Tunneling spectroscopy of YBa2Cu3O6.5+x and Y0.95Al0.05Ba2Cu3O6.5+x with use of a scanning-tunneling microscope

Abstract

We have made gap measurements at 4.2 K on the high-$T_c$ superconductor ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.5+\mathrm{x}}$ by electron tunneling using a low-temperature scanning-tunneling microscope. In all the samples studied we observe large variations in the energy gap. We observe a very sharp gap in all the samples with 2Δ=5 meV yielding a value of 2Δ/$k_B$ $T_c$=0.7, far below the weak-coupling limit. The largest gap we measure has a value of 2Δ=95 meV and indicates an extremely strong coupling value of 2Δ/$k_B$ $T_c$=13. The large gap may be due to tunneling between a series of superconducting regions within the sample. This possibility and other results are discussed in terms of tunneling into a glassy superconductor.