  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. By assuming the enhancement of
electron-acoustic phonon supercollisions from the localized defect at the
photothermoelectric current-generating interface, we provide a broader
perspective to the ultrafast photoresponse of graphene, highlighting the
previously overlooked effect of the interface for cooling dynamics. We show
that the transient photothermoelectric response, which has been attributed
exclusively to supercollisions, can be successfully explained without rejecting
the established optical phonon relaxation pathway, demonstrating that the two
cooling mechanisms are not mutually exclusive but complement each other.
