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Register a new userAn ab-initio evaluation of the local effective interactions in the $rm Na_{x} Co O_2$ familly
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Added by
Marie-Bernadette Lepetit
Publication date
2010
fields
Physics
and research's language is
English
Authors
Sylvain Landron
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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 $U/tsim 15$, 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.
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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 $U/tsim 26$ 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.
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