Generalized fresnel-floquet equations for driven quantum materials
Physical Review B American Physical Society 105:17 (2022) 174301
Abstract:
Optical drives at terahertz and midinfrared frequencies in quantum materials are increasingly used to reveal the nonlinear dynamics of collective modes in correlated many-body systems and their interplay with electromagnetic waves. Recent experiments demonstrated several surprising optical properties of transient states induced by driving, including the appearance of photo-induced edges in the reflectivity in cuprate superconductors (SCs), observed both below and above the equilibrium transition temperature. Furthermore, in other driven materials, reflection coefficients larger than unity have been observed. In this paper we demonstrate that unusual optical properties of photoexcited systems can be understood from the perspective of a Floquet system, a system with periodically modulated parameters originating from pump-induced oscillations of a collective mode. These oscillations lead to an effective Floquet system with periodically modulated parameters. We present a general phenomenological model of reflectivity from Floquet materials, which takes into account parametric generation of excitation pairs. We find a universal phase diagram of drive-induced features in reflectivity which evidence a competition between driving and dissipation. To illustrate our general analysis, we apply our formalism to two concrete examples motivated by recent experiments: A single plasmon band, which describes Josephson plasmons (JPs) in layered SCs, and a phonon-polariton system, which describes upper and lower polaritons in materials such as insulating SiC. Finally, we demonstrate that our model can be used to provide an accurate fit to results of phonon-pump–terahertz-probe experiments in the high-temperature SC YBa(sub>2CU3O6.5. Our model explains the appearance of a pump-induced edge, which is higher in energy than the equilibrium JP edge, even if the interlayer Josephson coupling is suppressed by the pump pulse.Nonlocal nonlinear phononics
Nature Physics Springer Nature 18 (2022) 457-461
Abstract:
Nonlinear phononics relies on the resonant optical excitation of infrared-active lattice vibrations to induce targeted structural deformations in solids. This form of dynamical crystal structure design has been applied to control the functional properties of many complex solids, including magnetic materials, superconductors and ferroelectrics. However, phononics has so far been restricted to protocols in which structural deformations occur within the optically excited volume, sometimes resulting in unwanted heating. Here, we extend nonlinear phononics to propagating polaritons, spatially separating the functional response from the optical drive. We use mid-infrared optical pulses to resonantly drive a phonon at the surface of ferroelectric LiNbO3. Time-resolved stimulated Raman scattering reveals that the ferroelectric polarization is reduced over the entire 50 µm depth of the sample, far beyond the micrometre depth of the evanescent phonon field. We attribute this effect to the anharmonic coupling between the driven mode and a polariton that propagates into the material. For high excitation amplitudes, we reach a regime in which the ferroelectric polarization is reversed, as revealed by a sign change in the Raman tensor coefficients of all the polar modes.Terahertz phase slips in striped La2−xBaxCuO4
Physical Review B American Physical Society 105:2 (2022) L020502
Abstract:
Interlayer transport in high-TC cuprates is mediated by superconducting tunneling across the CuO2 planes. For this reason, the terahertz frequency optical response is dominated by one or more Josephson plasma resonances and becomes highly nonlinear at fields for which the tunneling supercurrents approach their critical value IC. These large terahertz nonlinearities are in fact a hallmark of superconducting transport. Surprisingly, however, they have been documented in La2-xBaxCuO4 (LBCO) also above TC for doping values near x=1/8 and interpreted as an indication of superfluidity in the stripe phase. Here, electric-field-induced second harmonic is used to study the dynamics of time-dependent interlayer voltages when LBCO is driven with large-amplitude terahertz pulses, in search of other characteristic signatures of Josephson tunneling in the normal state. We show that this method is sensitive to the voltage anomalies associated with 2πJosephson phase slips, which near x=1/8 are observed both below and above TC. These results document a regime of nonlinear transport that shares features of fluctuating stripes and superconducting phase dynamics.Phase diagram for light-induced superconductivity in κ−(ET)2−X
Physical Review Letters American Physical Society 127:19 (2021) 197002
Abstract:
Resonant optical excitation of certain molecular vibrations in κ−(BEDT−TTF)2Cu[N(CN)2]Br has been shown to induce transient superconductinglike optical properties at temperatures far above equilibrium Tc. Here, we report experiments across the bandwidth-tuned phase diagram of this class of materials, and study the Mott insulator κ−(BEDT−TTF)2Cu[N(CN)2]Cl and the metallic compound κ−(BEDT−TTF)2Cu(NCS)2. We find nonequilibrium photoinduced superconductivity only in κ−(BEDT−TTF)2Cu[N(CN)2]Br, indicating that the proximity to the Mott insulating phase and possibly the presence of preexisting superconducting fluctuations are prerequisites for this effect.Engineering crystal structures with light
Nature Physics Springer Nature 17:10 (2021) 1087-1092