Mapping momentum-dependent electron-phonon coupling and non-equilibrium phonon dynamics with ultrafast electron diffuse scattering

TL;DR

This paper demonstrates that ultrafast electron diffuse scattering (UEDS) can directly map momentum-dependent electron-phonon and phonon-phonon coupling across the Brillouin zone, revealing detailed phonon dynamics in graphite.

Contribution

The study introduces a novel application of UEDS to obtain momentum-resolved EPC and PPC information without energy resolution, providing new insights into phonon relaxation pathways.

Findings

UEDS maps EPC and PPC strength through phonon population changes.

Strong EPC observed at the K-point transverse optical phonon and along the Γ-K branch.

Phonon relaxation occurs in three stages involving decay, intraband thermalization, and interband relaxation.

Abstract

Despite their fundamental role in determining material properties, detailed momentum-dependent information on the strength of electron-phonon and phonon-phonon coupling (EPC and PPC, respectively) across the entire Brillouin zone (BZ) has proved difficult to obtain. Here we demonstrate that ultrafast electron diffuse scattering (UEDS) directly provides such information. By exploiting symmetry-based selection rules and time-resolution, scattering from different phonon branches can be distinguished even without energy resolution. Using graphite as a model system, we show that UEDS patterns map the relative EPC and PPC strength through their profound sensitivity to photoinduced changes in phonon populations. We measure strong EPC to the $K$-point transverse optical phonon of $A_1'$ symmetry ($K-A_1'$) and along the entire longitudinal optical branch between $\Gamma-K$, not only to the $\Gamma-E_{2g}$ phonon as previously emphasized. We also determine that the subsequent phonon relaxation pathway involves three stages; decay via several identifiable channels to transverse acoustic (TA) and longitudinal acoustic (LA) phonons (1-2 ps), intraband thermalization of the non-equilibrium TA/LA phonon populations (30-40 ps) and interband relaxation of the LA/TA modes (115 ps). Combining UEDS with ultrafast angle-resolved photoelectron spectroscopy will yield a complete picture of the dynamics within and between electron and phonon subsystems, helping to unravel complex phases in which the intertwined nature of these systems have a strong influence on emergent properties.