Tunneling measurements on the high-Tc superconductors La1.85Sr0.15CuO4−δ and YBa2Cu3O7−δ

Abstract

We have measured the superconducting properties of the new high-$T_c$ superconductors ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ by electron tunneling. The measured I-V curves can be described neatly with simple tunneling theory, using the Bardeen-Cooper-Schreiffer density of states for the superconductor. For the La-Sr compound we find an energy gap ranging between 10 and 20 meV. By exerting a large pressure on the sample the gap value could be increased to 35 meV. From these measurements we extract 2Δ/$k_B$ $T_c$=3.5–4. For the Y-Ba compound we find a pressure-independent gap 2Δ=23±2 meV or 2Δ/$k_B$ $T_c$=3.2±0.4.