Andreev spectroscopy of FeSexTe1−x

•We fabricated FeSe0.2Te0.8/oxide/Pb and FeSe0.2Te0.8/Al/Al2O3/Pb junctions.•These junctions show anomalous conductance spectra and no Josephson current.•These spectra are explained by S/N interface model with extraordinarily gaps.•Except for anomalous gap values, the result is consistent with s+− symmetry.

We fabricated two types of junctions on the c-axis plane of FeSe0.2Te0.8 (Fe[Se,Te], Tc ∼ 13 K) crystals. In the superconductor/oxide/superconductor heterojunctions with a Pb film as a counter electrode, we found a large conductance peak centered at 0 V, and a gap-like feature, which has a much higher energy scale than those reported as a gap energy of Fe[Se,Te]. In spite of a large energy scale, their temperature dependence approximately follows a BCS curve, indicating they come from superconductivity of Fe[Se,Te]. Since these structures can be understood as the results of Andreev bound state at the S/N boundary and the interference effect in the normal metal [1], we tried to identify the effect of a normal layer by fabricating Fe[Se,Te]/Al/Al2O3/Pb junction. Here, we could realize a clean controlled S/N interface and the tunnel junction to investigate electronic properties of the Al slab. We found similar conductance spectra as those junctions without Al layer, giving a support that the anomalous conductance spectra come from the effect of the S/N interface. As expected, the energy scale of the observed features was reduced when the thickness of the normal metal was increased. The absence of Josephson current and the existence of an Andreev bound state may be a signature of the sign-reversal paring in Fe[Se,Te].