Unconventional superconductivity and moderate spin fluctuations with gap at low energies in intercalated iron selenide superconductor Li_x(NH₃)_yFe_2-δSe₂ probed by ⁷⁷Se NMR

We report ⁷⁷Se NMR measurements of high-quality polycrystals on the intercalated iron selenide superconductor Li_x(NH₃)_yFe₂-_δSe₂, which has a high superconducting transition temperature T_c of 44K. In the superconducting state, the temperature (T) dependence of the nuclear spin relaxation rate 1/T₁ decreases sharply without a coherence peak below T_c; this behavior can be reproduced by unconventional superconducting states with sign-reversal gap function. In the normal state, the Knight shift (K) and 1/T₁T decrease significantly upon cooling, which is characteristic of heavily electron-doped FeSe-based compounds. A comparison of the T dependences of K and 1/T₁T reveals that moderate spin fluctuations appear at high temperatures and are gradually suppressed upon cooling below 200K, suggesting a spin-gap feature in the spin fluctuation spectrum at low energies. We note that these features are widely observed in many heavily electron-doped high-T_c FeSe-based superconductors that possess the characteristic electronic configuration dominated by large electron Fermi surfaces and a sinking hole-like pocket (or incipient band) around the Fermi level. It is in contrast to the typical Fe-based compounds characterized by hole and electron FSs with similar sizes, such as bulk FeSe, where spin fluctuations are significantly enhanced at low energies toward low temperatures. We discuss the universality and diversity of the relationships between the characteristics of the spin fluctuations and superconductivity in Fe-based compounds.