Temperature dependence of gap related structures in YBa2Cu3O7-delta break junctions

We have applied the break junction technique to highly epitaxial c-axis oriented YBaCuO thin films with T-c(rho =0)=91K deposited on (001) SrTiO3 or LaAlO3 substrates by a high oxygen pressure d.c. sputtering technique. The film thickness was about 1500 Angstrom and a photolithographic process was used to reduce to 100 mum the junction width across the fracture. By this procedure, tunable resistance break junctions with tunneling current favored dong the ab-planes have been realized. The junctions were formed at low temperatures with freshly fractured surfaces and an inert tunnel barrier was created by helium gas or liquid. A good stability was obtained with the normal-state resistance R-N changing about 15% in the temperature range between 4.2 K and 100 K. We have measured the temperature dependence of the conductance maxima that are related to superconducting energy gap at the Fermi level.