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

A 2D quantum antiferromagnet with a four-fold degenerate valence-bond-solid ground state

We study the competition between antiferromagnetic order and valence bond solid formation in a two-dimensional frustrated spin-1/2 model. The J1-J2 model on the square lattice is further frustrated by introducing products of three-spin projectors which stabilize four dimer-product states as degenerate ground states. These four states are reminiscent of the dimerized singlet ground state of the Shastry-Sutherland model. Using exact diagonalizations, we study the evolution of the ground state by varying theratio of interactions. For a large range of parameters (J2 > 0.25J1), our model shows a direct transition between the valence-bond-solid phase and the collinear antiferromagnetic phase. For small values of J2, several intermediate phases appear which are also analyzed

Charge order in the incommensurate compounds Sr_14−xCa_xCu_24O_41Sr\_{14-x}Ca\_xCu\_{24}O\_{41}

The present paper studies, using ab-initio calculations, the influence of the incommensurate structural modulations on the low energy physics of the $Sr\_{14-x}Ca\_xCu\_{24}O\_{41}$ oxides. On-site, nearest-neighbor and next-nearest-neighbor effective parameters were computed within a $t-J+V$ model based on the copper magnetic orbitals. The structural modulations appear to be the key degree of freedom, responsible for the low energy properties such as the electron localization, the formation of dimers in the $x=0$ compound or the anti-ferromagnetic order in the $x=13.6$ compound

The role of dynamical polarization of the ligand to metal charge transfer excitations in {\em ab initio} determination of effective exchange parameters

The role of the bridging ligand on the effective Heisenberg coupling parameters is analyzed in detail. This analysis strongly suggests that the ligand-to-metal charge transfer excitations are responsible for a large part of the final value of the magnetic coupling constant. This permits to suggest a new variant of the Difference Dedicated Configuration Interaction (DDCI) method, presently one of the most accurate and reliable for the evaluation of magnetic effective interactions. This new method treats the bridging ligand orbitals mediating the interaction at the same level than the magnetic orbitals and preserves the high quality of the DDCI results while being much less computationally demanding. The numerical accuracy of the new approach is illustrated on various systems with one or two magnetic electrons per magnetic center. The fact that accurate results can be obtained using a rather reduced configuration interaction space opens the possibility to study more complex systems with many magnetic centers and/or many electrons per center.Comment: 7 pages, 4 figure

Étude ab initio de la structure électronique des composés d'oxyde de cuivre incommensurables Sr(14-x)Ca(x)Cu(24)O(41).

This work is divided into two parts, the first one is devoted to methodological developments, the second to the ab-initio study, using an embedded fragment spectroscopy method, of Sr(14-x)Ca(x)Cu(24)O(41) compounds.The methodological part proposes a systematic and general improvement of the Evjen's Madelung potential calculation method, as well as a simplification of the ab-initio computation method allowing an important gain in computational resources without loss of accuracy.The study of the Sr(14-x)Ca(x)Cu(24)O(41) (x=0 et x=13.6) compounds shows the influence of the structural incommensurate modulations on the low energy properties (dimerisation, antiferromagnetic order) of both their chain and ladder sub-systems. t-J+V complete models are derived from ab-initio calculations as a function of the structural incommensurate parameter.Ce travail comporte deux partie, l'une de développement méthodologique, l'autre concerne l'étude ab-initio, par une méthode de spectroscopie de fragments immergés dans un bain, des composés Sr(14-x)Ca(x)Cu(24)O(41).Le travail méthodologique propose une généralisation et systématisation du calcul du potentiel de Madelung ainsi qu'une simplification de la méthode de calcul ab-initio permettant un gain de ressources sans perte de précision.L'étude des composés Sr(14-x)Ca(x)Cu(24)O(41) (x=0 et x=13.6) montre l'influence des modulations structurales incommensurables des deux sous-systèmes (chaînes et échelles de spins) sur la physique de basse énergie des composés (dimérisation, ordre anti-ferromagnétique). Des modèles t-J+V ont été déterminés à partir des calculs ab-initio en fonction des paramètre incommensurables

Coherent phonon dynamics in laser excited bismuth from first principles

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Influence of the structural modulations and the Chain-ladder interaction in the Sr14−xCaxCu24O41Sr_{14-x}Ca_{x}Cu_{24}O_{41} compounds.

8 octobre 2004International audienceWe studied the effects of the incommensurate structural modulations on the ladder subsystem of the $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$ family of compounds using ab-initio explicitly-correlated calculations. From these calculations we derived $t-J$ model as a function of the fourth crystallographic coordinate $\tau$ describing the incommensurate modulations. It was found that in the highly calcium-doped system, the on-site orbital energies are strongly modulated along the ladder legs. On the contrary the two sites of the ladder rungs are iso-energetic and the holes are thus expected to be delocalized on the rungs. Chain-ladder interactions were also evaluated and found to be very negligible. The ladder superconductivity model for these systems is discussed in the light of the present results

Charge order in the incommensurate compounds Sr14−xCaxCu24O41Sr_{14-x}Ca_xCu_{24}O_{41}

The present paper studies, using ab-initio calculations, the influence of the incommensurate structural modulations on the low energy physics of the $Sr_{14-x}Ca_xCu_{24}O_{41}$ oxides. On-site, nearest-neighbor and next-nearest-neighbor effective parameters were computed within a $t-J+V$ model based on the copper magnetic orbitals. The structural modulations appear to be the key degree of freedom, responsible for the low energy properties such as the electron localization, the formation of dimers in the $x=0$ compound or the anti-ferromagnetic order in the $x=13.6$ compound

Influence of the structural modulations and the Chain-ladder interaction in the Sr14−xCaxCu24O41Sr_{14-x}Ca_{x}Cu_{24}O_{41} compounds.

8 octobre 2004International audienceWe studied the effects of the incommensurate structural modulations on the ladder subsystem of the $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$ family of compounds using ab-initio explicitly-correlated calculations. From these calculations we derived $t-J$ model as a function of the fourth crystallographic coordinate $\tau$ describing the incommensurate modulations. It was found that in the highly calcium-doped system, the on-site orbital energies are strongly modulated along the ladder legs. On the contrary the two sites of the ladder rungs are iso-energetic and the holes are thus expected to be delocalized on the rungs. Chain-ladder interactions were also evaluated and found to be very negligible. The ladder superconductivity model for these systems is discussed in the light of the present results

Effective valence-bond theory for strongly correlated systems

International audienceThe present paper describes how to extract accurate effective Valence-Bond models for strongly correlated compounds. It details how, in difficult cases, it is possible to choose the model space supporting the effective Hamiltonian using a joint diagonalization of the target states density matrices. The information is reduced from the target MRCI states to the effective model using the effective Hamiltonians concepts. The problems of non-orthogonality and accuracy are discussed. The methodology is briefly illustrated by a couple of examples of applications