Comment on "Absence of spin liquid in non-frustrated correlated systems"

In a recent Letter, Hassan and S\'en\'echal [1] discussed the existence of a spin-liquid phase of the half-filled Hubbard model on the honeycomb lattice. Using schemes, such as the variational cluster approximation (VCA) and the cluster dynamical mean field theory (CDMFT) in combination with exact diagonalization (ED), they argued that a single bath orbital per site of the six-atom unit cell is insufficient and leads to the erroneous conclusion that the system is gapped for all nonzero values of the onsite Coulomb interaction $U$. In contrast, we point out here that, in the case of the honeycomb lattice, six bath levels per six-site unit cell are perfectly adequate for the description of short-range correlations. Instead, we demonstrate that it is the violation of long-range translation symmetry inherent in CDMFT-like schemes which opens a gap at Dirac points. The gap found at small $U$ therefore does not correspond to a Mott gap. As a result, present CDMFT schemes are not suitable for the identification of a spin-liquid phase on the honeycomb lattice. [1] S.R. Hassan and D. S\'en\'echal, Phys. Rev. Lett. 110, 096402 (2013).Comment: one page, no figure

Quasi-particle spectra of perovskites: Enhanced Coulomb correlations at surfaces

Photoemission spectra of the perovskites Ca$_x$Sr$_{1-x}$VO$_3$, Ca$_x$La$_{1-x}$VO$_3$, and SrRuO$_3$ indicate that Coulomb correlations are more pronounced at the surface than in the bulk. To investigate this effect we use the dynamical mean field theory combined with the Quantum Monte Carlo technique and evaluate the multi-orbital self-energy. These systems exhibit different degrees of band filling and range from metallic to insulating. The key input in the calculations is the layer dependent local density of states which we obtain from a tight-binding approach for semi-infinite cubic systems. As a result of the planar character of the perovskite $t_{2g}$ bands near the Fermi level, the reduced coordination number of surface atoms gives rise to a significant narrowing of the surface density of those subbands which hybridize preferentially in planes normal to the surface. Although the total band width coincides with the one in the bulk, the effective band narrowing at the surface leads to stronger correlation features in the quasi-particle spectra. In particular, the weight of the quasi-particle peak near $E_F$ is reduced and the amplitude of the lower and upper Hubbard bands is enhanced, in agreement with experiments

Subband filling and Mott transition in Ca_{2-x}Sr_xRuO_4

A new concept is proposed for the paramagnetic metal insulator transition in the layer perovskite Ca_{2-x}Sr_xRuO_4. Whereas the pure Sr compound is metallic up to very large Coulomb energies due to strong orbital fluctuations, structural changes induced by doping with Ca give rise to a interorbital charge transfer which makes the material extremely sensitive to local correlations. Using dynamical mean field theory based on finite temperature multi-band exact diagonalization it is shown that the combination of crystal field splitting and onsite Coulomb interactions leads to complete filling of the d_xy band and to a Mott transition in the half-filled d_xz,yz bands.Comment: 4 pages, 3 figure

Quantum phase transition in the two-band Hubbard model

The interaction between itinerant and Mott localized electronic states in strongly correlated materials is studied within dynamical mean field theory in combination with the numerical renormalization group method. A novel nonmagnetic zero temperature quantum phase transition is found in the bad-metallic orbital-selective Mott phase of the two-band Hubbard model, for values of the Hund's exchange which are relevant to typical transition metal oxides.Comment: 4 pages, 4 eps figures, revised version, to appear in Phys. Rev. Let

Single Mott Transition in Multi-Orbital Hubbard Model

The Mott transition in a multi-orbital Hubbard model involving subbands of different widths is studied within the dynamical mean field theory. Using the iterated perturbation theory for the quantum impurity problem it is shown that at low temperatures inter-orbital Coulomb interactions give rise to a single first-order transition rather than a sequence of orbital selective transitions. Impurity calculations based on the Quantum Monte Carlo method confirm this qualitative behavior. Nevertheless, at finite temperatures, the degree of metallic or insulating behavior of the subbands differs greatly. Thus, on the metallic side of the transition, the narrow band can exhibit quasi-insulating features, whereas on the insulating side the wide band exhibits pronounced bad-metal behavior. This complexity might partly explain contradictory results between several previous works.Comment: 8 pages, 11 figure