Localized defect-assisted acoustic phonon scattering of hot carriers in graphene

The broadband and ultrafast photoresponse of graphene has been extensively studied in recent years, although the photoexcited carrier dynamics is still far from being completely understood. Different experimental approaches imply either one of two fundamentally different scattering mechanisms for hot electrons. One is high-energy optical phonons, while the other is disorder-driven supercollisions with acoustic phonons. However, the concurrent relaxation via both optical and acoustic phonons has not been considered so far, hindering the interpretation of different experiments within a unified framework. Here we expand the optical phonon-mediated cooling model, to include electron scattering with the acoustic phonons, as well as the non-zero graphene Fermi level. By assuming the contribution of electron-acoustic phonon scattering enhanced by the localized defect at the photothermoelectric current-generating interface, we highlight the previously overlooked effect of the interface for cooling dynamics, and provide a theoretical basis for the ultrafast photoresponse of graphene. We show that the transient photothermoelectric response in graphene, which has been attributed exclusively to supercollisions, can be successfully explained without appealing to the intrinsic disorder of graphene. By resolving the inconsistency between the previously suggested two models, and explaining all scenarios in a single theoretical framework, the proposed model will propel the practical investigation of graphene photoresponse in general, and assist the study of hot carrier dynamics in particular.