Comment on "Precision global measurements of London penetration depth in FeTe$_{0.58}$Se$_{0.42}$"

TL;DR

This paper critically examines previous TDO measurements of London penetration depth in Fe(Se,Te) superconductors, highlighting inconsistencies with the claimed nodeless two-gap pairing and questioning the robustness of earlier conclusions.

Contribution

It provides a critical analysis of prior TDO data, emphasizing the incompatibility of their models with observed pair breaking effects and questioning surface condition effects.

Findings

Previous data inconsistent with clean limit two-gap model

Strong pair breaking effects indicated by T^n dependence

Surface condition effects may influence TDO measurements

Abstract

Cho et al. [Phys. Rev. B, 84, 174502 (2011)] have reported on the temperature dependence of the London penetration depth deduced from Tunnel Diode Oscillator (TDO) measurements in optimally doped Fe(Se,Te) single crystals. According to their analysis, these measurements chould suggest a nodeless two-gap pairing symmetry with strong pair breaking effects. However, to reach this conclusion, the authors fit the temperature dependence of the superfluid density with a two band {\it clean} limit model which is incompatible with the presence of strong pair breaking effects, deduced from the $T^n$ temperature dependence of the London penetration depth below $T_c/3$. Moreover they claim that their results are also ruling out the suggestion that surface conditions can significantly affect the TDO data but this conclusion is based on one very specific damaging process, and is completely ignoring the large dispersion in the previously published TDO data.