  Experimental signatures of charge density waves (CDW) in high-temperature
superconductors have evoked much recent interest, yet an alternative
interpretation has been theoretically raised based on electronic standing waves
resulting from quasiparticles scattering off impurities or defects, also known
as Friedel oscillations (FO). Indeed the two phenomena are similar and related,
posing a challenge to their experimental differentiation. Here we report a
resonant X-ray diffraction study of ZrTe$_3$, a model CDW material. Near the
CDW transition, we observe two independent diffraction signatures that arise
concomitantly, only to become clearly separated in momentum while developing
very different correlation lengths in the well-ordered state. Anomalously slow
dynamics of mesoscopic ordered nanoregions are further found near the
transition temperature, in spite of the expected strong thermal fluctuations.
These observations reveal that a spatially-modulated CDW phase emerges out of a
uniform electronic fluid via a process that is promoted by self-amplifying FO,
and identify a viable experimental route to distinguish CDW and FO.
