Conductance spectrum and superconducting gap structures observed in c-axis FeSe0.3Te0.7/Au junctions

The electric transport properties of superconducting FeSe0.3Te0.7/Au c-axis (S/N) junctions, fabricated by using Pulsed Laser Deposition technique, have been investigated in the temperature range of 2 K to the superconducting transition temperature TC ~ 13.5 K, and in the presence of applied magnetic fields from 0 to 9 T. A large zero bias conductance peak has always been observed in the conductance spectra of several batches of junctions. In addition, we have also found several gap-like features. Using the extended BTK theory with the currently favored nodeless S+- wave symmetry, our conductance spectrum can be reproduced quite well in the low bias range with Δ1 = 4 meV, and Δ2 = 6 meV at 2 K. However the experimental conductance spectrum is substantially below the calculated one in the high bias range. Furthermore, there is a conductance minimum at about 20 meV, which may be the reason for the discrepancy.

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