Hopping matrix elements from first-principles studies of overlapping fragments: Double exchange parameters in manganites

We recently developed a scheme for first-principles calculations of hopping matrix elements between localized states in extended systems. We apply the scheme to the determination of double exchange (DE) parameters in lightly hole-doped LaMnO3 and electron-doped CaMnO3. DE is one of the important factors for understanding the properties of doped manganites. The calculations are based on the construction of wave functions for localized hole states or localized electron states for large embedded clusters. The wave functions of these clusters are expressed in terms of localized orbitals, obtained from calculations on smaller units, or fragments, centered around a transition metal ion. The starting point of electronic states expressed in terms of localized orbital sets is conceptually attractive. It also allows for a rigorous treatment of local electron correlation and electronic relaxation effects. In the present study, the fragments are embedded [MnO6] units. The large clusters contain either two or four Mn ions and all neighboring oxygen ligands. The results are compared with conventional embedded cluster calculations. In both compounds, the effective hopping matrix elements, or double exchange (DE) parameters, in the ab planes (in the Pbnm space group) are larger than along the c axes. We found nearly perfect agreement with the Anderson-Hasegawa model for the spin dependence of the DE parameters. Nearest-neighbor parameters are more than one order of magnitude larger than next nearest-neighbor parameters. In LaMnO3 the DE in the ab planes is approximate to -0.26 eV. If there were no Jahn-Teller distortion present in the material, it would have been twice as large. In CaMnO3, the corresponding nearest-neighbor DE parameter for hopping of a doped electron in the ab planes is only approximate to -0.17 eV, due to the antiparallel spin coupling. However, since this interaction is much larger than the exchange coupling, we suggest that it induces local ferromagnetic clusters around the doped electrons. (C) 2006 Wiley Periodicals, Inc.