van der Waals driven anharmonic melting of the 3D charge density wave in VSe2

Abstract

Understanding of charge-density wave (CDW) phases is a main challenge in condensed matter due to their presence in high-Tc superconductors or transition metal dichalcogenides (TMDs). Among TMDs, the origin of the CDW in VSe2 remains highly debated. Here, by means of inelastic x-ray scattering and first-principles calculations, we show that the CDW transition is driven by the collapse at 110 K of an acoustic mode at qCDW = (2.25 0 0.7) r.l.u. The softening starts below 225 K and expands over a wide region of the Brillouin zone, identifying the electron-phonon interaction as the driving force of the CDW. This is supported by our calculations that determine a large momentum-dependence of the electron-phonon matrix-elements that peak at the CDW wave vector. Our first-principles anharmonic calculations reproduce the temperature dependence of the soft mode and the TCDW onset only when considering the out-of-plane van der Waals interactions, which reveal crucial for the melting of the CDW phase.

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.…”

The stochastic self-consistent harmonic approximation: calculating vibrational properties of materials with full quantum and anharmonic effects

“…Frequency of the (μ, q) static approximation phonon from D (F) Page 15-1 Ω μ (q), Γ μ (q) Frequency and linewidth of the (μ, q) anharmonic phonon in the Lorentzian approximation equation (81) As made explicit in equation (7), only thermal effects on the ions are taken into account so far, whereas the electrons are considered at zero temperature. However, at very high temperatures the entropy associated to electrons may be important.…”

“…The force calculations needed for the SSCHA variational minimization can be performed at different theoretical levels or at different thermodynamic conditions, disentangling the driving forces of the instability. For instance, calculations within the SSCHA have enlightened the sensitivity of CDW transitions in monolayer TMDs to strain [51] and doping [54], the difference (or similarities) between the CDW transitions in bulk and the corresponding two-dimensional structures [51,54], as well as the importance of Van der Waals forces in the melting of CDW transitions [81]. Consequently the SSCHA program is expected to have a large impact on theoretical studies of CDW transitions for many type of materials, not just TMDs.…”

Observation of orbital order in the Van der Waals material 1T-TiSe2

“…Quasi two-dimensional Van der Waals (VdW) materials are layered solids with strong in-plane covalent bonding and weak interlayer VdW interactions, that have become a focal area for materials research in recent years [1][2][3][4][5][6][7][8][9][10]. The success of graphene -which stems from the paradigmatic VdW material graphitein particular, triggered a search for similar VdW materials that can be exfoliated to the monolayer limit, but which harbour physical properties beyond those of graphene [11,12].…”