Precision global measurements of London penetration depth in FeTe0.58Se0.42

Abstract

We report tunnel-diode resonator (TDR) measurements of in-plane London penetration depth, $\ensuremath{\lambda}(T)$, in optimally-doped single crystals of FeTe${}_{0.58}$Se${}_{0.42}$ with ${T}_{c}\ensuremath{\sim}14.8$ K. To avoid any size-dependent calibration effect, six samples of different sizes and deliberately introduced surface roughness were measured and compared. The power-law behavior, $\ensuremath{\Delta}\ensuremath{\lambda}(T)=A{T}^{n}$, was found for all samples with the average exponent ${n}_{\text{avg}}=2.3\ifmmode\pm\else\textpm\fi{}0.1$ and the prefactor ${A}_{\text{avg}}=1.0\ifmmode\pm\else\textpm\fi{}0.2$ nm/K${}^{2.3}$. The average superfluid density is well described by the self-consistent two-gap $\ensuremath{\gamma}$ model resulting in ${\ensuremath{\Delta}}_{{}_{I}}(0)/{k}_{B}{T}_{c}=1.93$ and ${\ensuremath{\Delta}}_{{}_{II}}(0)/{k}_{B}{T}_{c}=0.9$. These results suggest the nodeless two-gap pairing symmetry with strong pair breaking effects. In addition, it is found from comparison among six different samples that, while the exponent $n$ remains virtually unchanged, the prefactor $A$ shows some variation, but stays within a reasonable margin, ruling out some recent suggestions that surface conditions can significantly affect the results. This indicates that the calibration procedure used to obtain $\ensuremath{\lambda}(T)$ from the measured TDR frequency shift is robust and that the uncertainty in sample dimensions and the nature of surface roughness play only a minor role.