Monolayer VTe2 : Incommensurate Fermi surface nesting and suppression of charge density waves

Abstract

We investigated the electronic structure of monolayer VTe2 grown on bilayer graphene by angleresolved photoemission spectroscopy (ARPES). We found that monolayer VTe2 takes the octahedral 1T structure in contrast to the monoclinic one in the bulk, as evidenced by the good agreement in the Fermi-surface topology between ARPES results and first-principles band calculations for octahedral monolayer 1T -VTe2. We have revealed that monolayer 1T -VTe2 at low temperature is characterized by a metallic state whereas the nesting condition is better than that of isostructural monolayer VSe2 which undergoes a CDW transition to insulator at low temperature. The present result suggests an importance of Fermi-surface topology for characterizing the CDW properties of monolayer TMDs.Layered transition-metal dichalcogenides (TMDs) are a promising candidate for realizing outstanding properties associated with two-dimensionalization since bulk TMDs are known to exhibit various physical properties such as magnetism, Mott-insulator phase, and charge density wave (CDW), besides the wide range of transport property (insulator, semiconductor, metal, and superconductor), most of which are prone to the change in the dimensionality of materials. When TMDs are thinned to a single monolayer (2D limit), they exhibit even more outstanding properties distinct from bulk, as represented by the room-temperature ferromagnetism in VSe 2 in contrast to the nonmagnetic nature of bulk [1] and the change in the band-gap property from indirect to direct transition in MoS 2 [2]. The CDW is a most pronounced phenomenon widely seen in both bulk and atomic-layer TMDs. In bulk TMDs, the interplay between the Fermisurface nesting and the energy-gap opening as well as its relationship to the strength of CDW properties such as the CDW transition temperature (T CDW ) has been a target of intensive studies [3].The role of dimensionality to the mechanism of CDW, in particular whether or not the CDW is more stable in the 2D limit, is now becoming a target of fierce debates, being stimulated by a recent success in fabricating various atomic-layer TMDs by exfoliation and epitaxial techniques. Recent studies on some TMDs such as TiSe 2 , VSe 2 , and NbSe 2 [4-8] have shown a marked increase in T CDW upon reducing the thickness down to a few monolayers. In contrast, it has been reported that the CDW vanishes in monolayer TaS 2 and TaSe 2 [9, 10], suggesting an important role of substrate and many-body effects. In 1T -VSe 2 , reducing the number of layers by exfoliating bulk crystal leads at first to gradual decrease of T CDW , but at a critical film thickness of ∼ 10 nm, the T CDW exhibits a characteristic upturn and reaches 140 K in a few monolayers, much higher than bulk T CDW (110 K) [11]. Such enhancement of T CDW in monolayer VSe 2 was also revealed by angle-resolved photoemission spectroscopy (ARPES) wherein the role of FS nesting and electronphonon coupling was intensively debated [4,6,7,[12][13][14]. However, the nature of 2D CDW is still far from reaching a con...

Polymorphic Spin, Charge, and Lattice Waves in Vanadium Ditelluride

“…Since the late 1950s, both CDW and magnetic orderings have been separately reported in VTe 2 of various thicknesses;15–19,21 in the former studies, however, the powder and thin film‐based VTe 2 materials showed conflicting features regarding their lattice structure at room temperature 19,20,22. Recent studies of thin VTe 2 films show that the interlayer interaction plays a role for the structural changes 18,23,24. However, no study on possible polymorphic quantum states from high‐quality VTe 2 single crystals has been performed.…”

Observation of pressure induced charge density wave order and eightfold structure in bulk VSe2

“…1 a,d. 1 T -VSe 2 undergoes an incommensurate CDW transition around 110 K and commensurate CDW transition around 80 K 19 driven by the conventional Fermi surface nesting mechanism 20 or the newly proposed electron–phonon coupling 21 , forming a 4a × 4a × 3c superstructure as shown in Fig. 1 b,e.…”

Electronic structure and charge-density wave transition in monolayer VS2

“…On the other hand, bulk VSe2 has a CDW phase (105 K) with 4 × 4 × 3 periodicity which is attributed to FS nesting [8], while bulk VTe2 has strong CDW phase (482 K) with 4 × 4 × 3 periodicity [9]. Recent extensive studies have revealed that VSe2 and VTe2 exhibits transition from 3D nesting vector to 2D one, depending on the thickness, while the detailed role of thickness, interface, or stoichiometry remained controversial [16][17][18][19] In fact, huge attention was focused on vanadium dichalcogenides due to its theoretically calculated ferromagnetism in their monolayer (ML) form [20,21]. While extensive theoretical results predicted both 1H and 1T phase for ML VX2 (X=S, Se, Te) with spin-polarized band structures, most experiments show the 1T phase ML with non-magnetic band structures in the case of VSe2 and VTe2.…”