Low-frequency relaxation rate in the superconducting YBa2Cu3O6.7

The submillimeter-wave 3 cm(-1) < nu < 40 cm(-1) complex conductivity of th e reduced YBa2Cu3O6.7 film (T-C = 56.5 K) was investigated for temperatures 4 K < T < 300 K. The frequency dependence of the effective quasiparticle s cattering rate 1/tau*(nu) was extracted from the spectra. 1/tau* is shown t o be frequency independent at low frequencies and high temperatures. On dec reasing temperature the scattering rate increases with increasing frequenci es. Finally, at 6 K it follows a power-law, 1/tau* proportional to nu(1.75/-0.3).