Current transport and electronic states in a,b-axis-oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$/${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ sandwich-type junctions

Precise measurement of the temperature and voltage dependence of junction conductance has been carried out for a,b-axis-oriented ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$/${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$/${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ sandwich-type junctions to investigate the possible origin of Josephson coupling in these junctions. Regardless of the presence or absence of the Josephson effect, most of the junctions exhibited a dip in conductance around zero voltage in their dI/dV profiles at low temperatures. This dI/dV anomaly was attributed to the existence of a minimum in the density of states due to electron-electron interaction in disordered metals in the vicinity of a tunneling barrier within the junctions. The complex temperature dependence of junction conductance was reproduced well by a theoretical model in which both tunneling conduction paths and variable range hopping paths were assumed to exist within the ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ barrier layer. No definite evidence of current transport through a small number of localized levels or a metallic conduction path in ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ has been confirmed, even for junctions with a 20-nm-thick barrier layer. © 1996 The American Physical Society.