London penetration depth and superfluid density in single crystals of Fe(Te,Se) and Fe(Te,S) superconductors

Abstract

The in-plane London penetration depth, $\hat{I}»(T)$, was measured in single crystals of the iron-chalcogenide superconductors Fe${}_{1.03}$(Te${}_{0.63}$Se${}_{0.37}$) and Fe${}_{1.06}$(Te${}_{0.88}$S${}_{0.14}$) by using a radio-frequency tunnel diode resonator. As is also the case for the iron-pnictides, these iron-chalcogenides exhibit a nearly quadratic temperature variation of $\hat{I}»(T)$ at low temperatures. The absolute value of the penetration depth in the $T\to 0$ limit was determined for Fe${}_{1.03}$(Te${}_{0.63}$Se${}_{0.37})$ by using an Al coating technique, giving $\hat{I}»(0)\approx 560\pm 20$ nm. The superfluid density $\ddot{I}{}_s(T)=\hat{I}{»}^2(0)/\hat{I}{»}^2(T)$ was fitted with a self-consistent two-gap ${\hat{I}}^3-$model. While two different gaps are needed to describe the full-range temperature variation of $\ddot{I}{}_s(T)$, a non-exponential behavior at low temperatures requires additional factors, such as scattering and/or significant gap anisotropy.