Temporal relaxation measurements of photoinduced nonequilibrium in superconductors

Abstract

A new technique, transient photoimpedance response (TPR), has been formulated and was used to measure the nonequilibrium relaxation process in YBCO and Niobium. The TPR method dispenses with the need for weak link structures and temporal relaxation measurements are made directly on unpatterned thin films. With a dc current bias, voltage signals are produced in response to photoabsorbed short (300 fs) laser pulses producing a transient impedance change. Nonequilibrium excitation lifetimes are related to the temporal dependence of the voltage signal produced by transient changes in the superconductor’s impedance. With the TPR technique excitation lifetimes in the normal, transition, and superconducting state are observed on each sample. Data interpretation is facilitated by measuring and comparing the TPR signal in all three states. Typically the normal- and transition-state TPR signals in YBCO and Nb are bolometric and correspond to resistive impedance changes. However, the relaxation signal’s time constants in the normal and transition states are different and are dominated by phonon trapping and the relaxation of the order parameter, respectively. In the zero-resistance superconducting state the TPR signal is due to kinetic inductance changes and the nonequilibrium’s relaxation is a manifestation of quasiparticle generation and recombination processes strongly influenced by phonon trapping in the sample. Qualitive and quantitative aspects of the TPR method are presented with some experimental results on YBCO and Nb.