Planar tunneling spectroscopy of Y_1-xPr_xBa₂Cu₃O₇ thin films as a function of crystallographic orientation

We present a systematic study of the charge transport and quasiparticle tunneling properties of Y_1-xPr_xBa₂Cu₃O₇ thin films. Pr doping increases the resistivity along the copper oxide planes and suppresses the superconducting critical temperature T_c, ultimately inducing a superconductor-insulator transition. The tunneling conductance is reproducible and correlated with the crystallographic film orientation. The crystallographic dependence can be divided into two distinct categories: Tunneling into (001)-oriented (c-axis) films and tunneling into (100)-, (110)-, and (103)-oriented (ab-oriented) films. c-axis tunneling data exhibit a conductance dip at zero bias and a broad temperature-dependent peak over ∼15-40 mV that decreases in magnitude but stays fixed in energy for increasing Pr doping levels, ab-plane tunneling data exhibit a zero-bias x conductance peak and a gaplike feature at an energy that scales roughly linearly with T_c for x=0.0, 0.2, and 0.4. When x=0.5, the resistivity is not linear in temperature and a zero-bias conductance dip is observed. The background conductance that ensures conservation of states in the low-temperature ab-plane data exhibits temperature- and doping-dependent structure over ∼15-40 mV that is very similar to the peak observed in c-axis tunneling. Finally, analysis of the temperature and magnetic field dependence of the zero bias conductance peak indicates that states are conserved to within ∼20%, supporting its interpretation as a feature of a superconducting density of states.