# Informal Friday Talks

### Friday, 15 February 2019, 2.30pm (26C 401)

Konstantin Nestmann:

How general are time-local master equations?

I will explore which evolutions of open quantum systems can be described using time-local master equations, specifically their relation to markovian dynamics and the question if genuinely non-markovian situations can be described using a time-local formalism.

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### Friday, 1 February 2019, 2.30pm (MBP2 116)

Feng Xiong:

Topological semimetals

Topological materials and their correspondent exotic physical properties have been explored widely not only in theory but also in experiments. This talk I want to share the main history of the discovery of topological family including topological insulator, semimetal and recently nodal line semimetals. By using symmetry classification, people can denote different system with certain topological invariant number. Most importantly, there is a close correspondence between bulk and surface states. At last, I also like to share model’s construction about nodal line semimetal to form various topologies in 3d BZ and discuss about what we can further explore in this field.

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### Friday, 18 January 2019, 2.30pm (MBP2 116)

Niclas Müller:

Topology in Floquet systems

Topological phases of matter have gained much interest in the last decade. These are systems whose electronic properties are dictated by the geometry of the (gapped) bandstructure. A topologically nontrivial bandstructure has no measurable consequences in the bulk of the system but leads to topologically protected exponentially localized edge states (TES) lying inside the bulk gap of a finite sample. Due to their unique properties the TES's are promising candidates for the realization of qubits in solid state systems.

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### Friday, 30 November 2018, 2.30pm (26C/401)

Yen-Ting Lin:

Topology and Majorana fermions in quantum wires

A review of A. Yu Kitaev Uspekhi Fizicheskikh Nauk . **44 **131 (2001)

### Friday, 9 November 2018, 2.30pm (26C/401)

Viktor Reimer:

What Kraus operators can tell us about open-system dynamics

I will talk about general open-system dynamics and in particularly focus on the Kraus operator-sum representation. Its general properties are reviewed and I establish an intuitive physical understanding directly related to measurement procedures. To give you some hands on experience, I present a derivation of Kraus representations for simple models. At the end, I give an overview about current research interests and issues yet to be resolved.

### Friday, October 19, 2018, 2pm (26C/401)

Jonas Becker:

One approach to paper quality plots with python's matplotlib

I will present only one of many possible approaches to plotting for journals. I will start with a few basics about image formats, fonts, LaTeX, pdf, etc. I will then go into boilerplate code I use for plotting. I look forward to hearing about your different approaches and discussing problems of mine. I will discuss concepts on the black board and perform some live coding on the projector.

### Friday, July 27, 2018, 2pm (MBPII 015)

Lukas Weber:

Surface critical behavior in the O(n) model with Landau theory and epsilon expansion

### Friday, June 1, 2018, 2pm (MBPII 015)

Xingjie Han:

A brief introduction to cuprates

### Friday, May 4, 2018, 2pm (MBPII 015)

Timo Reckling:

Doping a Mott insulator: physics of high-temperature superconductivity

The seminar will be based on a review paper (same title) by P. Lee, N. Nagaosa and X. Wen, which is published as: Rev. Mod. Phys. 78, 17 (2006).

We will cover chapters I-V of the paper.

### Friday, April 20, 2018, 2pm (MBPII 015)

Jonas Becker:

Fractionalized Excitations in Topological Spin Liquids

The seminar will be based on the review paper "Exotic quantum phases and phase transitions in correlated matter" by Fabien Alet, Aleksandra M. Walczak, Matthew P.A. Fisher, arXiv:cond-mat/0511516

Published in https://doi.org/10.1016/j.physa.2006.04.003 (Elsevier)

### Friday, April 6, 2018, 2pm (26 C 401)

Giulio Schober:

Ab initio materials physics and microscopic electrodynamics of media

We scrutinize the traditional macroscopic theory of electrodynamics in media from a modern first-principles point of view. Our main result is that the traditional, phenomenological theory has to be replaced with a theory which is derived from first principles (i.e., many-body quantum mechanics), and which is consistent with the common practice in ab-initio materials science.

### Friday, February 9, 2018, 2pm (26 C 401)

Jan Diekmann:

The seminar will be based on the historical paper by J. Kondo: Resistance Minimum in Dilute Magnetic Alloys, Prog. Theor. Phys., **32**:37, 1964

### Friday, February 2, 2018, 2pm (26 C 401)

Jovan Odavić:

Statistical tools for detecting power law distributions

We present a set of statistical tools based on maximum likelihood estimators and Kolmogorov-Smirnov statistics that give a statistically accurate measure of power law distributions in empirical data. These methods supersede common graphical identification of such distributions.The seminar will be based on the paper: SIAM Review **51**, 661-703 (2009)

### Friday, January 12, 2018, 2pm (26 C 401)

Lisa Markhof:

Functional renormalization group in one dimension with MPI

### Friday, December 8th, 2017, 2pm (26 C 401)

Niklas Dittmann:

Introduction to time-dependent density-functional theory

### Friday, December 1st, 2017, 2pm (26 C 401)

Carsten Lindner:

Time dynamics of open quantum system

### Friday, November 10th, 2017, 2pm (26 C 401)

Timo Reckling:

Functional renormalization group in 2D lattice Hubbard models

### Friday, October 27th, 2017, 2pm (26 C 401)

Stephan Heßelmann:

Determinantal Quantum Monte Carlo simulations for fermionic lattice models

I will review the basic formulation of Determinantal Quantum Monte Carlo (DQMC), which is based on the introduction of bosonic auxilliary fields via a discrete Hubbard-Stratonovitch decomposition to decouple the fermionic interaction term. I will discuss all the necessary numerical steps and use the one band Hubbard model as prototypical example for a fermionic lattice model of interest, as well as comment on the advantages and limitations of DQMC compared to other available methods.

### Friday, October 20th, 2017, 2pm (26 C 401)

Jannis Ehrlich:

Introduction to the GW approximation in many-body perturbation theory

During the last decade the GW approximation has been applied successfully to a broad range of real materials, resulting in more realistic band structures in contrast to DFT. While the latter is a mean field theory, GW originates from an expansion of the self energy of the many body system in terms of the one particle Green function.

In this talk I will derive the set of the Hedin Equations which are a self consistent set of equations describing the full electronic problem. By one iteration of those equations the GW-self energy can be obtained. I will further give a rough description how it is implemented in the Jülich GW-code "spex".

### Friday, September 8th, 2017, 2pm (26C 402)

Julian Mußhoff:

Introduction to Dynamical Mean-Field Theory (DMFT)

### Friday, August 25th, 2017, 2pm (26C 402)

Patrick Emonts:

Programming experience (makefiles/debuggers etc.)

### Friday, August 11th, 2017, 2pm (26C 402)

Kim Lind Pedersen:

The physics behind www.quantumgraphs.com

### Friday, July 28th, 2017, 2pm (26C 402)

Philippe Suchsland:

Applying neural networks in condensed matter physics

### Friday, July 14th, 2017, 2pm (26C 402)

Jovan Odavić:

Density-functional theory for lattice systems

### Friday, June 23rd, 2017, 2pm (26C 402)

Gregor Michalicek:

Introduction to density-functional theory (DFT)

### Friday, June 2nd, 2017, 2pm (26C 402)

Andishe Khedri:

Introduction to the numerical renormalization group (NRG)

### Friday, May 19th, 2017, 2pm (26C 402)

Jonas Becker:

Concluding remarks on the stochastic series expansion (SSE)

### Friday, May 5th, 2017, 2pm (26C 402)

Cornelie Koop:

Measurement in the stochastic series expansion (SSE)

### Friday, April 21st, 2017, 2pm (26C 402)

Patrick Emonts:

Introduction into stochastic series expansion (SSE)

### Friday, March 3rd, 2017, 2pm (26C 402)

Katharina Eissing :

Presentation of FRG (functional renormalization group) in a simple and instructive way