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 $^{77}\mathrm{Se},^{7}\mathrm{Li}$, and $^{1}\mathrm{H}$ nuclear magnetic resonance (NMR) study of the powdered hyper-interlayer-expanded ${\mathrm{Li}}_{x}{({\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})}_{y}{\mathrm{Fe}}_{2\ensuremath{-}z}{\mathrm{Se}}_{2}$ with a nearly optimal ${T}_{\mathrm{c}}=45$ K. The absence of any shift in the $^{7}\mathrm{Li}$ and $^{1}\mathrm{H}$ NMR spectra indicates a complete decoupling of interlayer units from the conduction electrons in FeSe layers, whereas nearly temperature-independent $^{7}\mathrm{Li}$ and $^{1}\mathrm{H}$ spin-lattice relaxation rates are consistent with the non-negligible concentration of Fe impurities present in the insulating interlayer space. On the other hand, the strong temperature dependence of $^{77}\mathrm{Se}$ NMR shift and spin-lattice relaxation rate, $1{/}^{77}{T}_{1}$, is attributed to the holelike 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 $1{/}^{77}{T}_{1}\ensuremath{\propto}{T}^{4.5}$ still complies with the Cooper pairing mediated by spin fluctuations.