  The microscopic mechanisms of the symmetry energy in nuclear matter are
investigated in the framework of the relativistic Brueckner-Hartree-Fock (RBHF)
model with a high-precision realistic nuclear potential, pvCDBonn A. The
kinetic energy and potential contributions to symmetry energy are decomposed.
They are explicitly expressed by the nucleon self-energies, which are obtained
through projecting the $G$-matrices from the RBHF model into the terms of
Lorentz covariants. The nuclear medium effects on the nucleon self-energy and
nucleon-nucleon interaction in symmetry energy are discussed by comparing the
results from the RBHF model and those from Hartree-Fock and relativistic
Hartree-Fock models. It is found that the nucleon self-energy including the
nuclear medium effect on the single-nucleon wave function provides a largely
positive contribution to the symmetry energy, while {the nuclear medium effect
on the nucleon-nucleon interaction, i.e., the effective $G$-matrices generates
the negative contribution}. The tensor force plays an essential role in the
symmetry energy around the density. The scalar and vector covariant amplitudes
of nucleon-nucleon interaction dominate the potential component of the symmetry
energy. Furthermore, the isoscalar and isovector terms in the optical potential
are extracted from the RBHF model. The isoscalar part is consistent with the
results from the analysis of global optical potential, while the isovector one
has obvious differences at higher incident energy due to the relativistic
effect.
