Growth, transport, and tunneling spectroscopy of Y1̳-̳x̳Prx̳Ba2̳Cu3̳07̳ thin films as a function of crystallographic orientation
The planar tunneling spectroscopy of superconducting, Pr-doped YBa2Cu301 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=O, 0.2, 0.4, and 0.5, corresponding to zero resistance superconducting transition temperatures (Tc's) of 90 K, 70 K, 45 K, and 20 K, respectively. The optimization ofthe PrBa2Cu30 7 template utilized when growing (100)- and (110)-oriented YBa2Cu301 thin films is reported. Extensive characterization of the films by resistivity and magnetic susceptibility measurements, x-ray diffraction, and scanning electron microscopy show they are of extremely high quality and reproducible. The superconducting and normal state properties are comparable to those measured in single crystals, and the (110)-oriented YBa2Cu307 films exhibit Tc's that are unsurpassed for this orientation. The conductance of Yt-xPrxBa2Cu30711/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)-, (11 0)-, and iii (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 aboriented films exhibit three important features. First, a gap-like feature is observed at an energy, eV-2.2ksTc, that approximately scales with the Tc 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 c and a zero bias conductance dip. The gap-like features for c-axis and ab-oriented films coincide at the roughly the same energy only for undoped films. There is also a high bias dip in the c-axis conductance at roughly the same energy as that observed in ab-oriented films. It is unknown if the high bias structure in the tunneling conductance of ab-oriented and c-axis films is related. The tunneling conductance within the gap-like feature energy for ab-oriented films is qualitatively different than that from conventional superconductors. Results are compared and contrasted to predictions for a superconducting gap with dx2 -/ symmetry.
tunneling spectroscopy, thin films, crystallographic orientation
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations