Quasiparticle Thermalization and Recombination in High-Temperature Superconductors Excited by Femtosecond Optical Pulses

Abstract

The discovery of high-temperature superconductors (HTS) and associated expectations of application of these materials in ultrafast electronics and optoelectronics has created an urgent need for a comprehensive experimental characterization of time-resolved dynamics of carriers in HTS and their response to pulsed, external optical perturbations. The above goal was accomplished via comprehensive transient photoexcitation measurements of light-induced nonequilibrium phenomena in high-quality, epitaxial YBa₂Cu₃O_{7−x (YBCO) thin-film microbridge samples. The photoresponse from < lOOfs-wide laser pulses was measured in the temperature range from 20 K to 80 K, using a subpicosecond electro-optic sampling system. The physical origin of the signal was attributed to the nonequilibrium electron heating effect, in which only electron states are perturbed by laser radiation, while the film phonons remain in thermal equilibrium. From the observed single-picosecond electrical transients, measured in the resistive state, we were able to extract, using the two-temperature model, the characteristic electron thermalization and electron-phonon relaxation time constants to be 0.56 ps and 1.1 ps, respectively. The nonequilibrium kinetic-inductive response was measured in the superconducting state, fitted into both the two-temperature and Rothwarf-Taylor models, and compared to the predictions of s- and d-wave pairing mechanism models. No phonon trapping effect (typical for low-temperature superconductors) was observed in YBCO; thus, the quasiparticle lifetime was given by the quasiparticle recombination time and estimated from the Rothwarf-Taylor equations to be well below lps, and approximately 1.8 ps from the two-temperature model. From the point of view of applications, the single-picosecond intrinsic response of a YBCO superconductor demonstrates that hot-electron HTS photodetectors should exhibit intrinsic bit rates exceeding 300 Gbit/s, making them one of the fastest optoelectronic switches, well suited for digital and communication applications}