Interorbital Charge Transfers and Fermi-Surface Deformations in Strongly Correlated Metals: Models, BaVS3and NaxCoO2

Fermi-surface deformations in strongly correlated metals, in comparison toresults from band-structure calculations, are investigated. We show thatcorrelation-induced interband charge transfers in multi-orbital systems maygive rise to substantial modifications of the actual Fermi surface. Dependingin particular on the relative strength of the crystal-field splitting and ofthe Hund's exchange coupling, correlations may either reinforce orbitalpolarization or tend to compensate differences in orbital occupancies, asdemonstrated by investigating a 2-band Hubbard model in the framework ofdynamical mean field theory (DMFT). The physical implications of suchinterorbital charge transfers are then explored in two case studies: BaVS$_3$and Na$_x$CoO$_2$. By means of the DMFT in combination with the local densityapproximation (LDA) to density functional theory (DFT), new insights in theunderlying mechanism of the metal-to-insulator transition (MIT) of BaVS$_3$ areobtained. A strong charge redistribution in comparison to LDA calculations,i.e., a depletion of the broader $A_{1g}$ band in favor of the narrower $E_g$bands just above the MIT is found. In addition, the intriguing problem ofdetermining the Fermi surface in the strongly correlated cobaltate systemNa$_{x}$CoO$_2$ is discussed.Comment: final versio