Ab initio study of the CE magnetic phase in half-doped manganites: Purely magnetic versus double exchange description

The leading electronic interactions governing the local physics of the CE phase of half-doped manganites are extracted from correlated ab initio calculations performed on an embedded cluster. The electronic structure of the low-energy states is dominated by double exchange configurations and O-2$p_{\sigma}$ to Mn-3d charge transfer configurations. The model spectra of both a purely magnetic non-symmetric Heisenberg Hamiltonian involving a magnetic oxygen and two non-symmetric double exchange models are compared to the \textit{ab initio} one. While a satisfactory agreement between the Heisenberg spectrum and the calculated one is obtained, the best description is provided by a double exchange model involving excited non-Hund atomic states. This refined model not only perfectly reproduces the spectrum of the embedded cluster in the crystal geometry, but also gives a full description of the local double-well potential energy curve of the ground state (resulting from the interaction of the charge localized electronic configurations) and the local potential energy curves of all excited states ruled by the double exchange mechanism

Electron correlations and bond-length fluctuations in copper oxides: from Zhang--Rice singlets to correlation bags

We perform first principles, multiconfiguration calculations on clusters including several CuO$_6$ octahedra and study the ground-state electron distribution and electron--lattice couplings when holes are added to the undoped $d^9 p^6$ configuration. We find that the so-called Zhang--Rice state on a single CuO$_4$ plaquette is nearly degenerate with a state whose leading configuration is of the form Cu $d^9$-- O $p^5$-- Cu $d^9$. A strong coupling between the electronic and nuclear motion gives rise to large inter-site charge transfer effects for half-breathing displacements of the oxygen ions. Under the assumption of charge segregation into alternating hole-free and hole-rich stripes of Goodenough \cite{jbg_02,jbg_03}, our results seem to support the vibronic mechanism and the traveling charge-density wave model from Refs.\cite{jbg_02,jbg_03} for the superconductivity in copper oxides.Comment: submitted to Phys. Rev.

Renormalization of the quasiparticle hopping integrals by spin interactions in layered copper oxides

Holes doped within the square CuO2 network specific to the cuprate superconducting materials have oxygen 2p character. We investigate the basic properties of such oxygen holes by wavefunction-based quantum chemical calculations on large embedded clusters. We find that a 2p hole induces ferromagnetic correlations among the nearest-neighbor Cu 3d spins. When moving through the antiferromagnetic background the hole must bring along this spin polarization cloud at nearby Cu sites, which gives rise to a substantial reduction of the effective hopping parameters. Such interactions can explain the relatively low values inferred for the effective hoppings by fitting the angle-resolved photoemission data. The effect of the background antiferromagnetic couplings of renormalizing the effective nearest-neighbor hopping is also confirmed by density-matrix renormalization-group model Hamiltonian calculations for chains and ladders of CuO4 plaquettes

Electron correlations and bond-length fluctuations in layered copper oxides: electron versus hole doping

We investigate the nature of the electronic ground state and electron-lattice couplings for doped chains of CuO_4 plaquettes or CuO_6 octahedra. The undoped configuration implies here Cu 3d^9 and O 2p^6 formal valence states. The results of multiconfiguration calculations on 4-plaquette (or 4-octahedra) linear clusters indicate strong electron-lattice interactions and polaronic behavior of the doped particles, for both electron and hole doping. For certain phases of the oxygen-ion half-breathing distortions a multi-well energy landscape is predicted. Since each well is associated to carriers localized at different sites, the half-breathing displacements induce charge transfer along the chain. In the case of hole-doping, the trends found by ab initio multiconfiguration calculations on 4-octahedra clusters are confirmed by density-matrix renormalization-group calculations for a p-d, extended Hubbard model with chains of few tens of CuO_4 plaquettes. Under the assumption of charge separation and the formation of 1/3-doped stripes, our results seem to support the vibronic mechanism and the traveling charge-density wave scenario proposed in some recent contributions for superconductivity in copper oxides.Comment: references added, typos correcte

Ab initio study of the magnetic interactions in the spin-ladder compound SrCu2O3

A wide range of experimental, semiempirical, and theoretical values have been reported in the literature for the magnetic coupling parameters of the two-leg ladder compound SrCu2O3. We apply quantum chemical and density functional techniques to calculate accurate N-electron wave functions or densities for two different Cu2O7 clusters that represent the leg (Ji) and rung (J') of the ladder. Our data indicate that Ji is slightly larger than J' (J' /Ji'0.9) with J'52139 meV ~21670 K! and Ji52156 meV ~21870 K!. Recent experimental data indicate a more strongly anisotropic ratio, J' /Ji'0.5. The origin of the difference is unclear, as our ab initio estimates of J' and Ji seem to be converged with respect to the size of the basis set, the level of electron correlation, and the size of the cluster. However, we also find a surprisingly strong ferromagnetic interladder interaction which may play a role in resolving this discrepancy