  Coherent phonons, light-induced coherent lattice vibrations in solids,
provide a powerful route to engineer structural and electronic degrees of
freedom using light. In this manuscript, we formulate an ab initio theory of
the displacive excitation of coherent phonons (DECP), the primary mechanism for
light-induced structural control in semimetals. Our study - based on the ab
initio simulations of the ultrafast electron and coherent-phonon dynamics in
presence of electron-phonon interactions - establishes a predictive
computational framework for describing the emergence of light-induced
structural changes and the ensuing transient band-structure renormalization
arising from the DECP mechanism. We validate this framework via a combined
theoretical and experimental investigation of coherent phonons in the elemental
semimetal antimony. Via a Fourier analysis of time- and angle-resolved
photoemission spectroscopy (tr-ARPES) measurements, we retrieve information
about transient spectral features and quasiparticle renormalization arising
from the coherent A1g phonon as a function of momentum, energy, time, and
fluence. The qualitative and quantitative agreement between experiment and
theory corroborates the first-principles approach formulated in this study.
Besides advancing the fundamental understanding of electron-phonon interactions
mediated by coherent phonons, this study opens new opportunities for
predictively engineering structural and electronic degrees of freedom in
semimetals via the DECP mechanism.
