  We report the results of a 3D particle-in-cell (PIC) simulation carried out
to study the early-stage evolution of the shock formed when an unmagnetized
relativistic jet interacts with an ambient electron-ion plasma. Full-shock
structures associated with the interaction are observed in the ambient frame.
When open boundaries are employed in the direction of the jet; the forward
shock is seen as a hybrid structure consisting of an electrostatic shock
combined with a double layer, while the reverse shock is seen as a double
layer. The ambient ions show two distinct features across the forward shock: a
population penetrating into the shocked region from the precursor region and an
accelerated population escaping from the shocked region into the precursor
region. This behavior is a signature of a combination of an electrostatic shock
and a double layer. Jet electrons are seen to be electrostatically trapped
between the forward and reverse shock structures showing a ring-like
distribution in a phase-space plot, while ambient electrons are thermalized and
become essentially isotropic in the shocked region. The magnetic energy density
grows to a few percent of the jet kinetic energy density at both the forward
and the reverse shock transition layers in a rather short time scale. We see
little disturbance of the jet ions over this time scale.
