Two-dimensional intrinsic ferrovalley Janus 2H-VSeS monolayer with high Curie temperature and robust valley polarization

Inspired by the unique physical properties of two-dimensional (2D) V-based Janus dichloride monolayers with intrinsic ferromagnetism and high Curie temperature $T_c$, we investigate the electronic structure, spin-valley polarization, and magnetic anisotropy of a Janus 2H-VSeS monolayer in detail using first-principles calculations. The results show that the Janus 2H-VSeS monolayer exhibits a large valley polarization of 105 meV, high $T_c$ of 278 K, and in-plane magnetocrystalline anisotropy contributed by the $d_{x^2-y^2}/d_{xy}$ orbitals of V atoms. The biaxial strain ($-8\%<\varepsilon <8\%$) can effectively tune the magnetic moments of the V atom, valley polarization $\mathrm{\Delta }E, T_c$, and magnetocrystalline anisotropy energy of the Janus 2H-VSeS monolayer. The corresponding $\mathrm{\Delta }E$ and $T_c$ are adjusted from 72 to 106.8 meV and from 180 to 340 K, respectively. Due to the broken space- and time-reversal symmetries, opposite valley charge carriers carry opposite Berry curvatures, which leads to prominent anomalous Hall conductivity at the $K$ and $K^{\prime }$ valleys. The maximum modulation of Berry curvature can reach to 45% and 9.5% by applying the biaxial strain and charge carrier doping, respectively. The stable in-plane magnetocrystalline anisotropy and robust spontaneous valley polarization make the ferromagnetic Janus 2H-VSeS monolayer a promising material for achieving spintronic and valleytronic devices.

Figure 1

(a) Top and side views of the Janus 2H-VSeS monolayer. The red, yellow, and green spheres represent V, S, and Se atoms, respectively. (b) The average electric potential along the $z$ axis and (c) the phonon spectrum for the Janus 2H-VSeS monolayer. (d) The spin-polarized band structures of the Janus 2H-VSeS monolayer. The red and blue lines represent the spin-up ($\alpha$) and spin-down ($\beta$) band structures. The inset is the first Brillouin zone containing the $K$ and $K^{\prime }$ points. (e) The band structures of the Janus 2H-VSeS monolayer with SOC. (f) The V $d$ orbital-resolved band structure of the Janus 2H-VSeS monolayer with SOC; the green and blue dots denote the contribution from the $d_{z^2}$ and $d_{x^2-y^2}/d_{xy}$ orbitals of the V atom, respectively. The arrows between the VBM and the CBM denote the valley-selective optical transitions induced by left and right circularly polarized light ${\sigma }^{+}$ and ${\sigma }^{-}$.

Figure 2

(a)–(c) The V $d$ and Se(S) $p$ orbital-resolved MAEs of the Janus 2H-VSeS monolayer. Dependence of (d) the magnetic moment, (e) magnetic susceptibility, and (f) specific heat capacity on temperature for the Janus 2H-VSeS monolayer.

Figure 3

(a) The band structure of the Janus 2H-VSeS monolayer calculated by MLWFs. Calculated Berry curvature of the Janus 2H-VSeS monolayer (b) over the 2D Brillouin zone and (c) along the high-symmetry lines. (d) Calculated anomalous Hall conductivity of the 2H-VSeS monolayer. The two parallel dashed lines indicate the two valley extremes.

Figure 4

(a) The total MAE for the Janus 2H-VSeS monolayer under various strains. (b) The V, Se, and S atomic-layer-resolved MAE for the Janus 2H-VSeS monolayer under various strains. The V $d$ and Se $p$ orbital-resolved MAE for the Janus 2H-VSeS monolayer at strains of (c) and (d) $-4\%$ and (e) and (f) 4%.

Figure 5

(a) The magnetic moment of the V atom and $T_c$ for the Janus 2H-VSeS monolayers under various strains. Dependence of (b) the magnetic moment and (c) specific heat capacity on the temperature for the Janus 2H-VSeS monolayer at strains of $-8\%, -4\%$, 4%, and 8%.

Figure 6

(a) Band structures of Janus 2H-VSeS monolayers under various strains ($\pm 4\%$) with SOC. (b) Dependence of $\mathrm{\Delta }K, \mathrm{\Delta }{K}^{\prime }$, and valley polarization $\mathrm{\Delta }E$ on the strain. (c) The Berry curvature of the Janus 2H-VSeS monolayers at strains of $-4\%, -2\%$, 0%, 2%, and 4%.

Figure 7

(a) The MAE and (b) exchange energy $J$ for the Janus 2H-VSeS monolayer with various charge carrier dopings. (c) The band structures and (d) Berry curvature for the Janus 2H-VSeS monolayer with 0.05 hole and 0.05 electron doping, respectively. The Fermi level is set to zero.