# Aktuelle Veranstaltungen

## Kolloquium

Thema:

### Why physics needs materials science and why materials science needs physics

Datum:

18.10.21

Uhrzeit:

16:15

Ort:

H4

Vortragender:

Prof. Gabi Schierning

Bielefeld University

Inhalt:

It is the beauty and strength of solid state physics to comprehensively describe matter with the help of both, experimentally accessible model systems and theory. Regrettably, the number of materials in which the pure models accurately describe the experimental findings is usually small, and very often these model systems have exotic compositions and no or only limited practical application. The strength of materials science lies in its pragmatic way of optimizing complex materials and alloys for applications, often – because of the application – with high significance and impact. But regrettably, too, this materials’ optimization has only limited guidance from a theoretical point of view. I will discuss this discrepancy between experimentally and/or theoretically accessible model systems in solid state physics and engineered materials for broad applications on the example of charge density wave (CDW) phases. Conceptually CDW phases are well understood. Their signatures have theoretically been proposed and experimentally been evidenced in a handful of materials, all of them exotic, and rarely relevant for application, but fascinating enough to give work to a large community of physicist. The most important features that give an experimental hint on the occurrence of such CDW phases are fermi surface nesting, phonon softening and with it the softening of the elastic constants, Kohn anomalies, transport anomalies, and most importantly, a non-diffusive structural phase transition. Non-diffusive structural phase transitions are well known in materials science. The most important one is the martensitic phase transition that occurs in steel or shape memory alloys and students of materials science learn it in their first semester. These non-diffusive structural phase transitions are important enough to give work to a large community of material scientists. Looking in more detail at them, it is found that they show fermi surface nesting, phonon softening and with it the softening of the elastic constants, Kohn anomalies and transport anomalies, all strong indications for CDW phases to occur. Nonetheless has theory never made an attempt to describe such applied engineering materials, neither has the engineering community ever tried to let their materials’ development be guided by theory. Maybe it is time that the theoretical concepts of charge ordering in matter meet the engineering of steel!

Ansprechpartner:

Dekan

## Kolloquium Mathematische Physik

Thema:

### Random matrices, spin glasses, and machine learning

Datum:

23.07.21

Uhrzeit:

16:15

Ort:

ZOOM/Konferenzschaltung

Vortragender:

Jon Keating

Oxford University

Inhalt:

I will describe some problems relating to machine learning and their connections to random matrix theory and spin glasses. These connections give a mathematical framework for understanding in qualitative terms the effectiveness of certain algorithms that are important in machine learning, but developing them into precise models remains a major challenge. I will reflect on the different roles played by models in computer science and physics, focussing on those involving random matrices.

Ansprechpartner:

G. Akemann

## Seminar Hochenergiephysik

Thema:

### Machine Learning for Thermodynamic Observables in Lattice Field Theories

Datum:

06.07.21

Uhrzeit:

14:15

Ort:

Online, via ZOOM

Vortragender:

Lena Funcke

Inhalt:

In this talk, I will discuss how applying machine learning techniques to lattice field theory is a promising route for solving problems where Markov Chain Monte Carlo (MCMC) methods are problematic. More specifically, I will show that deep generative models can be used to estimate thermodynamic observables like the free energy, which contrasts with existing MCMC-based methods that are limited to only estimate free energy differences. I will demonstrate the effectiveness of the proposed method for two-dimensional $\phi^4$ theory and compare it to MCMC-based methods in detailed numerical experiments.

Ansprechpartner:

G. Endrödi

## Seminar Kondensierte Materie

Thema:

### 14:30 Untersuchung von frustrierten Spin-1/2-Systemen mit Hilfe von quantum-three-coloring am Beispiel des Kuboktaeders

Datum:

14.10.21

Uhrzeit:

14:30

Ort:

Hybrid - Zoom/D5-153

Vortragender:

Florian Brökemeier

Universität Bielefeld

Inhalt:

Ansprechpartner:

Jürgen Schnack

## Seminar Mathematische Physik

Thema:

### On Non-Hermitian Beta-Ensembles

Datum:

14.10.21

Uhrzeit:

16:00

Ort:

D5-153

Vortragender:

Patricia Päßler

Universität Bielefeld

Inhalt:

Log-gases with inverse temperature beta are systems with many applications in physics, for example in the theory of superconductors or the fractional quantum Hall effect. For some specific values of beta a correspondence to random matrix theory (RMT) is well established. The advantage of this connection is the usage of the RMT methods in the study of those systems. The goal of this talk is the discussion of Log-gases in two dimensions, i.e. in the non-Hermitian case, for more general values of the inverse temperature. Therefore, we study in the first part a model of normal 2 × 2 matrices with beta in [0,2] and discuss whether we find a surmise for the nearest-neighbour spacing distribution of large matrices. In the second part of the talk we introduce the study of symmetry classes in non-Hermitian RMT. We conjecture that the classes of complex symmetric and complex quaternion matrices can be effectively described by Log-gases in two dimensions with non-integer inverse temperatures.

Ansprechpartner:

Gernot Akemann

## Seminar Bielefeld-Melbourne Zufallsmatrizen

Thema:

### Central Limit Theorems to Stable and Invariant Random Matrices

Datum:

20.10.21

Uhrzeit:

09:00

Ort:

ZOOM / Konferenzschaltung

Vortragender:

Mario Kieburg

Melbourne University

Inhalt:

Heavy-tailed random matrices have surprising and novel effects that can be hardly seen with the classical ensembles. For instance, in recent years it was shown that heavy-tailed Wigner matrices can exhibit localised eigenvector statistics for the eigenvalues in the tail while everything stays the same as we know it for the bulk statistics of a GUE. This effect, some intriguing as well as real world applications, and some own numerical experiments have motivated us to study invariant heavy-tailed random matrices. One of the questions we have addressed has been about central limit theorems at fixed matrix dimensions and invariant random matrices that are stable when adding independent copies of the random matrix under consideration. I will report on our new findings and will sketch the main ideas of their proofs in the present talk. These projects have been carried out in collaboration with Jiyuan Zhang and Adam Monteleone.

Ansprechpartner:

Mario Kieburg

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