Observation of Multiple Charge Density Wave Phases in Epitaxial Monolayer 1T-VSe₂ Film

Abstract As a special order of electronic correlation induced by spatial modulation, the charge density wave (CDW) phenomena in condensed matters attract enormous research interests. Here, using scanning-tunneling microscopy in various temperatures, we discover a hidden incommensurate stripe-like CDW order besides the ($\sqrt 7 $×$\sqrt 3 $) CDW phase at low-temperature of 4 K in the epitaxial monolayer 1T-VSe2 film. Combining the variable-temperature angle-resolved photoemission spectroscopic (ARPES) measurements, we discover a two-step transition of an anisotropic CDW gap structure that consists of two parts Δ1 and Δ2. The gap part Δ1 that closes around ~150 K is accompanied with the vanish of the ($\sqrt 7 $×$\sqrt 3 $) CDW phase. While another momentum-dependent gap part Δ2 can survive up to ~340 K, and is suggested to the result of the incommensurate CDW phase. This two-step transition with anisotropic gap opening and the resulted evolution in ARPES spectra are corroborated by our theoretical calculation based on a phenomenological form for the self-energy containing a two-gap structure Δ1+Δ2, which suggests the different forming mechanism between the ($\sqrt 7 $×$\sqrt 3 $) and the incommensurate CDW phases. Our findings provide significant information and deep understandings on the CDW phases in monolayer 1T-VSe2 film as a 2D material.