Interorbital charge transfers and Fermi-surface deformations in strongly correlated metals: models, BaVS$_3$ and Na$_{x}$CoO$_2$

TL;DR

This paper investigates how strong electron correlations cause interband charge transfers and Fermi-surface deformations in multi-orbital metals, with case studies on BaVS$_3$ and Na$_x$CoO$_2$, revealing new insights into their electronic properties.

Contribution

It demonstrates that correlation-induced interband charge transfers significantly modify Fermi surfaces, using DMFT and LDA+DFT to analyze BaVS$_3$ and Na$_x$CoO$_2$.

Findings

Charge redistribution in BaVS$_3$ affects the Fermi surface near the MIT.

Interorbital charge transfers influence the electronic structure of Na$_x$CoO$_2$.

Correlations can either enhance or compensate orbital polarization.

Abstract

Fermi-surface deformations in strongly correlated metals, in comparison to results from band-structure calculations, are investigated. We show that correlation-induced interband charge transfers in multi-orbital systems may give rise to substantial modifications of the actual Fermi surface. Depending in particular on the relative strength of the crystal-field splitting and of the Hund's exchange coupling, correlations may either reinforce orbital polarization or tend to compensate differences in orbital occupancies, as demonstrated by investigating a 2-band Hubbard model in the framework of dynamical mean field theory (DMFT). The physical implications of such interorbital 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 density approximation (LDA) to density functional theory (DFT), new insights in the underlying mechanism of the metal-to-insulator transition (MIT) of BaVS$_3$ are obtained. 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 of determining the Fermi surface in the strongly correlated cobaltate system Na$_{x}$CoO$_2$ is discussed.