Possible Superconducting Symmetry and Magnetic Correlations in K0.8Fe2Se2: A 77Se-NMR Study

We report Se-77-NMR measurements in the recent discovered K0.8Fe2Se2 with the superconducting transition temperature T-c = 32 K. The characteristic point of this system is that the high density of state is located in the vicinity of the Fermi level. The comparison between Knight shift and nuclear spin lattice relaxation rate 1/T-1 shows that the antiferromagnetic spin correlation is not strong in this system, but it is reasonable to consider that its development toward low temperatures occurs. 1/T-1 in the superconducting state can be reproduced well by an s(+/-)-wave model, but a d-wave model also roughly reproduces the data. Non exponential behavior in 1/T-1 at low temperatures disagrees with a single isotropic gap.