Measurements of the superconducting gap of La-Sr-Cu-O with a scanning-tunneling microscope

We report the first tunneling measurements made on the new high-transition-temperature perovskite superconductors. These were obtained using an ultrahigh-vacuum low-temperature scanning-tunneling microscope. The superconducting gap of a ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ sample is 12±3 mV, and the onset $T_c$ is 40 K, from which 2Δ/$k_B$ $T_c$=7±2. The possibility that this surprisingly high ratio may be due to a mechanism other than superconductivity is discussed.