  Besides the giant peak of the nonlocal resistance $R_{NL}$, an anomalous
negative value of $R_{NL}$ has been observed in graphene systems, while its
formation mechanism is not quite understood yet. In this work, utilizing the
non-equilibrium Green's function method, we calculate the local-current flow in
an H-shaped non-interacting graphene system locating in the ballistic regime.
Similar to the previous conclusions made from the viscous regime, the numerical
results show that a local-current vortex appears between the nonlocal measuring
terminals, which induces a backflow current and a remarkable negative voltage
drop at the probe. Specifically, the stronger the vortex exhibits, the more
negative $R_{NL}$ manifests. Besides, a spin-orbital coupling is added as an
additional tool to study this exotic vortex, which is not a driving force for
the arising vortex at all. Moreover, a breakdown of the nonlocal
Wiedemann-Franz law is obtained in this ballistic system, and two experimental
criteria are further provided to confirm the existence of this exotic vortex.
Notably, a discussion is made that the vortex actually originates from the
collision between the flowing current and the boundaries, due to the long
electron mean free path and the consequent ballistic transport caused by the
specific linear spectrum of graphene.
