Planar Tunneling Into YBCMO (M = Zinc, Nickel) Thin Films

Abstract

113 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2001.Planar tunneling spectroscopy of Zn and Ni-doped Y1Ba 2Cu3O7-delta thin films is reported. The tunneling conductance is studied as a function of crystallographic orientation, doping concentration, temperature and applied magnetic field. Tunnel junctions are fabricated with either Bi or Pb as a counter-electrode, ex-situ, on thin films grown by off-axis DC magnetron sputter deposition from single stoichiometric targets. Films are grown in different crystallographic orientations, classified into ab-plane and c-axis. Doping concentrations of 0.01, 0.02, 0.24 (Ni) and 0.01, 0.02, 0.08 (Zn), corresponding to zero-resistance superconducting transition temperatures (Tc's) of 84K, 81K, 77K (Ni) and 84K, 81K, 60K (Zn), respectively, are studied. Extensive characterization by resistivity vs. temperature measurements, scanning electron microscopy (SEM), X-ray Diffraction (XRD), Rutherford back-scattering (RBS) and magnetic susceptibility, shows reproducible and high quality films Resistivity vs. temperature measurements show good agreement with data reported in literature for single crystals. Tunnel junction characteristics are reproducible. Single step elastic tunneling is the predominant transport mechanism across the barrier, as shown by many standard quality checks, including the quality of the well studied Pb and YBCO density of states. The ab-plane tunneling conductance of Zn and Ni doped YBCO films shows distinct features. A gap-like feature that decreases in energy with increasing doping concentration is observed, together with the zero bias conductance peak, comprised of surface-induced Andreev bound states (ABS). The ABS are a direct consequence of the superconducting order parameter of YBCO being of d-wave symmetry. The magnitude of the ABS decreases as the doping concentration is increased. In the case of Zn doped films, the ABS are quenched for the highest doping concentration used. Applied magnetic fields split the ABS, consistent with the Doppler-shift model for ABS transport along an interface. Results are discussed in the light of theoretical models of d-wave superconductors with impurities, and available data on similar systems. Related experiments, that are relevant to the main subject of the thesis, which include a study of a phase transition into a broken-time reversal symmetry state, the magnetic field orientation dependence and magnetic hysteresis of the ABS, are also discussed. Finally, experiments aimed to further investigate the effect of Zn and Ni-doping on the low-energy DoS of YBCO are presented.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD