  The self-energy of nucleons in asymmetric nuclear matter is evaluated
employing different realistic models for the nucleon-nucleon interaction.
Starting from the Brueckner Hartree Fock approximation without the usual
angle-average in the two-nucleon propagator the effects of the hole-hole
contributions are investigated within the self-consistent Green's function
approach. Special attention is paid to the isospin-dependence of correlations,
which can be deduced from the spectral functions of nucleons in asymmetric
matter. The strong components of the proton - neutron interaction lead in
neutron-rich matter to a larger depletion for the occupation probability of
proton states below the Fermi momentum.
