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 Ts but starts to be suppressed below T*, well above the superconducting transition temperature Tc(H), resulting in a broad maximum of (T1T)−1 at Tp(H). This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because T* and Tp(H) decrease in the same manner as Tc(H) with increasing H, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above Tc(H).