The schematic geometry for Hall measurements in ambipolar
materials such as graphene is presented in Fig. 1. A longitudinal
(x-directed) electric current density Jx is imposed in the rectangular
sample, and a normal (z-directed) magnetic field Bz is applied.
Electrons and holes are deflected in the y-direction as shown in
the diagram, until a y-directed electric field Ey is built up to counter
their net lateral flow.