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.

Charge density waves in two-dimensional transition metal dichalcogenides

“…The change of the CDW vector is due to the formation of the perfect 2D FS nesting condition driven by enhanced electron-electron correlations in ML 1T-VSe 2 [106,111]. Moreover, the CDW gap is clearly obtained in ML 1T-VSe 2 at perfectly nested FS sections with gap size 60 ∼90 meV, suggesting the FS nesting picture as a CDW driving mechanism in ML 1T-VSe 2 [101,[106][107][108][109].…”

“…Thinned down to an atomically thin 2D limit, CDW order and driving mechanism can be modified due to the change of the electronic structure as well as the absence of the k z dispersion [101,[106][107][108][109][110]. Indeed, the epitaxially grown ML 1T-VSe 2 exhibits [109], in contrast to the 4×4 for the bulk.…”

“…Thinned down to an atomically thin 2D limit, CDW order and driving mechanism can be modified due to the change of the electronic structure as well as the absence of the k z dispersion [101,[106][107][108][109][110]. Indeed, the epitaxially grown ML 1T-VSe 2 exhibits [109], in contrast to the 4×4 for the bulk. The change of the CDW vector is due to the formation of the perfect 2D FS nesting condition driven by enhanced electron-electron correlations in ML 1T-VSe 2 [106,111].…”

“…Despite the clear signature of the CDW superstructure and gap in ML 1T-VSe 2 , discrepancies exist in determining the T CDW . While low energy electron diffraction (LEED) [107] and STM measurements [106,109] extracted T CDW = 140-150 K, the ARPES results show a very broad range of T CDW = 110-340 K by fitting the temperature-dependence of CDW gap to a BCS model [101,106,109]. Moreover, ARPES reveals an anisotropic two-gap structure in ML 1T-VSe 2 , where the band near Γ starts opening the gap below ∼150 K while another band near M shows a two-step transition of the gaps at 150 K and 340 K, respectively [109].…”

“…While low energy electron diffraction (LEED) [107] and STM measurements [106,109] extracted T CDW = 140-150 K, the ARPES results show a very broad range of T CDW = 110-340 K by fitting the temperature-dependence of CDW gap to a BCS model [101,106,109]. Moreover, ARPES reveals an anisotropic two-gap structure in ML 1T-VSe 2 , where the band near Γ starts opening the gap below ∼150 K while another band near M shows a two-step transition of the gaps at 150 K and 340 K, respectively [109]. Since the √ 7 × √ 3 CDW superstructure is only obtained below 150 K, several possible origins of the high-temperature gap structure have been proposed, including the substrate effect [106], pseudogap phase by charge and spin fluctuation [108], and hidden incommensurate CDW formation at high temperature [109].…”