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.
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We acknowledge helpful discussions with K. W. Post and A. Charnukha. This research was supported through DOE-BES Grant No. DE-FG02-00ER45799. D.N.B. is the Moore Foundation Investigator in quantum materials GBMF4533. Sample fabrication at Columbia University was supported by the AFOSR (Grant No. FA9550-16-1-0601, A.N.P.) and from shared facilities supported by the NSF MRSEC (Grant No. DMR-1420634). D.Z.-R.W. acknowledges support from NSF IGERT (Grant No. DGE-1069240). Y.L., W.L., and Y.S. acknowledge support from the National Key Research and Development Program under Contract No. 2016YFA0300404, the National Nature Science Foundation of China under Contracts No. 11674326 and No. U1232139.
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