Growth, Transport, and Tunneling Spectroscopy of Y(1-X) Pr(x) Ba(2) Cu(3) O(7) Thin Films as a Function of Crystallographic Orientation
The planar tunneling spectroscopy of superconducting, Pr-doped YBa$\rm\sb2Cu\sb3O\sb7$ thin films is reported. The tunneling conductance is studied as a function of both crystallographic orientation and Pr concentration. Tunnel junctions are fabricated on thin films grown by off-axis magnetron sputter deposition from single, stoichiometric targets. Four crystallographic orientations are grown: (100), (110), (103), and (001). Four different Pr concentrations are studied where x = 0, 0.2, 0.4, and 0.5, corresponding to zero resistance superconducting transition temperatures (T$\rm\sb{c}$'s) of 90 K, 70 K, 45 K, and 20 K, respectively. The superconducting and normal state properties of the thin films are comparable to those measured in single crystals, and the (110)-oriented $\rm YBa\sb2Cu\sb3O\sb7$ films exhibit T$\rm\sb{c}$'s that are unsurpassed for this orientation. The conductance of $\rm Y\sb{1-x}Pr\sb{x}Ba\sb2Cu\sb3O\sb7$/I/Pb planar tunnel junctions, where I is an insulator formed at the interface between the two materials, is measured as a function of temperature and magnetic field. The junctions are reproducible and well characterized. Tunneling is the dominant transport mechanism through the junctions, as verified by the observation of a well-defined Pb superconducting density of states. For a fixed Pr concentration, the same features are observed for (100)-, (110)-, and (103)-oriented (ab-oriented) films, and these features are qualitatively different than those measured for (001)-oriented (c-axis) films. The tunneling conductance data from ab-oriented films exhibit three important features. First, a gap-like feature is observed at an energy, eV $\rm\sim2.2k\sb{B}T\sb{c},$ that approximately scales with the T$\rm\sb{c}$ of the film. Second, there is a temperature and magnetic field dependent zero-bias conductance peak that disappears for the highest Pr concentration studied. Third, a conductance dip at 45 meV that is asymmetric with respect to bias is observed in undoped films. In contrast, the tunneling conductance data for c-axis films exhibit a gap-like feature at a fixed energy that is independent of T$\rm\sb{c}$ and a zero bias conductance dip. There is also a high bias dip in the c-axis conductance that is coincident with the energy of the dip observed in ab-oriented films. Results are compared and contrasted to predictions for a superconducting gap with $\rm d\sb{x}\sp2\sb{-y}\sp2$ symmetry.
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University of Illinois at Urbana-Champaign