Directional Tunneling and Andreev Reflection onYBa2Cu3O7−δSingle Crystals: Predominance ofd-Wave Pairing Symmetry Verified with the Generalized Blonder, Tinkham, and Klapwijk Theory

Abstract

We report directional tunneling and point-contact spectroscopy measurements on the ${100}$, ${110}$, and ${001}$ faces of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals at 4.2 K. The conductance spectra show fully developed quasiparticle tunneling, Andreev reflection, and zero-bias peak characteristics, depending systematically on the junction orientation and impedance. Quantitative spectral analysis using the generalized formalism of Blonder, Tinkham, and Klapwijk (BTK) indicates a predominantly ${d}_{{x}^{2}{\ensuremath{-}y}^{2}}$ pairing symmetry, with less than 5% $s$-wave component in either the $d+s$ or $d+is$ scenario.