Quantum and classical mode softening near the charge-density-wave/superconductor transition of Cu$_{x}$TiSe$_{2}$: Raman spectroscopic studies

TL;DR

This study uses Raman spectroscopy to investigate how charge density wave modes soften near the transition in Cu_xTiSe2, revealing universal scaling behavior and evidence of a quantum critical point within the superconducting phase.

Contribution

It demonstrates that mode softening follows a universal power-law scaling with temperature and Cu content, and links x-dependent softening to lattice expansion and a quantum critical point.

Findings

Mode frequency scales as (1 - p)^0.15 with p = T/T_CDW or x/x_c

X-dependent softening caused by reduced electron-phonon coupling

Evidence of a quantum critical point within the superconducting phase

Abstract

Temperature- and x-dependent Raman scattering studies of the charge density wave (CDW) amplitude modes in CuxTiSe2 show that the amplitude mode frequency omega_o exhibits identical power-law scaling with the reduced temperature, T/T_CDW, and the reduced Cu content, x/x_c, i.e., omega_o ~ (1 - p)^0.15 for p = T/T_CDW or x/x_c, suggesting that mode softening is independent of the control parameter used to approach the CDW transition. We provide evidence that x-dependent mode softening in CuxTiSe2 is caused by the reduction of the electron-phonon coupling constant lambda due to expansion of the lattice, and that x-dependent `quantum' (T ~ 0) mode softening reveals a quantum critical point within the superconductor phase of CuxTiSe2.