Possible nodal superconducting gap in Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$) single crystals from ultra-low temperature penetration depth measurements

TL;DR

This study uses ultra-low temperature measurements of the London penetration depth in Fe(Te,Se) crystals to investigate the superconducting gap structure, revealing potential nodal behavior in some samples.

Contribution

It provides evidence for possible nodal superconducting gaps in Fe(Te,Se) crystals through detailed low-temperature penetration depth measurements.

Findings

Power-law behavior of penetration depth with exponent ~2

Observation of linear temperature dependence in some samples

Indication of nodal superconducting gap structure

Abstract

Using a radio frequency tunnel diode oscillator technique, we measured the temperature dependence of the in-plane London penetration depth $\Delta\lambda_{ab}(T)$ in Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$ single crystals, down to temperatures as low as 50 mK. A significant number of samples, with nominal Se concentration $x$=0.36, 0.40, 0.43 and 0.45 respectively, were studied and in many cases we found that $\Delta\lambda_{ab}(T)$ shows an upturn below 0.7 K, indicative of a paramagnetic type contribution. After subtracting the magnetic background, the low temperature behavior of penetration depth is best described by a power law with exponent $n\approx2$ and with no systematic dependence on the Se concentration. Most importantly, in the limit of T$\rightarrow$0, in some samples we observed a narrow region of linear temperature dependence of penetration depth, suggestive of nodes in the superconducting gap of Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$.