Giant magneto-photoelectric effect in suspended graphene

We study the optical response of a suspended graphene field-effect transistor in magnetic fields of up to 9 T (quantum Hall regime). With an illumination power of only 3 {\mu}W, we measure a photocurrent of up to 400 nA, corresponding to a photo-responsivity of 0.14 A/W, which we believe to be the highest value ever measured in single-layer graphene. We estimate that every absorbed photon creates more than 8 electron-hole pairs, which demonstrates highly effective carrier multiplication. As suggested by the dependence of the photocurrent on gate voltage and magnetic field, we propose a ballistic two-stage mechanism where the incident photons create primary charge carriers which then excite secondary charge carriers in the chiral edge states via Auger-type inelastic Coulomb scattering processes at the graphene edge.