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 omega(0) exhibits identical power-law scaling with the reduced temperature T/T-CDW and the reduced Cu content x/x(c), i.e., omega(0)similar to(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 associated with the reduction of the electron-phonon coupling constant, and that x-dependent quantum (T similar to 0) mode softening suggests the presence of a quantum critical point within the superconductor phase of CuxTiSe2.