Thin film growth of cuprate and ruthenate perovskites and electronic characterization of the electron-doped superconductor niodium cerium copper oxide.

摘要

The electron-doped cuprate Nd2-xCexCuO4-delta (NCCO) appears to differ significantly from the hole-doped cuprates in its superconducting and normal state properties. A number of experiments have indicated NCCO may have a more isotropic order parameter than the hole-doped cuprates, which display excited states at all finite energies and a sign change in the order parameter. The normal state resistivity of NCCO does not exhibit the linear temperature dependence shown by the hole-doped cuprates, and the optimally-doped material has a negative Hall coefficient.;This work encompasses thin film growth of cuprates and ruthenates by pulsed laser deposition and electronic characterization of NCCO using transport and tunneling techniques. Growth procedures for the deposition of thin films of YBa2CU3O7-delta (YBCO) and NCCO with excellent superconducting properties are described. Growth procedures are also given for the deposition of metallic thin films of Sr2RuO 4, a ruthenate that is isostructural with La2CuO4 and which superconducts at T = 1.35 K in single crystals.;Resistivity and Hall measurements were conducted on a series of ion-irradiated NCCO thin films to probe how superconductivity is destroyed by point defect disorder. Results indicate the reduction of Tc with residual resistivity is very similar in NCCO and YBCO. The behavior of both compounds could be explained by invoking the effects of disorder on a 2D superconductor. Hall data suggests that NCCO is a compensated metal, with both electron and hole carriers.;C-axis tunnel junctions between NCCO and the conventional superconductor Pb were fabricated and display a nonzero Josephson current, evidence that NCCO has an s-wave component to its superconducting order parameter. The Josephson coupling was much smaller than predicted if NCCO were a pure, isotropic s-wave superconductor. Single particle tunneling spectroscopy showed a zero-bias semi-gap with zero-voltage conductance above 50% of high-voltage conductance. High-voltage features in the density of states occurred at known NCCO phonon energies.;NCCO bicrystal grain boundary junctions were fabricated and displayed Josephson supercurrents, with IcRN ≈ 100 muV. The zero-bias semi-gap feature measured by single particle spectroscopy in the bicrystal junctions agrees well with that measured in the Pb/NCCO junctions.