Na Induced Correlations in Nax_xCoO2_2

Increasing experimental evidence is building which indicates that signatures of strong correlations are present in the Na rich region of Na$_x$CoO$_2$ (ie. $x\approx0.7$) and absent in the Na poor region (ie. $x\approx0.3$). This is unexpected given that NaCoO$_2$ is a band insulator and CoO$_2$ has an integer filled open shell making it a candidate for strong correlations. We explain these experimental observations by presenting a minimal low-energy Hamiltonian for the cobaltates and solving it within LDA+DMFT. The Na potential is shown to be a key element in understanding correlations in this material. Furthermore, LDA calculations for the realistic Na ordering predict a \emph{binary} perturbation of the Co sites which correlates with the Na$_1$ sites (ie. Na sites above/below Co sites)

Orbital selective and tunable Kondo effect of magnetic adatoms on graphene: Correlated electronic structure calculations

We have studied the effect of dynamical correlations on the electronic structure of single Co adatoms on graphene monolayers with a recently developed novel method for nanoscopic materials that combines density functional calculations with a fully dynamical treatment of the strongly interacting 3d-electrons. The coupling of the Co 3d-shell to the graphene substrate and hence the dynamic correlations are strongly dependent on the orbital symmetry and the system parameters (temperature, distance of the adatom from the graphene sheet, gate voltage). When the Kondo effect takes place, we find that the dynamical correlations give rise to strongly temperature-dependent peaks in the Co 3d-spectra near the Fermi level. Moreover, we find that the Kondo effect can be tuned by the application of a gate voltage. It turns out that the position of the Kondo peaks is pinned to the Dirac points of graphene rather than to the chemical potential.Comment: 12 pages, 7 figure

Phase diagram, energy scales and nonlocal correlations in the Anderson lattice model

We study the Anderson lattice model with one f-orbital per lattice site as the simplest model which describes generic features of heavy fermion materials. The resistivity and magnetic susceptibility results obtained within dynamical mean field theory (DMFT) for a nearly half-filled conduction band show the existence of a single energy scale $T^*$ which is similar to the single ion Kondo temperature $T_K^o$. To determine the importance of inter-site correlations, we have also solved the model within cellular DMFT (CDMFT) with two sites in a unit cell. The antiferromagnetic region on the phase diagram is much narrower than in the single-site solution, having a smaller critical hybridization $V_c$ and N\'eel temperature $T_N$. At temperatures above $T_N$ the nonlocal correlations are small, and the DMFT paramagnetic solution is in this case practically exact, which justifies the ab initio LDA+DMFT approach in theoretical studies of heavy fermions. Strong inter-site correlations in the CDMFT solution for $T<T_N$, however, indicate that they have to be properly treated in order to unravel the physical properties near the quantum critical point.Comment: 10 page

Cluster Dynamical Mean-Field Theory of the density-driven Mott transition in the one-dimensional Hubbard model

The one-dimensional Hubbard model is investigated by means of two different cluster schemes suited to introduce short-range spatial correlations beyond the single-site Dynamical Mean-Field Theory, namely the Cluster-Dynamical Mean-Field Theory and its periodized version. It is shown that both cluster schemes are able to describe with extreme accuracy the evolution of the density as a function of the chemical potential from the Mott insulator to the metallic state. Using exact diagonalization to solve the cluster impurity model, we discuss the role of the truncation of the Hilbert space of the bath, and propose an algorithm that gives higher weights to the low frequency hybridization matrix elements and improves the speed of the convergence of the algorithm.Comment: 6 pages, 4 figures, minor corrections in v

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

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.Comment: 9 pages, 7 figure