Microwave Spectroscopy of Thermally Excited Quasiparticles in   YBa_2Cu_3O_{6.99}
  We present here the microwave surface impedance of a high purity crystal of $YBa_2Cu_3O_{6.99}$ measured at 5 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 lifetimes, due to both the collapse of inelastic scattering below $T_c$ and the very weak impurity scattering in the high purity $BaZrO_3$-grown crystal used in this study. Above 20 K, the scattering increases dramatically, initially at least as fast as $T^4$. 
A. Hosseini, R. Harris, Saeid Kamal, P. Dosanjh, J. Preston, Ruixing   Liang, W.N. Hardy and D.A. Bonn