New lithium- and diamine-intercalated superconductors Li_x(C_nH₂n₊₄N₂)_yMoSe₂ (n = 2, 6)

We have succeeded in synthesizing new intercalation superconductors Li_x(C₂H₈N₂)_yMoSe₂ and Li_x(C₆H₁₆N₂)_yMoSe₂ with T_c = 4.2 and 3.8–6.0 K, respectively, via the co-intercalation of lithium and ethylenediamine or hexamethylenediamine into semiconducting 2H-MoSe₂. It has been found that the T_c values are related not to the interlayer spacing between MoSe₂ layers so much but to the electronic density of states (EDOS) at the Fermi level. Moreover, only Li-intercalated Li_xMoSe₂ with a small interlayer spacing has been found to be non-superconducting. Accordingly, it has been concluded that not only a sufficient amount of EDOS at the Fermi level due to the charge transfer from intercalated Li to MoSe₂ layers but also the enhancement of the two-dimensionality of the crystal structure and=or electronic structure due to the expansion of the interlayer spacing between MoSe₂ layers is necessary for the appearance of superconductivity in MoSe₂-based intercalation superconductors. The pairing mechanism and the analogy to the superconductivity in the electric double-layer transistors of 2H-MoX₂ (X = S, Se, Te) are discussed.