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

TL;DR

This study reveals that hyper-interlayer-expanded FeSe exhibits a significantly increased superconducting transition temperature of 45 K, with suppressed nematic order and spin fluctuations, yet still consistent with spin-fluctuation-mediated pairing.

Contribution

It demonstrates that interlayer expansion enhances Tc in FeSe without inducing nematic order or strong spin fluctuations, challenging existing theories on their roles in superconductivity.

Findings

Superconducting Tc reaches 45 K in hyper-interlayer-expanded FeSe.

No evidence of electronic nematic order or enhanced spin fluctuations in the normal state.

Spin-fluctuation-mediated pairing remains plausible despite suppressed nematicity and spin fluctuations.

Abstract

The superconducting critical temperature, $T_{\rm c}$, 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 $T_{\rm c}=45$~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 $T_{\rm c}$. 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.