Non-Equilibrium Phenomena in Conventional and High-Temperature Superconductors Probed by Time-Domain Terahertz-Spectroscopy

Abstract

In the last decades, time-resolved studies on superconductors after optical excitation aimed at understanding the interplay between quasiparticles, phonons and the superconducting condensate in non-equilibrium after optical excitation. Past experiments have measured the effect of the superconducting gap and the normal state pseudogap on the carrier relaxation dynamics, the absence or presence of the phonon-bottleneck in these classes of superconductors and the photoinduced melting with intense optical pulses. So far, most of this work has been focused on high-Tc cuprates and still, there are unsolved issues with regard to condensate relaxation and the optical energy needed to drive the superconductor-metal phase-transition in these compounds. In contrast, there are only two experiments on electron-doped cuprates, and only a few optical studies so far addressed the transient dynamics after optical perturbation in conventional superconductors. So far, a detailed study of the non-equilibrium dynamics with picosecond time-resolution even in conventional superconductors is still missing.The present thesis measured the timescales of Cooper-pair breaking (CPB) after optical perturbation and subsequent condensate relaxation using two different experimental techniques, namely optical-pump terahertz-probe (OPTP) and terahertz-pump terahertz-probe (TPTP) spectroscopy. Specifically, this study focused on a conventional superconductor Niobiumnitride (NbN) and an electron-doped cuprate Pr1.85Ce0.15CuO4-δ (PCCO) at optimal doping. In Addition, the scaling of condensate depletion as a function of excitation density is measured using both measurement techniques, in order to extract the optical energy needed to suppress superconductivity.