The zone-center $E_{2g}$ modes play a crucial role in ${\mathrm{MgB}}_2$, controlling the scattering mechanisms in the normal state as well the superconducting pairing. Here, we demonstrate via first-principles quantum-field theory calculations that, due to the anisotropic electron-phonon interaction, a hot-phonon regime where the $E_{2g}$ phonons can achieve significantly larger effective populations than other modes, is triggered in ${\mathrm{MgB}}_2$ by the interaction with an ultrashort laser pulse. Spectral signatures of this scenario in ultrafast pump-probe Raman spectroscopy are discussed in detail, revealing also a fundamental role of nonadiabatic processes in the optical features of the $E_{2g}$ mode.

• Received 6 April 2019
• Revised 25 October 2019
• Accepted 28 January 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.077001