Infrared nanoimaging of the metal-insulator transition in the charge-density-wave van der Waals material $1T\text{−}{\mathrm{TaS}}_2$

Using scanning near-field optical microscopy at cryogenic temperatures, we explored the first-order metal-insulator transition of exfoliated $1T\text{−}{\mathrm{TaS}}_2$ microcrystals on a ${\mathrm{SiO}}_2/\mathrm{Si}$ substrate. We clearly observed spatially separated metallic and insulating states during the transition between commensurate and nearly commensurate charge-density-wave phases. The capability to probe electrodynamics on nanometer length scales revealed temperature-dependent electronic properties of the insulating and metallic regions near the transition temperature. At fixed temperature, a remarkably broad spatial boundary between insulating and metallic regions was observed, across which the nano-optical signal smoothly evolved over a length scale of several hundred nanometers. To understand these observations, we performed Ginzburg-Landau calculations to determine the charge-density-wave structure of the domain boundary, which revealed the existence of an intermediate electronic phase with unique properties distinct from the bulk thermodynamic phases.

Figure 1

Imaging the metal-to-insulator transition in a thin microcrystal of $1T\text{−}{\mathrm{TaS}}_2$: (a) Resistivity and CDW phase diagram of bulk $1T\text{−}{\mathrm{TaS}}_2$. (b) Schematic of the near-field imaging experiment. (c) Sample topography showing the $\approx 12$ nm (20 layer) thick microcrystal (the yellow curve shows the outline of the area displayed in the nano-IR images). (d–o) Nano-IR images of metal-to-insulator transition at incident light frequency $\omega =1155$ ${\mathrm{cm}}^{-1}$. Insulating regions (blue) first appear around the edges of the microcrystal and grow inwards until the central metallic NC area (yellow and red) vanishes and the entire piece has entered the insulating C phase.

Figure 2

Quantitative analysis of nanoimaging from Fig. 1: (a) Selected histograms for the cooling transition, showing the evolution of the normalized nano-IR signal with temperature. The bimodal distributions reflect the first-order nature of the transition, while the shift in the peak centers indicates that the underlying electronic properties of the two phases change with temperature. (b) Nano-IR signal as a function of temperature for insulating (blue) and metallic (orange) phases extracted by fitting the histogram distributions, illustrating the change in local optical properties. Inset: insulating fill fraction versus temperature.

Figure 3

Domain boundary analysis: (top panel) High-resolution nano-IR image of phase coexistence during the cooling cycle showing the diffuse boundary. (bottom panel) Line cut across the insulator-to-metal boundary (white line in the top panel). The dotted black lines show the fitted FWHM of the transition region ($w\approx 450$ nm), while the dotted red lines show the estimated radius of our AFM tip ($r\approx 27$ nm). Insets: C-CDW (left) and NC-CDW (right) structures. Each star-of-David represents one CDW cluster.

Figure 4

Nonequilibrium CDW at a phase boundary: (top panel) Landscape of Landau free energy (arbitrary units, relatively offset) minimized for fixed values of $\mathit{Q}$, as identified by their relative angle ${\theta }_Q$ in the Brillouin zone. For $T<T_c$, four distinct phases are metastable as indicated by the colored dots. With decreasing temperature, the $\mathit{Q}$ of lowest energy are ${\mathit{Q}}_I$, ${\mathit{Q}}_{\mathrm{NC}}$, and ${\mathit{Q}}_{\mathrm{C}}$, respectively. ${\mathit{Q}}_{\mathrm{NE}}$ denotes a nonequilibrium (NE) CDW state interpolating the NC and C phases (see text). (bottom panel) Numerical minimization of Landau free energy across an inter-phase boundary between C and NC phases. The local orientation of the CDW-vector ${\theta }_Q$ and the local CDW amplitude $\langle \psi \rangle$ are plotted versus distance $x$ from the boundary location, measured in units of $\zeta$, a characteristic length scale [38]. The NE CDW spontaneously stabilizes in proximity to the interphase boundary.