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

Abstract

ABSTRACT: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 LaMnO 3 and electron-doped CaMnO 3 . 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 [MnO 6 ] 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. Nearestneighbor parameters are more than one order of magnitude larger than next nearestneighbor parameters. In LaMnO 3 the DE in the ab planes is Ϸ Ϫ0.26 eV. If there were no Jahn-Teller distortion present in the material, it would have been twice as large. In CaMnO 3 , the corresponding nearest-neighbor DE parameter for hopping of a doped electron in the ab planes is only Ϸ Ϫ0.17 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.

Fermiology of cuprates from first principles: From small pockets to the Luttinger Fermi surface

“…22͒ and the double-exchange in mixed-valence systems. 23 For undoped cuprates, with formally one 3d x 2 −y 2 electron per Cu site, the CAS wave function is similar to the variational wave function used in numerical studies of the 2D, one-band Hubbard model. 24 The main difference is that all integrals, including intersite Coulomb and exchange terms, are computed here in a totally ab initio way.…”

Ab initioabsorption spectrum of NiO combining molecular dynamics with the embedded cluster approach in a discrete reaction field

“…In the (effective) one-electron tight-binding models, the variation of the inphase and out-of-phase overlap of the atomic orbitals that form the Bloch orbitals causes a dispersion of the orbital energies, while in explicit n-electron treatments of the band structure, band dispersion arises from the variations of the hopping amplitudes along the different crystal directions. [112][113][114][115] Here, the broadening of the valence and conduction bands is caused by the inhomogeneity of the structure due to the movement of the nuclei, which introduces variations in the relative energies of the relevant n-electron states. Furthermore, the DRF method showed as an adequate approach to treat the polarization in crystalline environments under the condition that the largest polarization effects are treated in the wave function, and that the artificial polarization of the ground state is removed from the system.…”

Study of the Electronic Structure of NiGa2S4 and Extraction of the Spin Hamiltonian Parameters from Ab Initio Calculations

“…From this identification, both the relevant operators and the values of their corresponding interactions can be determined rigorously and univocally. Such a procedure has been used successfully to extract subtle interactions such as electron transfer, Coulomb repulsion, isotropic magnetic coupling, zero‐field splitting (ZFS) parameters, double exchange interactions, exchange transfer terms,, , and three‐ and four‐body terms , …”