Pseudogap behavior of the nuclear spin-lattice relaxation rate in FeSe probed by ⁷⁷Se-NMR

We conducted ⁷⁷Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature (T₁T)^-1 increases below the structural transition temperature T_s but starts to be suppressed below T^∗, well above the superconducting transition temperature T_c(H), resulting in a broad maximum of (T₁T)^-1 at T_p(H). This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because T^∗ and T_p(H) decrease in the same manner as T_c(H) with increasing H, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above T_c(H).