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

Abstract

Recently, by intercalating organic ions into bulk FeSe superconductors, two kinds of layered FeSe-based superconductors [(TBA) x FeSe and (CTA) x FeSe] with superconducting transition temperatures (T c) above 40 K have been discovered. Due to the large interlayer distance (∼ 15 Å), these new layered superconductors have a large resistivity anisotropy analogous to bismuth-based cuprate superconductors. Moreover, remarkable pseudogap behavior well above T 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) x FeSe, with a reduced interlayer distance (∼ 10 Å) compared to (TBA) x FeSe and (CTA) x FeSe. By performing 77Se NMR and transport measurements, we observe a similar pseudogap behavior well above T c of ∼ 40 K and a large resistivity anisotropy of ∼10 4 in (PY) x FeSe. All these facts strongly support a universal pseudogap behavior in these layered FeSe-based superconductors with quasi-two-dimensional electronic structures.