Precursor region with full phonon softening above the charge-density-wave phase transition in 2H-TaSe2

Abstract

Research on charge-density-wave (CDW) ordered transition-metal dichalcogenides continues to unravel new states of quantum matter correlated to the intertwined lattice and electronic degrees of freedom. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of the canonical CDW compound 2H-TaSe₂ complemented by angle-resolved photoemission spectroscopy and density functional perturbation theory. Our results rule out the formation of a central-peak without full phonon softening for the CDW transition in 2H-TaSe₂ and provide evidence for a novel precursor region above the CDW transition temperature T_CDW, which is characterized by an overdamped phonon mode and not detectable in our photoemission experiments. Thus, 2H-TaSe₂ exhibits structural before electronic static order and emphasizes the important lattice contribution to CDW transitions. Our ab-initio calculations explain the interplay of electron-phonon coupling and Fermi surface topology triggering the CDW phase transition and predict that the CDW soft phonon mode promotes emergent superconductivity near the pressure-driven CDW quantum critical point.

The authors study the charge-density-wave (CDW) compound 2H-TaSe₂ by inelastic x-ray scattering combined with photoemission spectroscopy. They find evidence for a precursor region above the CDW transition temperature, which is characterized by an overdamped phonon mode and is not detectable by photoemission.