Session S04: Light-Induced Phases of Matter: Floquet Manipulation and Coherent Control

Theory of Time-Resolved Responses in Transient Floquet Systems

Time-resolved spectroscopy has emerged as a powerful technique to investigate far-from-equilibrium physics. Motivated by pump-probe experiments, we develop a formalism to calculate the time-resolved response of a material quenched by Floquet driving with light or a phonon mode and then probed by an ultrafast pulse. In contrast to previous studies, we do not assume the system has reached a steady state when the probe starts, allowing us to track the spectroscopic response as the system prethermalizes. We find that the response can have spectral peaks which do not correspond to the probe frequency nor the harmonics of the drive and which depend on the time of the probe pulse. These findings are illustrated in the optical response of a transition metal oxide driven by a phonon mode. We also discuss the selection rules originating from dynamical symmetries in optical responses for non-steady state systems.   

Predicted Novel Type of Photoinduced Topological Phase Transition Accompanied by Collision and Collapse of Dirac-cone Pair in Organic Salt α-(BEDT-TTF)2I3

Photoinduced topological phase transitions in the Dirac-electron systems have attracted intensive research interest since its theoretical prediction in graphene, where the application of circularly polarized light opens a gap at the Dirac points and renders the system a topologically nontrivial Chern insulator phase. However, most of the previously studied phenomena in two-dimensional Dirac systems are basically based on the same physical mechanism, i.e., the gap opening in the Dirac electron bands with circularly polarized light. In my talk, we theoretically predict a novel type of photoinduced topological phase transition accompanied by collision and collapse of gapped Dirac points in the organic salt α-(BEDT-TTF)₂I₃. By constructing the Floquet theory for this compound, we demonstrate that the irradiation of elliptically polarized light causes collision of the Dirac points through the photoinduced band deformation and their collapse, which eventually results in the topological phase transition from a topological semimetal with gapped Dirac cones to a normal insulator when the elliptical axis is oriented at a specific angle with respect to the crystallographic axes. We argue that this novel photoinduced phase transition can be experimentally detected by the measurement of Hall conductivity. The present work enriches the fundamental physics of photoinduced topological phase transitions and thus contribute to development of this rapidly growing research field.   

The Ultra-critical Floquet Non-Fermi Liquid

We demonstrate the existence of a quantum non-equilibrium steady state of periodically driven fermions coupled to a bosonic bath that has no counterpart in equilibrium. This state features multiple sharp Fermi surfaces which are the locus of higher order cusp-like non-analyticities in the momentum occupation, but without an associated jump or quasiparticle residue, therefore making these states non-equilibrium non-fermi liquids. More intriguinly, these non-analyticities survive even when the bath is at finite temperature. This is a property that has no analogue in known equilibrium fermi or non-fermi liquids, because for these equilibrium states temperature invariably acts as a relevant perturbation which smears out the sharpness of the jump or the non-analyticities of the occupation function and endows the fluid with a finite correlation length, beyond which it behaves classically. We discuss how monocromatic radiation in the microwave range impinging in high quality electronic systems is a promising regime to detect these states in experiments.   

Time evolution of the elementary excitations in a photo-excited electron-doped cuprate via tr-RIXS

Investigating the ultrafast dynamics in photo-excited quantum materials is a topic of strong interest in materials science research. To date, available time-resolved probes have revealed information about the lattice, symmetry, and electronic band structure. However, the information about the collective excitations in energy-momentum space, such as magnons and plasmons, are largely unavailable due to the lack of a suitable method. The advent of high repetition rate x-ray free-electron lasers enables time-resolved resonant inelastic scattering (RIXS) experiments, which accesses elementary excitations associated with the magnetic, charge, orbital, and lattice degrees of freedom, thus providing the previously missing pieces of information. In this presentation, I will present our tr-RIXS measurement on the electron-doped cuprates Nd_2-xCe_xCuO₄. A photoexcitation-induced, change of the paramagnon, plasmon, and orbital excitations spectrum were observed. The momentum and time dependence will be discussed.    

Quantum geometric guiding principles for optically controlling competing superconducting phases

We have shown that in centrosymmetric superconductors with strong spin-orbit interactions, there can be a sub-gap Bardasis-Schrieffer mode that when driven strongly permits switching between singlet and triplet superconducting states [1]. This can be seen from a generalized Ginzburg-Landau theory of competing superconducting orders in which a linear coupling to light is symmetry-allowed between even-parity and odd-parity orders. When calculating this key coupling parameter from a lattice model, we find that it is dominated by a gauge-invariant quantum geometric quantity even for dispersive bands. This quantity is linear in the non-Abelian Berry connection and has a natural interpretation as a matrix element between neighboring Wannier states. This establishes spin-orbit-coupled materials with nontrivial band geometry, such as moiré heterostructures, as prime candidates for experiments looking to probe and control this collective mode. It also has ramifications for the supercurrent in mixed-parity superconductors. Our findings offer a richer understanding of the interplay between quantum geometry and competing superconducting phases.

[1] S. Gassner, C. S. Weber, and M. Claassen. arXiv:2306.13632 (2023).   

Chirping of superconducting order parameter using compression-based nonequilibrium Greens function

We study photodoped superconductors using dynamical mean-field theory with second Born approximation as the impurity solver. We excite the superconducting system with a laser pulse and show that the order parameter decays super-exponentially for strong pulses, and oscillates for weak pulses. We attribute the oscillation to the amplitude mode excitations of the order parameter, which slows down as the system evolves, a phenomenon known as chirping. The chirping gets more enhanced the more we approach to the dynamical critical point. Finally, we show that the chirping phenomenon can be measured using optical pump-probe experiments in photodoped superconductors. To integrate our results to the long time scales required to study this phenomenon, we have developed a generelization of the compressed representation integration method that allows for matrix valued Green's functions.   

Gauge Invariant Truncated Models in Cavity Quantum Electrodynamics

Resolving gauge ambiguities for truncated matter models coupled to quantum light has received a lot of attention recently. Equivalence of the dipole gauge and the coulomb gauge Hamiltonians was achieved by properly constructing unitary operators in the truncated matter subspace. We revisit these ideas and discuss gauge invariance in a general setting, not necessarily restricted to the dipole or the coulomb gauges.   

Quantum Monte Carlo study of the cavity-coupled interacting electron gas

Using the auxiliary-field quantum Monte Carlo method, we study the interacting electron gas coupled to a cavity. While in the homogeneous case and the dipole approximation, the light-matter coupling in this model can still be solved analytically, we study the effect of a modulating external potential and - within an approximation - multiple photon modes carrying finite momentum. Our results inform ongoing efforts in developing improved functionals for the quantum electrodynamical density functional theory (QEDFT) and may shed light on the shortcomings of the dipole approximation in extended systems.   

Probing the polaritonic potential energy surface via Raman spectroscopy

Controlling chemical reactions using light has been a long-standing dream for chemists. Recently it has been shown that the rates of chemical reactions can be changed using light-matter hybrid particles. These light-matter hybrid particles, known as polaritons, form when there is a strong coupling between light and molecular excitation inside an optical cavity. These states have entirely new optical properties and have been used to show a wide range of phenomena such as exciton-polariton condensate, enhanced charge and long-range energy transport. However, the mechanism which is responsible for the changes brought upon by polariton formation in the reaction dynamics remains unclear. In order to understand the complete mechanism, which includes structure-function relationship, we must understand the changes in molecular structure in polariton potential energy surface. To study the effects of strong light-matter coupling in the excited state dynamics of photochemical reactions we have used advanced Raman techniques on pentacene exciton-polariton cavities. We have observed that the molecular structure changes upon polariton formation and the amount of change in the molecular structure can be tuned by various degrees of freedom provided by the cavity. Overall, our probe of polaritonic potential energy surfaces will enable us to tune the molecular structure in the polaritonic states which will help us in making efficient devices and control chemical reactions.   

Using photon correlations to observe quantum phase transitions in strongly interacting cavity-embedded materials

We show that photon correlation measurements can probe emergent magnetic properties of Mott insulators strongly coupled to a cavity photon mode, establishing a new quantum-optical tool for observing quantum phase transitions in strongly-correlated materials. When placed in a cavity, light-matter coupling entangles cavity photons to magnetic excitations, imprinting the photons with information about magnetic correlations. Remarkably, this same information about magnetic correlations is transmitted to the coherence properties of an easily observable output field, which we prove by generalizing quantum optical input-output relations to many body quantum systems. To illustrate this, we consider a ladder Mott insulator in a cavity and demonstrate using DMRG that spin dimer correlations reveal a gapless mode at the quantum phase transition to a dimerized state. Our work shows that this gapless mode can be observed in quantum photon correlations of the output field, providing a new experimental tool for interrogating quantum materials and their phase transitions.