Quantum and classical mode softening near the charge-density-wave- superconductor transition of CuxTiSe2

Temperature- and x-dependent Raman scattering studies of the charge-density-wave (CDW) amplitude modes in CuxTiSe2 show that the amplitude mode frequency ω0 exhibits identical power-law scaling with the reduced temperature T/TCDW and the reduced Cu content x/xc, i.e., ω0∼(1-p)0.15 for p=T/TCDW or x/xc, 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 associated with the reduction of the electron-phonon coupling constant, and that x-dependent "quantum" (T∼0) mode softening suggests the presence of a quantum critical point within the superconductor phase of CuxTiSe2.