Thema: 
Soft Xray Spectroscopy of Quantum Materials 
Datum: 
15.11.22 
Uhrzeit: 
14:15 
Ort: 
H6 
Vortragender: 

Universität Kiel 

Inhalt: 

Ansprechpartner: 
Thema: 
tba 
Datum: 
04.11.22 
Uhrzeit: 
16:15 
Ort: 
D5153 
Vortragender: 

Universität Mainz 

Inhalt: 

Ansprechpartner: 
Thema: 
Mitigating the Hubbard Sign Problem. A Novel Application of Machine Learning 
Datum: 
07.11.22 
Uhrzeit: 
16:15 
Ort: 
D6135 
Vortragender: 
Marcel Rodekamp 
FZ Jülich 

Inhalt: 
Many fascinating systems suffer from a severe (complex action) sign problem preventing us from simulating them with Markov Chain Monte Carlo. One promising method to alleviate the sign problem is the transformation towards Lefschetz Thimbles. Unfortunately, this suffers from poor scaling originating in numerically integrating of flow equations and evaluation of an induced Jacobian. In this talk we present a Neural Network architecture based on complexvalued affine coupling layers. This network performs such a transformation efficiently, ultimately allowing simulation of systems with a severe sign problem. We test this method within the Hubbard Model at finite chemical potential, modelling strongly correlated electrons on a spatial lattice of ions. 
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Thema: 
Assigning Temperatures to Eigenstates 
Datum: 
04.11.22 
Uhrzeit: 
14:15 
Ort: 
D5153 
Vortragender: 
Masudul Haque 
TU Dresden 

Inhalt: 
In formulating the statistical mechanics of isolated quantum systems, an inescapable issue is the definition of temperature, which is not a priori defined within closedsystem quantum mechanics. We examine and compare different possible ways of assigning temperatures to energies, or equivalently, to eigenstates. The commonly used assignment of temperature is based on the canonical energytemperature relationship, which depends only on energy eigenvalues and not on the structure of eigenstates. For eigenstates, we consider defining temperature by minimizing the distance between (full or reduced) eigenstate density matrices and canonical density matrices. We show that for full eigenstates, the minimizing temperature depends on the distance measure chosen, and matches the canonical temperature for the trace distance; however the two matrices are not close. With reduced density matrices, the minimizing temperature has fluctuations that scale with subsystem and system size but is apparently independent of distance measure, and in particular limits the two matrices become equivalent. 
Ansprechpartner: 
Thema: 
Manyparticles diffusing with resetting: study of the largedeviation properties of the flux distribution 
Datum: 
05.05.22 
Uhrzeit: 
16:00 
Ort: 
D5153 
Vortragender: 
Costantino Di Bello 
Inhalt: 
In this paper we studied a model of noninteracting particles moving on a line following a common dynamics. In particular we considered either a diffusive motion with Poissonian resetting, and a runandtumble motion with Poissonian resetting. We were interested in studying the distribution of the random variable $Q_t$ defined as the flux of particles through origin up to time $t$. We used the notation $P(Q,t)$ to identify the probability $\mathbb{P}\{Q_t=Q\}$. We considered particles initially located on the negative half line with a fixed density $\rho$. In fully analogy with disordered systems, we studied both the annealed and the quenched case for initial conditions. In the former case we found that, independently from the specific dynamics, $P_\mathrm{an}(Q,t)$ has a Poissonian shape; while in the latter case, for what concerns the diffusive dynamics with resetting, the large deviation form of the quenched distribution reads $P_\mathrm{qu}(Q,t)\sim \exp\left[r^2t^2 \Psi_\mathrm{diff}\left(\dfrac{Q}{\rho t}\right)\right]$ with the large deviation function $\Psi_\mathrm{diff}(x)$ exhibiting a discontinuity in the third derivative, hence aiming, despite the simplicity of the model, at the exhistence of a dynamical phase transition. The quenched distribution for the runandtumble dynamics, instead, does not exhibit any kind of phase transition. Importance sampling Monte Carlo simulations were performed to prove the analytical results. References: Current fluctuations in noninteracting runandtumble particles in one dimension Tirthankar Banerjee, Satya N. Majumdar, Alberto Rosso, and Grégory Schehr, Phys. Rev. E 101, 052101 https://doi.org/10.1103/PhysRevE.101.052101 Current Fluctuations in One Dimensional Diffusive Systems with a Step Initial Density Profile B. Derrida and A. Gerschenfeld, J. Stat. Phys. 137, 978 (2009) https://doi.org/10.1007/s1095500998301 
Ansprechpartner: 
Thema: 
Unitary matrix integrals, spectral form factors, and long range random walk models 
Datum: 
05.10.22 
Uhrzeit: 
09:00 
Ort: 
ZOOM / Konferenzschaltung 
Vortragender: 

University of Amsterdam 

Inhalt: 
Unitary matrix integrals over symmetric functions have a wide variety of applications, including quantum chaos, random processes, enumerative combinatorics, and number theory. In this talk, we derive various novel identities on such integrals, demonstrating universality of the spectral form factor for a broad class of matrix models. We then extend these results and apply them to correlation functions of longrange random walk models, leading to various surprising relations and dualities between them, as well convenient methods for their computation. 
Ansprechpartner: 