Probing the Superconducting Gap from Tunneling Conductance on NdFeAsO0.7 with TC=51 K

We report the tunneling spectroscopy of an iron-based oxypnictide NdFeAsO0.7 with T-C = 51 K, measured by a mechanical point contact technique. Mainly two kinds of tunneling spectra have been observed reproducibly. One is tunneling conductance displaying sharp superconducting gap peaks at 6.0 +/- 1.0 mV, in which hump (or kink) structures are also observed at 20-30 mV. Another is that showing dominantly the larger superconducting gap Delta(L) with sharp conductance peaks at 14 +/- 1.0 meV, in which the trace of a smaller gap (Delta(S) = 5-7 meV) is simultaneously observed. Our results give direct evidence for the existence of multiple gaps in the quasiparticle excitation spectrum of this multiband system, although the origin of the hump at 20-30 mV is still unclear.