Dynamical Control of Quantum Materials

The theoretical understanding of hidden phases in strongly correlated quantum systems has been a challenge, because of the large separation of timescales between the intertwined dynamics of electrons, the lattice, and relevant order parameters. To overcome this limitation, we develop a theoretical approach build on a non-perturbative quasi-steady description of the electronic state. In a first application, we found that even within a minimal Holstein model for a charge density wave material, photo-excitation can result in an in-homogeneously disordered phase, with profound effect on the long-time dynamics.

Transition metal dichalcogenides (TMDs) are an exciting model system to study ultrafast energy dissipation pathways, and to create and tailor new emergent quantum phases [1,2]. The versatility of TMDs results from the confinement of optical excitations in two-dimensions and the concomitant strong Coulomb interaction that leads to excitonic quasiparticles with binding energies in the range of several 100 meV. In TMD stacks consisting of at least two layers, the interlayer interaction can be precisely controlled by manipulating the twist angle: The misalignment of the crystallographic directions leads to a momentum mismatch between the high symmetry points of the hexagonal Brillouin zones. This strongly impacts the interlayer wavefunction hybridization, and, moreover, adds an additional moiré potential. Crucially, in this emergent energy landscape, dark intra- and interlayer excitons dominate the energy dissipation pathways. While these dark excitonic features are hard to access in all-optical experiments, time-resolved momentum microscopy [3] can provide unprecedented insight on these quasiparticles [4]. Here we show the ultrafast formation dynamics of dark interlayer excitons in twisted WSe2/MoS2 heterostructures in space and time. First, we identify a hallmark signature of the moiré superlattice that is imprinted onto the momentum-resolved interlayer exciton photoemission signal. With this data, we reconstruct the electronic part of the exciton wavefunction, and relate its extension to the moiré wavelength of the heterostructure. Second, we find that interlayer excitons are effectively formed via exciton-phonon scattering, and subsequent interlayer tunneling at the interlayer hybridized ΣW valleys on the sub-50 fs timescale. Our work provides direct access to interlayer exciton formation dynamics in space and time and reveals opportunities to study correlated moiré and exciton physics for the future realization of exotic quantum phases of matter. References [1] Wang et al., Rev. Mod. Phys. 90, 021001 (2018). [2] Wilson et al., Nature 599, 383 (2021). [3] Keunecke et al., Rev. Sci. Ins. 91, 063905 (2020). [4] Schmitt et al., Nature 608, 499-503 (2022).

Harmonic generation is a general characteristic of driven nonlinear systems. We report on terahertz-field driven high-harmonic generation in the three-dimensional Dirac semimetal Cd3As2 [1]. The yield of the harmonic generation can be sensitively controlled by tuning the driving-pulse ellipticity [2]. The experimental results can be understood by terahertz field-driven nonlinear kinetics of the Dirac electrons. [1] S. Kovalev, R. M. A. Dantas et al, Nature communications 11, 2451 (2020). [2] S. Germanskiy, R. M. A. Dantas et al, Physical Review B 106, L081127 (2022).

Topological Photonics is an emerging and novel field of research, adapting concepts from condensed matter physics to photonic systems adding new degrees of freedom. After the first demonstations of topological photonic insulators [1], the field has moved on to study and exploit the inherent non-hermiticity of photonic systems and the interplay with their topological nature. In my talk I will discuss novel photonic lattice devices resulting from the coupling of individual vertical III-V semiconductor microresonators, forming what can be more broadly descibed as synthetic matter. Here, the so-called exciton-polaritons – hybrid states of light and matter – can emerge in the strong coupling regime. By chosing precise lattice geometries we are able to taylor optical band structures realizing novel photonic lattice. Here, the specific geometry as well as the hybrid light-matter nature allow for ways to break time-reversal symmetry and implement topologically non-trivial systems. Here, we were able to experimentally demonstrate the first exciton-polariton topological insulator, manifesting in chiral, topologically protected edge modes [2]. In order to study topological effects in combination with optical non-linearities, so-called topological lasers have been envisaged and realized. They exploit topological effects to efficiently couple and phase-lock extended arrays of lasers to behave like one single coherent laser. The major drawback so far is that the emission appears in the plane of the topologically protected light propagation, thus hindering light extraction. Here, we have presented the first experimental demonstration of a topological insulator vertical cavity laser array [3], using the so-called crystalline topological insulator model. Starting for the above mentioned examples, I will give an overview of the field of topological optical lattices and lasers and give an outlook on emerging novel materials beyond III-V semiconducturs, such as organic materials, transistion metal dichalcogenides and perovskites [4,5]. [1] Rechtsman et al. Nature 496, 196–200 (2013); Hafezi et al., Nat. Photon. 7, 1001–1005 (2013).\\ [2] Klembt et al., Nature 562, 552–556 (2018).\\ [3] Dikopoltsev et al., Science 373, 1514–1517 (2021).\\ [4] Dusel et al. Nano Lett. 21, 6398–6405 (2021).\\ [5] Shan et al., Nat. Commun. 12, 6406 (2021).

The ultrafast light-matter interaction is a powerful tool to control quantum materials’ properties and induce nontrivial dynamical states of matter such as transient superconductivity and Floquet topological phases. Crucial to the identification of these emergent phenomena is the concept of quantum coherence and our ability to distinguish genuine quantum correlations from fluctuations of randomized charge, spin, and orbital degrees of freedom. In certain systems, such as superconductors or certain quantum magnets, we can discriminate between incoherent and coherent processes by measuring specific order parameters. However, this approach cannot be easily generalized to more challenging forms of many-body entangled phases, such as quantum spin liquids. Is there an alternative way to diagnose the presence of quantum correlations in a light-driven material? In this talk, I will discuss how recent developments in the use of quantum metrology operators, called entanglement witnesses, open the possibility to probe many-body entanglement across photoinduced phase transitions. I will outline recent experimental and theoretical progress in quantifying spin entanglement in condensed matter systems and discuss how entanglement witnesses can be used to diagnose entanglement in systems with dynamically tuned electronic interactions. Finally, I will discuss how these methods can be applied to ultrafast optical and resonant inelastic x-ray scattering experiments, thus establishing a pathway to quantitative entanglement spectroscopy in quantum materials.

When a continuous symmetry of a physical system is spontaneously broken, two types of collective modes emerge: the amplitude and phase modes of the order-parameter fluctuation. For superconductors, the amplitude mode is the so called “Higgs mode”, the condensed-matter analogue of a Higgs boson in particle physics. Recent advances in THz technology allow to excite such Higgs modes in superconductors. I will review how this leads to a “Higgs Spectroscopy” that reveals the internal structure and dynamics of a superconducting condensate. In unconventional superconductors a complicated interplay of competing or intertwined orders obscures the view on the possible pairing mechanism. I will discuss how a phase resolved nonlinear THz spectroscopy paves the way to such a Higgs spectroscopy directly probing the collective condensate dynamics and exposing its coupling to external collective modes or quasiparticle excitations. Finally, extending the Higgs Spectroscopy into optical pump-drive spectroscopy also allows investigating transient dynamics of light driven superconducting states and disentangling the condensate dynamics from excited quasiparticles. [1] Hao Chu et al., Phase-resolved Higgs response in superconducting cuprates, Nature Commun. 11, 1793 (2020). [2] Lukas Schwarz et al., Classification and characterization of nonequilibrium Higgs modes in unconventional superconductors, Nature Commun. 11, 287 (2020). [3] Hao Chu et al., Fano interference of the Higgs mode in cuprate high-Tc superconductors, arXiv:2109.09971 [4] L. Feng et al. Dynamical interplay between superconductivity and charge-density-wave: a nonlinear terahertz study of coherently-driven 2H-NbSe2 and La2-xSrxCuO4, arxiv:2211.10947

Light-induced superconductivity in stripe-ordered phases of cuprate superconductors was initially reported in La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_4$ [1], and subsequently in La$_{2-x}$Ba$_x$CuO$_4$(LBCO) [2]. The mechanism was interpreted as the revival of inter-layer Josephson coupling which is frustrated in equilibrium[3]. To have a deeper understanding on the interplay between the charge/spin stripes and the superconductivity, we investigated another typical stripe-ordered superconductor, La$_{2-x-y}$Nd$_y$Sr$_x$CuO$_4$ where the correlation length of stripes is shown to be short-ranged compared to that of LBCO [4]. Upon the irradiation of ultrafast near-infrared light pulses, a clear plasma edge-like behavior is identified in the c-axis terahertz reflectivity below the onset temperature of the charge stripe. Detailed analysis of the optical spectra will be discussed. References 1. D. Fausti, R. I. Tobey, N. Dean, S. Kaiser, A. Dienst, M. C. Hoffmann, S. Pyon, T. Takayama, H. Takagi, and A. Cavalleri, Science 331, 189 (2011). 2. D. Nicoletti, E. Casandruc, Y. Laplace, V. Khanna, C. R. Hunt, S. Kaiser, S. S. Dhesi, G. D. Gu, J. P. Hill, and A. Cavalleri, Phys. Rev. B 90, 100503(R) (2014). 3. E. Berg, E. Fradkin, E.-A. Kim, S. A. Kivelson, V. Oganesyan, J. M. Tranquada, and S. C. Zhang, Phys. Rev. Lett. 99, 127003 (2007). 4. S. B. Wilkins, M. P. M. Dean, Jörg Fink, Markus Hücker, J. Geck, V. Soltwisch, E. Schierle, E. Weschke, G. Gu, S. Uchida, N. Ichikawa, J. M. Tranquada, and J. P. Hill, Phys. Rev. B 84, 195101 (2011).

Higgs spectroscopy is a new and emergent field [1-3] that allows to classify and determine the superconducting order parameter by means of ultra-fast optical spectroscopy. At present, there are two established ways to activate the Higgs mode in superconductors, namely a single-cycle ‘quench’ or an adiabatic, multicycle ‘drive’ pulse, both illustrated in Figure 1. In the talk I will report on the latest progress on Higgs spectroscopy, in particular on the role of the third-harmonic-generation (THG) [4-6] and the possible IR-activation of the Higgs mode by impurities or external dc current [7,8]. I also provide new predictions for time-resolved ARPES experiments in which, after a quench, a continuum of Higgs mode is observable and a phase information of the superconducting gap function would be possible to extract [9]. Finally, I show that the Higgs mode can be seen directly in tr-Raman scattering experiments [10,11]. [1] L. Schwarz, D. Manske et al., Nat. Commun. 11, 287 (2020). [2] L.Schwarz and D. Manske, Phys. Rev. B 101, 184519 (2020). [3] H.Chu, S. Kaiser, D. Manske et al., Nat. Commun. 11, 1793 (2020). [4] L. Schwarz. R. Haenel, and D. Manske, Phys. Rev. B 104, 174508 (2021). [5] H.Chu, S. Kaiser, D. Manske et al., submitted to Nature Materials. [6] M.-J. Kim, S. Kaiser, D. Manske et al., submitted to Nature Photonics. [7] M. Puviani, L. Schwarz, X.-X. Zhang, S. Kaiser, and D. Manske, Phys. Rev. B 101, 220507 (2020). [8]. R. Haenel, P. Froese, D. Manske, and L. Schwarz, Phys. Rev. B 104, 134504 (2021). [9] L.Schwarz, B. Fauseweh, and D. Manske, Phys. Rev. B 101, 224510 (2020). [10] M. Puviani, R. Hackl, D. Manske et al., Phys. Rev. Lett. 127, 197001 (2021). [11] M. Rübhausen, D. Manske et al., in preparation.

Optically driven quantum materials exhibit a variety of non-equilibrium functional phenomena, which have been primarily characterized with ultrafast optical, photoemission, X-Ray scattering and spectroscopy methods. However, little has been done to characterize their transient electrical responses, which are directly associated with the functionality of these materials. Especially interesting are the associated nonlinear transport characteristics, which are not easily measured at picosecond temporal resolution. Here, we report on ultrafast transport measurements in photo-excited K3C60, achieved by connecting thin films of this compound to photo-conductive switches with co-planar waveguides. We observe characteristic signatures of a photo-induced granular superconductivity, including nonlinear current-voltage response that reveal the presence of excited weak links between transiently superconducting grains. Ultrafast nonlinear transport provides new access to the physics of driven quantum materials and enables integration of non-equilibrium functionalities into ultrafast optoelectronic platforms.

We study the direct laser drive of infrared-active phonons that are quadratically coupled to a fermion chain. Considering the backaction of phononic excitations on the electronic dispersion, we uncover a novel phase transition in the non-equilibrium steady state. This transition has first order characteristics such as metastability, which enables preparation of a highly excited phonon state via a chirp protocol. Similar non-linearities will occur in other materials with strong quadratic phonon-electron coupling, including irradiated high-temperature superconductors, for which the driven phonons can play an important role in dynamically tunable pairing.

Active optical control of materials a rapidly growing field with prospects for developing novel functional materials and devices. In recent years, there has been considerable interest in using light to manipulate electronic phases, charge ordering, and interlayer correlations in low-dimensional materials, such as the transition-metal dichalcogenides (TMDCs). These approaches promise ultrafast, reversible, and non-invasive control of material properties. In this context, our research group studies fundamental mechanisms of light-induced phase transitions by developing ultrafast imaging and diffraction techniques using electrons and photons. We recently studied the coherent control over metal-to-insulator transitions in low-dimensional systems, which are important for their ultrafast changes to electronic and lattice properties. In particular, we demonstrated the role of vibrational coherence in controlling a metal-insulator structural phase transition in a quasi-one-dimensional solid-state surface system [1]. Utilizing a femtosecond double-pulse excitation scheme to switch the system to a metastable metallic state, we observed delay-dependent oscillations of the relevant collective amplitude modes, which suggest a ballistic component of the transition. Using Ultrafast Transmission Electron Microscopy (UTEM), we track the evolution of the order parameter in charge-density wave (CDW) domains of 1T-TaS$_2$ with simultaneous femtosecond temporal and nanometer spatial resolution. Specifically, a tailored dark-field scheme allows for the observation of relaxation pathways and the dynamics of phase boundaries [2]. Tilt-series ultrafast nanobeam electron diffraction in the transformed regions yields a reconstruction of CDW rocking curves at high momentum resolution, revealing an intermittent suppression of out-of-plane structural correlations. This dimensional crossover coincides with a loss of in-plane translational order, suggesting the formation of a transient hexatic state [3]. Finally, recent single-pulse quench experiments on the low-temperature commensurate CDW phase in 1T-TaS$_2$ reveal the coexistence of opposing chirality as a new structural characteristic of the light-induced hidden state [4]. We found that a single-pulse quench produces a state with long-range order and both chiral enantiomorphs of the low-temperature CDW. The coexistent long-range-order of both chiralities suggests extended interfaces between twisted charge-density wave layers, resulting in a higher-level Moiré structure, which is predicted to exhibit metallic conductivity. References: [1] "Coherent control of a surface structural phase transition", J. G. Horstmann, H. Böckmann, B. Wit, F. Kurtz, G. Storeck and C. Ropers, Nature 583, 232–236 (2020). [2] "Ultrafast nanoimaging of the order parameter in a structural phase transition", Th. Danz, T. Domröse, C. Ropers, Science 371, 371-374 (2021) [3] "Light-induced hexatic state in a layered quantum material", T. Domröse, T. Danz, S. F. Schaible, K. Rossnagel, S. V. Yalunin, C. Ropers, arXiv:2207.05571, (2022) [4] W. C.-W. Huang et al., unpublished (2023)

Optical driving of materials has emerged as a promising tool to engineer new phases of matter. In this talk I present a promising microscopic mechanism for efficiently photo-inducing superconductivity. We investigate an attractive electron-electron interaction mediated by a boson coupling to an electronic transition between two bands separated by a band gap. While this attraction is small in equilibrium, we find that it can be increased by several orders of magnitude when the bosons are driven into a non-thermal state. We investigate the potential of this mechanism to increase the superconducting transition temperature, and find that pairing in the gap equation is resonantly amplified when the bosons are in a non-thermal state. We argue that this mechanism provides a simple prescription for designing new platforms for photo-induced superconductivity -- already providing a promising, experimentally accessible candidate material.

An active research topic in condensed matter physics is the nonequilibrium control of material properties. In recent years, striking examples of ultra-fast control by laser driving have been demonstrated in experiments and in numerical studies. Especially in the case of photo-doped Mott systems, interesting mechanisms underlying the appearance of nonthermal electronic orders have been identified and systematically explored. I will present an overview of the insights which have been gained in model studies based on nonequilibrium dynamical mean field theory.

Many ultrafast excitation pathways in materials are inherently nonthermal in nature. Femtosecond diffuse scattering techniques have recently emerged as powerful methods to directly access nonthermal phonon populations in photo-excited materials, owing to their unique sensitivity to both electron-phonon and phonon-phonon interactions with momentum resolution. Here we perform femtosecond electron diffuse scattering (FEDS) experiments to study the microscopic energy flow in two prototypical 2D materials, black phosphorus and MoS2. In both cases, the measurements reveal the emergence of highly anisotropic nonthermal phonon populations persisting for several picoseconds after exciting the electrons with a light pulse. The experiments are complemented by ab initio simulations. Specifically, we carry out first-principles calculations of the coupled electron–phonon dynamics based on the time-dependent Boltzmann equations. This enables us to determine the impact of electron–phonon and phonon–phonon scattering on the ultrafast dynamics of electrons and phonons with momentum resolution. Furthermore, to facilitate direct comparison with the experimental data, we perform computations of the structure factor. Specifically, we introduce an efficient first-principles methodology for the calculation of the all-phonon inelastic scattering in solids, and demonstrate its broad applicability to other 2D materials. Our joint experimental-theoretical approach provides a detailed picture of nonthermal phonons in photo-excited materials.

Lightwave electronics has pushed the control of condensed matter to unprecedented time scales. By harnessing the carrier wave of intense light as an alternating voltage, electrons can be driven faster than a cycle of light, opening a fascinating quantum world full of promise for future quantum technologies. I will discuss prominent examples of lightwave-driven dynamics in solids, ranging from Bloch oscillations via all-optical band structure reconstruction to first attosecond clocking of correlations between Bloch electrons. Utilizing topologically non-trivial trajectories of quasi-relativistic electrons, we optically engineer Floquet-Bloch bands and directly watch their birth, rise and collapse in the first-ever sub-cycle band structure videography. Moreover, we combine lightwave electronics with scanning tunneling microscopy to take atom-scale slow-motion movies of an individual vibrating molecule and exert femtosecond atomic forces, which choreograph quantum motion of a single molecule. This concept offers a radically new way of directly watching and controlling key elementary dynamics in nature or steer chemical reactions, on their intrinsic spatio-temporal scales.

Photocurrents in topological materials have been associated with topological properties of the electronic bands, such as the Berry connection. However, despite some experimental demonstrations, it has remained unclear what part the topology actually plays in the process. In this talk I will discuss our use of time- and angle-resolved photoemission spectroscopy (trARPES) to resolve photocurrents in the excited electronic states of a topological insulator. By analyzing the rise times of the population following the optical excitation, we gained a complete view of the occupied and unoccupied electronic states, and how they are coupled by the light. Our work provides a microscopic understanding of how to control photocurrents in materials with spin-orbit coupling and broken inversion symmetry, and paves the way to control of currents in topological states. I will also briefly describe trARPES experiments on a magnetic Weyl semimetal. * Soifer et al, PRL 2019

Ultrashort light pulses play a critical role in our quest to observe and exploit ever-faster physical phenomena. In particular, few-cycle lasers with frequencies in the visible range enable the visualization and control of chemical and physical processes occurring on femto to attosecond timescales. I will discuss how the interaction of these intense and ultrafast light fields with matter can be used to guide electrons in matter and generate bursts of currents on ultrafast femtosecond timescales, an emerging direction of research called lightwave electronics. Specifically, I will discuss how in the context of graphene-based nanojunctions it is possible to disentangle the ultrafast laser-induced currents into contributions by real and virtual carriers and use this augmented to control landscape to design petahertz electronic logical circuits elements that operate 106 times faster than present-day capabilities [1]. [1] T. Boolakee, C. Heide, A. Garzón-Ramírez, H. B. Weber, I. Franco and P. Hommelhoff, Nature 605, 251-255 (2022)

When coupling strong periodic driving fields with quantum materials, new photon-dressed states emerge. By using time-reversal symmetry breaking mid-infrared light, the topological state of quantum materials can be changed or manipulated. We characterize the non-equilibrium transport properties of these photon-dressed states using ultrafast optoelectronic circuits. In the type-II Weyl semimetal MoTe2, we find direct evidence of these photon-dressed states in the nonperturbative linear-in-E scaling of circularly dichroic photocurrents. Furthermore, we find that when pumping the material strongly enough, the interlayer shear phonon couples to the transport properties of the out-of-equilibrium states. Pumped above a certain threshold, this phonon changes the location of the Weyl nodes in the Brillouin zone, and the material can even undergo a reversible structural phase change that also changes the band topology. Our results suggest multiple ways in which light can manipulate the topological states of technologically relevant quantum materials.

Phase transitions, ubiquitous in nature, are characterized by breaking of characteristic material symmetries, which can be described by an order parameter. The phase of a material is then determined by the shape and the minima of the free energy surface. While in thermal equilibrium, a system occupies the phase space close to these minima, and phase transitions occur as function of thermodynamic state variables, ultrafast laser excitation allows modifying the energy surfaces on timescales faster than the response time of the system, thereby driving ultrafast phase transitions, and triggering coherent dynamics within the transient energy surfaces. This approach not only provides information about the microscopic couplings behind a phase transition, but also allows for coherent manipulation of material properties. In my talk, I will discuss recent experiments employing time- and angle-resolved photoemission spectroscopy (trARPES), unearthing rich dynamics of photoinduced phase transitions in prototypical charge-density wave materials, including a transition into a hidden, metastable state.

In the search for realizations of Quantum Spin Liquids (QSL), it is essential to understand the interplay between inherent structural disorder and long-range spin entanglement. H$_3$LiIr$_2$O$_6$ is regarded as a spin liquid proximal to the Kitaev-limit in which bond disorder and stacking faults are known to be present [1]. These sources of random disorder have been invoked to account for the non-zero NMR relaxation rate and divergent specific heat, which are not expected within the Kitaev QSL model [2]. As such, the nature of H$_3$LiIr$_2$O$_6$ remains unknown. Resonant inelastic X-ray scattering (RIXS) is the only technique able to resolve the magnetic excitations in iridium base Kitaev materials and is amenable to being extended into the time domain. In this talk, I will show that a broad bandwidth and momentum-independent continuum of magnetic excitations characterize the low-temperature low-energy excitations in H$_3$LiIr$_2$O$_6$. Our data support an interpretation of H$_3$LiIr$_2$O$_6$ as a disordered topological spin liquid in close proximity to bond-disordered versions of the KQSL [3]. I will also discuss preliminary time-resolved RIXS data in the context of Floquet engineering of magnetic exchange in Kitaev materials. [1] Kitagawa et al., Nature 554, 341 (2018) [2] Knoelle et al., Phys. Rev. Lett. 122, 047202 (2019) [3] de la Torre et al., submitted

Lead halide perovskites have recently emerged as promising semiconductor materials for next-generation optoelectronic devices such as solar cells or LEDs. Their outstanding optoelectronic properties have been linked to a delicate interplay of their complex lattice structure and electronic degrees of freedom. So far, to tailor material properties, the community focused mainly on changing the static design of the perovskite lattice by tuning their chemical composition or morphology. Meanwhile, the full potential for dynamic phonon-driven ultrafast material control, as demonstrated for oxide perovskites, has not been exploited yet. Here we demonstrate the nonlinear control of the structural order parameter in form of the octahedral twist mode via intense single-cycle THz fields exceeding 1.5 MV/cm. We probe the resulting coherent lattice response in the orthorhombic phase via the THz-induced Kerr effect (TKE) in hybrid organic-inorganic MAPbBr3 and all-inorganic CsPbBr3 perovskites. Additionally, we find the dynamically disordered cubic phase to be dominated by a strong THz polarizability leading to a nonlinear THz refractive index on the order of 10^-14 cm²/W. We discuss possible nonlinear phononic and photonic excitation pathways and provide a comprehensive THz-THz-VIS four wave-mixing model to simulate the highly dispersive TKE response. This work opens the door to lattice-driven material control in complex hybrid and organic semiconductors, which might lead to a better understanding concerning the role of phase fluctuations and dynamic disorder for this emerging class of defect tolerant semiconductors.

The rich phase diagrams of many transition metal oxides (TMOs) is the result of the intricate interplay between electrons, phonons, and magnons. This makes TMOs very susceptible to external parameters such as pressure, doping, magnetic field, and temperature which in turn can be used to finely tune their properties. The same susceptibility makes TMOs the ideal playground to design experiments where the interaction between tailored electromagnetic fields and matter can trigger the formation of new, sometimes exotic, physical properties. This aspect has been explored in time domain studies [1] and has led to the demonstration that ultrashort mid-IR light pulses can “force” the formation of quantum coherent states in matter, disclosing a new regime of physics where thermodynamic limits may be bridged and quantum effects can, in principle, appear at ambient temperatures. In this presentation, I will review our recent results in archetypal strongly correlated cuprate superconductors and demonstrate the feasibility of a light-based control of quantum phases in real materials [2,3,4]. I will then introduce our new approaches to time domain spectroscopy going beyond mean photon number observables [5-10] and show that the statistical features of light can provide information on superconducting fluctuations that are invisible to standard linear and non-linear optical spectroscopies[11]. Finally, I will elaborate on our current directions on leveraging both the electromagnetic field fluctuations and the strong driving of materials to control the onset of quantum coherent states in complex materials. [1] Advances in physics 65, 58-238, 2016 [2] Science 331, 189-191 (2011) [3] Phys. Rev. Lett. 122, 067002 (2019) [4] Nature Physics 17, 368–373 (2021) [5] Phys. Rev. Lett. 119, 187403 (2017) [6] New J. Phys. 16 043004 (2014) [7] Nat. Comm. 6, 10249 (2015) [8] PNAS March 19, 116 (12) 5383-5386 (2019) [9] J. of Physics B 53, 145502 (2019) [10] Optics Letters 45, 3498 (2020) [11] Light Sci Appl 11, 44 (2022)

TBA

Interactions among the charge, spin, and lattice degrees of freedom in complex materials occur on energy scales of few meV to several 100 meV, i.e. on pico- to femtosecond timescales. Extending the element- and site-specificity of soft x-ray absorption spectroscopy into the time domain provides detailed insight into the elementary interactions leading to, e.g., energy transfer dynamics across interfaces in [Fe/MgO]n heterostructures [1,2] and, in combination with time-dependent density functional theory, into the correlation of local electron correlations and spin dynamics in fcc Ni [3]. While the high soft x-ray flux at x-ray free electron lasers can be a challenge due to sample damage it can also be exploited in the analysis of non-linear effects to analyze elementary interactions [4]. The use of circularly polarized soft x-ray pulses extends these capabilities to studies of the transient magnetization and high frequency spin fluctuations interacting with the crystal lattice. These findings are based on a continuous development of the experimental endstations at the user facilities [5,6] and are currently extended to oxides and organic materials. These activities were conducted in a large collaboration, involving the SCS beamline at the European X-FEL, Hamburg; the Femtospex beamline at BESSY II, Berlin; the MeV ultrafast electron diffraction facility at SLAC, Stanford; Uppsala University; Konstanz University, University of Duisburg-Essen and many others, see the references below. Funding by the Deutsche Forschungsgemeinschaft through CRC 1242 is gratefully acknowledged. References 1. Rothenbach, N. et al., Phys. Rev. B 100, 174301 (2019). 2. Rothenbach, N. et al., Phys. Rev. B 104, 144302 (2021). 3. Lojewski, T. et al., arXiv 2210.13162 (2022). 4. Engel, R., et al., arXiv 2211.17008 (2022). 5. Holldack, K., et al., J. Synchrotron Rad. 21, 1090 (2014). 6. Le Guyader, L., et al., arXiv 2211.04265 (2022).

Over the past decade, the condensed matter community has been consumed by the possibility of intense light creating superconductivity at room temperature. The excitement has been mainly driven by advances in pump and probe experiments, where pumping materials such as K3C60 and kappa-salts of the family κ-(ET)2-X by strong Mid-IR laser light has shown optical signatures consistent with a long-lived and a short-lived superconducting state respectively. In this talk, I will present a microscopic mechanism based on a boson coupled to electronic band transitions as the most natural way to achieve photo-induced superconductivity. I will report on recent work that shows that the boson-interband coupling mechanism leads to light induced superconductivity when driven that can be resonantly enhanced when the boson matches the electron gap gap energy. Finally, I will discuss how such a photo-induced state could be long lived long after a driving pulse has left the material.

New dynamical functions of quantum materials can be realized by Floquet engineering. Heterodyning is a signal processing technique that creates output signals by mixing the input signal with the dynamics of the multiplier, i.e., periodically driven Floquet state [1,2]. In Ref. [2], we discovered a new Landau quantization of two-dimensional electron gas (2DEG) induced by oscillating magnetic fields. This state shows a heterodyne Hall response that is quantized when a “magic condition” is fulfilled. We extended this theory to Dirac electrons in oscillating magnetic fields. In contrast to the 2DEG case, Landau-quantization occurs generically without enforcing any “magic condition”[3]. [1] A. Kumer, M. Rodriguez-Vega, T. Pereg-Barnea, B. Seradjeh, Phys. Rev. B 101, 174314 (2020). [2] T. Oka, L. Bucciantini, Phys. Rev. B 94, 155133 (2016). [3] S. Kitamura, T. Oka, in preparation.

Floquet or periodically driven systems show topological phases that are qualitatively different from their static counterparts. I will show that the edge modes encountered in certain free fermion Floquet systems are remarkably robust to adding interactions, even in disorder-free systems where generic bulk quantities can heat to infinite temperature due to the driving. This robustness of the edge modes to heating can be understood in the language of strong modes for free fermion chains, and almost strong modes for interacting chains. I will outline a tunneling calculation for extracting the long lifetimes of these edge modes by mapping the Heisenberg time-evolution of the edge operator to dynamics of a single particle in Krylov subspace. Following this I will introduce the concept of topological defects: non-local operators that can be deformed in the space and time direction without changing the physics. One of these topological defects is the "duality defect" that implements the Kramers-Wannier duality transformation. I will highlight the consequence of the duality defect on Floquet time-evolution.

The recent technological development of THz spectroscopy makes it possible to probe properties of quantum matter, which cannot be observed in equilibrium. This is of considerable interest in the field of unconventional superconductivity, where controlled probing of the relaxation dynamics yields access to understanding ground state properties of the underlying system. Motivated by the recent development of terahertz pump-probe experiments, I will discuss in my talk the short-time dynamics in superconductors with multiple attractive pairing channels and competing nematic instability. Studying a single-band and multiband superconductors, we analyze the signatures of collective excitations of the pairing symmetries (known as Bardasis-Schrieffer modes) as well as the order parameter amplitude (Higgs mode) in the short-time dynamics of the spectral gap and quasiparticle distribution after an excitation by a pump pulse. We show that the polarization and intensity of the pulse can be used to control the symmetry of the non-equilibrium state as well as frequencies and relative intensities of the contributions of different collective modes[1-2]. Finally, I address the question of whether pump-probe technique can be used to reveal an interplay between various collective modes visible in the superconducting state and to distinguish the Pomeranchuk nematic collective mode from the BS mode due to the subdominant Cooper-pairing channel[3-4]. References [1] Marvin A. Müller, Pavel A. Volkov, Indranil Paul, Ilya M. Eremin, Phys. Rev. B 100, 140501(R) (2019) [2] Marvin A. Müller, Pavel A. Volkov, Indranil Paul, Ilya M. Eremin, Phys. Rev. B 103, 024519 (2021) [3] Marvin A. Müller and Ilya M. Eremin, Phys. Rev. B 104, 144508 (2021). [4] Luke C. Rhodes, Jakob Böker, Marvin A. Müller, Matthias Eschrig and Ilya M. Eremin, npj Quantum Mater. 6, 45 (2021)

The chiral anomaly underlies various phenomena, including dissipationless transport currents in topological systems. While many investigations have explored the chiral anomaly in various closed topological systems, much less attention has been given to exploring this anomaly in open quantum systems. By employing the non-Hermitian description of open quantum systems in this talk, I will address whether there are anomalous conservation laws that remain unaccounted for. For this purpose, I will present a unified formulation to calculate anomalous responses in Hermitianized, anti-Hermitianized, and non-Hermitian systems of massless electrons with complex Fermi velocities coupled to non-Hermitian gauge fields. Using this formulation, I will show that the quantum conservation laws of chiral currents of non-Hermitian systems are not related to those in Hermitianized and anti-Hermitianized systems, as would be expected classically, due to novel anomalous terms that we derive. I will further present some physical consequences of our non-Hermitian anomaly that may have implications for a broad class of emerging experimental systems described by non-Hermitian Hamiltonians.

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.