Poster Presentation
Poster Presentation Details
On the evening of the first workshop day, a poster presentation will be held in front of the Physics Lecture Hall. Participants of the workshop will present scientific information on the following topics:


Spinvalley magnetism on the triangular moiré lattice with SU(4) breaking interactions

Momentumselective pair creation of spin excitations in dipolar bilayers

Edge mode dynamics in topological magnets

Magnomechanics for quantum information

Towards cavitymediated optical coupling of confined excitons in 2D materials

Abstract:
The exchange of quantum information beyond nearest neighbors is a main challenge in quantum computing. Heterostructures of 2D materials offer the possibilities to combine electronic with optical degrees of freedom as well as the incorporation of such samples inside optical cavities. We want to assess the possibilities to generate a scalable qubit platform combined with coupled quantum emitters.
This project focuses on the coupling of several emitters to the same optical cavity. As emitters, we plan to use neutral excitons inside transition metal dichalcogenides (TMDs) that can be confined via inhomogeneous inplane electric fields and offer tunability of their energy and oscillator strength. We want to reach strong coupling of such emitters with intermediate linewidths to fiberbased microcavities and demonstrate photon blockade as well as controllable emitteremitter coupling via the cavity. We will present our progress on this project.Authors: Andrea Bergschneider, Moritz Scharfstädt, Max Wegerhoff, Hubert Dulisch, David Tebbe, Timo Reinartz, Bernd Beschoten, Michael Köhl, Dante Kennes, Silvia V. Kusminskiy, Lutz Waldecker, Christoph Stampfer, Stefan Linden, Annika Kurzmann


Quantum Monte Carlo simulation of the Ising model in a lightinduced quantized transverse field

Abstract:
We study the Ising model in a lightinduced quantized transverse field (qTFIM) [1, 2] using quantum Monte Carlo to investigate the influence of lightmatter interactions on correlated quantum matter. To avoid a direct sampling of the photons, we develop a quantum Monte Carlo algorithm based on the recently introduced wormhole algorithm for spinboson systems [3], in which the bosonic degrees of freedom are integrated out analytically. By this means, the bosons induce a retarded spinspin interaction in imaginary time [3]. In contrast to the Ising interaction inherent to the model, this induced interaction is also nonlocal in space.
The method is applied to the unfrustrated antiferromagnetic qTFIM in the presence of a longitudinal field, for which meanfield considerations [2] suggest a rich quantum phase diagram including continuous phase transitions and a nontrivial intermediate phase. Our numerical findings confirm the presence of this intermediate phase. However, the extent of this phase is much smaller than anticipated and certain phase transitions turn out to be of first order rather than of second order.
[1] J. Rohn et al., Phys. Rev. Research 2, 023131 (2020)
[2] Y. Zhang et al., Sci Rep 4, 4083 (2014)
[3] M. Weber et al., Phys. Rev. Lett. 119, 097401 (2017)
Author: Anja Langheld (presenting), Kai Philipp Schmidt

Abstract:

Systematic Analysis of Crystalline Phases in Bosonmi Lattice Models with Algebraically Decaying DensityDensity Interaction

Abstract:
We propose a general approach to analyse diagonal ordering patterns in bosonic lattice models with algebraically decaying densitydensity interactions on arbitrary lattices. The key idea is a systematic search for the energetically best order on all unit cells of the lattice up to a given extent. Using resummed couplings we evaluate the energy of the ordering patterns in the thermodynamic limit using finite unit cells. We apply the proposed approach to the atomic limit of the extended BoseHubbard model on the triangular lattice at fillings $f=1/2$ and $f=1$. We investigate the groundstate properties of the antiferromagnetic longrange Ising model on the triangular lattice and determine a sixfold degenerate plainstripe phase to be the ground state for finite decay exponents. We also probe the classical limit of the FendleySenguptaSachdev model describing Rydberg atom arrays. We focus on arrangements where the atoms are placed on the sites or links of the Kagome lattice. Our method provides a general framework to treat cristalline structures resulting from longrange interactions.
Literature: https://arxiv.org/abs/2212.02091
Authors: Jan Alexander Koziol (presenting), Antonia Duft, Giovanna Morigi, Kai Phillip Schmidt

Abstract:

Quantum criticality of the frustrated traversefield Ising model in a triangula bialyer using directly evaluated enhanced perturbative continuous unitary transformations

Abstract:
Ising models in a transverse field are paradigmatic models for quantum phase transitions of various universality classes which occur depending on the lattice geometry and the choice or antiferromagnetic of ferromagnetic coupling. We investigate the quantum phase diagram of the antiferromagnetic transversefield Ising model (TFIM) on a triangular bilayer with an Ising interlayer coupling. Without a field, the model is known to host a classically disordered ground state, and in the limit of decoupled layers it exhibits the 3dXY transition of the corresponding single layer model. Our starting point for the unknown parts of the phase diagram is a highorder perturbative calculation from the limit of isolated dimers. Enhanced perturbative continuous unitary transformations (epCUTs) are used to derive series expansions for the groundstate energy and the energy gap. These are refined by directly evaluated epCUTs (deepCUTs) which provide estimates which coincide with the perturbative series up to its respective order and add a nonperturbative correction. These allow to draw conclusions about the nature of occurring quantum phase transitions.Ising models in a transverse field are paradigmatic models for quantum phase transitions of various universality classes which occur depending on the lattice geometry and the choice or antiferromagnetic of ferromagnetic coupling. We investigate the quantum phase diagram of the antiferromagnetic transversefield Ising model (TFIM) on a triangular bilayer with an Ising interlayer coupling. Without a field, the model is known to host a classically disordered ground state, and in the limit of decoupled layers it exhibits the 3dXY transition of the corresponding single layer model. Our starting point for the unknown parts of the phase diagram is a highorder perturbative calculation from the limit of isolated dimers. Enhanced perturbative continuous unitary transformations (epCUTs) are used to derive series expansions for the groundstate energy and the energy gap. These are refined by directly evaluated epCUTs (deepCUTs) which provide estimates which coincide with the perturbative series up to its respective order and add a nonperturbative correction. These allow to draw conclusions about the nature of occurring quantum phase transitions.
Authors: Lukas Schamriß (presenting), Matthias R. Walther, DagBjörn Hering, Kai P. Schmidt

Abstract:

Quantumcritical properties of random transversefield Ising models extracted by quantum Monte Carlo methods

Abstract:
The transversefield Ising model with quenched disorder is studied in one and two dimensions at zero temperature by stochastic series expansion quantum Monte Carlo simulations. Using a samplereplication method we are able to determine distributions of pseudocritical points, from which critical shift and width exponents $\nu_{s/w}$ are extracted by finitesize scaling. The critical points extrapolated to infinite systems are confirmed and refined by an analysis of averaged binder ratios. Scaling of the averaged magnetisation at the critical point is used further to determine the orderparameter critical exponent $\beta$ and the critical exponent $\nu_{av}$ of the average correlation function. The dynamical scaling in the Griffiths phase is investigated by measuring the local susceptibility in the disordered phase and the critical exponent $z′$ is extracted.
Authors: Calvin Krämer, Anja Langheld, Jan A. Koziol, Max Hörmann, Kai Phillip Schmidt

Abstract:

Realfrequency quantum fields theory applied to the singleimpurity Anderson model

Abstract:
A major challenge in the field of correlated electrons is the computation of dynamical correlation functions. For comparisons with experiment, one is interested in their realfrequency dependence. This is difficult to compute, as imaginaryfrequency data from the Matsubara formalism require analytic continuation, a numerically illposed problem. Here, we apply quantum field theory to the singleimpurity Anderson model (AM), using the Keldysh instead of the Matsubara formalism with direct access to the selfenergy and dynamical susceptibilities on the realfrequency axis. We present results from the functional renormalization group (fRG) at oneloop level and from solving the selfconsistent parquet equations in the parquet approximation. In contrast to previous Keldysh fRG works, we employ a parametrization of the fourpoint vertex which captures its full dependence on three realfrequency arguments. We compare our results to benchmark data obtained with the numerical renormalization group and to secondorder perturbation theory. We find that capturing the full frequency dependence of the fourpoint vertex significantly improves the fRG results compared to previous implementations, and that solving the parquet equations yields the best agreement with the NRG benchmark data, but is only feasible up to moderate interaction strengths. Our methodical advances pave the way for treating more complicated models in the future.
Authors: Anxiang Ge, Nepomuk Ritz, Elias Walter, Santiago Aguirre, Jan von Delft and Fabian B. Kugler

Abstract:

Simulating infinite temperature spin synamics by a spin dynamic meandfield theory

Abstract:
Spins are ideal candidates for quantum bits, the basic building block for quantum computing and quantum information technology. It is crucial to understand a spins dynamics if it is coupled to an environment which often consists of spins itself. We develop a dynamic meanfield theory for spin systems at infinite temperature (spinDMFT) [1]. The idea is to replace the large environment of a spin by a dynamic meanfield which displays a random Gaussian temporal evolution. Its autocorrelations are selfconsistently linked to the quantum mechanic
expectation values of spinspin correlations. This approach becomes exact in the limit of large lattice coordination numbers.
We improve the approach by considering spin clusters quantummechanically (CspinDMFT)[3]. The extended model is able to describe dynamic spin correlations measured in recent experiments [2][3] where an inhomogeneous spin$\tfrac12$ ensemble on a diamond surface is probed using nitrogenvacancy centers as sensors.[1] T. Gräßer et al., Phys. Rev. Research 3, 043168 (2021).
[2] K. Rezai et al., arXiv:2207.10688 (2022).
[3] T.Gräßer et al., arXiv:2307.14188 (2023).

Abstract:

Formation of Exceptional Points in PseudoHermetian Systems

Abstract
One of the main drivers of the research on nonHermitian systems is the occurence of special degeneracies in the spectrum of nonHermitian Hamiltonians called Exceptional Points (EPs). At these points, two or more eigenvalues and corresponding eigenvectors overlap rendering the Hamiltonian defective. EPs can be utilized for quantum sensing and for adiabatic state conversion, to name a few applications. Experimental realization of devices based on EPs is complicated by the need to tune the system to the vicinity of an EP. At the same time, the number of independent parameters required for tuning is diminished in the presence of symmetries. I consider the systems with PseudoHermitian symmetry, which is closely related to the usually employed PTsymmetry. I characterize separate nondegenerate levels with a Z2 topological index, corresponding to the signs of the pseudometric operator eigenvalues in the absence of Hermiticitybreaking terms. After that I show that the formation of secondorder EPs is governed by this topological index: EPs are provided only by pairs of levels with opposite indices. To demonstrate the approach, I consider transversefield Ising chain with longitudinal staggered gain and loss, which is pseudoHermitian with respect to parity. Using the integrability of the model in the absence of Hermiticitybreaking terms, I compute all the topological indices analytically and then use them to analyze the formation of second and thirdorder Exceptional points. As a side note, I also consider the ground state quantum Phase transitions in the thermodynamic limit of the model.

Abstract

Periodically Driven Heavy Fermion Systems

Abstract
In this work we study the effects of light irradiation on heavyfermion systems. A typical model for such systems is the periodic Anderson model where strongly repulsive, localized electrons in the 4f shell of rareearth ions hybridize with a sea of conduction electrons.The Kondo effect induces a new flat band of heavyfermions, near the Fermi energy. Applying a stationary light field induces a timeperiodic hybridization between the conduction and the 4f electrons, rather than a modulation of the onsite 4f energy, due to the dipole selection rules.On one hand, the Floquet theorem predicts that the periodic driving produces replicas of the Kondo resonance, centered around multiples of the driving frequency. However, the light field could also break up the Kondo singlets, thereby destroying the heavyfermion state altogether.

Abstract

Switching the magnetization in quantum antiferromagnets

Abstract:
The orientation of the order parameter of quantum magnets can be used to store information in a dense and efficient way. Switching this order parameter corresponds to writing data. To understand how this can be done, we study a precessional reorientation of the sublattice magnetization in an (an)isotropic quantum antiferromagnet induced by an applied magnetic field. introduce a description including the leading quantum and thermal fluctuations, namely Schwinger boson meanfield theory, because this theory allows us to describe both ordered phases and the phases in between them, as is crucial for switching. An activation energy has to be overcome requiring a minimum applied field ht which is given essentially by the spin gap. It can be reduced significantly for temperatures approaching the N'eel temperature facilitating switching. The time required for switching diverges when the field approaches ht which is the signature of an inertia in the magnetization dynamics. The temporal evolution of the magnetization and of the energy reveals signs of dephasing. The switched state has lost a part of its coherence because the magnetic modes do not evolve in phase.

Abstract:

Catalyzation of supersolidity in binary dipolar condensates

Abstract
Breakthrough experiments have newly explored the fascinating physics of dipolar quantum droplets and supersolids. The recent realization of dipolar mixtures opens further intriguing possibilities. We show that under rather general conditions, the presence of a second component catalyzes droplet nucleation and supersolidity in an otherwise unmodulated condensate. For miscible mixtures, droplet catalyzation results from the effective modification of the relative dipolar strength, and may occur even for a surprisingly small impurity doping. We show that different groundstates may occur, including the possibility of two coexisting interacting supersolids. The immiscible regime provides a second scenario for double supersolidity in an array of immiscible droplets. Further we will discuss how the superfluidity of this mixture can be tested.
(10.1103/PhysRevA.107.L021302)

Abstract

Anomalous scaling corrections and quantum phase diagram of the Heisenberg antiferromagnet on the spatially anisotropic honeycomb lattice

Transient transport spectroscopy of an interacting quantum dot with superconducting proximity

On the topological Josephson junctions in transverse magnetic field
 Leadinglogarithmic approximation by oneloop renormalization group within Matsubara formalism