Anharmonicity Reveals the Tunability of the Charge Density Wave Orders in Monolayer VSe₂

Abstract

VSe 2 is a layered compound that has attracted great attention due to its proximity to a ferromagnetic state that is quenched by its charge density wave (CDW) phase. In the monolayer limit, unrelated experiments have reported different CDW orders with different transition temperatures, making this monolayer very controversial. Here we perform first-principles nonperturbative anharmonic phonon calculations in monolayer VSe 2 in order to estimate the CDW order and the corresponding transition temperature. They reveal that monolayer VSe 2 develops two independent charge density wave orders that compete as a function of strain. Variations of only 1.5% in the lattice parameter are enough to stabilize one order or the other. Moreover, we analyze the impact of external Lennard-Jones interactions, showing that these can act together with anharmonicity to suppress the CDW orders. Our results solve previous experimental contradictions, highlighting the high tunability and substrate dependency of the CDW orders of monolayer VSe 2 .

2D Vanadium Sulfides: Synthesis, Atomic Structure Engineering, and Charge Density Waves

“…They underscore both the ubiquity of CDWs and structural phase transitions in low-dimensional vanadium chalcogenides and the metastability of any particular periodicity. The situation seems to be analogous to the case of few-layer compounds of the vanadium-based materials VSe 2 − and VTe 2 , , which host a plenitude of CDW phases (e.g., 4 × 4, , 5 × 1, and ), depending on the substrate and the number of layers.…”

Electronic structure and lattice dynamics of 1T−VSe2 : Origin of the three-dimensional charge density wave

“…At pressures P > 13 GPa, a structural transition towards a C2/m symmetry phase has been reported, concomitant with the emergence of SC [45]. The complex picture reported in bulk VSe 2 extends to the monolayer limit, where distinct competing charge orders with a significant range of transition temperatures have been observed [46,47], a behavior that is explained by the subtle anharmonic condensation of different phonon modes [48]. Interestingly, the formation of the CDW in the VSe 2 monolayer also competes with ferromagnetism [49].…”

“…All functionals yield accurate frequencies for the ω 2 phonon branch. The behavior of the frequencies with respect to the choice of functionals resembles the situation in the monolayer limit [48], in which the inclusion of the nonlocal vdW interactions in the exchange-correlation functional was mandatory to obtain a good description of the transition to the CDW phase.…”

“…The, a priori, more sophisticated vdW-DF functional proposed by Dion et al [79], which is truly nonlocal, is plotted with red squares and succeeds in melting the CDW, but overestimates the critical temperature by approximately 80 K. The lower critical temperatures obtained with Grimme's semiempirical vdW interactions might be due to the fact that it provides larger forces than the nonlocal vdW functional. It is interesting to remark, however, that the nonlocal vdW functional reproduces the CDW physics much better than the semiempirical approach in the VSe 2 monolayer [48], which is unable to predict the existence of the CDW transitions. The reason may be that Grimme's semiempirical approach is rather elemental and is probably introducing some arguably spurious intralayer contributions in supercell forces and energies that is slightly overestimating the contribution of vdW corrections.…”

Self-doped flat band and spin-triplet superconductivity in monolayer 1T-TaSe2−x Te x

“…The electronic filling of this flat band can be controlled with an external electric gate between the layers, allowing the exploration of a rich phase diagram where unconventional superconducting states emerge at certain fillings [18,19]. Apart from that, the low dimensionality of van der Waals materials enhances the emergence of symmetry-broken orders such as charge density waves (CDWs) that can be coupled to other phases [20][21][22]. Remarkably, CDW phases can induce reconstructions in the energy levels and create flat bands.…”