# Gäste

### Freitag, 11. Juni 2021, 14:00 Uhr

Kilian Fraboulet (Université Paris-Saclay)

Path-integral approaches to strongly-coupled quantum many-body systems: applications to a (0+0)-D O(N) model

As many-body physicists, we aim at designing reliable and computationally affordable methods to describe many-body quantum systems. The path-integral formulation of quantum field theory provides us with plenty of techniques to achieve this. We will particularly focus on its ability to describe strongly-coupled many-body systems of finite size. In particular, collective behaviors can be efficiently described in such systems through the exploitation of spontaneous symmetry breaking (SSB) in mean field approaches. However, for mesoscopic systems, the fluctuations of order parameters associated with broken symmetries can not be neglected and tend to radiatively restore such symmetries. Hence, the efficiency of theoretical approaches in the treatment of finite-size quantum systems can notably be studied via their ability to restore spontaneously broken symmetries.

In this respect, a zero-dimensional O(N) model is taken as a theoretical laboratory to perform a comparative study of many state-of-the-art path-integral techniques combined or not with Hubbard-Stratonovich transformations: perturbation theory with various resummation methods (Padé-Borel, conformal mapping, Meijer-G), enhanced versions of perturbation theory (transseries derived via Picard-Lefschetz theory, optimized perturbation theory), self-consistent perturbation theory based on effective actions (CJT formalism, 4PPI effective action,...), functional renormalization group (FRG) techniques (FRG based on the Wetterich equation, DFT-FRG, 2PI-FRG). Some connections between these methods will be emphasized and their performances in the strongly-coupled regime will be examined in detail.

This seminar takes place online.

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### Dienstag, 18. Mai 2021, 16:00 Uhr

Eslam Khalaf (Harvard University)

The theory of Moiré materials, such as twisted bilayer graphene

This seminar takes place online.

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### Freitag, 16. April 2021, 16:30 Uhr

Erik van Loon and Tim Wehling (University of Bremen)

Random Phase Approximation for gapped systems: role of vertex corrections and applicability of the constrained random phase approximation

This seminar takes place online.

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### Dienstag,16. März 2021, 16:00 Uhr

Alexandre René (University of Ottawa)

Adapting deep learning to close the theory-experiment gap in neuroscience

This seminar takes place online.

### Mittwoch,17. Februar 2021, 15:30 Uhr

Michael Scherer (University of Cologne)

Flatten the band: Novel quantum materials with a twist

This seminar takes place online.

### Mittwoch,27. Januar 2021, 17:00 Uhr

Shibabrata Nandi (FZ Jülich)

Structure, magnetism and superconductivity in Fe-based superconductors

In iron-based high-temperature superconductors, magnetic fluctuations and magneto-elastic effects are believed to be important for the superconducting electron pairing mechanism. To gain insight into the interplay between the different ordering phenomena and the underlying couplings we studied the magnetic order and lattice distortion on *A*Fe_{2}As_{2 } (*A* = Ca, Sr, Ba, Eu) single crystals by neutron and x-ray diffraction. High-resolution x-ray diffraction and neutron scattering measurements reveal an unusually strong response of the lattice and ordered magnetic moment to superconductivity in Co-doped BaFe_{2}As_{2}. We propose that the coupling between lattice and superconductivity is indirect and arises due to the magnetoelastic coupling, in the form of emergent nematic order, and the strong competition between the magnetism and superconductivity. In contrast to the coexistence of superconductivity and antiferromagnetism in the Co-doped “122” sample, we show that the superconductivity and ferromagnetism coexist in the P-doped Eu based “122” Fe-pnictides. Coexistence between these two antagonistic phenomena are puzzling but can be explained in terms of the formation of a spontaneous vortex state.

This seminar takes place online.

### Dienstag, 26. Mai 2020, 11:00 Uhr

Idan Tamir (FU Berlin)

Low-dimensional superconductors, STM and beyond

In my talk I will discuss the sensitivity of the superconducting state in thin films and will present new spectroscopic results obtained from such films**.**

This seminar takes place in room **MBP1 015**.

### Dienstag, 18. Februar 2020, 11:00 Uhr

Peizhe Tang (MPI for the Structure and Dynamics of Matter, Hamburg)

Phase transitions and electronic tuning in magnetic topological materials

The interplay between magnetism and topology brings rich physics in the condensed matter physics in recent years. Many exotic phenomena have been observed in related systems, including the quantum anomalous Hall (QAH) effect, Weyl fermions, and antiferromagnetic (AFM) Dirac fermions. In this seminar, I will talk about three topics. The first one is about the magnetic phase transition driven by topological phase transition in magnetically doped topological insulator thin films, such as Cr doped Bi2(SexTe1-x)3 thin films [1]. In the second part, I will expand the notion of Dirac fermions into AFM system, in which both time reversal symmetry (T) and inversion symmetry (P) are broken but their combination PT is survived [2]. The third one is about the QAH phase with large Chern number driven by electric field in MnBi2Te4 thin film, whose 3D bulk state is reported as an AFM TI and thick film as axion insulator [3]. Our results provide several possible platforms to study the interplay of topological physics and magnetisms.

[1] Jingsong Zhang, Cuizu Chang, Peizhe Tang, et.al., Science **339**, 1582 (2013)

[2] Peizhe Tang, Quan Zhou, Gang Xu, Shou-Cheng Zhang, Nature Physics **12**, 1100 (2016)

[3] Shiqiao Du, et.al., arXiv:1909.01194 (2019)

This seminar takes place in room **26C 401.**

### Donnerstag, 13. Februar 2020, 15:00 Uhr

Johannes Lischner (Imperial College, London)

Controlling the electronic structure of 2d materials by twisting and defects

This seminar takes place in room **MBP1 026.**

### Freitag, 13. Dezember, 2019, 13:30 Uhr

Nagamalleswara Rao Dasari (Universität Erlangen-Nürnberg)

Ultrafast dynamics of strongly correlated systems

Recent studies on correlated systems have taken a new direction with the availability of ultrashort laser pulses. Using these pulses, we can excite and probe the physical properties of quantum materials on their intrinsic time scales before the system returns to the thermal equilibrium. Such experiments offer us an opportunity to explore hidden quantum states of matter and possible transient enhancement of collective orders in the correlated systems. In the first part of my talk, I will discuss how to uncover local interactions of Mott insulators, for example, Hubbard U and Hund's coupling J, using subcycle terahertz pulse. The second part of my talk will focus on controlling of non-local fluctuations in the low-dimensional systems using asymmetric light pulses.

[1] Nagamalleswararao Dasari, Jiajun Li, Philipp Werner and Martin Eckstein, "Revealing Hund’s multiplets in Mott insulators under strong electric fields". arXiv:1907.00754

[2] Nagamalleswararao Dasari and Martin Eckstein, "Ultra-fast electric field controlled spin correlations in the Hubbard model". Phys. Rev. B **100**, 121114 (R) 2019

This seminar takes place in room **26C 401.**

### Donnerstag, 14. November 2019, 10:00 - 11:40 Uhr

Bassano Vacchini (University of Milan)

Master equations for the description of non-Markovian dynamics in open quantum system theory

Open quantum system theory deals with the dynamics of non-isolated quantum systems. Their interaction with other quantum degrees of freedom, typically called environment, is effectively taken into account, giving rise to effects not appearing in a unitary evolution. The dynamics of open quantum systems can in particular be non-Markovian, i.e. feature memory effects. In recent years a large amount of research work has been devoted to define and characterize non-Markovian quantum dynamics.This tutorial lecture provides an introduction to this research field.

The first part of the presentation will motivate and introduce the basic concepts in the description of Markovian open quantum system dynamics. We will introduce the notion of completely positive quantum dynamical map for the evolution of a system affected by a quantum environment, and consider Lindblad master equations giving rise to quantum dynamical semigroups. An approach to the characterization of non-Markovian quantum dynamics based on the behavior of the trace distance as quantifier of distinguishability between states will further be introduced. We will then consider the main projection operator techniques, allowing to consider memory effects and leading to master equations in time-local or memory kernel form. We will finally construct classes of memory kernels that can be linked to a collisional dynamics and do provide well-defined complete positivity time evolutions.

This seminar takes place in room **MBP2 116.**

### Mittwoch, 11. September 2019, 10:00 Uhr

Takuya Okogawa (Technical University of Denmark):

Helical edge states coupled to localized spins

We research on the electronic and the transport properties of the helical edge state in Quantum Spin Hall insulator coupled to an environment localized spin: spin bath. We calculate the density of states and the current of this system using the equilibrium and non-equilibrium Green’s function formalism, respectively. Our result of the current correction agrees with the one derived from the Fermi Golden rule. Furthermore, the calculation is also performed for the system with an additional external magnetic field.

This seminar takes place in room **26C 401.**

### Dienstag, 10. September 2019, 15:30 Uhr

Karsten Held (TU Vienna):

Spatio-temporal electronic correlations: From quantum criticality to pi-tons

Electronic correlations give rise to fascinating physical phenomena such as high-temperature superconductivity and (quantum) criticality, but their theoretical description remains a grand challenge. Dynamical mean field theory has been a big step forward: it accurately describes the local electronic correlations including their quantum, temporal

dynamics. In recent years diagrammatic extensions of dynamical mean field theory, such as the dynamical vertex approximation, have been developed. These methods not only include the dynamics but also non-local correlations on all length scales [1].

After a brief introduction to these methods, I will present some recent highlights: the discovery of a new universality class of quantum critical exponents in the Hubbard model [2], the description of quantum criticality in the periodic Anderson model [3], and the discovery of new polaritons in strongly correlated electron systems, coined $\pi$-tons[4].

[1] G. Rohringer, H. Hafermann, A. Toschi, A. A. Katanin, A. E. Antipov, M. I. Katsnelson, A. I. Lichtenstein, A. N. Rubtsov, and K. Held, Rev. Mod. Phys. **90**, 025003 (2018)

[2] T. Schäfer, A. A. Katanin, K. Held, and A. Toschi Phys. Rev. Lett. **119**, 046402 (2017).

[3] T. Schäfer, A. A. Katanin, M. Kitatani, A. Toschi, and K. Held Phys. Rev. Lett. (2019) accepted [arXiv:1812.03821].

[4] A. Kauch, P. Pudleiner, K. Astleithner, T. Ribic, and K. Held [arXiv:1902.09342]

This seminar takes place in room **MBP1 026.**

### Donnerstag, 05. September 2019, 13:00 Uhr

Lara Ortmanns (TU Delft)

Magnons in Bilayers of van der Waals Materials

Van der Waals magnets are materials that are composed of 2D-layers bounded to each other through weak van der Waals interactions. We have calculated monolayer and bilayer dispersions for different types of exchange couplings and anisotropies. We discuss energy gaps, cases of degeneracy, their origin and possible lifting and explain how we derived analytic expressions for a bilayer with ferromagnetic intra-and antiferromagnetic interlayer coupling. We conclude with an outlook to possible further extensions of the project.

This seminar takes place in room **MBP1 026.**

### Donnerstag, 05. September 2019, 14:30 Uhr

David Schlegel (University of Göttingen)

Time-periodic Structure in Open Quantum Systems

Motivated by the idea of quantum time crystals in which systems exhibit time-translation symmetry breaking, we explore a novel approach to achieve time-periodic structure in open quantum systems instead of

closed systems with many-body interactions. We employ the method of quantum trajectories to simulate the dynamics of the open quantum system as stochastic processes. Applying this method to a system of

non-interacting Fermions on a ring coupled to an environment modeling local measurements, we reveal interrupted time-periodic structure in individual quantum trajectories.

In my talk about my masterproject, I will outline the fundamental ideas behind quantum time crystals, the theory of open quantum systems with focus on the quantum trajectory method, and show recent results for the considered open quantum system.

This seminar takes place in room **MBP1 026.**

### Dienstag, 29. Januar 2018, 16:00 Uhr

Ka Chun Chan (University of Freiburg)

Heat current and seebeck effect through single molecular junction

This seminar takes place in room** 26C 401.**

### Donnerstag, 29. November 2018, 11:00 Uhr

James Freericks (Georgetown University, Washington DC)

The Keldysh-ETH approach to quantum computing

It is well known that thermal state preparation at low-temperature is a challenge for current quantum computers. Yet, such an initial state is required for many different applications including simulating Green's functions. Here, we propose an alternative that is based on the eigenstate thermalization hypothesis for equilibrium systems and on Keldysh's nonequilibrium formulation for driven dissipative systems. We show how each can be employed within more conventional algorithms to simulate strongly correlated condensed matter systems on quantum computers.

This seminar takes place in room** MBP2, 116.**

### Mittwoch, 24. Oktober 2018, 10:30 Uhr

Luca Binci (University of Rome):

Ab-initio frequency dependent Born effective charges

High temperature superconductivity reached a new record with the discovery of H3S. In this material it has been found the highest critical temperature, even though at huge pressures. An interesting fact is that, in the normal phase, the ions of this system exhibit remarkable high effective charges. The effective charge is a well-known quantity in first principles calculations. It describes the polarization induced by the collective displacements of nuclei belonging to a given sublattice. This quantity is well defined for insulating crystals; however in metals it needs a generalization at finite frequency since, for this kind of systems, the static polarization is not defined.

In this thesis work we plan to develop a method to calculate ab-initio the Born effective charge tensor at finite frequencies. The final goal is to reproduce the infrared absorption spectrum of H3S.

This seminar takes place in room** 26C 401.**

### Dienstag, 18. September 2018, 16 Uhr

Yasuhiro Takura (University of Tsukuba, Japan):

Excess entropy production in quantum system

This seminar takes place in room** 26C 401.**

### Mittwoch, 29. August 2018, 14 Uhr

Ronald Starke (TU Bergakademie Freiberg):

Relativistic covariance of electrodynamics in media

This seminar takes place in room** 26C 401.**

### Freitag, 06. Juli 2018, 13 Uhr

Konstantin Nestmann (TU Dresden):

Time-convolutionless master equation: series expansions and convergence:

The talk's topic concerns the formally exact time-convolutionless master equation describing the dynamics of open quantum systems out of equilibrium. New series expansions for the master equation’s generator are presented and compared to existing series expansions. One of these derived series is then used to describe the stationary states for a quantum dot model.

This seminar takes place in room ** MBP1 015.**

### Mittwoch, 20. Juni, 2018, 10 Uhr

Konstantinos Ladovrechis (Institute of Theoretical Physics, IFW Dresden):

Anomalous Floquet topological crystalline insulators

Periodically driven systems can host so-called anomalous topological phases, in which protected boundary states coexist with topologically trivial Floquet bulk bands. An anomalous version of reflection symmetry protected topological crystalline insulators is introduced, obtained as a stack of weakly-coupled two-dimensional layers. The system has tunable and robust surface Dirac cones even though the mirror Chern numbers of the Floquet bulk bands vanish. The protection of boundary modes is discussed by adapting the scattering theory of topological invariants to mirror symmetry protected topological phases.

This seminar takes place in room** MBP2 015.**

### Dienstag, 19. Juni, 2018, 16 Uhr

Hernán Calvo (Instituto de Física Enrique Gaviola (CONICET) and FaMAF, Universidad Nacional de Córdoba, Argentina)

Quantum-dot based nanomotors with strong Coulomb interactions

In recent years there has been increasing excitement regarding nanoelectromechanical systems (NEMS) and particularly current-driven nanomotors [1]. Despite the broad variety of stimulating results found, the regime of strong Coulomb interactions has not been fully explored for this application. In this talk, we consider NEMS composed of a set of coupled quantum dots interacting with mechanical degrees of freedom taken in the adiabatic limit and weakly coupled to electronic reservoirs. A real-time diagrammatic approach [2] is used to derive general expressions for the current-induced forces, friction coefficients, and zero-frequency force noise in the Coulomb blockade regime of transport. We show our expressions obey Onsager’s reciprocity relations and the fluctuation-dissipation theorem for the energy dissipation of the mechanical modes [3]. The obtained results are illustrated with a nanomotor consisting of a double quantum dot capacitively coupled to rotating charges. We analyze the dynamics and performance of the motor as a function of the applied voltage and loading force for trajectories encircling different triple points in the charge stability diagram.

[1] R. Bustos-Marún, G. Refael, and F. von Oppen, Phys. Rev. Lett. **111**, 060802 (2013).

[2] J. Splettstoesser, M. Governale, J. König, and R. Fazio, Phys. Rev. B **74**, 085305 (2006).

[3] H. L. Calvo, F. D. Ribetto, and R. A. Bustos-Marún, Phys. Rev. B **96**, 165309 (2017).

This seminar takes place in room** 26C 401.**

### Dienstag, 16. Januar, 2018, 16 Uhr

Eugene Kogan (Department of Physics, Bar-Ilan University Israel):

RKKY interaction in graphene

We consider RKKY interaction between two magnetic impurities in graphene at a finite temperature. The consideration is based on the perturbation theory for the thermodynamic potential in the imaginary timerepresentation. We analyze the symmetry of the RKKY interaction on the bipartite lattice at half filling. Our analytical calculation of the interaction is based on direct evaluation of real space spin susceptibility.

This seminar takes place in room** 26C 401.**

### Montag, 15. Januar, 2018, 14 Uhr

Eugene Kogan (Department of Physics, Bar-Ilan University Israel):

Spin-anisotropic magnetic impurity in a Fermi gas: Integration of poor man’s scaling equations

We consider a single magnetic impurity described by the spin-anisotropic s-d(f ) exchange (Kondo) model and formulate a scaling equation for the spin-anisotropic model when the density of states (DOS) of electrons is a power-law function of energy (measured relative to the Fermi energy).We solve this equation containing terms up to the second order in coupling constants in terms of elliptic functions. From the obtained solution we find the phases corresponding to the infinite isotropic antiferromagnetic Heisenberg exchange, to the impurity spin decoupled from the electron environment (only for the pseudogap DOS), and to the infinite Ising exchange (only for the diverging DOS). We analyze the critical surfaces, corresponding to the finite isotropic antiferromagnetic Heisenberg exchange for the pseudogap DOS.

This seminar takes place in room** 26C 401.**

### Mittwoch, 20. Dezember, 2017, 10:00 Uhr

Karel Temmink (Institute for Theoretical Physics (ITFA) and Anton Pannekoek Institute for Astronomy (API), University of Amsterdam):

Tensor Network Methods for Open Quantum Systems

Presently, Tensor Network (TN) methods have firmly established themselves as reliable, efficient, and extremely powerful tools for quantum calculations. TNs have proven especially successful in regimes where the time-evolution is unitary and/or entropy obeys an area law, such as ground state calculations in closed quantum systems.

However, less has been accomplished for open quantum systems, where the time-evolution generated by the Lindblad master equation is no longer unitary, and dissipation and Hamiltonian interactions compete. These systems, which are often relatively poorly understood analytically, are also notorious in computational physics, as they tend to cause all sorts of numerical issues, with the most well-known being that simulated density operators often lose positivity and therefore cease being physical.

In my talk, I will introduce the general framework of TNs (Matrix Product States/-Operators) for closed quantum system ground state calculations, show how they can be extended to open quantum systems non-equilibrium steady state (NESS) calculations, and end with an example calculation of the NESS of a dissipative XXX Heisenberg spin chain.

This seminar takes place in room** 26C 401.**

### Montag 18. Dezember, 2017, 16:00-17:00 Uhr, Mittwoch 20. Dezember, 2017, 16:00-17:00 Uhr, Donnerstag 21. Dezember, 2017, 15:00-16:00 Uhr

Thomas C. Lang (Universität Innsbruck):

Diagrams, world lines, auxiliary fields and pumpkin spice - a basic introduction into stochastic flavors for simulating quantum many body systems

These seminars take place in room** 26C 401.**

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### Donnerstag, 05. Oktober, 2017, 10:00 Uhr

Ronald Starke (TU Freiberg):

Refractive index and dielectric tensor

The standard ab initio calculation of the refractive index is based on its identification with the root of the scalar dielectric function, a treatment which cannot be generalized directly to the case of frequency- and wavevector-dependent dielectric tensors. We discuss this problem on a fundamental level starting from the microscopic electromagnetic wave equation in materials, which was recently developed within the Functional Approach to electrodynamics in media. In particular, we investigate under which conditions the standard treatment can be justified, but we then provide a more general method of calculating the frequency- and direction-dependent refractive indices by means of a (2 × 2) complex-valued “optical tensor”. In principle, this method allows for the ab initio prediction of such diverse optical properties as birefringence and optical activity.

This seminar takes place in room** 26C 401.**

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### Dienstag, 26. September, 2017, 16:00 Uhr

Ribhu Kaul (University of Kentucky):

Quantum phase transitions in two dimensional SU(N) and SO(N) magnets

I will discuss the phases and phase transitions in some simple SU(N) and SO(N) quantum spin models, studied both using ideas from quantum field theory and with large scale numerical simulations. These models provide interesting examples where the emergence of gauge fields, both at critical points and extended phases, can be studied in quantum spin systems.

This seminar takes place in room** 26C 401.**

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### Montag, 25. September, 2017, 16:00 Uhr

Ribhu Kaul (University of Kentucky):

A lecture on deconfined quantum criticality

This seminar takes place in room** 26C 401.**

### Freitag, 7. Juli, 2017, 11:00 Uhr

Tommaso Roscilde (Laboratoire de Physique, Ecole Normale Supérieure de Lyon):

Quantum critical phenomena through the lens of quantum correlations

In quantum systems correlations can take forms which are impossible in classical mechanics. The most famous, yet elusive form of quantum correlation is represented by entanglement, a property well defined and investigated for pure states, and envisioned as a resource for nearly all technological tasks harnessing quantum many-body physics. In the real life of mixed states, on the other hand, incoherent fluctuations appear in the game, making the distinction of quantum vs. classical correlations less sharp. Being able to discern the “quantumness" of correlations in mixed states, and to identify many-body regimes in which correlations have a pronounced quantum character, represents a formidable question of both fundamental as well as technological nature.

In this seminar I will provide an overview of the theoretical importance of quantum correlations, starting from their very definition - to which we contributed recently with a statistical physics approach allowing to calculate them in generic systems, and potentially to measure them for a large class of quantum many-body systems relevant to experiments in AMO physics and solid-state physics. Furthermore I will discuss the centrality of quantum correlations inthe phase diagram of quantum critical phenomena - using the transverse-field Ising model as paradigmatic example I will show that quantum correlations at finite temperature provide an unprecedented insight, of purely quantum nature, into the various phases and their mutual crossovers. In particular the quantum critical enhancement of quantum correlations can be paired up with their metrological importance, opening the appealing perspective of "quantum critical metrology", which envisions a possible technological use of one of the pillars of modern quantum condensed matter.

This seminar takes place in room** 26C 402.**

### Freitag, 23. Juni 2017, 11:00 Uhr

Sudipto Singha Roy (Harish-Chandra Research Institute, Allahbad, India):

Doped resonating valence bond states: a quantum information study

Resonating valence bond states have played a crucial role in the description of exotic phases in strongly correlated systems, especially in the realm of Mott insulators and the associated highTc superconducting phase transition. In particular, RVB states are considered to be an important system to study the ground state properties of the doped quantum spin1/2 ladder. It is therefore interesting to understand how quantum correlations are distributed among the constituents of these composite systems. In this regard, we formulate an analytical recursive method to generate the wave function of doped shortrange resonating valence bond (RVB) states as a tool to efficiently estimate multisite entanglement as well as other physical quantities in doped quantum spin ladders. Importantly, our results show that within a specific doping concentration and model parameter regimes, the doped RVB state essentially characterizes the trends of genuine multiparty entanglement in the exact ground states of a Hubbard model with large onsite interactions. Moreover, we consider an isotropic RVB network of spin1/2 particles with a finite fraction of defects, where the corresponding wave function of the network is rotationally invariant under the action of local unitaries. By using quantum informationtheoretic concepts like strong subadditivity of von Neumann entropy and approximate quantum telecloning, we prove analytically that in the presence of defects, caused by loss of a finite fraction of spins, the RVB network sustains genuine multisite entanglement, and at the same time may exhibit finite moderaterange bipartite entanglement, in contrast to the case with no defects.

This seminar takes place in room** 26C 402.**

### Mittwoch, 05. April 2017, 9:15 Uhr

Fabian Kugler (LMU Munich):

Multiloop functional renormalization group that sums up all parquet diagrams

We present a multiloop flow equation for the four-point vertex in the functional renormalization

group (fRG) framework. The multiloop flow consists of successive one-loop calculations and sums up

all parquet diagrams to arbitrary order. This provides substantial improvement of fRG computations

for the four-point vertex and, consequently, the self-energy. Using the X-ray-edge singularity as

an example, we show that solving the multiloop fRG flow is equivalent to solving the (first-order)

parquet equations and illustrate this with numerical results.

This seminar takes place in room** 26C 401.**

### Dienstag, 21. Februar 2017, 16:00 Uhr

Björn Sbierski (FU Berlin):

Functional RG approach to spinless fermions in one dimension

This seminar takes place in room** 26C 401.**

### Freitag, 03. Februar 2017, 14:00 Uhr

Bruce Normand (Paul Scherrer Institute, Villingen, Switzerland):

Gapless spin-liquid ground state in the S = 1/2 kagome antiferromagnet

The defining problem in the field of frustrated quantum magnetism is the ground state of the nearest-neighbour S = 1/2 antiferromagnetic Heisenberg model on the kagome lattice. Despite the simplicity of the Hamiltonian, the solution has defied all theoretical and numerical methods employed to date. We apply the formalism of tensor-network states (TNS), specifically the method of projected entangled simplex states (PESS), whose combination of a correct accounting for multipartite entanglement and infinite system size provides qualitatively new insight. By studying the ground-state energy, the staggered magnetization we find at all finite tensor bond dimensions and the effects of a second-neighbour coupling, we demonstrate that the ground state is a gapless spin liquid. We discuss the comparison with other numerical studies and the physical interpretation of the gapless ground state.

This seminar takes place in room** 26C 402.**

### Montag, 19. Dezember 2016, 14:00 Uhr

Hannes Pichler (ITAMP, Harvard University):

The quantum stochastic Schrödinger equation with time delays: a MPS approach

We study the dynamics of photonic quantum circuits consisting of nodes coupled by quantum channels. We are interested in the regime where the time delay in communication between the nodes is significant. This includes the problem of quantum feedback, where a quantum signal is fed back on a system with a time delay. We formulate the quantum stochastic Schrödinger equation for problems with time delays and develop a matrix product state approach to solve it, which accounts in an efficient way for the entanglement between the emitted photons in the waveguide, and thus the non-Markovian character of the dynamics. We illustrate this approach with two paradigmatic quantum optical examples: two coherently driven distant atoms coupled to a photonic waveguide with a time delay, and a driven atom coupled to its own output field with a time delay as an instance of a quantum feedback problem.

This seminar takes place in room **26C 401.**

### Freitag, 01. Juli 2016, 10:30 Uhr

Dante Kennes (Department of Physics, Columbia University):

Entanglement scaling in many-body localized systems

We study the properties of excited states in one-dimensional many-body localized (MBL) sys-

tems using a matrix product state algorithm. First, the method is tested for a large disordered

non-interacting system, where for comparison we compute a quasi-exact reference solution via a

Monte Carlo sampling of the single-particle levels. Thereafter, we present extensive data obtained

for large interacting systems of L ∼ 100 sites and large bond dimensions χ ∼ 1700, which allows us

to quantitatively analyze the scaling behavior of the entanglement S in the system. The MBL phase

is characterized by a logarithmic growth S(L) ∼ log(L) over a large scale separating the regimes

where volume and area laws hold. We check the validity of the eigenstate thermalization hypothesis.

Our results are consistent with the existence of a mobility edge.

This seminar takes place in room **MBP2 015.**

### Donnerstag, 02. Juni, 2016, 10:30 Uhr

Leeor Kronik (Weizmann Institute of Science, Israel)*:*

Electronic structure from density functional theory: challenges and progress

https://www.weizmann.ac.il/materials/Leeor/

This seminar takes place in room **MBP1 026.**

### Mittwoch, 16. März, 2016, 15:00 Uhr

Imke Schneider (Fachbereich Physik, TU Kaiserslautern)*:*

Spin-charge-separated quasi-particles in one-dimensional quantum fluids

One-dimensional quantum fluids are prominent examples of systems in which the Fermi liquid paradigm of electron-like quasi-particles is known to break down. Instead Luttinger liquid theory predicts a low-energy spectrum described by two decoupled free bosonic fields associated with collective spin and charge degrees of freedom, respectively.

Here, we revisit the problem of dynamical response in these systems arguing that, as a result of spectral nonlinearity, long-lived excitations are best understood in terms of generally strongly interacting fermionic holons and spinons. This has far reaching ramifications for the construction of mobile impurity models used to determine threshold singularities in dynamical response functions. We formulate and solve the appropriate mobile impurity model describing the spinon threshold in the single-particle Green’s function. Our formulation further raises the question whether it is possible to realize a model of noninteracting fermionic holons and spinons in microscopic lattice models of interacting spinful fermions. We investigate this issue in some detail by means of density matrix renormalization group (DMRG) computations.

This seminar takes place in room **MBP1 026.**

### Donnerstag, 25. Februar, 2016, 10:30 Uhr

Miguel Martín-Delgado (Theoretical Physics 1 Department, Universidad Complutense de Madrid)*:*

Modern Aspects of Quantum Physics and Topology

In recent years, topological effects have found a variety of remarkable applications in quantum physics. A conceptual insight as to why topology plays a role in quantum physics is presented. This review includes basic explanations of how topology is a solution for quantum information and computation. New forms of quantum matter have appeared in condensed matter such as topological insulators and superconductors. They will be described in a broad context and emphasis is given on the classification of topological orders as new forms of quantum entanglement, highlighting their similarities and differences. A glimpse into the possible future developments will be commented in the outlook.

http://pendientedemigracion.ucm.es/info/giccucm/PersonalMAMD/

This seminar takes place in room **26C 401.**

### Mittwoch, 13. Mai, 2015, 12:45 Uhr

Michael Thoss (Institute for Theoretical Physics and Interdisciplinary Center for Molecular Materials, University of Erlangen):

Simulation of quantum dynamics and transport using multiconfiguration wave-function methods

The accurate theoretical treatment and simulation of quantum dynamical processes in many-body systems is a central goal in chemical and condensed matter physics. In this talk, the multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) method [1] is discussed as an example of an approach that allows an accurate description of quantum dynamics and transport in systems with many degrees of freedom. The ML-MCTDH method is a variational basis-set approach, which uses a multiconfiguration expansion of the wave function employing a multilayer representation and time-dependent basis functions. It extends the original MCTDH method [2] to significantly larger and more complex systems. Employing the second quantization representation of Fock space, the ML-MCTDH method can also be used to treat the dynamics of indistinguishable particles [3,4]. Illustrative applications of the methodology to models for charge transfer and transport are discussed, including electron transport in molecular junctions.

[1] H. Wang and M. Thoss, J. Chem. Phys. **119**, 1289 (2003).

[2] H.-D. Meyer, U. Manthe, and L.S. Cederbaum, Chem. Phys. Lett. **165** , 73 (1990); H.-D. Meyer, F. Gatti, and G.A. Worth (Eds.), Multidimensional Quantum Dynamics: MCTDH Theory and Applications, Wiley-VCH, Weilheim, 2009.

[3] H. Wang and M. Thoss, J. Chem. Phys. **131**, 024114 (2009).

[4] E. Wilner, H. Wang, G. Cohen, M. Thoss, E. Rabani, Phys. Rev. B **88**, 045137 (2013); **89**, 205129 (2014).

This seminar takes place in room **MBP2 117.**

### Donnerstag, 07. Mai 2015, 13 Uhr

Audrey Cottet (Laboratoire Pierre Aigrain, Département de Physique de l’Ecole Normale Supérieure):

Mesoscopic Quantum Electrodynamics with a single spin

A new type of experiments combining microwave cavities and mesoscopic circuits gathering nanoconductors and fermionic reservoirs has recently appeared [1,2,3]. This mesoscopic Quantum Electrodynamics (QED) offers many new possibilities like for instance quantum computing schemes based on localized electronic spins, or a powerful photonic study of electronic transport.

In the first part of this seminar, I will introduce a general theoretical framework to describe these experiments. This task faces two challenges. First, one has to quantize the electromagnetic field properly by taking into account electromagnetic boundary conditions which are naturally omitted in atomic cavity QED, due to the smallness of an atom. Second, in the nanocircuits, one has to take into account collective plasmonic modes, as well as electronic quasiparticle states which are absent from circuit QED performed with superconducting quantum bits. I will present a description of mesoscopic QED experiments which takes into account these specificities [4].

In the second part of this seminar, I will present experimental results demonstrating the coherent coupling of a single spin to photons stored in a microwave resonator. Using a circuit design based on a nanoscale spin-valve [5], we coherently hybridize the individual spin and charge states of a double quantum dot while preserving spin coherence. This scheme allows us to increase by five orders of magnitude the natural (magnetic) spin-photon coupling, up to the MHz range at the single spin level. Our coupling strength yields a cooperativity which reaches 2.3, with a spin coherence time of about 60ns [6]. We thereby demonstrate a mesoscopic device which could be used for non-destructive single spin read-out and distant spin/spin coupling via virtual cavity photons.

[1] M. R. Delbecq, V. Schmitt, F. D. Parmentier, N. Roch, J. J. Viennot, G. Fève, B. Huard, C. Mora, A. Cottet, and T. Kontos, Phys. Rev. Lett. **107**, 256804 (2011).

[2] T. Frey, P. J. Leek, M. Beck, A. Blais, T. Ihn, K. Ensslin, & A. Wallraff, Phys. Rev. Lett. **108**, 046807 (2012).

[3] K. D. Petersson, L. W. McFaul, M. D. Schroer, M. Jung, J. M. Taylor, A. A. Houck & J. R. Petta, Nature **490**, 380 (2012)

[4] A. Cottet, T. Kontos & B. Douçot, arXiv:1501.00803

[5] A. Cottet & T. Kontos, Phys. Rev. Lett. **105**, 160502 (2010).

[6] J.J. Viennot, M.C. Dartiailh, A. Cottet & T. Kontos, submitted

This seminar takes place in room **MBP1 026.**

### Dienstag, 24. Februar 2015, 16 Uhr

Takeo Kato (Institute of Solid State Physics, University of Tokyo):

Kondo signature in heat transport via a local two-state system

Heat and electric transport have several similarities as well as dissimilarities. Fourier's law in heat transport corresponds to Ohm's law in electric transport, and these laws are commonly categorized as diffusive transport. Ballistic transport leads to the quantization of conductance in electric as well as heat transport. The conductance quantum was measured in mesoscopic electric conduction in 1988 [1], and much later, the version of heat transport was also measured [2]. Recently, the concept of thermal diode has also been discussed, and an experiment has been conducted for demonstrating this [3]. Recent progress in transport studies strongly indicates that heat transport analogue exists for many categories of electric transport.

In this talk, we present theoretical study of the Kondo effect in heat transport via a local two-state system [4]. This system is described by the spin-boson Hamiltonian with Ohmic dissipation, which can be mapped onto the Kondo model with anisotropic exchange coupling. We derive the exact formula of thermal conductance, and evaluate it by the Monte Carlo method. Thermal conductance has a scaling form indicating the universal behavior characteristic of the Kondo effect. Below the Kondo temperature, conductance follows the universal temperature dependence proportional to T^3, showing nontrivial enhancement. This is a manifestation of strong correlation between system and reservoirs, which is analogous to the Kondo effect in electric transport. We also discuss coupling dependence of heat conductance.

[1] B. J. Wees et al., Phys. Rev. Lett. **60**, 848 (1988).

[2] K. Schwab et al., Nature (London) **404**, 974 (2000); H.-Y. Chiu et al., Phys. Rev. Lett. **95**, 226101 (2005).

[3] N. Li, J. Ren, L. Wang, G. Zhang, P. Hänggi and B. Li, Rev. Mod. Phys. **84**, 1045 (2012); C. W. Chang, D. Okawa, A. Majumdar and A. Zettl, Science **314**, 1121 (2006).

[4] K. Saito and T. Kato, Phys. Rev. Lett. **111**, 214301 (2013)

This seminar takes place in room **26C 401.**

### Dienstag, 10. Februar, 2015, 16 Uhr

Takafumi Suzuki (Institute of Solid State Physics, University of Tokyo):

Photon-assisted current noises through a quantum dot system

Photon-assisted transport through mesoscopic conductors has attracted much attention because the quantum nature of transport processes is significantly modified by time-dependent fields. In recent years, the scattering theory has revealed that current noises provide information about the photon-assisted transport of noninteracting electrons. For example, Levitov and Lesovik showed that photon-assisted current noises can detect the phase of the transmission amplitudes induced by the external time-dependent field [1]. Studying the effect of the Coulomb interaction is an important next step to discuss interesting physics, such as the Coulomb blockade and the Kondo effect.

In this talk, I will discuss the photon-assisted transport in an interacting quantum dot system under a periodically oscillating gate voltage [2]. Photon-assisted current noises in the presence of the Coulomb interaction are calculated based on a gauge-invariant formulation of time-dependent transport. The behavior of the vertex corrections under the AC field will be discussed within the self-consistent Hartree-Fock approximation.

The present result provides a useful viewpoint for understanding photon-assisted transport in interacting electron systems.

[1] G. B. Lesovik and L. S. Levitov, PRL **72**, 538 (1994)

[2] T. J. Suzuki and T. Kato, arXiv:1411.3520

This seminar takes place in room **26C 401.**