  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.
