  Raman scattering in the three-dimensional Dirac semimetal Cd3As2 shows an
intricate interplay of electronic and phonon degrees of freedom. We observe
resonant phonon scattering due to interband transitions, an anomalous
anharmonicity of phonon frequency and intensity, as well as quasielastic (E~0)
electronic scattering. The latter two effects are governed by a characteristic
temperature scale T* ~ 100 K that is related to mutual fluctuations of lattice
and electronic degrees of freedom. A refined analysis shows that this
characteristic temperature corresponds to the energy of optical phonons which
couple to interband transitions in the Dirac states of Cd3As2. As
electron-phonon coupling in a topological semimetal is primarily related to
phonons with finite momenta, the back action on the optical phonons in only
observed as anharmonicities via multi-phonon processes involving a broad range
of momenta. The resulting energy density fluctuations of the coupled system
have previously only been observed in low dimensional or frustrated spin
systems with suppressed long range ordering.
