Exact Diagonalization Dynamical Mean Field Theory for Multi-Band Materials: Effect of Coulomb correlations on the Fermi surface of Na0.3CoO2

Dynamical mean-field theory combined with finite-temperature exact diagonalization is shown to be a suitable method to study local Coulomb correlations in realistic multiband 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 quasiparticle spectra and self-energies at real frequencies. As a first application the correlation induced charge transfer between t2g bands in Na0.3CoO2 is investigated. For both Hund and Ising exchange the small eg Fermi surface hole pockets are found to be slightly enlarged compared to the noninteracting limit, in agreement with previous quantum Monte Carlo dynamical mean-field calculations for Ising exchange, but in conflict with photoemission data.