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

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

Novel Mott Transitions in Non-Isotropic Two-Band Hubbard Model

The Mott transition in a two-band Hubbard model involving subbands of different widths is studied as a function of temperature using dynamical mean field theory combined with exact diagonalization. The phase diagram is shown to exhibit two successive first-order transitions if the full Hund's rule coupling is included. In the absence of spin-flip and pair-exchange terms the lower transition remains first-order while the upper becomes continuous.Comment: 4 pages, 4 figures improved results for n_s=

Non-Fermi-liquid phases in the two-band Hubbard model: Finite-temperature exact diagonalization study of Hund's rule coupling

The two-band Hubbard model involving subbands of different widths is investigated via finite-temperature exact diagonalization (ED) and dynamical mean field theory (DMFT). In contrast to the quantum Monte Carlo (QMC) method which at low temperatures includes only Ising-like exchange interactions to avoid sign problems, ED permits a treatment of Hund's exchange and other onsite Coulomb interactions on the same footing. The role of finite-size effects caused by the limited number of bath levels in this scheme is studied by analyzing the low-frequency behavior of the subband self-energies as a function of temperature, and by comparing with numerical renormalization group (NRG) results for an effective one-band model. For half-filled, non-hybridizing bands, the metallic and insulating phases are separated by an intermediate mixed phase with an insulating narrow and a bad-metallic wide subband. The wide band in this phase exhibits different degrees of non-Fermi-liquid behavior, depending on the treatment of exchange interactions. Whereas for complete Hund's coupling, infinite lifetime is found at the Fermi level, in the absence of spin-flip and pair-exchange, this lifetime becomes finite. Excellent agreement is obtained both with new NRG and previous QMC/DMFT calculations. These results suggest that-finite temperature ED/DMFT might be a useful scheme for realistic multi-band materials.Comment: 15 pages, 17 figure

Coulomb correlations in the honeycomb lattice: role of translation symmetry

The effect of Coulomb correlations in the half-filled Hubbard model of the honeycomb lattice is studied within the dynamical cluster approximation (DCA) combined with exact diagonalization (ED) and continuous-time quantum Monte Carlo (QMC). The important difference between this approach and the previously employed cluster dynamical mean field theory (CDMFT) is that DCA preserves the translation symmetry of the system, while CDMFT violates this symmetry. As the Dirac cones of the honeycomb lattice are the consequence of perfect long-range order, DCA yields semi-metallic behavior at small onsite Coulomb interactions $U$, whereas CDMFT gives rise to a spurious excitation gap even for very small $U$. This basic difference between the two cluster approaches is found regardless of whether ED or QMC is used as the impurity solver. At larger values of $U$, the lack of translation symmetry becomes less important, so that the CDMFT reveals a Mott gap, in qualitative agreement with large-scale QMC calculations. In contrast, the semi-metallic phase obtained in DCA persists even at $U$ values where CDMFT and large-scale QMC consistently show Mott insulating behavior.Comment: 10 pages, 10 figure