  Photocurrent measurements provide a powerful means of studying the spatially
resolved optoelectronic and electrical properties of a material or device.
Generally speaking there are two classes of mechanism for photocurrent
generation: those involving separation of electrons and holes, and
thermoelectric effects driven by electron temperature gradients. Here we
introduce a new member in the latter class: the photo-Nernst effect. In
graphene devices in a perpendicular magnetic field we observe photocurrent
generated uniformly along the free edges, with opposite sign at opposite edges.
The signal is antisymmetric in field, shows a peak versus gate voltage at the
neutrality point flanked by wings of opposite sign at low fields, and exhibits
quantum oscillations at higher fields. These features are all explained by the
Nernst effect associated with laser-induced electron heating. This photo-Nernst
current provides a simple and clear demonstration of the Shockley-Ramo nature
of long-range photocurrent generation in a gapless material.
