Exact Diagonalization Dynamical Mean Field Theory for Multi-Band Materials: Effect of Coulomb correlations on the Fermi surface of Na_0.3CoO_2

TL;DR

This paper demonstrates that combining dynamical mean field theory with finite-temperature exact diagonalization effectively studies local Coulomb correlations in multi-band materials, enabling analysis of low-temperature properties and Fermi surface modifications.

Contribution

It introduces an efficient exact diagonalization approach for multi-band systems within DMFT, reducing finite-size effects and allowing detailed low-temperature studies of Coulomb correlations.

Findings

Small eg' Fermi surface hole pockets are slightly enlarged due to correlations.

The method yields smooth quasi-particle spectra at real frequencies.

Results agree with some previous calculations but conflict with photoemission data.

Abstract

Dynamical mean field theory combined with finite-temperature exact diagonalization is shown to be a suitable method to study local Coulomb correlations in realistic multi-band materials. By making use of the sparseness of the impurity Hamiltonian, exact eigenstates can be evaluated for significantly larger clusters than in schemes based on full diagonalization. Since finite-size effects are greatly reduced this approach allows the study of three-band systems down to very low temperatures, for strong local Coulomb interactions and full Hund exchange. It is also shown that exact diagonalization yields smooth subband quasi-particle spectra and self-energies at real frequencies. As a first application the correlation induced charge transfer between t2g bands in Na_0.3CoO_2 is investigated. For both Hund and Ising exchange the small eg' Fermi surface hole pockets are found to be slightly enlarged compared to the non-interacting limit, in agreement with previous Quantum Monte Carlo dynamical mean field calculations for Ising exchange, but in conflict with photoemission data.