Dichotomy of saddle points in energy bands of a monolayer NbSe${}_2$

Abstract

We theoretically show that two distinctive spin textures manifest themselves around saddle points of energy bands in a monolayer NbSe${}_2$ under external gate potentials. While the density of states at all saddle points diverge logarithmically, ones at the zone boundaries display a windmill-shaped spin texture while the others unidirectional spin orientations. The disparate spin-resolved states are demonstrated to contribute an intrinsic spin Hall conductivity significantly while their characteristics differ from each other.Based on a minimal but essential tight-binding approximation reproducing first-principles computation results, we established distinct effective Rashba Hamiltonians for each saddle point, realizing the unique spin textures depending on their momentum. Energetic positions of the saddle points in a single layer NbSe${}_2$ are shown to be well controlled by a gate potential so that it could be a prototypical system to test a competition between various collective phenomena triggered by diverging density of states and their spin textures in low-dimension.

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• Symmetry manipulation of nonlinear optical effect for metallic TMDC
• Nonlinear optical Hall effect of few-layered NbSe2
• Construction of optimized tight-binding models using \textit{ab initio} Hamiltonian: Application to monolayer $2H$-transition metal dichalcogenides