Planar tunnel junctions on 90 K and 60 K YBCO single crystals. Superconducting and normal state properties

We have studied the tunneling characteristics of planar junctions made on YBCO single crystals with T_c=90 K and T_c=60 K. Natural barriers of good quality have been obtained on both compounds. In comparison with the 90 K phase, the superconducting conductance curves of the 60 K phase show gap-like structures reduced in amplitude and shifted towards higher energies. For the higher T_c material, a gap opening, at about 90 K, is observed. Normal state conductances, measured at T>T_c, are highly linear in voltage on both compounds and have steeper slopes on the 60 K phase. The slopes are quite insensitive to temperature variations, and do not depend on the junction resistances nor on the materials used as counterelectrodes.