Nominal Effect of Mg Intercalation on the Superconducting Properties of 2H-NbSe.

2H-NbSe is a phonon-mediated, Fermi-surface topology-dependent multiband superconductor with an incommensurate charge-density wave (CDW) that coexists at a local level with superconductivity. Usually, the intercalation in 2H-NbSe enriches the CDW, enhances the -axis lattice parameter, and distorts the Fermi surface, which result in a decrease in the superconducting transition temperature (). The rate of decrease of depends on the electronic structure, size, valence, magnetic nature, and electronegativity of the intercalating species. Herein, we report an unusual effect of Mg intercalation on the superconductivity of 2H-MgNbSe ( = 0.0, 0.02, 0.06, 0.08, 0.10, and 0.12) synthesized by a high-temperature solid-state reaction method. Unlike other s- and p-block elements/species as intercalants (Rb, Sn, Ga, and Al) that have a sharp detrimental effect on the of 2H-NbSe within 1-5% of intercalation, Mg is found to be an exception. Upon Mg intercalation up to = 0.06, no remarkable changes in as compared to the parent 2H-NbSe ( ∼ 6.7 K) are observed, and further intercalation results in a small decrease in (for = 0.12, = 6.2 K). From heat-capacity measurements, it is inferred that superconducting Mg-intercalated 2H-NbSe exhibits strong electron-phonon coupling. Electronic structure calculations on two s-block element intercalated compounds of formula MNbSe ( = Mg, Rb) show that Rb s-, p-, and d-states completely overlap with the Nb d states, while the Mg s states lie in a low-energy region as compared to Nb d states, indicating a weak interaction between the intercalant and the Nb sublattice in MgNbSe as compared to RbNbSe. These results suggest that the electronic states of the Nb network in 2H-NbSe are least altered with Mg intercalation, which could be one of the reasons for the minimal effect on the with intercalation.