Leggett mode in cuprate superconductors and the electromagnetic properties of Josephson junctions

The effect of multiple Andreev reflections in YBa$_{2}$Cu$_{3}$O$_{7-x}$ and Tl$_{2}$Ba$_{2}$Ca$_{2}$Cu$_{3}$O$_{10-\delta}$ has been studied. Several independent subharmonic gap structures associated with the multigap character of these superconductors have been revealed on the $I(V)$ and $dI/dV$ characteristics of Andreev contacts. An additional fine structure has been observed, which can be attributed to the emission of Leggett plasmons. The electromagnetic properties of Josephson junctions at a microcrack in Tl$_{2}$Ba$_{2}$Ca$_{2}$Cu$_{3}$O$_{10-\delta}$ have been examined. The magnetic-field dependences of the critical current, as well as Fiske geometric resonances, have been studied in the Josephson regime (SIS type). The fundamental frequency of resonances and the Swihart velocity have been determined.