Oxygen Stripes inLa0.5Ca0.5MnO3fromAb InitioCalculations

We investigate the electronic, magnetic, and orbital properties of ${\mathrm{L}\mathrm{a}}_{0.5}{\mathrm{C}\mathrm{a}}_{0.5}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ 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 antiferromagneticaly 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.