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

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 $n\approx 2$ and with no systematic dependence on the Se concentration. Most importantly, in the limit of T$\to$0, in some samples we observed a narrow region of linear temperature dependence of penetration depth, suggestive of nodes in the superconducting gap of Fe${}_{1+y}$(Te${}_{1-x}$Se${}_x)$.

7 pages, 5 figures