Unique Gap Structure and Symmetry of the Charge Density Wave in Single-Layer VSe2

Abstract

Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the quasi-twodimensional nature of TMDCs; a different CDW symmetry is unexpected. We report herein the successful creation of pristine single-layer VSe2, which shows a (√7 × √3) CDW in contrast to the (4 × 4) CDW for the layers in bulk VSe2. Angle-resolved photoemission spectroscopy (ARPES) from the single layer shows a sizable (√7 × √3) CDW gap of ~100 meV at the zone boundary, a 220 K CDW transition temperature twice the bulk value, and no ferromagnetic exchange splitting as predicated by theory. This robust CDW with an exotic broken symmetry as the ground state is explained via a first-principles analysis. The results illustrate a unique CDW phenomenon in the two-dimensional limit.

Proximity-induced ferromagnetism and chemical reactivity in few-layer VSe2 heterostructures

“…Among dimensionality effects observed in TMDs, metallic VSe2 is a paradigmatic case. While in its bulk form it displays the onset of charge density waves (CDW) at 110 K [12][13][14], in the 2D limit a CDW with enhanced TC and coupling strength is observed [15][16][17] (sometimes even considered a metal-insulator transition [15]), and with a different pattern of atomic displacements to the bulk [18,19]. Moreover, its dimensionalitydependent magnetic properties are still under debate.…”

Correlation between crystal purity and the charge density wave in 1T−VSe2

“…Control of the distorted lattice periodicity is also possible by strain engineering, and an unconventional (4a x √ 3a) CDW has been observed by STM [19]. At the monolayer limit, there have been reports of possible ferromagnetism [10][11][12], strongly enhanced CDW order evidenced by a fully-gapped Fermi surface [13], and an increase of the transition temperature up to T CDW = 220 K [14].…”

Novel Superstructure-Phase Two-Dimensional Material 1T-VSe2 at High Pressure

“…Furthermore, in low-dimensional systems, the formation of a super-lattice is usually associated with either structural distortion (e.g., electron-phonon coupling) or Fermi-surface nesting. At ambient conditions, vanadium diselenide possesses a type II CDW with structural distortion (electron-phonon coupling) 18,31 . However, the high-pressure magneto-resistance and…”