short time scales, which made it possible not only to observe and describe but also to design systems with new remarkable properties by coupling them to external electromagnetic fields.


To study these phenomena, I am using using the non-equilibrium Green’s function theory (NEGF) approach, which is an extremely versatile formalism. Even though NEGF has been successfully used for many decades, there are still conceptual problems to be solved such as the generalization of the kinetic Kadanoff­ Baym equations (KBE) to the electron systems coupled with long­-range orders and with collective excitations [1], and dealing with high numerical complexity of the ab initio­-based approach. I present two examples.


The time non-­locality of the scattering term (rhs. of KBE) represents the major difficulty for the full two-­times propagation, the scaling is at least cubic with the physical propagation time making it very difficult to resolve smaller energy scales associated with phonons, magnons, etc. This can be handled with the so­-called Generalized Kadanoff­ Baym Ansatz (GKBA) allowing one to limit the propa­gation to the time-­diagonal. This implies a quadratic scaling. A tremendous progress has been recently achieved in further reducing the scaling to linear and establishing that the method works for all commonly used diagrammatic approx­imations. We have managed to extend GKBA to electron-boson systems [2].


Strong electronic correlations bring about a tantalizing variety of phenomena, such as metal ­to­ Mott ­insulator transitions. If such a system is driven out of equilibrium, even richer physics is expected. Recently we reported a remarkable observation of coherent electron­ spin dynamics on extremely long time­scale in NiO — a well­ studied transition metal oxide system [3]. Persistent THz oscillations (at 4.2THz) in the two­ photon photoemis­sion signal are so unexpected that they can be mistakenly taken for the magnetization dynamics, which is usually decoupled from the electronic subsystem. In fact, they can be at­tributed to the dynamics of triplon excitations produced by the pump pulse and propagating in the antiferromagnetic background.


[1] M. Schüler, J. Berakdar and Y. Pavlyukh, Time‐dependent many‐body treatment of electron‐boson dynamics: Application to plasmon‐accompanied photoemission, Phys. Rev. B. 93, 054303 (2016).

[2] D. Karlsson, R. van Leeuwen, Y. Pavlyukh, E. Perfetto, and G. Stefanucci, Efficient non-equilibrium Green's function simulations of correlated electron-boson systems, arXiv:2006.14965 (2020) .

[3] K. Gillmeister, D. Golež, C.‐T. Chiang, N. Bittner, Y. Pavlyukh, J. Berakdar, P. Werner, W. Widdra, Ultrafast coupled charge and spin dynamics in strongly correlated NiO, Nature Commun. 11, 4095 (2020).

The EuFe₂As₂-based compounds exhibiting 3d and/or 4f magnetic order were investigated by means of ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectroscopy [1-3]. It was found that spin-density-wave order of the Fe itinerant moments is suppressed by the chemical doping and in many cases the superconductivity is achieved. The Eu localized moments usually order regardless of the dopant concentration x, but undergo a spin reorientation with increasing x from the alignment parallel to the a-axis in the parent compound, toward crystallographic c-axis. The change of the 4f spins ordering from antiferromagnetic to ferromagnetic takes place simultaneously with a disappearance of the 3d spins order. The Fe nuclei experience the transferred hyperfine magnetic field due to the Eu²⁺ ordering for sufficiently substituted compounds, while the transferred field is undetectable in EuFe₂As₂ and for compound with a low substitution level. It seems that the 4f ferromagnetic component arising from a tilt of the Eu²⁺ moments to the crystallographic c-axis leads to the transferred magnetic field at the Fe atoms, even in the superconducting state.

[1] A. Błachowski, K. Ruebenbauer, J. Żukrowski, Z. Bukowski, K. Rogacki, P. J. W. Moll, and J. Karpinski, Phys. Rev. B 84, 174503 (2011).

[2] K. Komędera, A. Błachowski, K. Ruebenbauer, J. Żukrowski, S. M. Dubiel, L. M. Tran, M. Babij, and Z. Bukowski, J. Magn. Magn. Mater. 457, 1 (2018).

[3] K. Komędera, J. Gatlik, A. Błachowski, J. Żukrowski, D. Rybicki, A. Delekta, M. Babij, and Z. Bukowski, arXiv:2103.12698 (2021).

Iron-based superconductors display various phases originating in the competition between electronic, orbital, and spin degrees of freedom. Prominent among these novel effects is the orbital-selective Mott phase (OSMP), where interactions acting on a multiorbital Fermi surface cause the selective localization of electrons on one of the orbitals. As a consequence, the system is in a mixed state with coexisting metallic and Mott-insulating bands. In my talk, I will show that the magnetic ordering associated with the OSMP is significantly different from that observed in cuprates. The competing energy scales present in the low-dimensional OSMP induce an exotic spin arrangement without any apparent frustration in the system. I will also discuss the electronic properties of the OSMP, i.e., the emergence of the new quasiparticles and topological properties associated with them.

Antymonen to nowy materiał zbudowany z pojedynczej warstwy atomów antymonu. Ze względu na swoją budowę jest traktowany jako odpowiednik grafenu, jednak przewagą antymonenu jest naturalna obecność przerwy energetycznej, która umożliwia regulowanie przepływu prądu w urządzeniach elektronicznych. Spośród wielu odmian alotropowych antymonenu struktury typu α i β są najbardziej stabilne. Odmiana α stanowi silnie pofałdowaną w skali atomowej warstwę atomów antymonu i posiada dwie wyraźne podsieci, natomiast typ β to struktura podobna do plastra miodu z mniejszym pofałdowaniem. Obie formy strukturalne antymonenu cechują się zupełnie różnymi właściwościami fizycznymi istotnymi z punktu widzenia potencjalnych zastosowań w elektronice i optoelektronice. Faza α antymonenu, w przeciwieństwie do odmiany β, wyróżnia się mniejszą przerwą energetyczną, większą mobilnością nośników oraz anizotropowymi właściwościami elektrycznymi i termicznymi ze względu na mniejszą symetrię swojej struktury. Z tych powodów α-antymonen cieszy się obecnie dużym zainteresowaniem, jednak eksperymentalne utworzenie tego materiału, w tym badanie i modyfikowanie jego właściwości elektronowych, jest wciąż dużym wyzwaniem badawczym.


Badania eksperymentalne mają na celu wytworzenie pojedynczej warstwy α-antymonenu i poznanie jego struktury elektronowej i krystalograficznej. Otrzymane dane dotyczące właściwości elektronowych zostaną porównanie z wynikami obliczeń. W związku z uzyskaniem ciągłej warstwy antymonenu rozważa się funkcjonalizację tego materiału poprzez adsorpcję atomów metali ciężkich, co może prowadzić do konwersji z jednej odmiany alotropowej w drugą oraz do licznych zmian właściwości elektronowych. Do najważniejszych należą: pojawienie się liniowej dyspersji pasm w pobliżu poziomu Fermiego, znaczny wzrost mobilności nośników, przejście przerwy wzbronionej ze skośnej do prostej i/lub zmniejszenie jej wielkości oraz zwiększenie oddziaływania spin-orbita.

Dynamical quantum phase transitions (DQPTs) have become an established extension of equilibrium phase transitions into the out-of-equilibrium realm. Understanding their origin and possible universality are key open areas. Using a truncated Jordan-Wigner truncation, exact diagonalization, and matrix product state techniques, we study the dynamics of one- and two-dimensional Ising models with varying interaction ranges and show a direct connection between the type of dominant quasiparticles in the spectrum of the quench Hamiltonian and the type of nonanalyticities occurring in the Loschmidt return rate, a dynamical analog of the free energy. Our results also show a clear connection between the type of nonanalyticities and the phase of the long-time steady state in addition to how the order parameter decays at intermediate times. In particular, we discuss anomalous nonanalyticities that occur with no underlying local signature in the order parameter dynamics, unlike the traditional regular nonanalyticities that always correspond to zero crossings of the order parameter. Finally, we will show results for universal behavior with exclusively out-of-equilibrium critical exponents in the vicinity of DQPTs in random Ising chains, and additionally discuss proposals to experimentally extract them in current Rydberg atom platforms.

References:
[1] JCH, M. Van Damme, V. Zauner-Stauber, and L. Vanderstraeten, Phys. Rev. Research 2, 033111 (2020)
[2] JCH, N. Yegovtsev, and V. Gurarie, arXiv:1903.03109
[3] D. Trapin, JCH, and M. Heyl, arXiv:2005.06481
[4] JCH, D. Trapin, M. Van Damme, and M. Heyl, arXiv:2010.07307

The transport of a particle in the presence of a potential that changes periodically in space and in time can be characterized by the amount of work needed to shift a particle by a single spatial period of the potential. In general, this amount of work, when averaged over a single temporal period of the potential, can take any value in a continuous fashion. Here, we present a topological effect inducing the quantization of the average work. We find that this work is equal to the first Chern number calculated in a unit cell of a space-time lattice. Hence, this quantization of the average work is topologically protected. We illustrate this phenomenon with the example of an atom whose center of mass motion is coupled to its internal degrees of freedom by electromagnetic waves.

Ref: Phys. Rev. Lett. 123, 020601 (2019)