Modulating charge density wave states in ${\mathrm{TaSe}}_2$ by an electride substrate

Modulating charge density wave (CDW) states in layered materials has both fundamental scientific value and application potential in future electronic devices. Based on first-principles electronic structure calculations, we have studied the modulation of the CDW states in ${\mathrm{TaSe}}_2$ by using a typical electride ${\mathrm{Ca}}_2\mathrm{N}$ as the substrate. We find that the ${\mathrm{Ca}}_2\mathrm{N}$ monolayer can donate 0.49 electrons/f.u. to the ${\mathrm{TaSe}}_2$ monolayer and meanwhile avoids the disorder effect in conventional chemical substitution approach. With the uniform electron doping from ${\mathrm{Ca}}_2\mathrm{N}$, the CDW order in $1H-{\mathrm{TaSe}}_2$ is completely suppressed; in comparison, the CDW period in $1T-{\mathrm{TaSe}}_2$ transforms from the Star of David pattern to a $\sqrt{3}\times \sqrt{3}$ triangular pattern. Our studies enrich the phase diagram of ${\mathrm{TaSe}}_2$ and highlight the effective manipulation of the CDW states via an electride substrate, which calls for future experimental verification.

Figure 1

Top and side views of (a) ${\mathrm{Ca}}_2\mathrm{N}$, (b)$1H\text{−}{\mathrm{TaSe}}_2$, and (c) $1T-{\mathrm{TaSe}}_2$ monolayers. The blue, red, orange, and green balls represent Ca, N, Ta, and Se atoms, respectively. (d) Two-dimensional Brillouin zone (BZ) of the unit cell.

Figure 2

(a) Top and side views of the optimized $1H-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructure. The unit cell is outlined by the black solid line. (b) Electron localization function (ELF) map for the side view of the $1H-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructure. The 0 and 1 values represent the completely delocalized and localized electrons, respectively. (c) Electronic band structure and (d) phonon dispersion of the $1H-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructure along the high-symmetry paths of the BZ. The sizes of the blue, red, orange, and green dots in the electronic band structure represent the orbital weights of Ca, N, Ta, and Se atoms, respectively.

Figure 3

(a) Top and side views of the optimized $1T-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructure. The unit cell is outlined by the black solid line in the top view. (b) ELF map for the side view of the $1T-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructure. (c) Phonon dispersion of the $1T-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructure along the high-symmetry paths of the BZ. (d) Displacement patterns of two doubly degenerated imaginary phonon modes at the $K$ point of the unit cell BZ [the $\mathrm{\Gamma }$ point in the BZ of the $\sqrt{3}\times \sqrt{3}$ supercell in Fig. 4].

Figure 4

(a) Top and side views of the optimized $\sqrt{3}\times \sqrt{3}$ supercell of the $1T-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructure. The $\sqrt{3}\times \sqrt{3}$ supercell is outlined by the black solid line in the top view. The BZs of the unit cell (black) and the $\sqrt{3}\times \sqrt{3}$ supercell (red) are shown in the right corner, while the $K$ point in the former one coincides with the $\mathrm{\Gamma }$ point in the latter one. (b) Electronic band structures along the high-symmetry paths of the two-dimensional (2D) BZ for the $\sqrt{3}\times \sqrt{3}$ supercell. The sizes of the blue, red, orange, and green dots represent the orbital weights of Ca, N, Ta, and Se atoms, respectively. (c) Density of states (DOS) of the optimized and unoptimized heterostructures in the $\sqrt{3}\times \sqrt{3}$ supercell. (d) Phonon dispersion of the optimized $\sqrt{3}\times \sqrt{3}$ supercell.

Figure 5

Top and side views of the $1H-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructure. The six configurations are labeled as H-1 to H-6. The blue, red, orange, and green balls represent the Ca, N, Ta, and Se atoms, respectively.

Figure 6

Top and side views of the $1T-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructure. The six configurations are labeled as T-1 to T-6.

Figure 7

(a)–(e) Phonon dispersions of the H-1, H-3, H-4, H-5, and H-6 configurations of the $1H-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructures, respectively. (f)–(j) Phonon dispersions of the T-1, T-2, T-4, T-5, and T-6 configurations of the $1T-{\mathrm{TaSe}}_2/{\mathrm{Ca}}_2\mathrm{N}$ heterostructures, respectively.