Charge density wave, superconductivity, and nontrivial topology in monolayer NbSi 2 As 4

Charge density wave (CDW), superconductivity, nontrivial topology, and their interplay have garnered significant attention in condensed matter physics. In this Letter, we predicted ${\ensuremath{\alpha}}₂\text{\ensuremath{-}}{\mathrm{NbSi}}₂{\mathrm{As}}₄$ as a ground-state CDW candidate, combining with superconductivity and nontrivial topology within the two-dimensional (2D) $M{A}₂{Z}₄$ family. Using first-principles calculations, we systematically investigated its electronic structure, vibrational properties, CDW instability, and superconductivity. The ${\ensuremath{\alpha}}₂\text{\ensuremath{-}}{\mathrm{NbSi}}₂{\mathrm{As}}₄$ monolayer shares a similar crystal field with $2H\text{\ensuremath{-}}{\mathrm{NbSe}}₂$, resulting in a comparable Fermi-surface topology that enables the formation of CDW. The CDW instability arises from the electron-phonon coupling between Nb-$d$ orbitals and longitudinal acoustic phonons from the Nb-$xy$ vibrational mode. Electron doping and tensile strain can suppress the CDW instability, leading to 2D superconductivity with critical temperatures of 5.73 and 5.11 K under 0.2 ${e}^{\ensuremath{-}}$/f.u. and 2.3% strain, respectively. The strong hybridization between Nb-$d$ and As-$p$ orbitals causes the metallic band crossing the Fermi level to intersect with lower bands, forming six topologically nontrivial Dirac points as described by a three-band tight-binding model. This nontrivial topology coexists with both CDW and superconductivity within the same metallic band. The ground-state CDW in ${\ensuremath{\alpha}}₂\text{\ensuremath{-}}{\mathrm{NbSi}}₂{\mathrm{As}}₄$ monolayer offers a potential platform for exploring the interplay of many-body instabilities and topological physics.