Giant Intrinsic Photoresponse in Pristine Graphene

When the Fermi level matches the Dirac point in graphene, the reduced charge screening can dramatically enhance electron-electron (e-e) scattering to produce a strongly interacting Dirac liquid. While the dominance of e-e scattering already leads to novel responses such as the electron hydrodynamic flow, further exotic phenomena have been predicted to arise specifically from the unique kinematics of e-e scattering in a strongly interacting massless Dirac system, although their experimental evidence remains lacking. Here, by using optoelectronic probes, which are highly sensitive to the kinematics of electron scattering, we uncover a large, previously uncharacterized photocurrent even in the absence of defined p-n junctions, which arises uniquely from the strong, unusual electron interaction in charge-neutral graphene. This photocurrent emerges exclusively at the charge neutrality point and vanishes abruptly at non-zero charge densities. It is observed at places with broken reflection symmetry, and is selectively brightened at free graphene edges with sharp bends. Our findings reveal that the photocurrent relaxation is strongly suppressed by a drastic change of fast photocarrier kinematics in graphene when its Fermi level matches the Dirac point.