Monolayer ${\mathrm{VTe}}_2$: Incommensurate Fermi surface nesting and suppression of charge density waves

We investigated the electronic structure of monolayer ${\mathrm{VTe}}_2$ grown on bilayer graphene by angle-resolved photoemission spectroscopy (ARPES). We found that monolayer ${\mathrm{VTe}}_2$ takes the octahedral $1T$ structure in contrast to the monoclinic one in the bulk, as evidenced by the good agreement in the Fermi surface topology between ARPES results and first-principles band calculations for octahedral monolayer $1T\text{−}{\mathrm{VTe}}_2$. We have revealed that monolayer $1T\text{−}{\mathrm{VTe}}_2$ at low temperatures is characterized by a metallic state whereas the nesting condition is better than that of isostructural monolayer ${\mathrm{VSe}}_2$ which undergoes a charge density wave (CDW) transition to an insulator at low temperatures. The present result suggests an importance of the Fermi surface topology for characterizing the CDW properties of monolayer transition-metal dichalcogenides.

Figure 1

(a) Crystal structure of monolayer $1T\text{−}{\mathrm{VTe}}_2$. (b), (c) RHEED patterns of bilayer (BL) graphene and monolayer (ML) ${\mathrm{VTe}}_2$ on BL graphene, respectively. (d) AFM image of monolayer ${\mathrm{VTe}}_2$. High (white), medium (blue), and low (dark) intensity regions are attributed to atoms/molecules adsorbed on ${\mathrm{VTe}}_2$, clean monolayer ${\mathrm{VTe}}_2$, and atom molecules adsorbed on a graphene substrate, respectively. The reason why we did not attribute the dominant white and blue areas to graphene or 2 ML ${\mathrm{VTe}}_2$ is because we performed in situ ARPES measurements with the same sample and observed a dominant contribution from the monolayer energy bands to the total ARPES intensity. (e) Height profile along a cut shown by the magenta solid line in (d). (f) Photon-energy dependence of the EDC at the $\mathrm{\Gamma }$ point in monolayer ${\mathrm{VTe}}_2$.

Figure 2

(a) Plot of ARPES intensity for monolayer ${\mathrm{VTe}}_2$ along the $\mathrm{\Gamma }M$ and $\mathrm{\Gamma }K$ cuts measured with $h\nu =56$ eV at $T=40$ K. (b) Band structure obtained from the first-principles band-structure calculations for monolayer $1T\text{−}{\mathrm{VTe}}_2$ with the input of an experimental in-plane lattice constant (3.35 Å). Overall calculated bands were contracted by $13\%$ in the energy axis to find a reasonable matching with the experiment. (c), (d) Experimental band structure near $E_{\mathrm{F}}$, measured along the $\mathrm{\Gamma }M$ and $\mathrm{\Gamma }K$ cuts, respectively. Red and purple dashed curves are a guide for the eyes to trace the Te $5p$ and V $3d$ bands, respectively.

Figure 3

(a)–(c) Plots of ARPES intensity at $T=40$ K as a function of 2D wave vectors, $k_x$ and $k_y$, at three representative energy slices at $E_{\mathrm{B}}=E_{\mathrm{F}}$, 0.2, and 0.4 eV, respectively. Energy contours were obtained by integrating the intensity within $\pm 50$ meV with respect to each $E_{\mathrm{B}}$. (d) Calculated FS obtained from the first-principles band-structure calculations for monolayer $1T\text{−}{\mathrm{VTe}}_2$. (e) ARPES-derived band structure along three representative $\mathit{k}$ cuts [cuts A–C in (a)] which cross the triangular FS. The systematic evolution of the $\mathsf{V}$-shaped band dispersions from cut A to cut C indicates the holelike nature of triangular FS.

Figure 4

(a), (b) Schematic FS together with a $\mathit{k}$ cut and $\mathit{k}$ points where high-resolution ARPES measurements were performed for monolayer ${\mathrm{VTe}}_2$ and ${\mathrm{VSe}}_2$, respectively. (c), (d) Plots of ARPES intensity as a function of the wave vector and $E_{\mathrm{B}}$ symmetrized with respect to $E_{\mathrm{F}}$, measured at $T=40$ K along a cut crossing the corner of triangular pocket (cut A/B) for monolayer ${\mathrm{VTe}}_2$ and ${\mathrm{VSe}}_2$, respectively. (e) EDCs near $E_{\mathrm{F}}$ at $T=40$ K for monolayer ${\mathrm{VTe}}_2$ measured at various $k_{\mathrm{F}}$ points in (a). (f) Same as (e) but symmetrized with respect to $E_{\mathrm{F}}$. Zero intensity for each spectrum is indicated by a dashed magenta line to highlight the absolute spectral weight. (g) Same as (f) but for monolayer ${\mathrm{VSe}}_2$ [6]. (h) Comparison of the FS topology between monolayer ${\mathrm{VTe}}_2$ and ${\mathrm{VSe}}_2$. Red and blue circles correspond to the $k_{\mathrm{F}}$ points for monolayer ${\mathrm{VTe}}_2$ and ${\mathrm{VSe}}_2$, respectively. Orange and light blue arrows indicate possible nesting vectors $q$ for monolayer ${\mathrm{VTe}}_2$ and ${\mathrm{VSe}}_2$, respectively.