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

Abstract

The superconducting critical temperature, Tc, of FeSe can be dramatically enhanced by intercalation of a molecular spacer layer. Here we report on a Se77,Li7, and H1 nuclear magnetic resonance (NMR) study of the powdered hyper-interlayer-expanded Lix(C2H8N2)yFe2−zSe2 with a nearly optimal Tc=45 K. The absence of any shift in the Li7 and H1 NMR spectra indicates a complete decoupling of interlayer units from the conduction electrons in FeSe layers, whereas nearly temperature-independent Li7 and H1 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 Se77 NMR shift and spin-lattice relaxation rate, 1/77T1, is attributed to the holelike bands close to the Fermi energy. 1/77T1 shows no additional anisotropy that would account for the onset of electronic nematic order down to Tc. Similarly, no enhancement in 1/77T1 due to the spin fluctuations could be found in the normal state. Yet, a characteristic power-law dependence 1/77T1∝T4.5 still complies with the Cooper pairing mediated by spin fluctuations.

Source-reported materials — not catalogue approval

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