  We study the ground-state properties of spin-1/2 fermionic atoms confined in
a one-dimensional optical superlattice with harmonic confinement by using the
density matrix renormalization group method. For this purpose, we consider an
ionic Hubbard model that has superlattice potentials with 2-site periodicity.
We find that several different types of insulating regimes coexist even if the
number of atoms at each site is not an integer, but its average within the unit
cell is an integer or half integer. This is contrasted to the coexisting phase
of the metallic and Mott-insulating regimes known for the ordinary Hubbard
model in an optical lattice. The phase characteristics are elucidated by
investigating the profiles of the atom density, the local density/spin
fluctuations, the double occupation probability and the spin correlations in
detail.
