Consistent LDA+DMFT approach to electronic structure of transition metal oxides: charge transfer insulators and correlated metals

We discuss the recently proposed LDA+DMFT approach providing consistent parameter free treatment of the so called double counting problem arising within the LDA+DMFT hybrid computational method for realistic strongly correlated materials. In this approach the local exchange-correlation portion of electron-electron interaction is excluded from self consistent LDA calculations for strongly correlated electronic shells, e.g. d-states of transition metal compounds. Then the corresponding double counting term in LDA+DMFT Hamiltonian is consistently set in the local Hartree (fully localized limit - FLL) form of the Hubbard model interaction term. We present the results of extensive LDA+DMFT calculations of densities of states, spectral densities and optical conductivity for most typical representatives of two wide classes of strongly correlated systems in paramagnetic phase: charge transfer insulators (MnO, CoO and NiO) and strongly correlated metals (SrVO3 and Sr2RuO4). It is shown that for NiO and CoO systems LDA+DMFT qualitatively improves the conventional LDA+DMFT results with FLL type of double counting, where CoO and NiO were obtained to be metals. We also include in our calculations transition metal 4s-states located near the Fermi level missed in previous LDA+DMFT studies of these monooxides. General agreement with optical and X-ray experiments is obtained. For strongly correlated metals LDA$^prime$+DMFT results agree well with earlier LDA+DMFT calculations and existing experiments. However, in general LDA+DMFT results give better quantitative agreement with experimental data for band gap sizes and oxygen states positions, as compared to the conventional LDA+DMFT.

Electronic Structure of New Multiple Band Pt-Pnictide Superconductors APt3P

We report LDA calculated band structure, densities of states and Fermi surfaces for recently discovered Pt-pnictide superconductors APt3P (A=Ca,Sr,La), confirming their multiple band nature. Electronic structure is essentially three dimensional, in contrast to Fe pnictides and chalcogenides. LDA calculated Sommerfeld coefficient agrees rather well with experimental data, leaving little space for very strong coupling superconductivity, suggested by experimental data on specific heat of SrPt3P. Elementary estimates show, that the values of critical temperature can be explained by rather weak or moderately strong coupling, while the decrease of superconducting transition temperature Tc from Sr to La compound can be explained by corresponding decrease of total density of states at the Fermi level N(E_F). The shape of the density of states near the Fermi level suggests that in SrPt3P electron doping (such as replacement Sr by La) decreases N(E_F) and Tc, while hole doping (e.g. partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E_F) and possibly Tc.

Consistent LDA+DMFT -- an unambiguous way to avoid double counting problem: NiO test

We present a consistent way of treating a double counting problem unavoidably arising within the LDA+DMFT combined approach to realistic calculations of electronic structure of strongly correlated systems. The main obstacle here is the absence of systematic (e.g. diagrammatic) way to express LDA (local density approximation) contribution to exchange correlation energy appearing in the density functional theory. It is not clear then, which part of interaction entering DMFT (dynamical mean-field theory) is already taken into account through LDA calculations. Because of that, up to now there is no accepted unique expression for the double counting correction in LDA+DMFT. To avoid this problem we propose here the consistent LDA+DMFT approach, where LDA exchange correlation contribution is explicitly excluded for correlated states (bands) during self-consistent band structure calculations. What is left out of Coulomb interaction for those strongly correlated states (bands) is its non-local part, which is not included in DMFT, and the local Hartree like contribution. Then the double counting correction is uniquely reduced to the local Hartree contribution. Correlations for strongly correlated states are then directly accounted for via the standard DMFT. We further test the consistent LDA+DMFT scheme and compare it with conventional LDA+DMFT calculating the electronic structure of NiO. Opposite to the conventional LDA+DMFT our consistent LDA+DMFT approach unambiguously produces the insulating band structure in agreement with experiments.

Electronic and Magnetic Structure of Possible Iron Based Superconductor BaFe2Se3

We present results of LDA calculations (band structure, densities of states, Fermi surfaces) for possible iron based superconductor BaFe2Se3 (Ba123) in normal (paramagnetic) phase. Results are briefly compared with similar data on prototype BaFe2As2 and (K,Cs)Fe2Se2 superconductors. Without doping this system is antiferromagnetic with T_N^{exp}~250K and rather complicated magnetic structure. Neutron diffraction experiments indicated the possibility of two possible spin structures (antiferromagnetically ordered plaquettes or zigzags), indistinguishable by neutron scattering. Using LSDA calculated exchange parameters we estimate Neel temperatures for both spin structures within the molecular field approximation and show tau_1 (plaquettes) spin configuration to be more favorable than tau_2 (zigzags).

Optical Sum Rule in Strongly Correlated Systems

We discuss the problem of a possible violation of the optical sum rule in the normal (non superconducting) state of strongly correlated electronic systems, using our recently proposed DMFT+Sigma approach, applied to two typical models: the hot - spot model of the pseudogap state and disordered Anderson - Hubbard model. We explicitly demonstrate that the general Kubo single band sum rule is satisfied for both models. However, the optical integral itself is in general dependent on temperature and characteristic parameters, such as pseudogap width, correlation strength and disorder scattering, leading to effective violation of the optical sum rule, which may be observed in the experiments.

Comparative study of electron and hole doped High-Tc compounds in pseudogap regime: LDA+DMFT+Sk approach

Pseudogap regime for the prototype high-Tc compounds hole doped Bi2Sr2CaCu2O8-x (Bi2212) and electron doped Nd2-xCexCuO4 (NCCO) is described by means of novel generalized LDA+DMFT+Sk approach. Here conventional dynamical mean-field theory (DMFT) equations are supplied with additional (momentum dependent) self-energy Sk. In the present case Sk describes non-local dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations. Material specific model parameters of two neighboring CuO2 layers of Bi2212 and single CuO2 layer of NCCO were obtained within local density approximation (LDA) and constrained LDA method. We show that Fermi surface in presence of the pseudogap fluctuations have perfectly visible hot-spots for NCCO while in Bi2212 there is just rather broad region with strong antiferromagnetic scattering. Results obtained are in good agreement with recent ARPES and optical experiments.