Condensed Matter Physics Seminar
2006/2007 | 2007/2008 | 2008/2009 | 2009/2010 | 2010/2011 | 2011/2012 | 2012/2013 | 2013/2014 | 2014/2015 | 2015/2016 | 2016/2017 | 2017/2018 | 2018/2019 | 2019/2020 | 2020/2021 | 2021/2022 | 2022/2023 | 2023/2024 | 2024/2025
2023-03-24 (Friday)
room 1.02, Pasteura 5 at 12:15 
Piotr Wrzosek (IFT UW)A hole in the Ising antiferromagnet on a square lattice: revisiting an old problem
The motion of a single hole in the t-J_z model on a square lattice is one of the oldest and (naively) best understood problems in the field of strongly correlated electrons. This is because such a problem can approximately be mapped onto the case of a single particle moving in the linear potential. The latter yields a series of eigenstates whose energies scale as (J_z/t)^2/3, leading to the onset of a so-called ladder spectral function. Here we show that such a spectrum is indeed realized on a Bethe lattice. However, on the square lattice the situation is far richer: besides the ladder spectrum, the spectral function of a t-J_z model contains also a substantial incoherent part as well as eigenstates whose energies scale linearly with J_z/t.
2023-03-17 (Friday)
room 1.02, Pasteura 5 at 12:15
Jeffrey Everts (FUW)Nematic electrolytes in and out of equilibrium: topological-defect control and spintronics-inspired electromotive forces
In this talk I will discuss the physics of nematic electrolytes: ions in nematic liquid crystals. Liquid crystals can flow like ordinary liquids, but there are also differences, such as the emergence of elasticity. Nematics can be seen as orientationally ordered anisotropic dielectrics that are flexoelectric - they can form a spontaneous polarisation upon straining the material which is not accounted for in the dielectric tensor. When convoluted with the ionic degrees of freedom, several interesting phenomena can occur. I will demonstrate that nematic electrolytes in some cases can be characterized by an anisotropic Debye screening length and how topological defects can become charged and/or cause ionic charge separation around them forming ionic multilayers (instead of the familiar double layers). Furthermore, the topological defects can alter the surface charge distribution of mesoscopic objects such as flat plates or spheres and alter the differential capacitance depending on its global orientation. Finally, I will discuss our recent work on the charge dynamics of a nematic electrolyte, which in a sense have many similarities with that of an Archimedes' screw: a hydraulic machine that can either pump or generate energy as a turbine. This leads to a coupling of the charge current to the winding density of a liquid crystal, which naturally leads to topological defects being driven by ionic current or being used as inductors.
2023-03-10 (Friday)
room 1.02, Pasteura 5 at 12:15
Hiroyuki Yamase (NIMS Tsukuba, Japan)Compton scattering study of Fermi surface in high-temperature cuprate superconductors
After reviewing x-ray Compton scattering, I explain a motivation to perform x-ray Compton scattering for cuprate superconductors even though the shape of the Fermi surface is believe to be established already in cuprates. Our new discovery is a deformation of the Fermi surface by the underlying electronic nematicity.
2023-01-27 (Friday)
room 1.02, Pasteura 5 at 12:15
Stefanos Papanikolaou (NOMATEN Centre of Excellence, National Centre for Nuclear Research, Otwock, Poland)Multiscale modeling of nanomechanical deformation in high entropy alloys
Nano- and micro- scale testings of materials have been a key tool for understanding the different mechanical properties of materials at small scale with respect to bulk ones. Experimental techniques such as nanoindentation, digital image correlation, and uniaxial pillar testing have provided a great wealth of information and insights on the investigation of size effects in mechanical behavior of materials, and a comparison to multiscale modeling approaches. However, a precise comparison to experiments, especially for advanced materials towards extreme conditions' applications, pose stringent requirements on accuracy of both experimental procedures and modeling predictions.In this work, I will focus on the multiscale modeling approaches of high entropy alloys. I will discuss density functional theory, molecular dynamics and continuum plasticity approaches to model these materials, with a focus on experimental comparisons. I will discuss the basic physics and usefulness of machine learning and statistical data science methods, for the applications of nanoindentation and digital image correlation, and several multicomponent alloys, such as equiatomic NiCoCr, FeNiCr, and NiCoCrFeMn.
2023-01-20 (Friday)
room 1.02, Pasteura 5 at 12:15
Francesco Ferrari (Goethe University, Frankfurt)Gapless spin liquids in disguise
Variational wave functions constitute a precious tool for the study of frustrated spin models, which represent a formidable challenge for most numerical methods. In this talk, we discuss how magnetic and nonmagnetic phases of spin systems can be described by Gutzwiller-projected fermionic wave functions. In addition to ground state properties, we describe a variational approach to compute dynamical spectra by means of suitable Ansätze for excited states. The analysis of the dynamical structure factor of frustrated systems allows us to show how gapless spin liquids can "disguise themselves" on cylindrical geometries.
2023-01-13 (Friday)
room 1.02, Pasteura 5 at 12:15
David Mitrouskas (IST Klosterneuburg, Austria)The Fröhlich polaron at strong coupling
Despite being introduced by Landau almost 90 years ago, there are still some basic aspects of the polaron that are not fully understood from a mathematical point of view. In particular, the connection between Pekar's semi-classical analysis, in which the field is treated as a classical variable, and the Fröhlich model at strong coupling has posed interesting mathematical problems. In this talk, we will present recent results concerning the spectrum of the Fröhlich Hamiltonian. These include an asymptotic formula for the energy momentum relation and the abundance of eigenvalues below the essential spectrum at fixed total momentum. If time permits, we will also discuss the dynamical properties of the polaron. For suitable initial conditions, the quantum dynamics can be approximated by the time-dependent Landau-Pekar equations, a set of coupled partial differential equations that describe the evolution of an electron in a slowly varying classical polarization field. The talk is based on joint work with J. Lampart, N. Leopold, K. Mysliwy, S. Rademacher, B. Schlein and R. Seiringer.
2022-12-16 (Friday)
room 1.02, Pasteura 5 at 12:15
Oleksandr Gamayun (IFT FUW)Exact results for one-dimensional mobile impurity
I will consider an integrable model of a mobile impurity propagating in a one-dimensional gas of free fermions. It is the simplest yet fundamental model capturing the peculiar physics and mathematics of the non-equilibrium processes. The integrability allows one to obtain a complete nonperturbative solution and express physical quantities in terms of the Fredholm determinants. After a detailed analysis of these determinants, I will discuss several striking physical phenomena, such as incomplete relaxation, Bloch oscillations, and momentum-dependent impurity statistics.
2022-12-09 (Friday)
room 1.02, Pasteura 5 at 12:15
Dieter Vollhardt (Center for Electronic Correlations and Magnetism, University of Augsburg)Why solve many-body problems in infinite dimensions?
A pedagogical introduction to solving classical and quantum mechanical interaction problems in infinite spatial dimensions is given. As a warm-up Bohr's atomic model is derived by solving the Schrodinger equation in infinite dimensions. Then we solve classical many-body problems (Ising model, Ising model with random coupling, hard sphere fluids) in this limit. Regarding quantum particles, the solution of the Hubbard model in infinite dimensions is discussed in detail. It corresponds to a dynamical mean-field theory, which allows to compute the properties of correlated electrons in models and materials. Finally, strategies to go beyond mean-field theory are presented.
2022-12-02 (Friday)
room 1.02, Pasteura 5 at 12:15
Przemysław Piekarz (IFJ Kraków)Kagome lattices and chiral phonons
In recent years, the chiral phonons with a quantized pseudoangular momentum were predicted theoretically and studied in various types of systems. During the seminar, I will present the calculation method, which enables to obtain the phonon dispersion relations and to analyze chiral properties of phonons. We applied this approach to investigate the lattice dynamics and structural phase transitions in the series of CoSn-like compounds containing kagome and honeycomb 2D lattices. I will discuss also the mechanism of charge ordering in the kagome superconductors AV3Sb5 (A=K,Cs,Rb).
2022-11-25 (Friday)
room 1.02, Pasteura 5 at 12:15
Adam Kłosiński (IFT UW)Topology of chalcogen chains
In this work we investigate the topological properties of selenium and tellurium chains. We postulate a realistic model which includes spin-orbit interaction. This model is topologically non-trivial, with a topological invariant protected by a crystalline symmetry. We describe the end states, which are orbitally polarized, with an orbital density modulation strongly peaked at the edge. Investigating further, we propose a model with a simplified but related geometry. This model, which is computationally simpler, decomposes into three SSH-3 chains allowing us to relate the invariant of a chalcogen chain to that of the SSH-3 model with an additional crystalline symmetry. We contrast this result with that recently obtained for a $p$-orbital zigzag chain on a plane -- the orbital SSH model.