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
Covington, Mark William
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https://hdl.handle.net/2142/80640
Description
Title
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
Author(s)
Covington, Mark William
Issue Date
1997
Doctoral Committee Chair(s)
Greene, Laura H.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
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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