Superconductivity in unconventional metals
Zhilong Yang, Haohao Sheng, Zhaopeng Guo, Ruihan Zhang, Quansheng Wu, Hongming Weng, Zhong Fang, Zhijun Wang
Abstract
Abstract Based on first-principles calculations, we demonstrate that 1H/2H-phase transition metal dichalcogenides M X 2 ( M = Nb, Ta; X = S, Se, Te) are unconventional metals, which have an empty-site band of $${A}_{1}^{{\prime} }@1e$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mi>A</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>′</mml:mo> </mml:mrow> </mml:msubsup> <mml:mi>@</mml:mi> <mml:mn>1</mml:mn> <mml:mi>e</mml:mi> </mml:mrow> </mml:math> elementary band representation at the Fermi level. The computed phonon dispersions indicate the stability of the system at high temperatures, while the presence of the soft phonon mode suggests a phase transition to the charge density wave state at low temperatures. Based on the Bardeen-Cooper-Schrieffer theory and computed electron-phonon coupling, our calculations show that the superconductivity (SC) in NbSe 2 is mainly attributed to the soft phonon mode due to the half filling of the empty-site band. Accordingly, the SC has been predicted in unconventional metals TaNS monolayer and 2H-TaN 2 bulk with computed T C = 10 K and 26 K respectively. These results demonstrate that the unconventional metals with partial filling of the empty-site band offer an attractive platform to search for superconductors.