Effects of Grain Boundaries on Microwave Surface Resistance of Superconducting Films

Abstract

Microwave surface resistance of superconducting films with grain boundaries (GBs) is theoretically investigated as the functions of the critical current density J/sub cj/ at GB junctions and of the film thickness d. The two-fluid model is used for electrical current in the grains, whereas the Josephson-junction model is adopted for tunneling current across GBs. Theoretical expressions of the surface resistance R/sub sf/ are obtained not only for weakly coupled GBs with small J/sub cj/ but also for strongly coupled GBs with large J/sub cj/. The results show that R/sub sf/ nonmonotonically depends on the critical current density J/sub cj/ at GB junctions, and R/sub sf/ of superconductors with GBs can be smaller than the surface resistance for ideal homogeneous superconductors without GB. The R/sub sf/ is proportional to 1/d for thin films of d/spl Lt/d/sub cr/ and R/sub sf/ is independent of d for thick slabs of d/spl Gt/d/sub cr/, where the crossover thickness d/sub cr/ is much larger than 2/spl lambda/ (where /spl lambda/ is the London penetration depth) for weakly coupled GBs.