Charge-density-wave quantum critical point under pressure in 2$H$-TaSe$_2$

TL;DR

This study investigates how applying pressure suppresses charge-density waves in 2H-TaSe2 and how this suppression relates to the enhancement of superconductivity near a quantum critical point, using advanced X-ray techniques and calculations.

Contribution

It provides detailed experimental and theoretical insights into the suppression of charge-density waves and the emergence of superconductivity near a quantum critical point in 2H-TaSe2.

Findings

Charge-density-wave state is suppressed at 19.9 GPa.

Quantum critical point is near the maximum superconducting temperature.

Ab-initio calculations support the experimental observations.

Abstract

Suppressing of an ordered state that competes with superconductivity is one route to enhance superconducting transition temperatures. Whereas the effect of suppressing magnetic states is still not fully understood, materials featuring charge-density waves and superconductivity offer a clearer scenario as both states can be associated with electron-phonon coupling. Metallic transition-metal dichalcogenides are prime examples for such intertwined electron-phonon-driven phases, yet, various compounds do not show the expected interrelation or feature additional mechanisms which makes an unambiguous interpretation difficult. Here, we report high-pressure X-ray diffraction and inelastic X-ray scattering measurements of the prototypical transition-metal dichalcogenide 2$H$-TaSe$_2$ and determine the evolution of the charge-density-wave state and its lattice dynamics up to and beyond its suppression at the critical pressure $p_c = 19.9(1)\,\rm{GPa}$ and at low temperatures. The high quality of our data allows the full refinement of the commensurate charge-density-wave superstructure at low pressure and we find the quantum critical point of the charge-density-wave to be in close vicinity to the reported maximum superconducting transition temperature $T_{sc} = 8.2\,\rm{K}$. $Ab-initio$ calculations corroborate that 2$H$-TaSe$_2$ is a reference example of order-suppressed enhanced superconductivity and can serve as a textbook case to investigate superconductivity near a charge-density-wave quantum critical point.