Nanoscale Conductivity Imaging of Correlated Electronic States in ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ Moiré Superlattices

We report the nanoscale conductivity imaging of correlated electronic states in angle-aligned ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ heterostructures using microwave impedance microscopy. The noncontact microwave probe allows us to observe the Mott insulating state with one hole per moiré unit cell that persists for temperatures up to 150 K, consistent with other characterization techniques. In addition, we identify for the first time a Mott insulating state at one electron per moiré unit cell. Appreciable inhomogeneity of the correlated states is directly visualized in the heterobilayer region, indicative of local disorders in the moiré superlattice potential or electrostatic doping. Our work provides important insights on 2D moiré systems down to the microscopic level.

Figure 1

(a) Schematic of the gated ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ superlattice device structure. The thicknesses of top and bottom HBN are 11 and 16 nm, respectively. (b) Optical microscopy image of the heterostructure device. The ${\mathrm{WSe}}_2$, ${\mathrm{WS}}_2$ monolayer flakes, FLG contact, and top gate are outlined in green, orange, white, and black, respectively. (c) Optical absorption spectrum in region II of the heterostructure in (b). The three peaks indicated by dashed lines are due to splitting of the ${\mathrm{WSe}}_2$ $A$ exciton, which is indicative of intralayer moiré excitons in the ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ superlattice. (d) ODRC signals at two different ac excitation frequencies as a function of the top-gate voltage and filling factors. The dashed line denotes the charge-neutral point ($\sim -0.3\mathrm{V}$) and the shaded areas correspond to the Mott insulator state at $n/n_0=-1$ and correlated insulating state at $-1/3$. The inset illustrates the Mott insulator state with one hole per moiré unit cell.

Figure 2

(a) Schematic diagram of the TF MIM measurements on the ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ heterobilayer device. The inset shows the SEM image of the etched tungsten tip. (b) FEA simulated MIM signals as a function of the sheet conductance of the buried TMD layers. The inset shows the same plot in the linear scale. (c) Measured room-temperature MIM signals on the ${\mathrm{WS}}_2$ monolayer [orange star in (a)] as a function of the back-gate voltage. The black dash-dotted line indicates the charge-neutral point for electrons at $\sim 0.3\mathrm{V}$, where the Fermi level resides at the conduction band minimum (inset).

Figure 3

(a) Topographic image of the ${\mathrm{WSe}}_2/{\mathrm{WS}}_2$ device taken by the tuning-fork feedback. The ${\mathrm{WSe}}_2$, ${\mathrm{WS}}_2$, and top FLG regions are labeled by green, orange, and black dashed lines, respectively. The green, orange, and red stars mark the locations where the MIM curves in (c) are taken. (b) MIM-Im image at $T=24\mathrm{K}$ and $V_{\mathrm{bg}}=0\mathrm{V}$ in the same area as (a). Inset: MIM-Im images in the blue dashed box in (b) taken at $V_{\mathrm{bg}}=-2\mathrm{V}$. Scale bars in (a) and (b) are $2\mu \mathrm{m}$. (c) MIM-Im signals at various locations and temperature. The two dash-dotted lines denote the charge-neutral points, $-0.7\mathrm{V}$ for holes in ${\mathrm{WSe}}_2$ and $+0.3\mathrm{V}$ for electrons in ${\mathrm{WS}}_2$. The shaded areas correspond to the Mott insulating states at $n/n_0=\pm 1$. The data at 295K HS is scaled by 0.2 for compact visualization.

Figure 4

(a) Topography and (b) MIM-Im images at $V_{\mathrm{bg}}=-2\mathrm{V}$ for a close-up view in the heterostructure region. (c) MIM-Im signals at three different locations labeled in (b) as a function of the back-gate voltage. The positions of $n/n_0=\pm 1$ are indicated by arrows for each curve. For line No. 3 where the $n/n_0=1$ state is not fully developed, the arrow denotes the location where the slope changes sign.