CURRENT TRANSPORT AND ELECTRONIC STATES IN A,B-AXIS-ORIENTED YBA2CU3O7 PRBA2CU3O7/YBA2CU3O7 SANDWICH-TYPE JUNCTIONS/

Precise measurement of the temperature and voltage dependence of junct ion conductance has been carried out for a,b-axis-oriented YBa2Cu3O7/P rBa2Cu3O7/YBa2CU3O7 Sandwitch-type junctions to investigate the possib le origin of Josephson coupling in these junctions. Regardless of the presence or absence of the Josephson effect, most of the junctions exh ibited a dip in conductance around zero voltage in their dI/dV profile s at low temperatures. This dI/dV anomaly was attributed to the existe nce of a minimum in the density of states due to electron-electron int eraction in disordered metals in the vicinity of a tunneling barrier w ithin the junctions. The complex temperature dependence of junction co nductance was reproduced well by a theoretical model in which both tun neling conduction paths and variable range hopping paths were assumed to exist within the PrBa2Cu3O7 barrier layer. No definite evidence of current transport through a small number of localized levels or a meta llic conduction path in PrBa2Cu3O7 has been confirmed, even for juncti ons with a 20-nm-thick barrier layer.