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 $\alpha$-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 $1\mathrm{/}{T}_1$ 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 $\alpha$-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.