Understanding electron-mediated photon-phonon interactions from first principles*

Excitations of materials using ultrafast light pulses are conducive to non-equilibrium ("hot") electron distributions that interact with the underlying atomic lattice. In the case of metals, Allen showed thirty years ago [1] that the effect of these "hot" electrons is, to a good approximation, to warm up phonons homogeneously with a single characteristic timescale controlled by electron-phonon interactions. In semiconductors and low-dimensional materials, the reduced dielectric screening, and, in some cases, the higher lattice thermal conductivity weaken this hypothesis, hence calling for a more detailed physical picture.

In this talk, I will discuss our recent efforts in modeling the effect of "hot" electrons on lattice dynamics using first-principles calculations. First, I will describe our Boltzmann transport framework solving the coupled electron and phonon equations of motion [2], using first-principles calculations of the electron-phonon and phonon-phonon couplings. Using this approach, I will identify materials descriptors controlling the deviation from lattice thermal equilibrium caused by “hot” electrons, as measured in recent experiments [3,4]. Finally, I will discuss our time-dependent density functional theory-based framework for modeling the generation of coherent phonons, and its applications to the light-induced dynamics of charge density waves [5].

[1] P. B. Allen, "Theory of Thermal Relaxation of Electrons in Metals", Phys. Rev. Lett. 59, 1460 (1987)
[2] S. Sadasivam, M. K. Y. Chan, P. Darancet, "Theory of Thermal Relaxation of Electrons in Semiconductors" Phys. Rev. Lett. 119, 136602 (2017)
[3] P. Guo, et al., Nature Communications volume 9, 2792 (2018)
[4] I. Tung, et al., Nat. Photonics, Nature Photonics volume 13, p425–430(2019)
[5] A. Haldar, et al. (in preparation).