Dynamical Control of Quantum Materials

Motivated by the recent experimental measurements of time-dependent ARPES of TI irradiated by short pulses with few oscillations, this work proposes a theoretical approach to describe the dynamics of these electronic systems within the Floquet-inspired t-t' formalism. In this formalism, an instantaneous Floquet basis is built as a function of the time and the pulse amplitude and the evolution of the system is consequently described within an effective Floquet picture. More precisely, we studied the effect of a linearly polarized pulse in 2D Dirac models in order to elucidate the nature of the sidebands that appear in the irradiated systems. We compare the results from the Floquet t-t' formalism with the exact solution of the evolution by the Schödinger equation to elucidate the limits and validity of this formalism. This way, we can gain more insight in the physical interpretation of such sidebands and the Floquet replicas in the case of a pulse driving.

We study the change in transport as well as spectral properties of a fermionic quantum-chain placed in a single-mode cavity. The model under consideration is a 1D thight-binding chain, which can be occupied by spinless fermions, coupled to non-interacting fermionic reservoirs on both sides. The chain is then placed in a single-mode cavity and two different boson-fermion couplings are investigated: The Peierls-substitution, coupling the photons to the hopping between quantum dots, and a local coupling to the on-site energies. For both these cases, integrating-out the bosonic degrees of freedom in steady-state leads to an effective long-range, frequency-dependent fermion-fermion interaction. The effects of this interaction are investigated in-and out-of-equilibrium by evaluating the truncated functional renormalization group flow as well as first order perturbation theory in the Matsubara formalism and on the Keldysh contour. Supplementary, exact diagonalization studies are conducted for the isolated system without reservoirs. The interaction leads to frequency-dependent self-energies, even in first-order truncation, introducing a non-trivial energy-dependence into the system. This affects e.g. the mean occupation of the separate sites of the chain and the current through the chain in non-equilibrium. Further, the effective long-range interaction opens-up additional transport-channels across the chain, which leads to Fano-like interference in the linear conductance for certain parameter-spaces. These effects can be understood by studying the effective energy spectrum or the spectral function of the system.

Strong-field terahertz (THz) radiation offers a direct way to resonantly drive highly relevant low-energy modes in matter, e.g., lattice vibrations, molecular rotations, spin precession and the motion of (quasi-)free electrons. Using these pulses as a pump in ultrafast experiments enables the study of dynamic processes in solid-state, chemical and eventually biological systems. The TELBE THz facility at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) is a strong-field THz-source offering multicycle, CEP stable THz pulses with an exceptionally high spectral density at a repetition rate up to 500 kHz. Therefore, TELBE is ideally suited for the research on THz-driven nonlinear dynamics and resonant control of a number of low-energy degrees of freedom. In this contribution, we present recent scientific results on nonlinear THz frequency conversion, spin dynamics and superconductor spectroscopy. In addition, we present the latest technical developments at the accelerator-based THz sources at HZDR for enabling novel high-field THz research. Finally, an outlook on future developments of highly intense and flexible THz sources within the Dresden Advanced Light Infrastructure (DALI) project is given.

Resonant excitation of certain phonon modes in high-Tc cuprates has been shown to induce THz optical properties reminiscent of superconductivity far above the equilibrium transition temperature. So far, nothing is known regarding the magnetic response of these non-equilibrium states. Do they also feature a Meissner effect and expel an external magnetic field? In this work, we study the out of equilibrium magnetic response of these materials. We make use of the Faraday effect in a magneto-optical crystal adjacent to the sample to reconstruct its position-dependent magnetic properties with sub-picosecond time resolution. We probe the presence of a Meissner effect by quantifying the degree of magnetic field expulsion after excitation. Studying its dependence on temperature and magnetic field provides new insights on the properties of the non-equilibrium state.

We study the non-equilibrium dynamics of the Yukawa-Sachdev-Ye-Kitaev-Model; a solvable prototypical model for unconventional superconductivity. It consists of $N$ free fermions randomly interacting with dispersionless bosons, and hosts a non-Fermi liquid normal state out of which superconductivity emerges at the lowest temperatures. Employing an exact field theoretic solution on the Keldysch-Contour, we study the non-equilibrium dynamics by integrating the Kadanoff-Baym equations, hence obtaining numerically exact solutions, arbitrarily far from equilibrium. Using this approach, we discuss the dynamical onset of pairing, and explain the anomalous heating dynamics of the superconducting state under periodic driving, highlighting difference and similarities to conventional superconductivity.

In order to systematically Floquet-engineer specific effective dynamics in quantum systems, it is important to have analytical methods available to treat the periodic drive of the system. For non-resonantly driven systems there are powerful perturbative methods, like the high-frequency expansion, which can be used to compute the Floquet-Hamiltonian in a simple and efficient manner in the limit of high-frequencies. For resonantly driven systems there are no such methods and in general there is little understanding about the effects of the driving. I will present a newly developed perturbative expansion method which allows to analytically compute the scattering at resonantly driven impurities and is based on different timescales/energy scales in the system. I will demonstrate this method on the example of a pair scattering at a resonantly driven impurity in an attractive Fermi-Hubbard chain. In this setting the technique shows that pair-breaking is suppressed in the strong coupling limit $J \ll |U|$ ($J$ hopping, $U$ Hubbard-interaction) and leads to analytical predictions for the pair transmission. Based on \url{https://arxiv.org/abs/2210.08380}

The FELBE FEL at the ELBE Center for High-Power Radiation provides ultrashort narrowband tunable THz pulses that are well suited for driving many types of nonequilibrium and nonlinear dynamics. The low energy photons can couple to charge carriers or directly to structural modes in matter. Recent experiments have revealed the nonlinear interaction of the THz field in both solid and liquid phase matter. Highlights from these studies are presented along with the potential for further exploration of THz control of matter with the FELBE FEL as well as with the advanced high-field THz sources at the heart of the proposed successor to ELBE. The proposed new facility, DALI, would achieve an increase in pulse energy of up to three orders of magnitude, while providing greater flexibility of experimental parameters along with new methods for probing THz-driven structural dynamics by UED or tr-ARPES.

It has long been thought that first-principles methods are incompatible with Floquet theory due to a lack of a unique ground-state definition of the Floquet systems [1]. We are thus only able to calculate periodically driven quantum systems from first principles using expensive propagation based methods, which cannot simulate beyond ps timescales in practical calculations. We propose a more robust definition of the Floquet states, based on the exact one-to-one correspondence between Floquet and Bloch theory [2]. From this we get a unique definition of the Floquet ground state, and a more efficient variational method to derive them. This is a first step towards reincorporating Floquet theory in first-principles calculation methods, a necessary tool for Floquet engineering realistic laser driven materials. 1. N. T. Maitra, K. Burke, "On the Floquet formulation of time-dependent density functional theory", Chem. Phys. Lett. 359, (2002), 237. 2. C. M. Le, R. Akashi, and S. Tsuneyuki, "Missing quantum number of Floquet states", Phys. Rev. A 105, (2022), 052213.

We study nickel-related color centers in diamond towards their application as a solid-state light-spin interface. These centers have strong spin-orbit interaction, thus are potentially suitable for high-temperature operation. Using polarized resonant excitation we observe signs of both spin and charge optical pumping at 10K. We study some other effects with the case of nickel in magnesium oxide as follows: We perform electron spin resonance spectroscopy and polarization-sensitive magneto fluorescence spectroscopy of a dense ensemble of these ions, and find that, (i) the ground-state electron spin stays coherent at liquid-helium temperatures for several microseconds, and, (ii) there exist energetically well-isolated excited states which can optically couple to two ground spin states in orthogonal polarizations. The latter implies that fast, coherent optical spin control is possible. We then propose schemes for optical initialization and control of the ground-state spin using polarized optical pulses, as well as two schemes for the implementation of a noise-free, broad-band quantum-optical memory at near-telecom wavelengths using this material system.

Information processing using preferential excitation of one of the two energy degenerate valleys in inversion symmetry broken graphene-like systems has been achieved by circularly polarized pulses. These pulses couple differentially to the valleys which have opposite orbital angular momentum, depending on their polarization . Recent studies have, however, shown that linearly polarised light pulses can generate appreciable valley polarization, even in pristine graphene, without breaking inversion symmetry at the Hamiltonian level. In our poster, we will shed some light on the general mechanisms of this process of valley polarization with ultrashort laser pulses. We also show results for the terahertz regime, in which graphene shows strong non linear behaviour,and discuss the role of electronic decoherences for such longer pulses. Work done with Prof. JM Rost and Ulf Saalmann

Comprehending far-from-equilibrium many-body interactions is one major goal of current ultrafast condensed matter physics research. A particularly interesting but barely understood situation occurs during strong optical excitation, where electron and phonon systems are significantly perturbed from their equilibrium and cannot be described by Fermi-Dirac or Bose-Einstein distributions, respectively. Here, we use time- and angle-resolved photoelectron spectroscopy (trARPES) to study such situation for the prototypical material graphene. We show that upon optical excitation, it exhibits a complex non-equilibrium many-body response by evaluating the Dirac state linewidth and thus the imaginary part of the quasiparticle self-energy $\mathrm{Im}\Sigma$ from spectrally deconvoluted trARPES data. By employing first-principles theoretical modeling, we find that the observed experimental features are caused by ultrafast NEQ scatterings between optical phonons and photoexcited charge carriers, active on timescales well below 100\,fs. \noindent Düvel, Merboldt \emph{et al.}, Nano Letters 2022, 22, 12, 4897–4904

Floquet topological insulators (FTI) have become ubiquitous in the pursuit to realize new phases of matter. In general, the momentum dependent quasi energy spectrum of single particle time evolution operators or Floquet Hamiltonians are used to identify the band topology. In the presence of many particle interactions, this single particle picture breaks down. To solve this issue, topological invariants of static systems have been formulated through their single particle Green’s functions. We explore the challenges of extending this formalism to Floquet Green's function invariants and demonstrate properties of driven interacting systems in exact diagonalization studies.

Doping charge carriers into Mott insulators provides a pathway to produce intriguing emergent phenomena. In equilibrium systems, the doping can be chemically controlled. On the other hand, photo-doping, where particles are excited across the Mott gap, provides an alternative way. Compared to chemical-doping, photo-doping creates a wider variety of carriers, which may lead to the emergence of fascinating nonequilibrium states. In particular, when the gap is large, the life-time of photo-carriers becomes exponentially enhanced, which can lead to a metastable states after the intraband cooling of photo-carriers occurs. In this talk, we reveal the peculiar features of such metastable states realized in the one-dimensional extended Hubbard model [1,2]. Namely, we show that the corresponding wave function in the larger on-site interaction limit can be expressed as $|\Psi\rangle =|\Psi_{\rm charge}\rangle|\Psi_{\rm spin}\rangle |\Psi_{\rm \eta- spin}\rangle$, which indicates the separation of spin, charge and η−spin degrees of freedoms. Here $\eta$−spin represents the type of the photo-carriers. This state is analogous to the Ogata-Shiba state of the doped Hubbard model in equilibrium. $|\Psi_{\rm charge}\rangle|$ is determined by spinless free fermions, $|\Psi_{\rm spin}\rangle$ by the isotropic Heisenberg model in the squeezed spin space, and $|\Psi_{\rm \eta- spin}\rangle$ by the XXZ model in the squeezed $\eta$-spin space. In particular, the metastable $\eta$-pairing and charge-density-wave (CDW) states correspond to the gapless and gapful states of the XXZ model. The specific form of the wave function allows us to accurately determine correlation functions, and suggests that the central charge of the $\eta$-pairing state is 3 and that of the CDW phase is 2. We also discuss the dynamical properties of these states. Our results demonstrate that the emergent degrees of freedom activated by photo-doping can lead to peculiar types of quantum liquids absent in equilibrium. [1] Y. Murakami, S. Takayoshi, T. Kaneko, Z. Sun, D. Gole\v{z}, A. J. Millis, P. Werner, Comm. Phys. 5, 23 (2022). [2] Y. Murakami, S. Takayoshi, T. Kaneko, A. Läuchli, P. Werner, arXiv:2212.06263.

We theoretically predict a new working principle for optical amplification, based on Weyl semimetals: when a Weyl semimetal is suitably irradiated at two frequencies, electrons close to the Weyl points convert energy between the frequencies through the mechanism of topological frequency conversion from [Martin et al, PRX 7 041008 (2017)]. Each electron converts energy at a quantized rate given by an integer multiple of Planck's constant multiplied by the product of the two frequencies. In simulations, we show that optimal, but feasible band structures can support topological frequency conversion in the "THz gap" at intensities down to $2{\rm W}/{\rm mm}^2$; the gain from the effect can exceed the dissipative loss when the frequencies are larger than the relaxation time of the system. Topological frequency conversion provides a new paradigm for optical amplification, and further extends Weyl semimetals' promise for technological applications.

In this poster, we explore the use of Floquet engineering [1], a concept developed in quantum many-body systems, to dynamically control Chemical Reaction Networks (CRNs). CRNs are networks of chemical reactions where each reaction triggers the next one, and their influence spreads or dies depending on network structure and reaction rates. We describe the reactions and diffusion using the master equation, which can be mapped to a "quantum" many-body Hamiltonian using the Doi-Peliti mapping [2]. This enables us to apply ideas from solid-state physics, such as phase transitions and Floquet engineering, and concepts like quantum geometry. We apply this approach to a prototypical problem of cell signal transduction stimulated by optogenetics (Fig.1), analyzing the time evolution from two species of GPCRs on the plasma membrane stimulated by two colors of light. Using the time-dependent mean-field approach, we show that we can perform topological pumping and control cAMP production. [1] T. Oka and S. Kitamura, Annu. Rev. Condens. Matter Phys. 10, 387 (2019). [2] U. C. Tauber, M. Howard and B. P. Vollmayr-Lee, Journal of Physics A: Mathematical and General 38(17), R79 (2005).

The talk is about the microscopic dynamics of competing ordered phases in a two-dimensional correlated electron model, which is driven with a pulsed electric field of finite duration. In order to go beyond a mean-field treatment of the electronic interactions we adopt a large-$N$ generalization of the Hubbard model and combine it with the semiclassical fermionic truncated Wigner approximation [1] as a time evolution method. This allows us to calculate dephasing corrections to the mean-field dynamics and to obtain stationary states, which we interpret as prethermal order. We use this framework to simulate the light-induced transition between two competing phases (bond density wave and staggered flux) and find that the post-pulse stationary state order parameter values are not determined alone by the amount of absorbed energy but depend explicitly on the driving frequency and field direction. While the transition between the two prethermal phases takes place at similar total energies in the low- and high-frequency regimes, we identify an intermediate-frequency regime for which it occurs with minimal heating [2]. [1] S. M. Davidson et al, Ann Phys 384, 128 (2017) [2] A. Osterkorn and S. Kehrein, PRB 106, 214318 (2022)

We utilize pump-probe spectroscopy to measure the quasiparticle relaxation dynamics of BaFe$_2$As$_2$ in a diamond anvil cell at pressures up to 4.4 GPa and temperatures down to 8 K. Tracing the amplitude of the relaxation process results in an electronic phase diagram that illustrates the variation of the spin-density wave (SDW) order across the whole range of the applied pressures and temperatures. We observe a slowing down of the SDW relaxation dynamics in the vicinity of the phase transition boundary. However, its character depends on the trajectory in the phase diagram: the slowing down occurs gradually for the pressure-induced transition at low temperatures and abruptly for the thermally-driven transition. Our results suggest that the pressureinduced quantum phase transition in BaFe$_2$As$_2$ is related to the gradual worsening of the Fermi-surface nesting conditions.

We present the analysis of the slowing down exhibited by stochastic dynamics of a ring-exchange model on a square lattice, by means of numerical simulations. We find the preservation of coarse- grained memory of initial state of density-wave types for unexpectedly long times. This behavior is inconsistent with the prediction from a low frequency continuum theory developed by assuming a mean field solution. Through a detailed analysis of correlation functions of the dynamically active regions, we exhibit an unconventional transient long ranged structure formation in a direction which is featureless for the initial condition, and argue that its slow melting plays a crucial role in the slowing-down mechanism. We expect our results to be relevant also for the dynamics of quantum ring-exchange dynamics of hard-core bosons.

Out-of-equilibrium effects provide an elegant pathway for probing and understanding the underlying physics of correlated materials. In particular, controlling electronic band structure properties using ultrafast optical pulses has shown promise for creating exotic states of matter by inducing charge density waves, or modifying the fermi velocities of Dirac particles. Of recent interest is band renormalization effects in square-net materials as they possess interesting spectral properties, e.g., nodal lines or axial-Higgs physics. Here we present a theoretical study of out-of-equilibrium effects in the family of nodal line semimetals featuring a square net of atoms. Specifically, we show that the renormalization of the kinetic energy of electrons due to the ultrafast pump field leads to the enhancement of the effective mass and to the shift of the resonant frequencies of the material. Finally, we discuss signatures of such modifications in transient Rayleigh and Raman scattering. Our study demonstrates the potential of this approach in creating photoinduced phases in topological quantum matter through an all-optical route.

Interaction of intense laser light with solids may lead to changes in the symmetry of the solids. It is known that the interplay between transiently-evolving dynamical symmetry and electronic dynamics dictates several physical, optical and chemical properties of solids. Thus, probing the dynamical symmetries is essential for our microscopic understanding of several ubiquitous phenomena and for predicting new concepts in solids. The interaction between intense laser and vibrating graphene is modeled by solving the density matrix based using the semiconductor Bloch equation. In this contribution, we demonstrate that high-harmonic spectroscopy is sensitive to the transiently-evolving dynamical symmetry in vibrating graphene with subcycle temporal resolution. It is found that the dynamical symmetries lead to the generation of higher-order sidebands in the high-harmonic spectrum, which is modulated by the frequency of the coherent vibration. Also, we show that the spectral positions and the polarization of the sideband emission offer a sensitive probe of the dynamical symmetries associated with the excited phonon modes. Our work brings the key advantage of high-harmonic spectroscopy—the combination of subfemtosecond to tens of femtoseconds temporal resolution—to the problem of probing phonon-driven electronic response and its dependence on the dynamical symmetries in solids. The present work opens a platform to probe phonon-driven processes in solids and nonlinear phononics with subcycle temporal resolution. Ref: High-harmonic spectroscopy of coherent lattice dynamics in graphene, Physical Review B 106, 064303 (2022).

Periodic driving has become an essential tool for controlling quantum systems. Besides it utmost importance in engineering Hamiltonians otherwise not experimentally accesible, periodic driving opens the possibility to realise novel, non-equilibrium phases of matter without analog in static systems. While much advance has been made in engineering desired Hamiltonians, studying the actual states of interest of these periodically driven systems still mainly relies on adiabatic state preparation. However, ideal adiabatic processes require very slow dynamics, leading to a compromise in time scales of heating and decoherence processes. In order to allow for faster state preparation, we extent the framework of counter-diabatic driving to the non-equilibrium regime. In particular, we introduce a variational principle allowing us to compute experimentally accesible approximate counter-diabatic protocols in complex quantum systems. We apply our scheme to speed up the state preparation in a Floquet topological pump, a system recently realised in experiments.

Magnetic Weyl semimetals are a new class of topological matter in which broken time reversal symmetry is responsible to the formation of Weyl points. Co3Sn2S2, a prototypical example for this class, has been under intense study since the observation of Weyl points in synchrotron-based ARPES experiments. DFT calculations predict the existence of two energy-separated sets of Weyl points belonging to the spin-up and spin-down bands. At 177K, it undergoes a phase transition to a paramagnetic phase, thus restoring time-reversal symmetry and prohibiting the existence of Weyl points. This implies that the magnetic phase transition is topological, meaning that Weyl points of opposite chiralities must merge and annihilate. However, there is no direct observation of the Weyl points merging and annihilating because they shift to energies above the Fermi energy and thus become inaccessible to equilibrium measurements. We explore this phase transition using time-resolved ARPES: electrons are excited to unfilled bands by an IR pulse and detected via photoemission induced by a UV probe pulse. We report unusual dynamics of the excited electrons as well as their very long lifetime.

Coherent State Steering in Condensed Matter Systems with Strong Light-Matter Engineering M.S. Spencer, J. Urban, M. Frenzel, S.F. Maehrlein Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany Physical properties of materials are derived largely from their chemical constituents, structural arrangement, and local properties such as temperature and dielectric environment. Next-generation materials science is increasingly focused on manipulation of structural properties in order to access material properties on-demand, in search of emergent, enhanced, or even hidden states of matter. One such method to transiently modify crystalline materials is the application of intense terahertz (THz) laser pulses. These pulses enable resonant and selective excitation of infrared-active vibrational modes (phonons), allowing for coherent and ultrafast modulation of condensed-matter systems. By placing materials within an electromagnetic cavity, it is possible to tune the vibrational frequencies driven by intense THz radiation. This expansion of THz driving into the strong-coupling regime will provide an avenue towards control over the excited state of the lattice, potentially allowing access to exotic, transient states not achievable from intrinsic material properties. I will present our first steps towards realizations of strongly-coupled phonons in crystalline materials within the THz frequency range. Furthermore, I will discuss novel measurement techniques for the detection of strong light-matter interactions utilizing the unique technical capabilities of time-domain terahertz spectroscopy.

We consider the Landau-Zener type quantum tunneling with the addition of the second order twist term, which causes the geometric effects in tunneling amplitude. We derive a formula for tunneling probability and show that the nonadiabatic geometric effects provoke anomalous phenomena: gapped perfect tunneling, rectification, and counterdiabaticity. These results can be applied to the condensed matter systems such as the particle-hole pair production in Dirac systems induced by strong rotating electric fields, and predict an optically induced valley polarization in 2D Dirac materials, and the generation of a nonlinear chiral or spin current in 3D Dirac materials. We also disucuss the extension of the theory to many-body systems such as interacting quantum spin systems.

Floquet engineering has become a powerful tool for understanding laser induced coherent phenomena in quantum materials[1]. It is noticed that an artificial axial gauge field (also known as chiral gauge field) can be realized[2] when emergent Floquet Weyl points appear in the Floquet spectrum[3]. Here, we concentrate on 3D Dirac semimetals irradiated by circularly polarized laser fields which lead to a Floquet Weyl pair. By utilzing the envelope profile of the laser field and considering the skin effect from the semimetal surface, nonzero axial electric and magnetic fields can be achieved. In this talk, we focus on physical consequences from the axial fields such as generation of currents and spectral features, and discuss their possible experimental signatures[4]. [1] T. Oka, S. Kitamura, Annual Rev. Cond. Matt. Phys. 10, 387, (2019). [2] S. Ebihara, K. Fukushima, T. Oka, Phys. Rev. B 93, 155107 (2016). [3] L. Bucciantini, S. Roy, S. Kitamura, T. Oka, Phys. Rev. B 96, 041126 (R) (2017). [4] H.-H. Teh, T. Numasawa, T. Oka, in progress.

Interaction of strong light with matter can completely change its electronic structure and resulting laser-dressed material can exhibit physical properties very different from their equilibrium properties. Specifically, when continuous wave time periodic laser is used to control physical properties of material, it is called Floquet engineering. In this work, we develop a generalized theory to calculate the optical absorption properties of solids dressed by strong periodic laser and show its application to compute the non-equilibrium absorption spectrum of a periodic model system. We simulate the optical absorption of laser-dressed solid by capturing all physical processes in the system that could occur due to the interaction of laser dressed system by a probe photon and compute the obtained two-time momentum momentum correlation function. We exploit the space and time periodicity of the laser-dressed solids by using the Floquet-Bloch states as our basis for the problem. We interpret the obtained final expressions for net absorption in terms of transitions occurring among the Floquet-Bloch states across different Floquet-Brillouin zones. We then proceed to apply our theory to compute optical absorption of laser-dressed periodic lattice system. We find that when the system is dressed with a low-frequency non-resonant laser it shows: replicas of the equilibrium absorption features as new features below the field-free band edge, blue-shift of the field-free band edge, and strong low frequency absorption and stimulated emission features, as the drive electric field strength is increased. We provide an explanation of all the observed phenomenon and show that a strong non-resonant light can completely transform the optical absorption of the material and transiently turn a wide band gap semiconductor into a broadband absorber.

Inducing superconductivity out of equilibrium is a grand challenge in the field of nonequilibrium condensed matter physics, which has been pursued in experimental studies on light-induced phenomena in the last decade or so. Strong driving inevitably injects energy to the system, which eventually turns to heat and suppresses superconductivity. So the question is how one can avoid rapid thermalization and maintain nonthermal high energy states for long time. Here we discuss two possible scenarios to achieve this: (i) Floquet superconducting states in the presence of dissipation (for a recent review of Floquet states, see [1]) and (ii) eta pairing states at high energies. In the former, we show that one can overcome heating effects in time-periodic steady states by employing a simple set up. In the latter, we discuss the effect of the coupling between eta pairing states and dynamical electromagnetic fields and how to induce eta pairing states by spontaneous light emission. [1] N. Tsuji, arXiv:2301.12676.

The manipulation of quantum materials by light is a fascinating issue spanning optics and condensed matter physics. Floquet engineering, which exploits the nontrivial electronic structure of systems subject to periodic driving (Floquet states), is the most promising candidate to achieve it. So far, the unique energy structure changes in the Floquet state induced by intense and ultrashort pulses have been studied intensively. On the other hand, such energy changes in time may trigger dynamic jumps between different Floquet states. As the energy shift becomes larger, the dynamical aspect of the Floquet state becomes non-negligible and has a significant impact on the resultant material properties. Here, by tuning the photon energy and the field strength of the driving light pulse, we have achieved dynamic jumps between Floquet states of excitons in monolayer WSe2 at room temperature, and successfully observed the dynamically engineered Floquet states using high-harmonic spectroscopy.

Operation of electronic devices in the picosecond regime is technically challenging and hitherto still largely unexplored. Here, we present an optoelectronic experiment for measurement of ultrafast memory switching, enabling accurate measurement of electrical switching parameters with 100 fs temporal resolution. Photoexcitation and electro-optic sampling on a CdMnTe substrate are used to generate and measure electrical pulse propagation along the transmission line. We demonstrate high contrast nonvolatile resistance switching from high to low resistance states of a 1T-TaS$_2$ device using single sub-2 ps electrical pulses. Using modeling we find that the switching energy density per unit area is exceptionally small, only 9.4 fJ/$\mu$m$^2$. The speed and energy efficiency of an electronic “write” process place the 1T-TaS$_2$ devices into a category of their own among new-generation nonvolatile memory devices.

Realizations of unconventional topological superconductors have been under debate and are still an open question of condensed matter research. In our work, we propose a light-induced mechanism to drive a conventional centrosymmetric superconductor into a metastable unconventional odd-parity superconducting state. First, we investigate in great detail what ingredients are needed to realize the existence of a metastable superconducting state and under which assumptions the subleading instabilities turn out to be unstable. We show that a magnetic field can be employed in order to stabilize an odd-parity state which would be unstable otherwise. We study this utilizing an algorithm based on Keldysh Green's functions that evolves the competing mean field order parameters in time, as well as perturbation theory. Then, we demonstrate with an exemplary light pulse how to drive the system from its conventional even-parity ground state into the desired extraordinary odd-parity state. Here, it is crucial that the pulse breaks inversion symmetry to introduce a coupling between the different symmetry sectors. This process reveals a completely new possibility to obtain topological superconductivity.

We describe coupled electron-phonon 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 self-consistent time evolution out to very long times (tens of picoseconds) enabling us to model pump-probe experiments of a charge density wave (CDW) material. Our system is a half-filled, one-dimensional (1D) Holstein chain that exhibits CDW ordering due to a Peierls transition. The chain is subjected to a time-dependent 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.

TMDC monolayers are atomically thin semiconductor materials, which, due to their reduced dimensionality and crystal structure, possess unique optical properties. In particular, they host bound electron-hole pairs, so-called excitons, with binding energies of several 100 meVs. Here, we report on pump-probe spectroscopy on excitons in a MoSe2 monolayer encapsulated in h-BN. We use spectrally broad probe pulses and a spectrometer to measure the transient differential reflectivity spectra. Based on a mode-locked Ti:Sa laser, a temporal resolution of down to 300fs is achieved. A frequency-doubled OPO can be used to generate non-resonant pump pulses in the visible wavelength range. Furthermore, probe pulses with a bandwidth of up to 70nm can be generated via supercontinuum generation in a photonic crystal fiber. The experiments are performed at 4K in a helium-flow cryostat. Within the first few picosecond after the pump pulse, we observe a blueshift of the exciton resonance. This shift depends on the wavelength of the pump pulses as well as the polarization of the pump- and probe pulses. Further experiments with an electrically gated heterostructure to control the doping of the monolayer are in progress.

The FELBE free-electron laser delivers intense THz and mid infrared pulses that are ideal to study nonlinear effects and carrier relaxation dynamics of low-energy excitations in solids [1]. We present the capabilities in the FELBE laboratories for degenerate and two-color pump-probe experiments, four-wave mixing and time-resolved near-field experiments. Exemplarily we show how nonlinear intraband transport in bilayer graphene manifests itself in polarization resolved pump-probe experiments [2]. Polarization resolved pump-probe experiments furthermore shed light into the microscopic scattering processes, allowing us to disentangle Coulomb and phonon-related scattering processes [3]. [1] M. Helm, S. Winnerl, A. Pashkin, J. M. Klopf, J.-C. Deinert, S. Kovalev, P. Evtushenko, U. Lehnert, R. Xiang, A. Arnold, A. Wagner, S. M. Schmidt, U. Schramm, T. Cowan & P. Michel, Eur. Phys. J. Plus 138, 158 (2023). [2] A. Seidl, R. Anvari, M. M. Dignam, P. Richter, T. Seyller, H. Schneider, M. Helm, and S. Winnerl Phys. Rev. B 105, 085404 (2022). [3] J. C. König-Otto, M. Mittendorff, T. Winzer, F. Kadi, E. Malic, A. Knorr, C. Berger, W. A. de Heer, A. Pashkin, H. Schneider, M. Helm, and S. Winnerl, Phys. Rev. Lett. 117, 087401 (2016).

We introduce a quantum optics platform featuring the minimal ingredients for the description of a spintronically pumped magnon condensate, where we study the driven-dissipative phase transitions. We consider a Dicke model weakly coupled to an out-of-equilibrium bath with a tunable spin accumulation. The latter is pumped incoherently in a fashion reminiscent of experiments with magnet-metal heterostructures. The core of our analysis is the emergence of a hybrid lasing-superradiant regime that does not take place in an ordinary pumped Dicke spin ensemble, and which can be traced back to the spintronics pumping scheme. We interpret the resultant non-equilibrium phase diagram from both a quantum optics and a spintronics standpoint. The outreach of our results concern dynamical control in magnon condensates and frequency-dependent gain media in quantum optics.