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

TL;DR

This study measures the London penetration depth in Fe(Te,Se) and Fe(Te,S) superconductors, revealing a quadratic temperature dependence and complex gap structure, advancing understanding of their superconducting properties.

Contribution

It provides the first detailed measurements of the penetration depth and superfluid density in these iron-chalcogenide superconductors, highlighting the need for a two-gap model and considering scattering effects.

Findings

Quadratic temperature dependence of λ(T) at low temperatures.

Superfluid density fits a two-gap γ-model with additional scattering or anisotropy.

Estimated λ(0) ≈ 560 nm for Fe(Te,Se) superconductor.

Abstract

The in-plane London penetration depth, $\lambda(T)$, 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 $\lambda(T)$ 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 $\lambda(0)\approx560 \pm 20$ nm. The superfluid density $\rho_s(T)=\lambda^2(0)/\lambda^2(T)$ was fitted with a self-consistent two-gap $\gamma-$model. While two different gaps are needed to describe the full-range temperature variation of $\rho_s(T)$, a non-exponential behavior at low temperatures requires additional factors, such as scattering and/or significant gap anisotropy.