Break-junction Tunneling Spectroscopy of Superconducting FeSexTe1-x

The energy gap ofFeSexTe1-x(x = 0.4-0.5) was investigated by break-junctiontunneling spectroscopy. We found two main kinds of the gap. The predominant one is aBardeen-Cooper-Schrieffersuperconducting gap structure with 4Δ (4 K) = 5 - 13 meV, corresponding to the SIS (superconductor-insulator-superconductor) junction, where 2Δ stands for the superconducting gap. The observed gap distribution is naturally attributed to local inhomogeneity in the tested crystalpatch. Another gap-likestructure appears as broadened gap peaks of Vp-p(4 K) ≈ 80 mV, which is consistent with the local large gapdetected by our scanning-tunneling-spectroscopy measurements using the same crystal batch. The possible origin of the gap is discussed bearing in mind the structural phase transitions occurring in FeSexTe1-x.