MICROWAVE RADIO PHYSICS OF UNCONVENTIONAL SUPERCONDUCTORS

The paper presents a review of main results obtained by the authors in the process of microwave (MW) investigations on unconventional superconductors and developments of MW devices based on cuprate high-temperature superconductors (HTS) carried out over the course of the past 10-15 years. Experimental investigations were carried out by the methods of impedance measurements of superconductor samples. For this purpose the authors have developed two measuring techniques in the mm wavelength range: on the base of quasioptical sapphire resonators and using a feature of the polarized wave p-field reflection from the superconductor surface at a grazing angle of incidence. Epitaxial films of the YBa2Cu3O7–δ cuprate superconductor and Fe-containing superconductors in the form of Ba(Fe0.926Co0.074)2As2 pnictide single crystals and FeSexTe1–x (x = 0.5 and 0.7) chalcogenide epitaxial films have been investigated. The results of the MW response of electrodynamic structures with the investigated samples were used as a base for finding the complex conductivity including the fluctuation one. In general, the results obtained confirm the scenario of the slit function d-wave symmetry for cuprate superconductors and s-wave symmetry for Fe-based superconductors. However, further investigations are required as there is a number of features and effects, namely: abnormal frequency dependence of the residual surface resistance in YBa2Cu3O7–δ in the form of ω3./2, increase of the quasi-particle conductivity with temperature decreasing after the critical one and avalanche-like transition from the superconducting state to the strongly dissipative state in the coplanar transmission line. New MW devices based on cuprate HTS films in the mm wave range 1) quasioptical sapphire resonator with radial slit and HTS end walls for investigation of Fe-based superconductors in the mm wave range: 1) quasioptical sapphire resonator with a radial slit and HTS end walls for investigation of Fe-based superconductors in the form of small (1-2 mm in the a-b plane) samples; 2) quasioptical planar resonator; 3) band-pass filter with an E-plane insertion in the cross-type waveguide. A possibility of noncontact testing the homogeneity properties of massive superconductors at room temperature is shown. The temperature dependence of the complex conductivity of YBa2Cu3O7-(Ba (Fe0.926Co0.074)2As2 and FeSexTe1-x (x = 0.5 and 0.7) superconductors as well as the corresponding physical quantities have been obtained, that enables one to judge about the evidence of relevant scenarios of the wave symmetry in the slit function of investigated superconductors. However, a number of revealed features and effects require further study. A possibility of developing large-scale MW HTS-based devices operating at frequencies up to 40 GHz was experimentally confirmed.