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

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 observe a new (2×1) CDW phase besides the ( CDW phase in epitaxial monolayer 1T-VSe 2 film. Combining the variable-temperature angle-resolved photoemission spectroscopic (ARPES) measurements, we discover an anisotropic CDW gap and a two-step transition associated with the different CDW phases, which were observed below 135 K for the ( ) CDW phase and between 135 K to 330 K for the (2×1) CDW phase respectively. CDW phase results a full gap, while the (2×1) CDW phase shows highly


momentum dependence and results a partial gap structure at the Fermi surface.
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. Our findings provide significant information and deep understanding on the CDW phases in monolayer 1T-VSe2 film as a 2D material.
Although CDWs are ubiquitous in some 3D and 2D materials, but the physical mechanism is still not received a unified explanation in VSe2.
Using in-situ variable-temperature angle-resolved photoemission spectroscopic (VT-ARPES) and scanning-tunneling microscopic (STM) techniques, here we investigate the CDW phase transition in the monolayer 1T-VSe2 film grown on bilayer graphene substrate by molecular beam epitaxial (MBE) method. We found a new CDW phase with (2×1) reconstruction undetected before, besides the (√7×√3) reconstruction in the monolayer 1T-VSe2. Through the analysis of the CDW gap evolutions at different momentum positions from the VT-ARPES spectra, we found that the CDW gap along the Γ-M direction exhibits a monotonic temperature dependence and vanishes at 135 K, associated with the disappearance of the (√7×√3) reconstruction observed by STM.
Along the M-K direction, the CDW gap is also reduced with temperature, but does not vanish at 135 K, instead it extends to 330 K. Interestingly, the gap in the temperature range of 135 K-330 K coincides the (2×1) reconstruction detected by STM. Combining with the theoretical calculations using a phenomenological form for the self-energy containing a two-gap structure, we show that the CDW gap exhibits highly anisotropic momentum and temperature dependences, and shows a two-step transition along the M- The monolayer 1T-VSe2 film was grown on bilayer graphene substrate, which was obtained by flash annealing the 4H-SiC(0001) wafer at 1250 ℃ for 60 cycles 23 . The X-ray photoelectron spectroscopy (XPS), VT-ARPES and room-temperature scanningtunneling microscopy (RT-STM) were performed in-situ. The ultra-low-temperature scanning-tunneling microscopy (ULT-STM) and variable-temperature scanningtunneling microscopy (VT-STM) were performed ex-situ at Nano-X, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), China. The first-principles calculations were performed using the QUANTUM ESPRESSO package base on density functional theory (DFT) 24 . The generalized gradient approximation with the Perdew-Burke-Ernzerhof functional 25 was used to describe the electron exchange and correlation effects. Detailed methods can be seen in the Supplementary Information.
The structure of the 1T-VSe2 unit cell is represented as a ball and stick model in Figure 1(a). The triangle formed by the top layer of Se atoms is rotated by 180° relative to the bottom Se layer. The reflection high-energy electron diffraction (RHEED) image of a monolayer 1T-VSe2 film grown on bilayer graphene substrate is shown in Figure   1(b). The sharp RHEED patterns prove that the film was well-crystalized. In Figure   1(c), the XPS spectrum shows the binding energies of Se 3d5/2 (~ 56 eV), Se 3d3/2 (~57 eV), V 2p3/2 (~ 512 eV) and V 2p1/2 (~ 520 eV) orbitals. To further determine the surface morphology of the sample, we took a 100×100 nm 2 STM image scanned at 7 K [ Figure   1(d)]. The grown 1T-VSe2 formed a large-scale flat single-layer film with a coverage of ~ 60%. Few bilayer islands were formed on the 1T-VSe2 surface, but they will not affect our VT-ARPES and VT-STM measurements due to their rather small sizes. To determine the CDW order of the 1T-VSe2 film in different temperature ranges, we took  Δ2). Notably, the Δ2 shows a highly anisotropic momentum dependence, which has no trance near the Γ point but can be clearly observed near the M point.
With the two-gap form, we can go further to make a comparison to the experimental ARPES spectra by using the phenomenological self-energy expression 31 developed originally for high-Tc cuprates, where∆ is the CDW gap, 1 the single-particle scattering rate, 0 the inverse particlehole pair lifetime, and ( ) the single-particle dispersion. Using Eq.
(2), we can calculate the single-particle spectral function ( , ) via the Green's function as Information [see Figure S1]. In addition, the shape of the constant energy mapping is also modified.
We note that by simply treating the scattering rates 1 and 0 as constants in our calculations using Eq. (2), we get a good agreement to the experimental data. It suggests that the single-particle scattering rate 1 and inverse pair lifetime 0 may show less or even no temperature dependence, and also affect less the CDW phase transitions and gap evolutions in monolayer 1T-VSe2. This is in contrast to the case in high-Tc cuprates 31 , where both 1 and 0 assumes a strong temperature dependence.
In summary, we found a new (2×1) CDW phase in the temperature range between 135 K to 330 K besides the (√7×√3) CDW phase existing below 135 K in the epitaxial monolayer 1T-VSe2 film. Combining our theoretical analysis, we found that these two CDW phases exhibit a two-step CDW gap transition. The one corresponding to the (√7×√3) reconstruction results a full gap. While the other corresponding to the (2×1) reconstruction shows a highly momentum dependence and results a partial gap structure at the Fermi surface. Our results illustrate an unusual CDW phenomenon in monolayer 1T-VSe2.