Thema: 
Electron dynamics in solids after ultrashort highenergy excitation 
Datum: 
18.11.19 
Uhrzeit: 
16:15 
Ort: 
H6 
Vortragender: 

Fachbereich Physik, TU Kaiserslautern 

Inhalt: 
Laser excitation of solid matter is a versatile tool for a broad range of fundamental studies as well as technological applications. Elementary processes within the material as electronphonon coupling or spincharge interaction can be studied on their intrinsic timescales. Extreme states of matter can be created with unknown thermodynamic properties. Possible technological applications include ultrafast switching of magnetization in hard drives, material removal with unprecedented precision, and compression of matter towards inertial fusion as a new controllable energy source. A joint starting point for such studies is the initial excitation. When an ultrashort laser pulse of visible light is absorbed by a solid, the electrons in the material are excited to a nonequilibrium state. A sequence of relaxation processes transfers the material into a new equilibrium. During this phase, common descriptions of the energy dissipation or electron density evolution fail, since averaged quantities like temperature lose their meaning. In this talk, I present our results on the interplay of microscopic collision processes determining the relaxation of nonequilibrium electron distributions. We apply complete Boltzmann collision integrals to study the mutual influence of different interaction and relaxation processes. We reveal that a partial nonequilibrium can disturb the thermalization of a subsystem with a faster intrinsic timescale. We provide corrections to temperaturebased approaches like modified and transiend electronphonon coupling parameters, and show the importance of the chemical potential relaxation for macroscopic quantitites like ferromagnetic magnetization. 
Ansprechpartner: 
Thema: 
Relativistic hydrodynamics, heavyion collisions, dynamical black holes and resurgent series 
Datum: 
18.10.19 
Uhrzeit: 
16:15 
Ort: 
V3201 
Vortragender: 

MaxPlanckInstitute for Gravitational Physics, Potsdam 

Inhalt: 
The past 12 years has constituted the golden age for theoretical studies of relatativistic hydrodynamics. The experimental motivation for these developments came from ultrarelativistic heavyion collision at RHIC and LHC accelerators in which the paradigm of stronglyinteracting medium modelled hydrodynamically became the working horse for explaining the data. These experimental and phenomenological developments have come handinhand with theoretical progress in understanding relativistic hydrodynamics as an effective description embedded in quantum field theory. In my colloquium I will review the line of thought based on AdS/CFT (holography), an approach to study stronglycoupled quantum field theories using gravitational techniques, focusing on understanding the limits of applicability of relativistic hydrodynamics in farfromequilibrium quantum field theory. A beautiful spinoff of this analysis is understanding hydrodynamic gradient expansion as a part of a transseries, which encodes, through resurgence, information about genuinely nonequilibrium excitations of a collective state of matter. Based on a series of works reviewed in arXiv:1610.02023 and arXiv:1707.02282, as well as some later / ongoing work. 
Ansprechpartner: 
Thema: 
Probing ALP dark matter through polarisation measurements towards a gravitational lensing galaxy. 
Datum: 
19.11.19 
Uhrzeit: 
14:15 
Ort: 
D6135 
Vortragender: 

Bielefeld 

Inhalt: 
Presence of dark matter in our Universe has been well established by astrophysical measurements. However, little is known of their nature. Of late, axionlike particles (ALPs) are emerging as one of the favoured candidate of dark matter. Because interaction of photons with ALPs induces birefringence amongst many other effects, propagation of linearly polarised electromagnetic signals through axion field imprint measurable signatures. In this seminar, I will discuss, from an observer's perspective, some of the astrophysical probes that have been used recently to constrain the mass of ALPs and their coupling with photons. I will present a novel technique to probe ALP dark matter which we are currently developing  using spectropolarimetric measurements at centimetrewavelengths towards gravitationally lensed polarised quasars. This technique is then applied to a lensed system where we obtain stringent constraints on the ALPs in a lensing galaxy far far away! 
Ansprechpartner: 
Thema: 
Topologically protected Landau level in the vortex lattice of a Weyl superconductor 
Datum: 
14.11.19 
Uhrzeit: 
16:00 
Ort: 
D5153 
Vortragender: 
Carlo Beenakker 
InstituutLorentz, Leiden University 

Inhalt: 
Good ideas in science may be rejected for a while, but they have a tendency to return when the time is right. This has happened with a 20year old conjecture that certain superconductors in a magnetic field would support a fieldindependent flatband in the middle of the gap. After Gorkov, Schrieffer, and Anderson proposed this idea of a superconducting Landau level, it was dismissed because it does not survive the broadening effects of the magnetic vortex lattice in a superconductor. We have discovered a way around this, by populating the Landau levels with Weyl fermions rather than with conventional electrons. Weyl fermions come in a lefthanded and in a righthanded variety and a Landau level contains only one of these two chiralities. The Landau level is protected from broadening by the vortex lattice because that needs to mix both chiralities in order to be effective. If this new twist on an old idea is borne out by experiments on Weyl superconductors, it would finally allow for quantum Hall physics to enter the superconducting domain. For example, the superconducting Landau level would have a quantized heat conductance parallel to the magnetic field. 
Ansprechpartner: 
Thema: 
Critical behaviour and characteristic polynomials of nonHermitian random matrices 
Datum: 
23.05.19 
Uhrzeit: 
16:15 
Ort: 
D5153 
Vortragender: 

University of Sussex 

Inhalt: 
I will discuss some recent developments regarding the normal matrix model. In particular my interest will be in certain critical models where the limiting support of the eigenvalues can radically change its topology by slightly adjusting an external parameter. I will discuss how aspects of the model can be explicitly mapped to the study of expectations of characteristic polynomials of nonHermitian random matrices (e.g. Ginibre or truncated unitary). Many of these averages are related to Painlevé transcendents, and by exploiting this, a precise and nontrivial asymptotic expansion of partition functions can be calculated in the critical models. This is joint work with Alfredo Deaño (University of Kent). 
Ansprechpartner: 
Thema: 
tba 
Datum: 
18.12.19 
Uhrzeit: 
16:15 
Ort: 
V3201 
Vortragender: 

University of Warwick 

Inhalt: 

Ansprechpartner: 