Enhanced superconducting transition temperature in hyper-interlayer-expanded FeSe despite the suppressed electronic nematic order and spin fluctuations

Abstract

The superconducting critical temperature, $T_{\mathrm{c}}$, of FeSe can be dramatically enhanced by intercalation of a molecular spacer layer. Here we report on a ${}^{77}$Se, ${}^7$Li and ${}^1$H nuclear magnetic resonance (NMR) study of the powdered hyper-interlayer-expanded Li${}_x($C${}_2$H${}_8$N${}_2$)${}_y$Fe${}_{2-z}$Se${}_2$ with a nearly optimal $T_{\mathrm{c}}=45$~K. The absence of any shift in the ${}^7$Li and ${}^1$H NMR spectra indicates a complete decoupling of interlayer units from the conduction electrons in FeSe layers, whereas nearly temperature-independent ${}^7$Li and ${}^1$H spin-lattice relaxation rates are consistent with the non-negligible concentration of Fe impurities present in the insulating interlayer space. On the other hand, strong temperature dependence of ${}^{77}$Se NMR shift and spin-lattice relaxation rate, $1{/}^{77}{T}_1$, is attributed to the hole-like bands close to the Fermi energy. $1{/}^{77}{T}_1$ shows no additional anisotropy that would account for the onset of electronic nematic order down to $T_{\mathrm{c}}$. Similarly, no enhancement in $1{/}^{77}{T}_1$ due to the spin fluctuations could be found in the normal state. Yet, a characteristic power-law dependence $1{/}^{77}{T}_1\propto T^{4.5}$ still comply with the Cooper pairing mediated by spin fluctuations.

7 pages, 6 figures

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• Unconventional density waves and superconductivities in Fe-based superconductors and other strongly correlated electron systems
• Superconductivity without hole-pocket in electron-doped FeSe: Analysis beyond the Migdal-Eliashberg formalism
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• Nearly ferromagnetic metal state in the collapsed tetragonal phase of YFe${}_2$(Ge,Si)${}_2$
• Crystal structures and the sign reversal Hall resistivity in iron-based superconductors Lix(C3H10N2)0.32FeSe (0.15<x<0.4)