97 research outputs found
Electronic structure of the 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
. As for the family the Fermi level orbitals
are the orbitals of the cobalt atoms ; the being always lower
in energy by more than 240\,meV. The electron correlation is found very large
as well as the parameters fluctuations as a function of the
structural modulation. The main consequences are a partial 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
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
superconducting compound. The effective ligand field splitting within the
orbitals was found to be , the
orbital being destabilized compared to the ones. The effective
Hund's exchange and Coulomb repulsion were evaluated to
and --. The NN hopping parameters were determined within
the three orbitals and found to be of the same order of magnitude as
the 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
Similarities between the and Hubbard models in weakly correlated regimes
We present a comparative study of the Hubbard and models far away from
half-filling. We show that, at such fillings the Hamiltonian can be seen
as an effective model of the repulsive Hubbard Hamiltonian over the whole range
of correlation strength. Indeed, the range of the
Hubbard model can be mapped onto the finite range of the
model, provided that the effective exchange parameter 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
and the infinitely correlated 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
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 ). The model is checked against experimental results and literature
analysis
An ab-initio evaluation of the local effective interactions in the familly
We used quantum chemical ab initio methods to determine the effective
parameters of Hubbard and models for the compounds (x=0
and 0.5). As for the superconducting compound we found the cobalt
orbitals above the ones by a few hundreds of meV due to the
-- hybridization of the cobalt orbitals. The correlation
strength was found to increase with the sodium content while the in-plane
AFM coupling decreases. The less correlated system was found to be the pure
, however it is still strongly correlated and very close to the Mott
transition. Indeed we found , which is the critical value for the
Mott transition in a triangular lattice. Finally, one finds the magnetic
exchanges in the layers, strongly dependant of the weak local
structural distortions
First principles calculation of the phonons modes in the hexagonal ferroelectric and paraelectric phases
The lattice dynamics of the 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 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
Fractionnally charged excitations in the charge density wave state of a quarter-filled t-J chain with quantum phonons
Elementary excitations of the 4k 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 . Relevance to the 4k
charge density wave instability in or recently discovered
in (TMTTF)X (X=PF, AsF) is discussed.Comment: 4 pages, 4 figure
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
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