
chair: Martin Eckstein

09:00  09:45

Ryo Shimano
(The University of Tokyo)
Lightinduced anomalous Hall effect in three dimensional Dirac electron systems
Floquet engineering has become a growing paradigm of nonequilibrium phenomena in condensed matter system. One fascinating aspect of the Floquet engineering is that it can realize novel quantum phases inaccessible in equilibrium. Striking example are the lightinduced chiral gauge field in Dirac electron systems and emergence of new topological phases such as FloquetWeyl states. In this talk, we report on the lightinduced anomalous Hall effect(AHE) in three dimensional Dirac electron systems probed by timeresolved terahertz Faraday effect. The possible origin of the observed AHE will be discussed in terms of the chiral gauge field.

09:45  10:20

Takahiro Morimoto
(The University of Tokyo)
Geometrical nonlinear optical effects in noncentrosymmetric crystals
The responses of materials to high intensity light, i.e., nonlinear optical responses, constitute a vast field of physics and engineering. One of nonlinear optical responses that are attracting a recent keen attention is a bulk photovoltaic effect called shift current which arises from a geometrical (Berry) phase of a Bloch wave function and has a close relationship to the modern theory of electric polarization [1]. While most previous studies of the bulk photovoltaic effects have focused on band insulators of noninteracting electrons, systems of correlated electrons have a potential to support a novel nonlinear functionality. In this talk, I will present our recent efforts in seeking nonlinear optical effects originating from unique excitations in interacting electron systems, including excitons in semiconductors [2], magnetic excitations in multiferroic materials [3], and phonon excitations in electronphonon coupled systems [4], where excitations below the electronic band gap generate novel photovoltaic effects through the shift current mechanism.
[1] T. Morimoto, and N. Nagaosa, Sci. Adv. 2, e1501524 (2016).
[2] T. Morimoto, and N. Nagaosa, Phys. Rev. B 94, 035117 (2016).
[3] T. Morimoto, S. Kitamura, S. Okumura, Phys. Rev. B 104, 075139 (2021).
[4] Y. Okamura et al. PNAS 119, e2122313119 (2022).

10:20  10:50

Coffee break


chair: Michael Sentef

10:50  11:25

JanPhilip Joost
(ChristianAlbrechtsUniversität Kiel)
Nonequilibrium Green Functions in Linear Time
The selfconsistent theoretical treatment of correlation and quantum effects in nonequilibrium beyond onedimensional systems is a particular challenge that has been successfully attacked with nonequilibrium Green functions (NEGF) methods [1]. However, NEGF simulations are hampered by a cubic scaling of the computation time with the number of time steps $N_t$. Recently, a dramatic acceleration has been achieved within the G1–G2 scheme [2] by transforming the NEGF equations, within the HartreeFock Generalized Kadanoff–Baym ansatz (GKBA) [3], to a timelocal form for the singleparticle and twoparticle Green functions. A detailed discussion of the method and its application to a variety of selfenergies including particleparticle and particlehole $T$ matrix, $GW$, and the dynamically screened ladder (DSL) was presented recently [4]. Due to its relation to the singletime BBGKY hierarchy, the G1–G2 scheme can benefit from a variety of wellestablished techniques of twoparticle reduced density matrix (2RDM) theory, such as enforcing contraction consistency or a purification of the dynamics, to further improve its accuracy and numerical stability [5]. A drawback of the G1–G2 scheme is the memory cost needed to store the twoparticle Green function. This can be significantly relieved by using a recently developed alternative stochastic approach to the G1–G2 scheme [6]. We present first results for the stochastic $GW$ approximation.
[1] N. Schlünzen et al., PRB 93, 035107 (2016)
[2] N. Schlünzen et al., PRL 124, 076601 (2020)
[3] P. Lipavský, V. Špička, and B. Velický, PRB 34, 6933 (1986)
[4] J.P. Joost et al., PRB 101, 245101 (2020)
[5] J.P. Joost et al., PRB 105, 165155 (2022)
[6] E. Schroedter et al., submitted to Cond. Matt. Phys. (2022), arXiv:2204.08250

11:25  12:00

Emanuel Gull
(University of Michigan, Ann Arbor)
From models to realistic systems – Integrating the Baym Kadanoff equations for molecules
This talk will focus on the timeintegration of the `threecontour' Baym Kadanoff equations and the calculation of secondorder selfenergies. Special emphasis is placed on obtaining results for multiorbital systems such as molecules with realistic Coulomb interactions, as well as couplings to a light field and singleparticle excitation spectra. We will examine the performance of numerical techniques such as efficient basis functions and adaptive compression schemes. Comparisons to real frequency spectra obtained with equilibrium methods and Nevanlinna continuation are shown.

12:00  13:00

Lunch break

13:00  14:00

Informal discussions


chair: Emanuel Gull

14:00  14:20

Michael Schüler
(Paul Scherrer Institut Villigen)
Probing Topological Floquet States by ultrafast Time and AngleResolved Photoemission Spectroscopy: fingerprints in circular dichroism and birth of Floquet states
Observing signatures of lightinduced Floquet topological states in materials has been shown to be very challenging. Angleresolved photoemission spectroscopy (ARPES) is well suited for the investigation of Floquet physics, as it allows to directly probe the dressed electronic states of driven solids. Depending on the system, scattering and decoherence can play an important role, which has hampered the observation of Floquet states. Another challenge is to disentangle Floquet side bands from laserassisted photoemission (LAPE), since both lead to similar signatures in ARPES spectra.
In this talk, we will discuss how to tackle these challenges with advances in timeresolved ARPES (trARPES). First, we investigate the emergence of Floquet state in the transition metal dichalcogenide 2HWSe$_2$, one of the most promising systems for observing Floquet physics. Based on fully ab initio treatment of the lightmatter interaction, we discuss how the Floquet topological state manifests in characteristic features in the circular dichroism in photoelectron angular distributions. Combining highly accurate modeling of the photoemission matrix elements with an ab initio description of the lightmatter interaction, we investigate regimes which can be realized in current stateoftheart experimental setups.
We also discuss the ultrafast “birth” of FloquetBloch bands from the topological surface state of Bi$_2$Te$_3$ probed by subcycle trARPES. The combination of experiment and theory paves the way for realizing Floquet states down to the ultimate fewcycle limit.

14:20  14:40

Tanay Nag
(Uppsala University)
Dynamical construction of Quadrupolar and Octupolar topological superconductors
We propose a threestep periodic drive protocol to engineer twodimensional~(2D) Floquet quadrupole superconductors and threedimensional~(3D) Floquet octupole superconductors hosting zerodimensional Majorana corner modes~(MCMs), based on unconventional $d$wave superconductivity. Remarkably, the driven system conceives four phases with only 0 MCMs, no MCMs, only anomalous $\pi$ MCMs, and both regular 0 and anomalous $\pi$ MCMs. To circumvent the subtle issue of characterizing 0 and $\pi$ MCMs separately, we employ the periodized evolution operator to architect the dynamical invariants, namely quadrupole and octupole motion in 2D and 3D, respectively, that can distinguish different higherorder topological phases unambiguously. Our study paves the way for the realization of dynamical quadrupolar and octupolar topological superconductors.
Ref: Phys. Rev. B 105, 155406 (2022)

14:40  15:00

Steven Tomsovic
(Washington State University)
PostEhrenfest Manybody Quantum Dynamics: a Semiclassical Approach
Interference in quantum dynamics cannot be delayed beyond an Ehrenfest time scale, which therefore delimits the quantumclassical correspondence. Nevertheless, for bosonic manybody quantum systems with a meanfield limit, advanced semiclassical methods do incorporate genuine manybody quantum interferences accurately for large particle numbers relying exclusively on such meanfield solutions. In this way, system specific nonequilibrium quantum dynamical features, which cannot be captured in detail within a truncated Wigner approximation, can still be understood with the knowledge of mean field solutions. We illustrate the predictive power of pre and postEhrenfest semiclassical dynamics with the example of coherent state propagation in a BoseHubbard model.

15:00  15:20

Michael Kajan
(University of Bonn)
Auxilliary particle field theory for generalised and nonMarkovian JaynesCummings Models
The interaction of photons with molecules or atoms coupled to vibrations are often described by JaynesCummings or spinboson models. Commonly, these models are treated via a rateequation approach [1] due to the noncanonical dynamics of the (spinlike) electronic excitations. However, this approach does not account for nonMarkovian dynamics of the internal vibrational states. We introduce a novel auxilliaryparticle formulation for the electronic and vibrational state of the molecules. This fieldtheoretical approach can be applied in and out of equilibrium and can capture nonMarkovian dynamics, large reservoir sizes as well as spontaneously broken U(1) symmetry due to BoseEinstein condensation. We present results for a pump cavity system filled with a dilute dye solution showing a BEC transition of photons [2]. Further generalisations of this formulation can be applied to various open or closed multilevel systems.
[1] P. Kirton, J. Keeling, Phys. Rev. Lett. 111, 100404 (2013).
[2] J. Klaers, J. Schmitt, F. Vewinger, M. Weitz, Nature 468, 545 (2010).

15:20  15:50

Coffee break


chair: Johann Kroha

15:50  16:10

Manuel Weber
(Max Planck Institute for the Physics of Complex Systems)
Theoretical description of timeresolved photoemission in chargedensitywave systems out to long times
We describe coupled electronphonon systems semiclassically—Ehrenfest dynamics for the phonons and quantum mechanics for the electrons—using a classical Monte Carlo approach that determines the nonequilibrium response to a large pump field. The semiclassical approach is quite accurate, because the phonons are excited to average energies much higher than the phonon frequency, eliminating the need for a quantum description. The numerical efficiency of this method allows us to perform a selfconsistent time evolution out to very long times (tens of picoseconds) enabling us to model pumpprobe experiments of a charge density wave (CDW) material. Our system is a halffilled, onedimensional (1D) Holstein chain that exhibits CDW ordering due to a Peierls transition. The chain is subjected to a timedependent electromagnetic pump field that excites it out of equilibrium, and then a second probe pulse is applied after a time delay. By evolving the system to long times, we capture the complete process of lattice excitation and subsequent relaxation to a new equilibrium, due to an exchange of energy between the electrons and the lattice, leading to lattice relaxation at finite temperatures. We employ an indirect (impulsive) driving mechanism of the lattice by the pump pulse due to the driving of the electrons by the pump field. We identify two driving regimes, where the pump can either cause small perturbations or completely invert the initial CDW order. Our work successfully describes the ringing of the amplitude mode in CDW systems that has long been seen in experiment.

16:10  16:30

Miroslav Hopjan
(Jozef Stefan Institute Ljubljana)
Realtime nonadiabatic dynamics in the onedimensional Holstein model: trajectorybased versus exact methods
We benchmark a set of quantumchemistry methods including the multitrajectory Ehrenfest, the fewestswitches surfacehopping, and the multiconfigurational Ehrenfest method against exact quantummanybody techniques by studying realtime dynamics in the Holstein model up to 51 sites. We address both the physics of single Holstein polarons and the dynamics of chargedensity waves at finite electron densities. For larger systems, we compare the quantumchemistry methods to exact dynamics obtained from timedependent density matrix renormalization group calculations with local basis optimization (DMRGLBO).
Our results show that the multitrajectory Ehrenfest method in general only captures the ultrashort time dynamics accurately and that the surfacehopping method with suitable corrections provides a much better description of the longtime behavior. We further show that the multiconfigurational Ehrenfest method yields a significant improvement over the multitrajectory Ehrenfest method and can be converged to the exact results in small systems with moderate computational efforts, however, for extended systems, this convergence is slower with respect to the number of configurations. Our benchmark study demonstrates that DMRGLBO is a useful tool for assessing the quality of the quantumchemistry methods.

16:30  16:50

David Jansen
(GeorgAugustUniversität Göttingen)
Dynamical properties in electronphonon systems at finite temperatures from matrixproduct state methods
We compute the optical conductivity for the Holstein polaron and bipolaron with dispersive phonons at finite temperature using a matrixproduct statebased method. We combine purification [1], to obtain the finitetemperature states [2], together with the parallel timedependent variational principle (pTDVP) [3] algorithm to compute the realtime currentcurrent correlation functions. The pTDVP algorithm utilizes local basis optimization [4] to efficiently treat the phononic degrees of freedom.
For the polaron, we find that the phonon dispersion alters the optical conductivity at several temperatures in the weak, intermediate, and strong coupling regimes. In the two first cases, we see that the spectrum goes from being continuous to discrete when going from an upwards to a downwards phonon dispersion relation. In the strong coupling regime, the dispersion leads to a shift of the center of the spectrum.
For the bipolaron, we also see that the dispersion shifts the spectrum. The results fit well with an analytical expression derived from the BornOppenheimer Hamiltonian.
[1] Verstraete et al., Phys. Rev. Lett. 93, 207204 (2004)
[2] Jansen et al., Phys. Rev. B 102, 165155 (2020)
[3] Secular et al., Phys. Rev. B 101, 235123 (2020)
[4] Zhang et al., Phys. Rev. Lett. 80, 2661 (1998)

16:50  17:10

Md Mursalin Islam
(Tata Institute of Fundamental Research Mumbai)
Nonequilibrium scalar field dynamics starting from Fock states: Absence of thermalization in one dimensional phonons coupled to fermions
We developed a new formalism to study nonequilibrium dynamics of scalar fields using Keldysh field theory. We use our formalism to study nonequilibrium dynamics of phonons coupled to thermal baths. When connected to an Ohmic bath, all phonon modes thermalize as expected. On the other hand when we couple the phonons to onedimensional fermionic bath, low wavelength modes fail to thermalize at all temperatures. At low temperatures, constraints from energymomentum conservation lead to a narrow bandwidth of particlehole excitations and the phonons effectively do not see this bath. On the other hand, the strong band edge divergence of particlehole density of states leads to an undamped "polaritonlike" mode of the dressed phonons above the band edge of the particlehole excitations. These undamped modes contribute to the lack of thermalization of long wavelength phonons at high temperatures. In higher dimensions, these constraints and the divergence of density of states are weakened and leads to thermalization at all wavelengths.

17:10  18:00

Informal discussions

18:00  19:00

Dinner

19:00  20:00

Informal discussions
