Oxygen stripes in La0.5Ca0.5MnO3 from ab initio calculations.

We investigate the electronic, magnetic, and orbital properties of La0.5Ca0.5MnO3 perovskite by means of an ab initio electronic structure calculation within the Hartree-Fock approximation. Using the experimental crystal structure reported by Radaelli et al. [Phys. Rev. B 55, 3015 (1997)]], we find a charge-ordering stripelike ground state. The periodicity of the stripes, and the insulating magnetic structure, consisting of antiferromagnetically coupled zigzag chains, are in agreement with neutron x-ray and electron diffraction experiments. However, the detailed structure is more complex than that envisaged by simple models of charge and orbital order on Mn d levels alone, and is better described as a charge-density wave of oxygen holes, coupled to the Mn spin/orbital order.