Infrared nanoimaging of the metal-insulator transition in the charge-density-wave van der Waals material 1T-TaS2

Using scanning near-field optical microscopy at cryogenic temperatures, we explored the first-order metal-insulator transition of exfoliated 1T-TaS2 microcrystals on a SiO2/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.