Reply to "Comment on \emph{"Precision global measurements of London penetration depth in Fe(Te$_{0.58}$Se$_{0.42}$)"}"

TL;DR

This paper defends the accuracy of London penetration depth measurements in FeTe$_{0.58}$Se$_{0.42}$ against critiques, clarifying measurement techniques and addressing surface roughness effects, emphasizing the importance of proper calibration and multi-band superconductor considerations.

Contribution

The authors demonstrate that surface roughness has minimal impact on penetration depth measurements and clarify the limitations of using the first penetration field for critical field estimation in multi-band superconductors.

Findings

Surface roughness effects are minor on measurements.

Calibration of the TDR technique is verified and reliable.

Universal superfluid density behavior does not apply to multi-band superconductors.

Abstract

We reply to the Comment by T. Klein, P. Rodi\`{e}re and C. Marcenat (arXiv:1205.2566), on our paper, "Precision global measurements of London penetration depth in FeTe$_{0.58}$Se$_{0.42}$", Phys. Rev. B 84, 174502 (2011). Our work was motivated by Klein \emph{et. al}, "Thermodynamic phase diagram of Fe(Se$_{0.5}$Te$_{0.5}$) single crystals in fields up to 28 Tesla", Phys. Rev. B 82, 184506 (2010). In their paper, Klein et al. have attributed a factor of five difference in the value of the London penetration depth obtained from their tunnel diode resonator (TDR) measurements and calculated from the "field of first penetration" to the surface roughness, although they have not verified it experimentally. In our paper, we have studied the effects of deliberately introduced surface roughness and found that its effects are minor and cannot be responsible for the difference of such magnitude. Instead, we suggest that the value of the "field of first penetration" measured with Hall - arrays cannot be used to extract a true lower critical field due to several reasons outlined in our Reply. We emphasize that the accuracy of the calibration procedure of the TDR technique has been carefully verified in several prior studies and our work on FeTe$_{0.58}$Se$_{0.42}$ further confirms it. We show that in their Comment, Klein \emph{et. al} use wrong arguments of the universal behavior of the superfluid density in the gapless limit, because it is inapplicable for the multi - band superconductors. We also discuss the applicability of the clean - limit $\gamma-$ model and the influence of the disorder on the obtained results.