Mixed state microwave response in superconducting cuprates

We study the microwave complex resistivity in the mixed state in several high-Tc superconductors: YBa2Cu3O7–d (YBCO), SmBa2Cu3O7–d (SmBCO) and Bi2Sr2CaCu2O8+x (BSCCO). Different setups and microwave techniques have been employed, in order to perform a systematic study as a function of the dc magnetic field and frequency (with resonant cavities at 21 and 48 GHz, and in the continuous spectrum between 5 and 20 GHz with a Corbino disk). Depending on the setup, moderate (up to 1.5 T) or high (up to 8 T) magnetic fields have been applied perpendicular to the (a,b) planes. In addition, in the 21 GHz setup we could vary the angle between the field and the microwave currents, rotating the field in the (a,b) plane. A systematic of the microwave response has been performed in YBCO, SmBCO and BSCCO with the field directed along the c axis. Combining different measurements we could determine the role of vortex motion in the high-frequency mixed state response. The field dependence of the 48 GHz complex resistivity indicates negligible effects of flux creep and flux pinning. This is directly confirmed by the measured frequency dependent resistivity of YBCO (Corbino disk, 6-20 GHz) which indicates a depinning frequency around 10 GHz at all fields (up to 8 T). However, all samples investigated present a clear sublinear dc field dependence in both real and imaginary part of the resistivity, that cannot be explained by free-flux flow motion alone. We show that these behaviours can be interpreted in the framework of increased quasiparticle response in superconductors with lines of nodes in the gap. Once the quasiparticle response is identified, the single vortex viscosity is extracted. We find common behaviours for all samples under study. Excellent agreement is found between the low field, high frequency data (cavity technique at 48 GHz, up to 1.5 T) and high field, low frequency data (Corbino disc, up to 8 T).