Scanning Tunneling Spectroscopy On Superconducting YBa2Cu3O7- δ Thin-Films: Effects Of Ca-Doping, Quasip Article Spin-Injection, Applied Current, And Magnetic Field.

Abstract

Low temperature scanning tunneling spectroscopy was performed on YBa2Cu3O 7-delta thin films as a function of Ca-doping, quasiparticle spin-injection, applied currents and magnetic fields. The purpose of these separate investigations was to gain insight on the microscopic pairing mechanism by examining how the superconducting order parameter (OP) evolves under these perturbations. Quasiparticle spin-injection experiments were performed on thin film heterostructures comprised of optimally doped YBa2Cu3O7-delta and ferromagnetic La0.66Ca0.33MnO3, using a technique which combines scanning tunneling spectroscopy and pulsed spin-injection. A spectral redistribution of the low-energy Andreev states was observed, consistent with an elevated effective quasiparticle temperature induced by spin-injection. Our results also indicate that spin-injection at moderate densities does not induce pair breaking or variation in the predominant d-wave pairing symmetry. Applied current experiments were performed on optimally doped (110)- and (001)-oriented YBa2Cu3O7-delta films using a pulsed technique. The current suppresses the phase-sensitive Andreev states, while leaving the pairing-dependent features largely unaffected. These results indicate that applied currents can weaken the phase of the superconducting order parameter, suggesting that phase-fluctuations can be easily induced in the cuprates. A doping dependent study was performed on (001)-oriented Y1- xCaxBa2Cu3O 7-delta thin films at x = 0, 0.05, 0.15 and 0.20 Ca levels, which enabled access to the overdoped regime. The measured spectra exhibited main-gap, subgap and satellite features which scale similarly in energy as a function of Ca-doping, suggesting that they are associated with a common pairing energy. The spectral data was analyzed using a multiband model which attributes the subgap features to the chain band and the main-gap and satellite features to the plane band for dx2--y 2 + s pairing symmetry. These results suggest that the superconductivity in YBa2Cu3O 7-delta involves multiple bands. Lastly, the evolution of the Andreev states in applied magnetic field was examined in Ca-doped (110)-oriented Y1-xCa xBa2Cu3O7-delta thin films. A splitting of the zero-bias conductance peak is observed in both zero and non-zero applied fields, indicating a lifting of the degeneracy of the Andreev states, consistent with the presence of a subdominant complex is/id' OP component near the (110) surface. However, local signatures of time-reversal symmetry breaking associated with the presence of a subdominant, complex OP component have yet to be observed. These results will be discussed within the theoretical framework of complex subdominant order parameter components in the high-Tc cuprates.