Microwave spectroscopy of thermally excited quasiparticles in YBa2Cu3O6.99

We present here the microwave surface impedance of a high-purity crystal of YBa2Cu3O6.99 measured at five frequencies between 1 and 75 GHz. This data set reveals the main features of the conductivity spectrum of the thermally excited quasiparticles in the superconducting state. Below 20 K there is a regime of extremely long quasiparticle scattering times, due to both the collapse of inelastic scattering below Tc and the very weak impurity scattering in the high-purity BaZrO3-grown crystal used in this study. Above 20 K, the scattering increases dramatically, initially at least as fast as T4.