FREQUENCY AND TEMPERATURE-DEPENDENCE OF THE MILLIMETER-WAVE CONDUCTIVITY OF EPITAXIAL YBA2CU3O7 FILMS

A millimeter wave spectrometer for frequencies between 100 and 350 GHz consisting of continuously tunable backward wave oscillators as sourc es and a quasioptical interferometer in the Mach-Zehnder configuration was used to measure the transmittivity in phase and amplitude of YBa2 Cu3O7 thin films on NdGaO3 substrates. From the measured spectra we de rived the real and imaginary part of the dynamic conductivity sigma = sigma(1) + i sigma(2) in the superconducting state as a function of te mperature. The sigma(1)(T) and sigma(2)(T) values at 300 GHz were comp ared to corresponding values at 19 GHz determined by surface impedance measurements of the same films using a shielded dielectric resonator. Our observed frequency dependence of both sigma(1)(T) and sigma(2)(T) is consistent with a strong reduction of the quasiparticle scattering rate tau(-1)(T) with decreasing temperature below T-c.