Tunneling measurements on the high-Tc superconductors La1.85Sr

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{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\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 2\ensuremath{\Delta}/${k}_{B}$${T}_{c}$=3.5--4. For the Y-Ba compound we find a pressure-independent gap 2\ensuremath{\Delta}=23\ifmmode\pm\else\textpm\fi{}2 meV or 2\ensuremath{\Delta}/${k}_{B}$${T}_{c}$=3.2\ifmmode\pm\else\textpm\fi{}0.4.