Nature of contact resistance in high-T c YBa2Cu3O7?? Superconductors

Abstract

The contacts between various metals and the high-Tc superconductor YBa2Cu3O7−δ are characterized by contact resistance,Rc, andI–V measurements from 300 K to 90 K. The contacts with bulk superconductor were made by vacuum deposition. Four metals, Au, Ag, Al, and Bi were investigated. The current transport across the contact is by carrier tunneling. All contacts were ohmic as theirI–V characteristics were symmetrical with respect to current direction.Rc values range between 10−2 to 101 Ωcm2 and increase linearly as the temperature is lowered. The contact resistance originates from two distinct physical processes. One is the modification of the carrier concentration at the interface by the contact metal. The second is the nature of carrier injection at the free surface of the superconductor. TheRc values depend on the contact metal-oxygen interaction parameter signifying the need for oxygen passivation for obtaining low contact resistances.