Lower critical field and SNS-Andreev spectroscopy of 122-arsenides: Evidence of nodeless superconducting gap

TL;DR

This study combines magnetic and spectroscopic techniques to investigate the superconducting gap structure of 122-arsenide crystals, providing evidence for a nodeless two-gap s-wave superconductivity with specific gap values.

Contribution

It presents a comprehensive analysis of the superconducting gap structure in 122-arsenides using critical field and Andreev spectroscopy, revealing a nodeless two-gap s-wave nature.

Findings

The London penetration depth fits an anisotropic s-wave or two-gap model.

The large gap ranges from 6-8 meV, small gap around 1.7 meV.

Evidence supports a nodeless two-gap s-wave superconducting state.

Abstract

Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field of a single crystal under static magnetic fields parallel to the axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic -wave-like gap or by a two-gap model, while a d-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound, with the same Tc. We estimated the range of the large gap value 6-8 meV (depending on small variation of and its a space anisotropy of about 30%, and the small gap 1.7 meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless s-wave two gaps.