Frequency dependent relaxation rate in the superconducting YBa2Cu3O{6+delta}

The submillimeter-wave 3 cm{-1} < nu < 35 cm{-1} complex conductivity of the reduced YBa2Cu3O{6+delta} film (Tc=56.5 K) was investigated for temperatures 4 K < T < 300 K and compared to the properties of the same film in the optimally doped state. The frequency dependence of the effective quasiparticle scattering rate 1/tau*(nu) was extracted from the spectra. 1/tau*(nu) is shown to be frequency independent at low frequencies and high temperatures. A gradual change to 1/tau*~nu^{1.5} law is observed as temperature decreases. In order to explain the observed temperature dependence of the low frequency spectral weight above Tc, the quasiparticle effective mass is supposed to be temperature dependent for T>Tc.