  We address the tunneling current in a graphene-hBN-graphene heterostructure
as function of the twisting between the crystals. The twisting induces a
modulation of the hopping amplitude between the graphene layers, that provides
the extra momentum necessary to satisfy momentum and energy conservation and to
activate coherent tunneling between the graphene electrodes. Conservation rules
limit the tunneling to states with wavevectors lying at the conic curves
defined by the intersection of two Dirac cones shifted in momentum and energy.
There is a critical voltage where the intersection is a straight line, and the
joint density of states presents a maximum. This reflects in a peak in the
tunneling current and in a negative differential conductivity.
