772 research outputs found
Quasi-particle spectra of perovskites: Enhanced Coulomb correlations at surfaces
Photoemission spectra of the perovskites CaSrVO,
CaLaVO, and SrRuO 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 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 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
. 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 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=
Correlation induced spin freezing transition in FeSe: a dynamical mean field study
The effect of local Coulomb interactions on the electronic properties of FeSe
is explored within dynamical mean field theory combined with finite-temperature
exact diagonalization. The low-energy scattering rate is shown to exhibit
non-Fermi-liquid behavior caused by the formation of local moments.
Fermi-liquid properties are restored at large electron doping. In contrast,
FeAsLaO is shown to be located on the Fermi-liquid side of this spin freezing
transition.Comment: 4 pages, 5 figure
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
Coulomb correlations do not fill the e'_g hole pockets in Na_{0.3}CoO_2
There exists presently considerable debate over the question whether local
Coulomb interactions can explain the absence of the small e'_g Fermi surface
hole pockets in photoemission studies of Na_{0.3}CoO_2. By comparing dynamical
mean field results for different single particle Hamiltonians and exact
diagonalization as well as quantum Monte Carlo treatments, we show that, for
realistic values of the Coulomb energy U and Hund exchange J, the e'_g pockets
can be slightly enhanced or reduced compared to band structure predictions, but
they do not disappear.Comment: 4 pages, 2 figure
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