Electrodynamic properties of oxide superconductors

The electrodynamic properties of oxide high temperature superconductors are reviewed. Most of the experimental data are from thin films and bulk single crystals of YBa_2Cu_3O_7_-_x, which - together with thin films of thallium-based oxide superconductors - provide the lowest microwave losses making epitaxially grown thin films attractive for applications in microwave technology. Among the various techniques for the determination of the electrodynamic response of superconducting thin films cavity, dielectric, and planar resonators are the most successful ones for the frequency range from 1 to 100 GHz. For the millimeter and submillimeter wave range nonresonant transmission techniques in frequency and time domain have been used successfully. Bulk single crystals have been studied by various types of cavity perturbation techniques. YBa_2Cu_3O_7_-_x bulk single crystals and epitaxial thin films exhibit a steep drop of the surface resistance R_s just below the transition temperature T_c to values at 77 K which are about 300 #mu##OMEGA# at 10 GHz. The observed dependence on the frequency f can be described by f"A with sample-dependent A values between 1.5 and 2. At lower temperatures R_s(T) is substantially different both from that predicted by BCS theory and observed experimentally for conventional superconductors. Typically, R_s(T) exhibits a plateau between 30 and 70 K corresponding to a frequency-dependent maximum in the real part of the conductivity. The observed correlation of the height of this maximum to the defect density indicates that above about 30 K the electrodynamic response is dominated by a strong decrease of the quasiparticle lifetime just below T_c. Below T_c/2 the situation is still quite unclear. results. In particular, gapless superconductivity can be easily achieved from magnetic pair breaking in the copper-oxygen chains. d-wave superconductivity, as expected for superconductivity mediated by spin fluctuations instead of phonons, leads to a gapless behavior as well. The observed temperature dependences of #lambda#_L and R_s can be explained to some extent within d-wave models. In contradiction to the hole doped cuprates (as e.g. YBa_2Cu_3O_7_-_x), the electron doped cuprates exhibit an electromagnetic response which is similar to predictions for an isotropic s-wave order parameter. (orig.)Available from TIB Hannover: RA 831(3773) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman