Possible nodal superconducting gap in Fe_(1+y)(Te_(1-x)Se_x) single crystals from ultralow temperature penetration depth measurements

摘要

Using a radio-frequency tunnel diode oscillator technique, we measured the temperature dependence of the in-plane London penetration depth Δλ_(ab)(T) in Fe_(1+y)Te_(1-x)Se_x) single crystals, down to temperatures as low as 50 mK. A significant number of samples, with nominal Se concentrations x = 0.36, 0.40, 0.43, and 0.45, respectively, were studied and in many cases we found that Δλ_(ab)(T) shows an upturn below 0.7 K, indicative of a paramagnetic-type contribution. After subtracting the magnetic background, the low-temperature behavior of penetration depth is best described by a power law with exponent n ≈ 2 and with no systematic dependence on the Se concentration. Most importantly, in the limit of T → 0, in some samples we observed a narrow region of linear temperature dependence of penetration depth, suggestive of nodes in the superconducting gap of Fe_(1+y)(Te_(1-x)Se_x).