Spin polarization driven by a charge-density wave in monolayer $1T\text{−}{\mathrm{TaS}}_2$

Using first-principles calculations, we investigate the electronic and vibrational properties of monolayer $T$-phase ${\mathrm{TaS}}_2$. We demonstrate that a charge-density wave is energetically favorable at low temperature, similar to bulk $1T\text{−}{\mathrm{TaS}}_2$. Electron-phonon coupling is found to be essential for the lattice reconstruction. The charge-density wave results in a strong localization of the electronic states near the Fermi level and consequently in spin polarization, transforming the material into a magnetic semiconductor with enhanced electronic correlations. The combination of inherent spin polarization with a semiconducting nature distinguishes the monolayer fundamentally from the bulk compound as well as from other two-dimensional transition metal dichalcogenides. Monolayer $T$-phase ${\mathrm{TaS}}_2$ therefore has the potential to enable two-dimensional spintronics.

Figure 1

Band structures and densities of states (per formula unit) of (a), (c) bulk $1T\text{−}{\mathrm{TaS}}_2$ and (b), (d) monolayer $T$-phase ${\mathrm{TaS}}_2$.

Figure 2

(a) Phonon frequency at $Q_{\text{CDW}}$ as a function of the Gaussian smearing for different $k$ meshes. (b) Phonon spectra of monolayer $T$-phase ${\mathrm{TaS}}_2$ for a $20\times 20\times 1k$ mesh and a Gaussian smearing of 0.01 Ry.

Figure 3

(a) Imaginary and (b) real part of the electronic susceptibility of ideal monolayer $T$-phase ${\mathrm{TaS}}_2$. White and black represent large and small values, respectively. The Brillouin zone boundaries are highlighted and arrows indicate $Q_{\text{CDW}}$.

Figure 4

Band structures and densities of states (per formula unit) of (a), (c) bulk $1T\text{−}{\mathrm{TaS}}_2$ and (b), (d) monolayer $T$-phase ${\mathrm{TaS}}_2$ in the CDW phase. The size of the circles in (b) is proportional to the $z$ component of the spin and the red (blue) color represents positive (negative) values.

Figure 5

(a) Charge-density difference between the ideal and CDW phases of monolayer $T$-phase ${\mathrm{TaS}}_2$. The isosurface value is 0.02 electrons/Å${}^3$. (b) Magnetization density $(z$ component of the spin) in the CDW phase. The isosurface value is 0.001 electrons/Å${}^3$. Ta and S atoms are shown in blue and yellow colors, respectively.