Charge-density-wave quantum critical point under pressure in 2H-TaSe₂

Superconductivity often emerges as a dome around a quantum critical point (QCP) where long- range order is suppressed to zero temperature, e.g., in high-temperature superconductors. Revitalized by the discovery of charge-density-wave (CDW) order in copper oxides, scientific interest focuses on the interplay between CDW order and superconductivity in transition-metal dichalcogenides. Here, I will present our study of the prototypical CDW compound 2H-TaSe₂ using high-pressure x-ray diffraction (XRD) and inelastic x-ray scattering (IXS) to determine the CDW phase diagram and study its lattice dynamical properties. Our experimental results are complemented by ab-initio lattice dynamical calculations based on density-functional perturbation theory. Results at ambient pressure provide first evidence for a full phonon softening at the CDW transition in 2H-TaSe₂ and reveal a novel precursor region above the CDW transition temperature T_CDW [1]. High-pressure XRD and IXS reveals the presence of a CDW quantum critical point whereas calculations show that electron-phonon coupling in 2H-TaSe₂ is mostly carried by the CDW soft phonon mode and can rationalize the reported superconducting transition temperatures at high pressures.

[1] Shen et al., Nat Commun 14, 7282 (2023).