Condensed-Matter Physics Seminar

In recent years numerous studies of non-equilibrium carrier dynamics in superconductors (SC) have been performed utilizing femtosecond real-time techniques. Research focused on the identification of relaxation processes and direct measurements of the relaxation times. One of the key issues at the moment is whether cuprates are in the so called phonon bottleneck regime as conventional SCs, or in the weak bottleneck regime, where relaxation is governed by the biparticle recombination kinetics. The theoretical model that has been most commonly used to interpret the dynamics is a phenomenological Rothwarf-Taylor (RT) model which describes the evolution of quasiparticle (QP) and high frequency phonon (HFP) populations via a set of two non-linear differential equations, which were shown recently to follow from the general set of kinetic equations for a SC. While the RT model has been known for almost 40 years, no rigorous attempt to solve it has been made thus far, and neither has a comparison to the experimental data been made.
In this seminar we will present a detailed study of the evolution of the SC state following excitation by ultrashort laser pulse using the RT model. The analytical solutions of the model have been obtained for all limiting cases and are in excellent agreement with numerical simulations. The analytical solutions enable comparison of the model to the experimental results. We show that RT model can account for most of the experimental observations in conventional as well as cuprate SC, and that both are in the strong bottleneck regime, where SC state recovery is governed by the HFP decay dynamics.