Evidence for Unconventional Superconductivity in Arsenic-Free Iron-Based Superconductor FeSe: A 77Se-NMR Study

We report the results of Se-77-nuclear magnetic resonance (NMR) in alpha-FeSe, which exhibits a similar crystal structure to the LaFeAsO1-xFx superconductor and shows superconductivity at 8 K. The nuclear-spin lattice relaxation rate 1/T-1 shows T-3 behavior below the superconducting transition temperature T-c without a coherence peak. The T1T = const. behavior, indicative of the Fermi liquid state, can be seen in a wide temperature range above T-c. The superconductivity in alpha-FeSe is also an unconventional one as well as LaFeAsO1-xFx and related materials. The FeAs layer is not essential for the occurrence of the unconventional superconductivity.