Superconductivity in unconventional metals

Abstract In this work, we define unconventional metals, where a set of low-energy bands belong to an elementary band representation (EBR) on an empty site. The characteristic could give rise to soft phonon modes and strong electron-phonon coupling, yielding charge-density wave (CDW) or/and superconductivity (SC). Based on the first-principles calculations, we demonstrate that 1H/2H-phase transition metal dichalcogenides MX2 (M = Nb, Ta; X = S, Se, Te) are unconventional metals, which have a single band of A′1@1e EBR at the Fermi level (EF). The computed phonon dispersions indicate the stability of the system at high temperatures, while the presence of soft phonon modes suggests a phase transition to the CDW state at low temperatures. Based on the Bardeen-Cooper- Schrieffer (BCS) theory and computed electron-phonon coupling, our calculations show that the SC in NbSe2 is mainly attributed to the soft phonon modes. In the end, the SC has been predicted in unconventional metal TaNS monolayer and 2H-TaN2 bulk with estimated TC = 10 K and 26 K respectively. These results demonstrate that the unconventional metals offer an attractive platform for studying the interplay between the empty-site EBR and correlated states.