Thema: 
Why physics needs materials science and why materials science needs physics 
Datum: 
18.10.21 
Uhrzeit: 
16:15 
Ort: 
H4 
Vortragender: 

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 nondiffusive structural phase transition. Nondiffusive 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 nondiffusive 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: 
Thema: 
Random matrices, spin glasses, and machine learning 
Datum: 
23.07.21 
Uhrzeit: 
16:15 
Ort: 
ZOOM/Konferenzschaltung 
Vortragender: 

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: 
Thema: 
Machine Learning for Thermodynamic Observables in Lattice Field Theories 
Datum: 
06.07.21 
Uhrzeit: 
14:15 
Ort: 
Online, via ZOOM 
Vortragender: 

Perimeter Institute, Ontario, Canada 

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 MCMCbased methods that are limited to only estimate free energy differences. I will demonstrate the effectiveness of the proposed method for twodimensional $\phi^4$ theory and compare it to MCMCbased methods in detailed numerical experiments. 
Ansprechpartner: 
Thema: 
14:30 Untersuchung von frustrierten Spin1/2Systemen mit Hilfe von quantumthreecoloring am Beispiel des Kuboktaeders 
Datum: 
14.10.21 
Uhrzeit: 
14:30 
Ort: 
Hybrid  Zoom/D5153 
Vortragender: 
Florian Brökemeier 
Universität Bielefeld 

Inhalt: 

Ansprechpartner: 
Thema: 
On NonHermitian BetaEnsembles 
Datum: 
14.10.21 
Uhrzeit: 
16:00 
Ort: 
D5153 
Vortragender: 

Universität Bielefeld 

Inhalt: 
Loggases 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 Loggases in two dimensions, i.e. in the nonHermitian 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 nearestneighbour spacing distribution of large matrices. In the second part of the talk we introduce the study of symmetry classes in nonHermitian RMT. We conjecture that the classes of complex symmetric and complex quaternion matrices can be effectively described by Loggases in two dimensions with noninteger inverse temperatures. 
Ansprechpartner: 
Thema: 
Central Limit Theorems to Stable and Invariant Random Matrices 
Datum: 
20.10.21 
Uhrzeit: 
09:00 
Ort: 
ZOOM / Konferenzschaltung 
Vortragender: 

Melbourne University 

Inhalt: 
Heavytailed 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 heavytailed 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 heavytailed 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: 