  Although charge density waves (CDWs) are omnipresent in cuprate
high-temperature superconductors, they occur at significantly different
wavevectors, confounding efforts to understand their formation mechanism. Here,
we use resonant inelastic x-ray scattering to investigate the doping- and
temperature-dependent CDW evolution in La2-xBaxCuO4 (x=0.115-0.155). We
discovered that the CDW develops in two stages with decreasing temperature. A
precursor CDW with quasi-commensurate wavevector emerges first at
high-temperature. This doping-independent precursor CDW correlation originates
from the CDW phase mode coupled with a phonon and "seeds" the low-temperature
CDW with strongly doping dependent wavevector. Our observation reveals the
precursor CDW and its phase mode as the building blocks of the highly
intertwined electronic ground state in the cuprates.
