POINT-CONTACT AND TUNNELING SPECTROSCOPY OF YBA2CU3O7-X AND YBBA2CU3O7-X (90-K PHASE) SINGLE-CRYSTALS

Point-contact and tunneling spectroscopy has been used to determine th e gap parameter Delta of single crystals of YBa2Cu3O7-x [28 meV less t han or equal to Delta (4.2 K) less than or equal to 33 meV] and YbBa2C u3O7-x [29 meV less than or equal to Delta (4.2 K) less than or equal to 32 meV], with transition temperatures 87 less than or equal to T-c less than or equal to 90 K. An additional fine structure has been obse rved on the current-voltage characteristics of point contacts containi ng a subharmonic structure [V-n = 2 Delta/(en)]. This additional struc ture may stem from a quantum size effect in the quasiparticle spectrum in the N transition region of the S-N-S point contact. Current-voltag e characteristics with a small excess current in the subgap region hav e been observed for S-I-S tunnel junctions. The shape of the current-v oltage characteristics of the S-I-S contacts can be described well by the semiempirical Dynes model. A ratio Gamma/Delta = 0.07, where Gamma is a smearing parameter (T = 4.2 K), has been found. This is a record -low value for high-T-c superconducting tunnel junctions. (C) 1994 Ame rican Institute of Physics.