Tunneling spectroscopy in thin films YBCO/Pb tunnel structures

Experimental results on electron tunneling into thin superconducting films of YBa/sub 2/Cu/sub 3/O/sub x/ in YBCO/Pb tunnel junctions are reported. Thin superconducting films of YBCO (thickness 0.7 mu m) were prepared by codeposition of Y, BaO, and Cu onto single-crystal substrates (sapphire, MgO, SrTiO/sub 3/). The maximum critical temperature at zero resistivity was T/sub ce/=85 K. The surface of films is smooth with pancake-shaped grains. The tunnel structure was completed by vacuum deposition of superconducting Pb film (thickness 0.3. mu m) onto YBCO film with either an uncleaned surface or after cleaning in Ar or O/sub 2/ plasma. Two mechanisms are presented which can lead to resonances in the tunneling current of high T/sub c/ superconductors. The first one is caused by geometrical localization of quasiparticles in normal regions in superconducting surroundings in the neighborhood of the counter electrode. It is shown that normal regions of the dimension about 10 nm lead to step distances obtained in experiments. The resonances above the gap can therefore occur not only in grainlike material but also in single crystals and in uniaxially aligned microcrystals with lamellar structure. The second mechanism is based on the model of localized baglike excitations by M. Weinstein (1987).