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

We report the results of 77 Se-nuclear magnetic resonance (NMR) in α-FeSe, which exhibits a similar crystal structure to the LaFeAsO 1- x F x 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 T 1 T = const. behavior, indicative of the Fermi liquid state, can be seen in a wide temperature range above T c . The superconductivity in α-FeSe is also an unconventional one as well as LaFeAsO 1- x F x and related materials. The FeAs layer is not essential for the occurrence of the unconventional superconductivity.