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

To characterize in detail the charge density wave (CDW) transition of 1⁢𝑇−VSe₂, its electronic structure and lattice dynamics are comprehensively studied by means of x-ray diffraction, muon spectroscopy, angle resolved photoemission (ARPES), diffuse and inelastic x-ray scattering, and state-of-the-art first-principles density functional theory calculations. Resonant elastic x-ray scattering does not show any resonant enhancement at either V or Se, indicating that the CDW peak at the 𝐾 edges describes a purely structural modulation of the electronic ordering. ARPES experiments identify (i) a pseudogap at 𝑇 > 𝑇⁢_CDW, which leads to a depletion of the density of states in the ML-M'L' plane at 𝑇 < 𝑇⁢_CDW, and (ii) anomalies in the electronic dispersion reflecting a sizable impact of phonons on it. A diffuse scattering precursor, characteristic of soft phonons, is observed at room temperature (RT) and leads to the full collapse of the low-energy phonon (𝜔₁) with propagation vector (0.25 0 −0.3) r.l.u. Here, we show that the frequency and linewidth of this mode are anisotropic in momentum space, reflecting the momentum dependence of the electron-phonon interaction (EPI), hence demonstrating that the origin of the CDW is, to a much larger extent, due to the momentum dependent EPI with a small contribution from nesting. The pressure dependence of the 𝜔₁ soft mode remains nearly constant up to 13 GPa at RT, with only a modest softening before the transition to the high-pressure monoclinic C2/m phase. The wide set of experimental data is well captured by our state-of-the art first-principles anharmonic calculations with the inclusion of van der Waals corrections in the exchange-correlation functional. The comprehensive description of the electronic and dynamical properties of VSe₂ reported here adds important pieces of information to the understanding of the electronic modulations in the family of transitio. Authors: Diego, Josu [Univ. of the Basque Country (UPV/EHU), San Sebastian (Spain); Centro de Física de Materiales (CSIC-UPV/EHU), San Sebastian (Spain)] (ORCID:0000000286592144); Subires, David [Donostia International Physics Center (DIPC), San Sebastian (Spain)]; Said, A. H. [Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)]; Chaney, D. A. [European Synchrotron Radiation Facility (ESRF), Grenoble (France)]; Korshunov, A. [European Synchrotron Radiation Facility (ESRF), Grenoble (France)]. DOE Contract: AC02-06CH11357. Subjects: 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY