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

Abstract

We report the results of ⁷⁷Se-nuclear magnetic resonance (NMR) in α-FeSe, which exhibits a similar crystal structure to the LaFeAsO_1−xF_x superconductor and shows superconductivity at 8 K. The nuclear-spin lattice relaxation rate 1⁄T₁ shows T³ 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 α-FeSe is also an unconventional one as well as LaFeAsO_1−xF_x and related materials. The FeAs layer is not essential for the occurrence of the unconventional superconductivity.