Planar tunneling spectroscopy ofY1−xPrxBa2Cu3O7thin films as a function of crystallographic orientation

Abstract

We present a systematic study of the charge transport and quasiparticle tunneling properties of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ 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 $\ensuremath{\sim}15--40$ mV that decreases in magnitude but stays fixed in energy for increasing Pr doping levels. $\mathrm{ab}$-plane tunneling data exhibit a zero-bias 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 $\mathrm{ab}$-plane data exhibits temperature- and doping-dependent structure over $\ensuremath{\sim}15--40$ 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 $\ensuremath{\sim}20%,$ supporting its interpretation as a feature of a superconducting density of states.