  The stationary Josephson current in a ballistic graphene system is
theoretically studied with focus on a planar junction consisting of a monolayer
graphene sheet on top of which a pair of superconducting electrodes is
deposited. To characterize such a planar junction, we employ two parameters:
the coupling strength between the graphene sheet and the superconducting
electrodes, and a potential drop induced in the graphene sheet by direct
contact with the electrodes. We derive a general formula for the Josephson
current by taking these parameters into account in addition to other basic
parameters, such as temperature and chemical potential. The resulting formula
applies to a wide range of parameters and reproduces previously reported
results in certain limits.
