Mixed state microwave resistivity of cuprate superconductors

We present an extended experimental investigation of the vortex-state complex resistivity at 48 GHz in several cuprate superconductors. Q-factor and frequency shift measurements of a resonant cavity in the end-wall configuration yielded the (a,b) plane complex resitivity r of YBa2Cu3O7–d, SmBa2Cu3O7–d and Bi2Sr2CaCu2O8+x highly oriented thin films. A moderate magnetic field (0-0.8 T) was applied along the c axis. We find that the experimental field dependence of r is made up by the sum of a relatively small linear (in the magnetic field B) term, and a substantial sublinear contribution in almost the full temperature range explored. We identify the linear term with the vortex motion contribution to the resistivity [1]: this yields absolute values of vortex viscosities that compare well with published data [1,2]. From the sublinear terms, we evaluate the field dependence of the quasiparticle and superfluid conductivities. We find that the superfluid conductivity ssf(T,B)is proportional to B1/2 in all materials investigated. This is in agreement with the predictions for superconductors with lines of nodes in the gap [3]. Moreover, the extension of the B1/2 dependence in nearly the full temperature range explored (T> 65 K) and up to temperatures close to Tc indicates that such pairing is not smeared out by the high operating temperatures.