Nodeless superconductivity of single-crystalline KxFe2-ySe2 revealed by the low-temperature specific heat

Low-temperature specific heat (SH) has been measured in KxFe2-ySe2 single crystals with T-c = 32 K. The SH anomaly associated with the superconducting transition is moderate and sharp, yielding a value of Delta C/T vertical bar T-c = 11.6 +/- 1.0 mJ/mol K-2. The residual SH coefficient gamma(0) in the superconducting state at T -> 0 is very small with a value of about 0.39 mJ/mol K-2. The magnetic-field-induced enhancement of the low-T SH exhibits a rough linear feature indicating a nodeless gap. This is further supported by the scaling based on the nodeless-gap approach of the low-T data at different magnetic fields. A rough estimate indicates that the normal-state SH coefficient gamma(n) is about 6 +/- 0.5 mJ/mol K-2, leading to Delta C/gamma T-n vertical bar T-c = 1.93 and placing this superconductor in the strong coupling region.