Excitonic Effects in the Photocarrier Dynamics of Two-Dimensional Materials.

Abstract

We investigate the role of excitonic correlations in shaping the ultrafast dynamics of photoexcited carriers in semiconductors. Conventional approaches describe relaxation within single-particle frameworks, where electron-electron and electron-phonon scattering drive thermalization toward Fermi-Dirac distributions, neglecting electron-hole correlations that dominate near band edges. We introduce a two-particle framework based on excitonic Bloch equations (XBEs) that captures carrier-phonon scattering and explicitly accounts for exciton formation. Applying this approach to nonresonantly photoexcited WSe₂ monolayers, we reveal qualitatively different carrier relaxation pathways: in contrast to state-of-the-art methods, XBEs predict enhanced intervalley scattering and dominant carrier population in Q valleys over K valleys, in agreement with time-resolved ARPES experiments. Moreover, the momentum distribution of thermalized carriers is shaped by exciton wave functions rather than by Fermi-Dirac statistics, signaling the formation of a correlated nonequilibrium state. These results establish excitonic correlations as a key mechanism governing photocarrier dynamics in excitonic materials.