Session W04: Light-Induced Manipulation of Mott Insulators and Strong Correlations

Light control of the optical nonlinearity of a driven quantum chain

Intense ultrashort electromagnetic fields have enabled the observation and control of spectacular light-driven emergent phenomena such as transient superconducting-like phases, charge ordering, and excitonic condensation. A particularly intriguing platform for such studies are quasi-one-dimensional Mott insulators, where light-matter interaction is predicted to modify effective many-body interactions and lead to exotic nonequilibrium ordering phenomena such as eta-pairing superconductivity. Here, we investigate a paradigmatic quantum chain, the quasi-one-dimensional cuprate compound Sr₂CuO₃. We find that intense mid-infrared excitation reshapes the many-body states contributing to its large optical nonlinearity, resulting in a dramatic transientreshaping of its harmonic emission spectrum. I will discuss how these observations may be rationalized by a light-driven renormalization of effective interactions, and its implications for the realization of long-range-ordered phases in light-driven quantum materials.   

Towards light-induced quantum spin liquidity in an organic Mott insulator

Quantum spin liquids (QSL) are systems exhibiting long-range entanglement and spin fractionalization, where magnetic order is prevented by frustrated interactions and quantum fluctuations. The potential applications in topologically-protected quantum computing are stimulating great interest in QSL, but solid state realizations of these states of matter are still elusive. Several materials are predicted to be proximate to a QSL state, but extra interactions often alter the correct balance of magnetic frustration. Here, we excite the candidate QSL material κ-(BEDT-TTF)₂Cu₂(CN)₃ with ultrashort mid-infrared light pulses with the aim to nudge it towards a QSL phase. This compound is a prototypical Mott-Hubbard system formed by dimers of BEDT-TTF molecules arranged in a triangular lattice, which at 6 K exhibits a transition to a valence bond state [1]. We tune the interdimer and intradimer Coulomb repulsion via coupling to molecular excitations and investigate the effects on the spin degrees of freedom by probing the continuum of fractionalized spinon excitations [2] with single-shot THz spectroscopy [3]. We observe a transient increase in the THz optical conductivity at low temperature which is suggestive of an enhanced spinon spectral weight. These results open the possibility of a photoinduced quantum spin liquid state in κ-(BEDT-TTF)₂Cu₂(CN)₃ with mid-infrared vibrational excitation and advance the pursuit of ultrafast control of long-range entanglement in driven quantum materials.

 

References:

[1] B. Miksch, et al., Science 372, 276 (2021).

[2] S. Elsässer, et al., Phys. Rev. B. 86, 155150 (2012).

[3] F.Y. Gao, et al., Opt. Lett. 47, 3479 (2022).   

Resonant Midinfrared Modulation of the Charge Transfer Band in an Organic Mott Insulator

The coupling between molecular vibrational modes and electrons is key to many light-induced phenomena in κ-type organic compounds, e.g., photomolecular high-temperature superconductivity [1]. κ-(ET)₂Cu₂(CN)_3, or (κ-CN), is a prototypical organic Mott insulator hosting genuine Mott-Hubbard physics on a triangular lattice [2]. Having a molecular (ET)₂ dimer, instead of an atom, behaving as one electron on each lattice site, κ-CN provides a flexible platform for tuning its Hubbard parameters. In this study, we interrogate how light-induced vibrations of the C=C bonds (ν₂₇) on the (ET)₂  dimers affect the charge degrees of freedom in κ-CN. Using ultrafast midinfrared pump pulses, we observed multi-component dynamics of the charge transfer band, including a fast response, a coherent phonon oscillation and a slow response that is resonantly amplified around the ν₂₇ mode. This indicates modulations of electronic properties in κ-CN due to local distortions of the (ET)₂ dimers. The stretching of the C=C bonds in a κ-type material was calculated to modulate the Hubbard U [1]. This result suggests an ability to modulate on-dimer electronic interactions, U, in κ-CN with light, perturbing its Hubbard Hamiltonian, leading to emergent states in the material.

 

[1]       M. Buzzi et al., Photomolecular High-Temperature Superconductivity, Phys. Rev. X 10, 031028 (2020).

[2]       Pustogow A., Thirty-Year Anniversary of κ-(BEDT-TTF)2Cu2(CN)3: Reconciling the Spin Gap in a Spin-Liquid Candidate, Solids 3(1):93-110 (2022).   

Subcycle pulse-induced nonequilibrium dynamics in one-dimensional Mott insulators

The elucidation of nonequilibrium states in strongly correlated systems holds the key to emergence of novel quantum phases. The nonequilibrium-induced insulator-to-metal transition is particularly interesting since it reflects the fundamental nature of competition between itinerancy and localization of the charge degrees of freedom. We investigate pulse-excited insulator-to-metal transition of the half-filled one-dimensional extended Hubbard model [1,2]. Calculating the time-dependent optical conductivity with the time-dependent density-matrix renormalization group, we find that broad mono- and half-cycle pulses inducing quantum tunneling strongly suppress spectral weights contributing to the Drude weight σ_D, even if we introduce a large number of carriers Δn_d [1]. This is in contrast to a metallic behavior of σ_D ∝ Δn_d induced by photon absorption and chemical doping. The strong suppression of σ_D in quantum tunneling is accompanied by electric polarization, which breaks inversion symmetry [2]. The breaking of inversion symmetries in a photoexcited state can be monitored by second harmonic generation. These findings provide a new methodology for designing the localization and symmetry of electronic states and open up a new field of subcycle-pulse engineering.

References:

[1] K. Shinjo, S. Sota, and T. Tohyama, Phys. Rev. Res. 4, L032019 (2022).

[2] K. Shinjo, S. Sota, S. Yunoki, and T. Tohyama, Phys. Rev. B 107, 195103 (2023).   

Anomalous suppression of photoinduced in-gap weight in the optical conductivity of a two-leg Hubbard ladder

Photoinduced nonequilibrium states in the Mott insulators reflect the fundamental nature of competition between itinerancy and localization of the charge degrees of freedom. The spin degrees of freedom will also contribute to the competition in a different manner depending on lattice geometry. We investigate pulse-excited optical responses of a half-filled two-leg Hubbard ladder [1]. Calculating the time-dependent optical conductivity by time-dependent density matrix renormalization group, we find that strong monocycle pulse inducing quantum tunneling gives rise to anomalous suppression of photo-induced in-gap weight, leading to negative weight. This is in contrast to finite positive weight in the Hubbard chain. The origin of this anomalous behavior in the two-leg ladder is attributed to photoinduced localized exciton in the time-dependent wavefunctions, which reflects strong spin-singlet dimer correlation in the ground state.

[1] T. Tohyama, K. Shinjo, S. Sota, and S. Yunoki, Phys. Rev. B 108, 035113 (2023).   

Witnessing Light-Driven Entanglement using Time-Resolved Resonant Inelastic X-ray Scattering

Quantum computers use entanglement to encode information which requires precise characterization and control of entanglement within their constituent materials. However, quantifying entanglement in many-body materials is a challenge in both theory and experiment. While entanglement witnesses can be extracted from spectroscopic data under equilibrium conditions, this approach cannot extend out of equilibrium and is therefore incompatible with laser control of materials. To address this limitation, we developed a systematic approach for characterizing time-dependent entanglement in nonequilibrium quantum materials using time-resolved resonant inelastic x-ray scattering (trRIXS). This is done by using quantum Fisher information (QFI) as a witness for the lower bound of entanglement depth, thus bypassing the requirement for a full tomography of the wavefunction. To showcase the efficiency of our approach, we applied it to a quarter-filled extended Hubbard model (EHM) and predicted light-enhanced quantum entanglement that we attribute to the proximity to a phase boundary. This advancement establishes the foundation for experimentally witnessing and manipulating entanglement in light-driven quantum materials via experimentally accessible ultrafast spectroscopic measurements.   

Manipulation of antiferrodistortive order in SrTiO3 via nonlinear phononics revealed by ultrafast X-ray diffraction

Ultrafast light illumination usually suppresses the low-temperature phases via increasing the kinetic energy of electrons. Coherent phonons generated by resonant infrared absorption have been shown as an alternative way to stabilize metastable phases inaccessible through external control such as strain, chemical doping, and temperature in equilibrium. SrTiO3 is a frustrated system where polar order and antiferrodistortive (AFD) rotations compete. Here we pump the SrTiO3 below the AFD transition temperature with intense 22THz mid-infrared pulses to resonantly drive IR active phonon modes and probe with diffraction from time-delayed femtosecond X-ray pulse from the LCLS x-ray free electron laser. At 101K, the Bragg peaks associated with the oxygen rotations of the AFD is enhanced within the first picosecond after the arrival of the pump pulse, followed by subsequent suppression at later times. This suggests the coherent excitation of phonons transiently enhances the long-range AFD orders. Fluence-dependent results at 75K suggest the transient enhancement shows quadratic dependence with the pump fluence while later-on intensity suppression follows a linear fluence dependence. Further temperature dependence data shows large suppression in the pump-probe signal as temperature decreases, indicating potential fluctuation contribution to the dynamical signal. Our results demonstrate the manipulation of AFD orders in SrTiO3 via resonant phonon excitation and provide new insights into the anharmonic response in quantum materials.   

Ultrafast infrared-light-driven symmetry control in crystalline materials

The interplay between structure, symmetry, and function is a fundamental and long-standing paradigm in materials physics. It dictates how we access physical properties, reveals hidden order, and guides both the search for, and the engineering of, new physical phenomena. Recent technological advances have enabled sub-picosecond manipulation of structure through the far-from-equilibrium drive of phonons — the mechanical modes of crystalline materials — using mid- and far-infrared light. How can we leverage these structural changes to induce new symmetries and functionalities?

In this talk, I will discuss our theoretical exploration of light-driven control of symmetry in crystalline materials via direct excitation of phonons. Focusing on strategies for tailored optical control of elusive symmetries and orders, I will demonstrate that new symmetries and length scales can emerge, that chemical environments not observed near equilibrium can be prepared, and that known or hidden phase boundaries may be traversed.   

Ultrafast domain wall motion in a polar oxide nanostructure

Domain wall propagation in conventional ferroelectric materials under an electric field is often hindered by disorder and hence its velocity is orders-of-magnitude lower than the speed of sound. The recent discovery of quasi-long-ranged topological polar textures in oxide superlattices — such as polar vortices and skyrmions — offers an alternative platform to investigate domain wall motion in ferroelectrics, where atomic-scale disorder plays a minor role in nanotextures with characteristic size on the order of 10 nm. Here, we study the domain dynamics of polar textures in a PbTiO₃/SrTiO₃ superlattice using time-resolved electron diffraction and microscopy. Following photoexcitation, we observed a rapid suppression of the polar texture within 1 ps, where textures within one domain are preferentially melted. The selective melting leads to a fast expansion of the other domain, whose boundary propagates near the sound speed characteristic of the collective motion of nanotextures. The spatiotemporal visualization of this ultrafast domain wall motion not only yields insights into the fundamental limit of its velocity in ferroelectric systems but also introduces new possibilities for memory devices based on topological polar textures.   

Enhanced magnetic stiffness in optically excited NdNiO3 thin film heterostructures

NdNiO₃ is a prototypical correlated system, exhibiting a sharp metal-insulator transition and concomitant para-antiferromagnetic transition. Experiments on confined NdNiO₃ heterostructures have shown that magnetic order weakens but persists down to two unit cell thickness, decoupling from the electronic order. A proposed explanation is the interfacial structure frustrates the octahedral breathing distortion which couples to the magnetic order in bulk. Here, we interrogate this picture by studying the ultrafast spin dynamics following infrared optical excitation. Time-resolved soft x-ray magnetic diffraction measurements were conducted on a series of NdNiO₃/NdAlO₃ superlattices with variable interface confinement, determined by the relative thickness of layers. We find the magnetic order parameter exhibits a pronounced stiffness under strong confinement: melting the order requires higher fluence, and recovery is delayed. At short times, the magnetic correlation length is enhanced, particularly in samples with weaker confinement. Energy scans show the dynamics are correlated with charge redistribution between long and short bond sites, relaxing the breathing distortion. These results point to the possibility of engineering femtosecond dynamics through heterostructuring.   

Modeling a phonon-driven lattice expansion in thin film LaAlO3

Advancements of high-power laser sources in the THz frequency range have opened up opportunities for coherent intense excitation of infrared(IR)-active phonons in crystalline materials. The nonlinear phononics mechanism utilizes anharmonic coupling between driven IR-active modes and other lattice modes to induce changes in crystal structure and functional properties on picosecond timescales. Recent experimental and theoretical works have noted the potential for phonon-induced strains on these short time scales [npj Quantum Mater. 5, 95 (2020) , PRL 129, 167401 (2022)]. A natural way to measure the response of the lattice due to these THz pulses is by employing X-ray diffraction to observe changes to the position and intensities of Bragg peaks.

We show, using theory and first-principle calculations, that these lattice anharmonicities are responsible for the expansion of the c-axis in a biaxially strained thin film of LaAlO₃ via a phonon-strain coupling. Our theoretical modeling agrees well with the  experimental x-ray diffraction results both in time-scales and unusual signals in the fourier transform.   

Phonon state tomography probes of optically excited phonon-induced electron dynamics

Optical driving of quantum materials has opened a door for new modalities for engineering and controlling novel states of matter enabled by access to the excitation spectrum far away from equilibrium. One important class of experiments involves the optical excitation of specific vibrational modes (phonons) which, because of the dipole selection rules, can only couple nonlinearly to the electron density. A theory of these systems requires understanding the combined electron and phonon subsystems. In this talk, we propose a method, dubbed phonon state tomography (PST), in order to statistically unravel the electronic dynamics in terms of contributions from different phonon configurations. To demonstrate the effectiveness of this approach, we consider a model of a photo-pumped metal whose phonons are excited at an initial time by an optical pulse and show that i) spin and charge exhibit opposite trends as a function of the average phonon number, and ii) charge correlations increase with raising the width of the pump pulse. Thus, this technique may serve as a diagnostic tool for the phonon-induced electron dynamics in pump-probe experiments.