Pseudogap Behavior of the Nuclear Spin-lattice Relaxation Rate in FeSe Probed by $^{77}$Se-NMR

TL;DR

This study uses $^{77}$Se-NMR to explore pseudogap behavior in FeSe, revealing superconducting fluctuations above $T_c$ similar to cuprates, linked to preformed pairs.

Contribution

It provides evidence of pseudogap behavior in FeSe linked to superconducting fluctuations and preformed pairs, expanding understanding of iron-based superconductors.

Findings

$(T_1T)^{-1}$ increases below $T_s$ but is suppressed below $T^*$

Broad maximum of $(T_1T)^{-1}$ at $T_p(H)$ indicates pseudogap

Pseudogap behavior diminishes with increasing magnetic field, consistent with superconducting fluctuations

Abstract

We conducted $^{77}$Se-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 $(T_1T)^{-1}$ increases below the structural transition temperature $T_\mathrm{s}$ but starts to be suppressed below $T^*$, well above the superconducting transition temperature $T_\mathrm{c}(H)$, resulting in a broad maximum of $(T_1T)^{-1}$ at $T_\mathrm{p}(H)$. This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because $T^*$ and $T_\mathrm{p}(H)$ decrease in the same manner as $T_\mathrm{c}(H)$ with increasing $H$, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above $T_\mathrm{c}(H)$.