Andrea Cavalleri

Abstract:

Crystal lattice fluctuations, which are known to influence phase transitions of quantum materials in equilibrium, are also expected to determine the dynamics of light-induced phase changes. However, they have only rarely been explored in these dynamical settings. Here we study the time evolution of lattice fluctuations in the quantum paraelectric SrTiO3, in which mid-infrared drives have been shown to induce a metastable ferroelectric state. Crucial in these physics is the competition between polar instabilities and antiferrodistortive rotations, which in equilibrium frustrate the formation of long-range ferroelectricity. We make use of high-intensity mid-infrared optical pulses to resonantly drive the Ti–O-stretching mode at 17 THz, and we measure the resulting change in lattice fluctuations using time-resolved X-ray diffuse scattering at a free-electron laser. After a prompt increase, we observe a long-lived quench in R-point antiferrodistortive lattice fluctuations. Their enhancement and reduction are theoretically explained by considering the fourth-order nonlinear phononic interactions to the driven optical phonon and third-order coupling to lattice strain, respectively. These observations provide a number of testable hypotheses for the physics of light-induced ferroelectricity.

Abstract:

Optically driven quantum materials exhibit a variety of non-equilibrium functional phenomena [1-11], which are potentially associated with unique transport properties. However, these transient electrical responses have remained largely unexplored, primarily because of the challenges associated with integrating quantum materials into ultrafast electrical devices. Here, thin films of K3C60 grown by Molecular Beam Epitaxy were connected by coplanar terahertz waveguides to a series of photo-conductive switches. This geometry enabled ultrafast transport measurements at high current densities, providing new information on the photo-induced phase created in the high temperature metal by mid-infrared excitation [12-16]. Nonlinearities in the current-voltage charactersitics of the transient state validate the assignment of transient superconductivity, and point to an inhomogeneous phase in which superconducting regions of the sample are connected by resistive weak links [17-23]. This work opens up the possibility of systematic transport measurements in driven quantum materials, both to probe their properties and to integrate them into ultrafast optoelectronic platforms

Abstract:

Photo-excitation at terahertz and mid-infrared frequencies has emerged as an effective way to manipulate functionalities in quantum materials, in some cases creating non-equilibrium phases that have no equilibrium analogue. In K3C60, a metastable zero-resistance phase was observed that has optical properties, nonlinear electrical transport and pressure dependencies compatible with non-equilibrium high-temperature superconductivity. Here we demonstrate a two-orders-of-magnitude increase in photo-susceptibility near 10 THz excitation frequency. At these drive frequencies, a metastable superconducting-like phase is observed up to room temperature. The discovery of a dominant frequency scale sheds light on the microscopic mechanism underlying photo-induced superconductivity. It also indicates a path towards steady-state operation, limited at present by the availability of a suitable high-repetition-rate optical source at these frequencies