  Using the Hubbard chain at quarter filling as a model system, we study the
ground state properties of highly doped antiferromagnets. In particular, the
Hubbard chain at quarter filling is unstable against 2k_F- and 4k_F-periodic
potentials, leading to a large variety of charge and spin ordered ground
states. Employing the density matrix renormalization group method, we compare
the energy gain of the ground state induced by different periodic potentials.
For interacting systems the lowest energy is found for a 2k_F-periodic magnetic
field, resulting in a band insulator with spin gap. For strong interaction, the
4k_F-periodic potential leads to a half-filled Heisenberg chain and thus to a
Mott insulating state without spin gap. This ground state is more stable than
the band insulating state caused by any non-magnetic 2k_F-periodic potential.
Adding more electrons, a cluster-like ordering is preferred.
