Precision global measurements of London penetration depth in FeTe(0.58)Se(0.42)

We report tunnel-diode resonator (TDR) measurements of in-plane London penetration depth, lambda(T), in optimally-doped single crystals of FeTe(0.58)Se(0.42) with T(c) similar to 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, Delta lambda(T) = AT(n), 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/K(2.3). The average superfluid density is well described by the self-consistent two-gap gamma model resulting in Delta(I) (0)/k(B)T(c) = 1.93 and 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 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.