NMR Evidence for Universal Pseudogap Behavior in Quasi-Two-Dimensional FeSe-Based Superconductors

Recently, by intercalating organic ions into bulk FeSe superconductors, two kinds of layered FeSe-based superconductors (TBA)_xFeSe and (CTA)_xFeSe with superconducting transition temperatures (T_\rm c) above 40 K have been discovered. Due to the large interlayer distance (\sim 15 Å), these new layered superconductors have a large resistivity anisotropy analogous to bismuth-based cuprate superconductors. Moreover, remarkable pseudogap behavior well above T_\rm c is revealed by nuclear magnetic resonance (NMR) measurements on ^77Se nuclei, suggesting a preformed pairing scenario similar to that of cuprates. Here, we report another new kind of organic-ion-intercalated FeSe superconductor, (PY)_xFeSe, with a reduced interlayer distance (\sim 10 Å) compared to (TBA)_xFeSe and (CTA)_xFeSe. By performing ^77Se NMR and transport measurements, we observe a similar pseudogap behavior well above T_\rm c of \sim 40 K and a large resistivity anisotropy of \sim10^\boldsymbol4 in (PY)_xFeSe. All these facts strongly support a universal pseudogap behavior in these layered FeSe-based superconductors with quasi-two-dimensional electronic structures.