Microwave surface impedance measurements on high-T c superconductors

We present measurements of the surface impedance of YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+y thin films in the THz regime. BelowT c , peaks in the real part of the couductivity are observed in both materials, but differ in the temperature dependence. Investigations of Pr-doped YBCO reveal deviations from theT-linear behavior of the resisitivity at characteristic temperatures that shift with the doping level and can be identified with the occurrence of a spin gap. The spin-gap temperature andT c as a function of doping span a Nagaosa-Lee-type phase diagram in the underdoped regime.