Dramatic electronic changes occur when atoms are pushed just a few picometers in a crystal. These periodic lattice distortions where atom positions distort by tiny amounts requires advanced higher-dimensional measurement using scanning / transmission electron microscopy. Sometimes these periodic distortions emerge spontaneously with the formation of charge density waves. Charge waves modulate the electron density with strong electron-lattice coupling. Strong evidence suggests that transformative correlated electron behavior may exist only in unrealized clean 2D materials such as 1T-TaS2. Unfortunately, experiment and theory suggest that extrinsic disorder in free standing 2D layers impedes correlation-driven quantum behavior.
Here we demonstrate a new route to realizing fragile 2D quantum states through epitaxial polytype engineering of van der Waals materials. The isolation of truly 2D charge density waves (CDWs) between metallic layers greatly stabilizes charge density waves long-range order. Lifting the coupling between neighboring CDW layers restore mirror symmetries via interlayer CDW twinning. The novel twinned-commensurate (tC-) CDW we report herein has a single metal–insulator phase transition at ~350 K as measured structurally and electronically. Here we show the critical temperature for spatially-coherent, commensurate (C-) CDW in 1T-TaS2 can be raised to well above room temperature (~150 K above the expected transition) by synthesizing clean interleaved 2D polytypic heterostructures. This stabilizes a collective insulating ground state (i.e. C-CDW) not expected to exist at room temperature. Herein we introduce epitaxial polytype engineering of van der Waals materials to access latent 2D ground states distinct from conventional 2D fabrication.
These results demonstrate endotaxial engineering as a route to isolating and stabilizing 2D collective quantum states in a well-defined extrinsic environment with identical chemistry but distinct band structure.