Precision global measurements of London penetration depth in FeTe _0.58Se_0.42

We report tunnel-diode resonator (TDR) measurements of in-plane London penetration depth, λ(T), in optimally-doped single crystals of FeTe _0.58Se_0.42 with T_c∼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, Δλ(T)=ATn, was found for all samples with the average exponent n_avg=2.3±0.1 and the prefactor A _avg=1.0±0.2 nm/K2.3. The average superfluid density is well described by the self-consistent two-gap γ model resulting in Δ_I(0)/k_BT_c=1.93 and Δ _II(0)/k_BT_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 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.