Coherent Phonons and Quasiparticle Renormalization in Semimetals from First Principles

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 theory of the displacive excitation of coherent phonons (DECP), the primary mechanism for light-induced structural control in semimetals. Our study—based on the simulations of the ultrafast electron and coherent-phonon dynamics in the 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 measurements, we retrieve information about transient spectral features and quasiparticle renormalization arising from the coherent <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>A</a:mi><a:mrow><a:mn>1</a:mn><a:mi>g</a:mi></a:mrow></a:msub></a:math> 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. We further apply this formalism to investigate the coherent-phonon dynamics in the topological Weyl semimetal -<c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi>WTe</c:mi></c:mrow><c:mn>2</c:mn></c:msub></c:mrow></c:math>. Besides reproducing the entire spectrum of coherent phonons observed in experiments, our simulations clearly indicate that the shear <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>A</e:mi><e:mrow><e:mn>1</e:mn><e:mi>g</e:mi></e:mrow></e:msub></e:math> mode—the mode orchestrating a light-induced phase transition in -<g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:msub><g:mrow><g:mi>WTe</g:mi></g:mrow><g:mn>2</g:mn></g:msub></g:mrow></g:math>—is strongly driven by the DECP mechanism and, thus, provide a conclusive explanation for the driving mechanism underpinning the phase transition. 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.