Electronic Structure and Enhanced Charge-Density Wave Order of Monolayer VSe₂

Abstract

How the interacting electronic states and phases of layered transition-metal dichalcogenides evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe grown on bilayer graphene/SiC. While the global electronic structure is similar to that of bulk VSe, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below T = 140 ± 5 K, concomitant with the emergence of charge-order superstructures evident in low-energy electron diffraction. These observations point to a charge-density wave instability in the monolayer that is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of X-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density functional theory. Our study thus points to a delicate balance that can be realized between competing interacting states and phases in monolayer transition-metal dichalcogenides.

Controlling the Charge Density Wave Transition in Monolayer TiSe2: Substrate and Doping Effects

“…In some cases, the monolayer of these materials may exhibit new properties compared to their bulk counterpart, for example, ferromagnetic ordering has been observed in monolayer VSe 2 while the bulk is paramagnetic to low temperatures . Moreover the CDW order temperature may be different for mono‐ and few‐layer samples compared to that of the bulk …”

Van der Waals magnets: Wonder building blocks for two‐dimensional spintronics?

“…Several scanning tunneling microscopy (STM) (Figure B) and angle‐resolved photoemission (ARPES) studies (Figure C) have consistently confirmed the CDW order in MBE‐grown monolayer VSe 2 . The main controversy is however centered around its intrinsic magnetic state.…”

“…B, Atomic resolution STM image of monolayer VSe 2 . C, ARPES intensity map of monolayer VSe 2 in comparison with theoretical spin‐resolved and nonmagnetic electronic bands . D, In‐plane magnetic hysteresis of monolayer VSe 2 grown on MoS 2 substrate .…”

“…26 Several scanning tunneling microscopy (STM) ( Figure 5B) and angle-resolved photoemission (ARPES) studies ( Figure 5C) have consistently confirmed the CDW order in MBE-grown monolayer VSe 2 . 98,99,101,102 The main controversy is however centered around its intrinsic magnetic state. Bonilla et al and Duvjir et al indicated that a ferromagnetic state persists up to room temperature in monolayer VSe 2 , 99,101 but their reported magnetizations of 15 and μ B per V atom remain to be debatable and disagree with the calculated value of~0.6 μ B .…”

“…Bonilla et al and Duvjir et al indicated that a ferromagnetic state persists up to room temperature in monolayer VSe 2 , 99,101 but their reported magnetizations of 15 and μ B per V atom remain to be debatable and disagree with the calculated value of~0.6 μ B . 26 On the other hand, Feng et al showed using ARPES and X-ray magnetic circular dichroism (XMCD) that there are no exchange-split V electronic bands and ferromagnetically coupled V atoms down to 10 K in an applied magnetic field of 9 T. 98 Fumega and Pardo 103 and Coelho et al 104 performed DFT calculations by considering the effect of CDW and concluded that the ferromagnetic ground state of monolayer VSe 2 is on the verge of instability. The authors demonstrated that the structural rearrangement derived from the CDW causes an energy-gap opening at the Fermi level and in turn the quenching of intrinsic ferromagnetism in the monolayer limit.…”