Electronic structure of the Ca3Co4O9\rm Ca_3Co_4O_9 compound from ab initio local interactions

We used fully correlated ab initio calculations to determine the effective parameters of Hubbard and t - J models for the thermoelectric misfit compound $\rm Ca_3Co_4O_9$. As for the $\rm Na_xCoO_2$ family the Fermi level orbitals are the $a_{1g}$ orbitals of the cobalt atoms ; the $e'_g$ being always lower in energy by more than 240\,meV. The electron correlation is found very large $U/t\sim 26$ as well as the parameters fluctuations as a function of the structural modulation. The main consequences are a partial $a_{1g}$ electrons localization and a fluctuation of the in-plane magnetic exchange from AFM to FM. The behavior of the Seebeck coefficient as a function of temperature is discussed in view of the ab initio results, as well as the 496\,K phase transition

An ab-initio evaluation of the local effective interactions in the superconducting compound Na_0.35CoO_2−1.3H_2O\rm Na\_{0.35} Co O\_2-1.3H\_2O

We used ab-initio quantum chemical methods, treating explicitly the strong correlation effects within the cobalt 3d shell, as well as the screening effects on the effective integrals, for accurately determining on-site and nearest-neighbor (NN) interactions in the $\rm Na\_{0.35} Co O\_2-1.3H\_2O$ superconducting compound. The effective ligand field splitting within the $t\_{2g}$ orbitals was found to be $\delta \sim 300 \rm meV$, the $a\_{1g}$ orbital being destabilized compared to the $e\_g^\prime$ ones. The effective Hund's exchange and Coulomb repulsion were evaluated to $J\_H\sim 280 \rm meV$ and $U\sim 4.1$--$4.8 \rm eV$. The NN hopping parameters were determined within the three $t\_{2g}$ orbitals and found to be of the same order of magnitude as the $t\_{2g}$ ligand field splitting, supporting the hypothesis of a three band model for this system. Finally we evaluated the NN effective exchange integral to be antiferromagnetic and $J=-66 \rm meV$

Similarities between the t−Jt-J and Hubbard models in weakly correlated regimes

We present a comparative study of the Hubbard and $t-J$ models far away from half-filling. We show that, at such fillings the $t-J$ Hamiltonian can be seen as an effective model of the repulsive Hubbard Hamiltonian over the whole range of correlation strength. Indeed, the $|t/U| \in [0,+\infty [$ range of the Hubbard model can be mapped onto the finite range $|J/t| \in [1, 0 ]$ of the $t-J$ model, provided that the effective exchange parameter $J$ is defined variationally as the local singlet-triplet excitation energy. In this picture the uncorrelated limit U=0 is associated with the super-symmetric point $J=-2|t|$ and the infinitely correlated $U=+\infty$ limit with the usual J=0 limit. A numerical comparison between the two models is presented using different macroscopic and microscopic properties such as energies, charge gaps and bond orders on a quarter-filled infinite chain. The usage of the $t-J$ Hamiltonian in low-filled systems can therefore be a good alternative to the Hubbard model in large time-consuming calculations.Comment: To be published in EPJB. 6 pages. 5 figure

Understanding interface effects in perovskite thin films

The control of matter properties (transport, magnetic, dielectric,...) using synthesis as thin films is strongly hindered by the lack of reliable theories, able to guide the design of new systems, through the understanding of the interface effects and of the way the substrate constraints are imposed to the material. The present paper analyses the energetic contributions at the interfaces, and proposes a model describing the microscopic mechanisms governing the interactions at an epitaxial interface between a manganite and another transition metal oxide in perovskite structure (as for instance $\rm SrTiO_3$). The model is checked against experimental results and literature analysis

Fractionnally charged excitations in the charge density wave state of a quarter-filled t-J chain with quantum phonons

Elementary excitations of the 4k$_F$ charge density wave state of a quarter-filled strongly correlated electronic one-dimensional chain are investigated in the presence of dispersionless quantum optical phonons using Density Matrix Renormalization Group techniques. Such excitations are shown to be topological unbound solitons carrying charge $e/2$. Relevance to the 4k$_F$ charge density wave instability in $\rm (DI-DCNQI)_2Ag$ or recently discovered in (TMTTF)$_2$X (X=PF$_6$, AsF$_6$) is discussed.Comment: 4 pages, 4 figure

An ab-initio evaluation of the local effective interactions in the NaxCoO2\rm Na_{x} Co O_2 familly

We used quantum chemical ab initio methods to determine the effective parameters of Hubbard and $t-J$ models for the $\rm Na_{x}CoO_2$ compounds (x=0 and 0.5). As for the superconducting compound we found the $a_{1g}$ cobalt orbitals above the $e_g^\prime$ ones by a few hundreds of meV due to the $e_g^\prime$--$e_g$ hybridization of the cobalt $3d$ orbitals. The correlation strength was found to increase with the sodium content $x$ while the in-plane AFM coupling decreases. The less correlated system was found to be the pure $CoO_2$, however it is still strongly correlated and very close to the Mott transition. Indeed we found $U/t\sim 15$, which is the critical value for the Mott transition in a triangular lattice. Finally, one finds the magnetic exchanges in the $\rm CoO_2$ layers, strongly dependant of the weak local structural distortions

First principles calculation of the phonons modes in the hexagonal YMnO3\rm YMnO_3 ferroelectric and paraelectric phases

The lattice dynamics of the $\rm YMnO_3$ magneto-electric compound has been investigated using density functional calculations, both in the ferroelectric and the paraelectric phases. The coherence between the computed and experimental data is very good in the low temperature phase. Using group theory, modes continuity and our calculations we were able to show that the phonons modes observed by Raman scattering at 1200K are only compatible with the ferroelectric $P6_{3} cm$ space group, thus supporting the idea of a ferroelectric to paraelectric phase transition at higher temperature. Finally we proposed a candidate for the phonon part of the observed electro-magnon. This mode, inactive both in Raman scattering and in Infra-Red, was shown to strongly couple to the Mn-Mn magnetic interactions

Fast calculation of the electrostatic potential in ionic crystals by direct summation metho

An efficient real space method is derived for the evaluation of the Madelung's potential of ionic crystals. The proposed method is an extension of the Evjen's method. It takes advantage of a general analysis for the potential convergence in real space. Indeed, we show that the series convergence is exponential as a function of the number of annulled multipolar momenta in the unit cell. The method proposed in this work reaches such an exponential xconvergence rate. Its efficiency is comparable to the Ewald's method, however unlike the latter, it uses only simple algebraic functions