  Properties of symmetric and asymmetric nuclear matter have been investigated
in the relativistic Dirac-Brueckner-Hartree-Fock approach based on projection
techniques using the Bonn A potential. The momentum, density, and isospin
dependence of the optical potentials and nucleon effective masses are studied.
It turns out that the isovector optical potential depends sensitively on
density and momentum, but is almost insensitive to the isospin asymmetry.
Furthermore, the Dirac mass $m^*_D$ and the nonrelativistic mass $m^*_{NR}$
which parametrizes the energy dependence of the single particle spectrum, are
both determined from relativistic Dirac-Brueckner-Hartree-Fock calculations.
The nonrelativistic mass shows a characteristic peak structure at momenta
slightly above the Fermi momentum $\kf$. The relativistic Dirac mass shows a
proton-neutron mass splitting of $m^*_{D,n} <m^*_{D,p}$ in isospin asymmetric
nuclear matter. However, the nonrelativistic mass has a reversed mass splitting
$m^*_{NR,n} >m^*_{NR,p}$ which is in agreement with the results from
nonrelativistic calculations.
