Andrea Cavalleri

Abstract:

Coherent terahertz (THz) emission is emerging as a powerful new tool to probe symmetry breakings in quantum materials. This method relies on second order optical nonlinearities and is complementary to second harmonic generation spectroscopy. Here, we report coherent THz emission from Josephson plasmons in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ , and find that the amplitude of the emitted field increases dramatically close to the superconducting transition temperature, $T_C$ . We show theoretically how emission is enhanced by critical superconducting fluctuations, a nonlinear analog of critical opalescence. This observation is expected to be of general importance for the study of many thermal and quantum phase transitions.

Abstract:

The nonlinear driving of collective modes in quantum materials can lead to a number of striking non-equilibrium functional responses, which merit a comprehensive exploration of underlying dynamics. However, the coherent coupling between nonlinearly-driven modes frequently involves multiple mode coordinates at once, and is often difficult to capture by one-dimensional pump probe spectroscopy. One example is phonon-mediated amplification of Josephson plasmons in YBa2Cu3O6+x, a phenomenon likely associated with the mysterious superconducting-like optical response observed in this material. Here, we report two-dimensional nonlinear spectroscopy measurements in driven YBa2Cu3O6+x. We excite apical oxygen phonons with pairs of mutually-delayed carrier envelope phase stable mid-infrared pump pulses, and detect time-modulated second-order nonlinear optical susceptibility. We find that the driven phonons parametrically amplify coherent pairs of fluctuating opposite-momentum Josephson plasma polaritons, corresponding to a squeezed state of the Josephson plasma.

Abstract:

A long-standing challenge in ultrafast magnetism and functional materials research, in general, has been the generation of a universal, ultrafast stimulus able to switch between stable magnetic states. Solving this problem would open up many new opportunities for fundamental studies, potentially impacting future data storage technologies. Ideally, step-like magnetic field transients with infinitely fast rise time would serve this purpose. Here we develop a new approach to generate ultrafast magnetic field steps by quenching supercurrents in a superconductor. We achieve magnetic field steps with millitesla amplitude, picosecond rise times and slew rates approaching 1 GT s–1. We test the potential of this technique by coherently rotating the magnetization in a ferrimagnet. Although in the current geometry, the magnetic field step is not sufficient to achieve complete switching, suitable improvements in the device geometry could make these magnetic steps both larger and faster. We foresee new applications ranging from quenches across phase transitions to complete switching of magnetic order parameters.

Abstract:

Inspired by the striking discovery of metastable superconductivity in K3C60 at 100K, far above Tc = 20 K, we discuss possible mechanisms for long-lived, photo-induced superconductivity. Starting from a model of optically-driven Raman phonons coupled to inter-band electronic transitions, we develop a microscopic mechanism for photo-controlling the pairing interaction. Leveraging this mechanism, we first investigate long-lived superconductivity arising from the thermodynamic metastable trapping of the driven phonon. We then propose an alternative route, where the superconducting gap created by an optical drive leads to a dynamical bottleneck in the equilibration of quasi-particles. We conclude by discussing the implications of both scenarios for experiments that can be used to discriminate between them. Our work provides falsifiable explanations for the nanosecond-scale photo-induced superconductivity found in K3C60, while simultaneously offering a theoretical basis for exploring metastable superconductivity in other quantum materials.