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

Abstract

We report tunnel diode resonator measurements of in-plane London penetration depth, λ(T ), in optimally-doped single crystals of Fe(Te0.58Se0.42) with Tc ∼ 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, ∆λ(T ) = AT n , was found for all samples with the average exponent navg = 2.3 ± 0.1 and the pre-factor Aavg = 1.0 ± 0.2 nm/K 2.3 . The average superfluid density is well described by the self-consistent two-gap γ model resulting in ∆ I (0)/kBTc = 1.93 and ∆ II (0)/kBTc = 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 pre-factor A shows some variation, but stays within reasonable margin ruling out some recent suggestions that surface conditions can significantly affect the results. This indicates that the calibration procedure used to obtain λ(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.

Evolution of London penetration depth with scattering in single crystals of K1−x Nax Kim1, Tanatar2, Liu3 et al. 2014Phys. Rev. BSelf Cite

“…[50] by the average and standard deviation of measurements on 12 crystals) for pure samples. The values at the lower boundary of the error bars provided the best agreement with TDR skin depth measurements, due to being less prone to cracks [51]. Within rather big error bars of the resistivity measurements, the resistivity value for Na-doped samples is indistinguishable from that of the pure material at high temperatures, so we adopted the same ρ(300 K).…”

Reply to “Comment on ‘Precision global measurements of London penetration depth in FeTe0.58Se0.42’”

“…Our answer is the following. We believe that the earlier data on Fe(Te,Se) crystals (including those of Klein et al) [2,12,21] represent the dirtier cases, whereas the sample in our more recent study [11] is closer to the clean behavior. Therefore, fitting the superfluid density with a clean -limit self -consistent two -gap model in the full temperature range is still justified at least in Ref.…”

“…London penetration depth was measured after each step. It was found that the penetration depths measured before and after razor-blade damaging didn't show any significant change and we concluded that roughness plays only a minor role, even for a very strong damage [11] …”

“…As shown in Ref. 11 it has nothing to do with the surface roughness, but reflects a real compositional difference between studied samples.…”

“…Therefore, fitting the superfluid density with a clean -limit self -consistent two -gap model in the full temperature range is still justified at least in Ref. 11 and results in a meaningful estimate of the interaction matrix and of the derived temperature -dependent superconducting gaps. On the other hand, the five -fold difference between "H c1 " estimated by Klein et al and their TDR data collected on the same crystal supports our conclusion that the quantity they measure is not a true H c1 .…”