Session F38: Light Induced Structural Control of Electronic Phases I

Nonperturbative Geometric Effects in Laser irradiated 3D Dirac Semimetals

Floquet engineering is a tool to understand and utilize coherent phenomena in quantum materials under laser irradiation [1]. This talk focuses on the Floquet states in 3D Dirac semimetals irradiated by circularly polarized laser fields (CPL). Within the pulse duration, artificial axial gauge fields (also known as chiral gauge fields) are realized [2], while Floquet Weyl points appear in the Floquet spectrum [3]. We will discuss the nonperturbative geometric effects [4,5] in this system and account for the recent ultrafast generation of charge current observed in bismuth.   

Quantum metric plasmons: quantum geometry enriched intrinsic bulk non-reciprocal plasmonics

Bloch quantum geometry holds immense potential to reveal new possibilities in light-matter interaction. Here, we unveil one such possibility by illustrating a new class of plasmons – quantum metric plasmons (QMPs), in strongly interacting Fermi liquids. QMPs are determined by Bloch wavefunction texture, and thus possess intrinsic non-reciprocity, i.e., ω(q) ≠ ω(-q), in the bulk. This is in contrast to currently available schemes utilizing out of equilibrium driving or magnetohydrodynamics for non-reciprocal plasmonic responses. We show that QMPs are passively generated by bulk directional currents in presence of broken parity and time reversal symmetries. Interestingly, we find that QMPs can even thrive in symmetric bands by responding to the parity and time reversal symmetry violations hidden in the Bloch wavefunction. We anticipate that QMPs can be realized in readily available parity-violating magnets, especially in moiré heterostructures where the quantum geometric responses are pronounced (e.g., twisted bilayer graphene heterostructures).   

Non-perturbative dynamics of correlated disordered flat-band system

We develop a numerical method for the time evolution of Gaussian wave packet on a flat-band-lattice in the presence of correlated disorder. We apply this to the one-dimension cross-stitch model and compare with the analytical results based on the linear band approximation. Reasonable agreement is found for the decay and dephasing process when the flat-band intersects with the dispersive band. Then we apply this method to investigate a two dimensional topological flat-band model and find quite different wave packet dynamics for different Berry phases.   

Kapitza stabilization of a quantum critical order

We explore the mechanism to control the quantum order near a quantum critical point (QCP) using a fast drive by an external field. To illustrate the proposed approach, we consider the ferroelectric QCP, where the externally applied electric field couples to the incipient ferroelectric order parameter P in SrTiO₃ (STO). To estimate the magnitude of the effect and its dependence on the polarization of the field, we use the Ginzburg-Devonshire free-energy anharmonic parameters. We find that STO irradiated by continuous-wave, off-resonant, coherent light with suitable polarization can induce ferroelectric order. We also suggest the generation of a second harmonic signal and X-ray diffraction measurements of the resulting strain as the experimental signature of the stabilized order. The approach can be viewed as a field-theory extension of mechanical control of the Kapitza pendulum where fast base oscillations stabilize the inverted pendulum position and are an example of Kapitza state engineering [1]. We also draw similarities and differences between the Kapitza approach and Floquet engineering.

[1] Kuzmanovski, D., Aeppli, G., Rønnow, H. M., & Balatsky, A. V. (2022). Kapitza stabilization of a quantum critical order. arXiv preprint arXiv:2208.09491.   

Giant Resonant Enhancement for Photo-induced Metastable Superconductivity in K3C60

The search for new non-equilibrium functional phases in quantum materials has become a central research theme in condensed matter physics. In the alkali doped fulleride K₃C₆₀, photo-excitation close to resonance of on-ball molecular vibrations has been shown to induce a transient state with superconducting-like optical properties up to 100K, far above the equilibrium transition temperature Tc = 20K.

In this talk I will first describe recent experiments which have extended the lifetime of the transient superconducting-like state up to several nanoseconds, rendering this state metastable. GHz-frequency electrical transport measurements carried out in the metastable regime revealed the onset of a state with vanishingly-low resistance.

Furthermore, a dominant energy scale for this phenomenon was recently discovered by tuning the drive frequency to 10THz. At this frequency, metastable photo-induced superconductivity is observed up to room temperature for fluences as low as 400 µJ/cm². This corresponds to a two order of magnitude enhancement in photo-susceptibility, allowing for a significant reduction in dissipation and tracing a path towards steady state operation.   

Analysis of Photo-Pumped Hubbard Model on a Square Lattice Using Randomized Singular Value Decomposition

We investigate the dynamics of photo-excited states of the Hubbard model on a square lattice by decomposing the solution of the time-dependent Schrödinger equation into eigenmodes obtained by the randomized singular value decomposition (RSVD) [1]. Using the RSVD utilizing the dimensionality compression, we are able to decompose huge matrices that cannot be treated by conventional singular value decomposition. For a half-filled Hubbard model in the strong coupling limit on a square lattice, the optical absorption spectrum of the Hubbard model has been reproduced almost exactly by incorporating small number of eigenmodes obtained by RSVD. Encouraged by this success of RSVD, we take a 16-site Hubbard model away from half filling (12.5% hole doping) and perform mode decomposition of time-dependent wave functions after photo pumping. Calculating spin correlations for each mode, we find that some eigenmodes with energy close to Mott gap at half filling exhibit characteristic anisotropic correlations different from spin correlations in other energy regions.

[1] N. Halko, P. G. Martinsson, and J. A. Tropp, SIAM Review 2011 53:2, 217-288   

Exciton-assisted low-energy magnetic excitations in a photoexcited Mott insulator on a square lattice

The photoexcitation of a Mott insulator on a square lattice weakens the intensity of both single- and two-magnon excitations as observed in time-resolved resonant-inelastic X-ray scattering and time-resolved Raman scattering, respectively. However, the spectral changes in the low-energy regions below the magnons have not yet been clearly understood. To uncover the nature of the photoinduced low-energy magnetic excitations of the Mott insulator, we numerically investigate the transient magnetic dynamics in a photoexcited half-filled Hubbard model on a square lattice [1]. After turning off a pump pulse tuned for an absorption edge, new magnetic signals clearly emerge well below the magnon energy in both single- and two-magnon excitations. We find that low-energy excitations are predominantly created via excitonic states at the absorption edge. These exciton-assisted magnetic excitations may provide a possible explanation for the low-energy spectral weight in a recent time-resolved two-magnon Raman scattering experiment on insulating YBa₂Cu₃O_6.1.

[1] K. Tsutsui, K. Shinjo, S. Sota and T. Tohyama, arXiv:2207.03141.

    

Emergent magnetism due to spin-phonon coupling

High intensity THz lasers allow for the coherent excitation of individual phonon modes. The ultrafast control of emergent magnetism by means of phonons opens up new tuning mechanisms for functional materials. While theoretically predicted phonon magnetic moments are tiny, recent experiments hint towards a significant magnetization in various materials. To explain these phenomena, we derive a coupling mechanism between the phonon angular momentum and the electron spin. This coupling introduces the transient level-splitting of spin-up and spin-down channels and a resulting magnetization. We estimate this magnetization on the example of the perovskite KTaO₃. Our results show an electronic magnetic moment of 0.2 μ_B per unit cell, depending on the doping level and electron temperature.   

Dynamics of photo-induced ferromagnetism in oxides with orbital degeneracy

By using intense coherent electromagnetic radiation, it may be possible to manipulate the properties of quantum materials very quickly, or even induce new and potentially useful phases that are absent in equilibrium. For instance, ultrafast control of magnetic dynamics is crucial for a number of proposed spintronic devices and can also shed light on the possible dynamics of correlated phases out of equilibrium. Inspired by recent experiments on spin-orbital ferromagnet YTiO3 we consider the nonequilibrium dynamics of Heisenberg ferromagnetic insulator with low-lying orbital excitations. We model the dynamics of the magnon excitations in this system following an optical pulse which resonantly excites infrared-active phonon modes. As the phonons ring down they can dynamically couple the orbitals with the low-lying magnons, leading to a dramatically modified effective bath for the magnons. We show this transient coupling can lead to a dynamical acceleration of the magnetization dynamics, which is otherwise bottlenecked by small anisotropy. Exploring the parameter space more we find that the magnon dynamics can also even completely reverse, leading to a negative relaxation rate when the pump is blue-detuned with respect to the orbital bath resonance. We therefore show that by using specially targeted optical pulses, one can exert a much greater degree of control over the magnetization dynamics, allowing one to optically steer magnetic order in this system. We conclude by discussing interesting parallels between the magnetization dynamics we find here and recent experiments on photo-induced superconductivity, where it is similarly observed that depending on the initial pump frequency, an apparent metastable superconducting phase emerges.   

Molecular van der Waals fluids in cavity quantum electrodynamics

Intermolecular van der Waals interactions are central to chemical and physical phenomena ranging from biomolecule binding to soft-matter phase transitions. However, there are currently very limited approaches to manipulate van der Waals interactions. In this work, we demonstrate that strong light-matter coupling can be used to tune van der Waals interactions, and, thus, control the thermodynamic properties of many-molecule systems. Our analysis reveals orientation-dependent intermolecular interactions between van der Waals molecules (for example, H₂) that depend on the distance between the molecules R as R⁻³ and R⁰. Moreover, we employ non-perturbative ab initio cavity quantum electrodynamics calculations to develop machine learning-based van der Waals interaction potentials for molecules inside optical cavities. By simulating fluids of up to 1,000 H₂ molecules, we demonstrate that strong light-matter coupling can tune the structural and thermodynamic properties of molecular fluids. In particular, we observe collective orientational order in many-molecule systems as a result of cavity-modified van der Waals interactions. These simulations and analyses demonstrate both local and collective effects induced by strong light-matter coupling and open new paths for controlling the properties of condensed phase systems.   

Nonperturbative treatment of the SSH model coupled to quantum light

We investigate how an optical cavity can modify electron-phonon coupling in the non-perturbative regime by developing a fully quantum-mechanical treatment of the structural and electronic properties of the Su-Schrieffer-Heeger model coupled to quantum light. We perform numerical calculations using the exact diagonalization method, and assess the effects of the cavity for both the ground and excited states. We show how including correlation between all the constituents of the system (electrons, nuclei, and photons) gives rise to so called phonoritons, and we compare our treatment to an approach based on the Peierls substitution method. Our results highlight some important factors to consider when treating light-matter interactions in the non-perturbative regime.