Ultrafast dynamics of nonequilibrium quasi-particles in high-temperature superconductors

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 better understanding of carrier dynamics in HTS, including the HTS electrical and optical properties and their response to pulsed, external electromagnetic perturbations. The above goals were accomplished via comprehensive transient photoexcitation measurements of light- induced nonequilibrium phenomena in high-quality, epitaxial YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-x</SUB> (YBCO) thin-film microbridge samples. The photoresponse from less than 100-fs laser pulses (390-nm wavelength) was measured using a subpicosecond electro-optic sampling system, in both the microbridge superconducting (flux-flow) and the resistive (switched) states, at temperatures ranging from 20 K to 80 K. 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, we were able to extract the characteristic electron thermalization and electron-phonon relaxation time constants to be 0.56 ps and 1.1. ps, respectively. The above quantities were essentially independent of temperature within our temperature-testing range, in agreement with the two- temperature model. The ratio between phonon and electron specific heats in YBCO was determined to be 38. The nonequilibrium kinetic-inductive response was also measured, 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 unphysical low, far below 1 ps, and approximately 2.5 ps from the two-temperature model. All the presented characteristic time constants must be regarded as the intrinsic response of a YBCO superconductor; thus, hot-electron HTS photodetectors should exhibit intrinsic bit rates exceeding 100 Gbit/s, making them one of the fastest optoelectronic switches, well suited for digital and communication applications.