270 research outputs found
Effect of covalency and interactions on the trigonal splitting in NaxCoO2
We calculate the effective trigonal crystal field Delta which splits the t2g
levels of effective models for NaxCoO2 as the local symmetry around a Co ion is
reduced from Oh to D3d. To this end we solve numerically a CoO6 cluster
containing a Co ion with all 3d states and their interactions included, and its
six nearest-neighbor O atoms, with the geometry of the system, in which the
CoO6 octahedron is compressed along a C3 axis. We obtain Delta near 130 meV,
with the sign that agrees with previous quantum chemistry calculations, but
disagrees with first-principles results in the local density approximation
(LDA). We find that Delta is very sensitive to a Coulomb parameter which
controls the Hund coupling and charge distribution among the d orbitals. The
origin of the discrepancy with LDA results is discussed.Comment: 6 pages, 2 figure
Comment on "Conductance scaling in Kondo-correlated quantum dots: Role of level asymmetry and charging energy"
In a recent work [L. Merker, S. Kirchner, E. Munoz, and T. A. Costi, Phys.
Rev. B 87, 165132 (2013)], the authors compared results of numerical
renormalization group and a perturbative approach for the dependence on
temperature T and magnetic field B of the conductance through a quantum dot
described by the impurity Anderson model, for small T and B. We show that the
equation used to extract the dependence on B from NRG results is incorrect out
of the particle-hole symmetric case. As a consequence, in the Kondo regime, the
correct NRG results have a weaker dependence on B and the disagreement between
both approaches increase.Comment: 3 pages, 1 figure, references update
Universal out-of-equilibrium transport in Kondo-correlated quantum dots
It is shown that the lesser and greater self energies and Green functions
calculated in a recent work on universal noequilibrium transport in interacting
quantum dots [E. Mu\~noz, C. J. Bolech, and S. Kirchner, Phys. Rev. Lett. 110,
016601 (2013)] are incorrect. The authors start from renormalized second-order
perturbation theory in the Coulomb repulsion in the particle-hole symmetric
impurity Anderson model. The reported lesser self-energy is compared with the
correct one, and with an analytic result valid for small frequency and applied
voltage. As a consequence of the mistakes, the conservation of the current is
not established and the results are unreliable.Comment: 1 page and a few lines, 1 figure, Comment on E. Mu\~noz, C. J.
Bolech, and S. Kirchner, Phys. Rev. Lett. 110, 016601 (2013). In version 3
further precisions are given regarding a simple mistake in the Letter (Ref.
2) and a comment to the use of Ward identities in Ref. 1 is adde
Scanning tunneling microscopy of adsorbed molecules on metalic surfaces for nearly localized atomic states
We consider a Hubbard-Anderson model which describes localized orbitals in
five different sites hybridized both among themselves and with a continuum of
extended states. A square planar geometry with an atom at the center is used to
represent TBrPP-Co molecules. When the renormalized effective hopping between
sites is small compared with a Kondo energy scale determined by the
sitecontinuum hybridization, the system can be described as a set of
independent Kondo resonances, rather than molecular states. We study the
crossover between both regimes and analyze the spectral density of conduction
electrons as a function of position. The results are in qualitative agreement
with measurements of the differential conductance in a system with TBrPP-Co
molecules adsorbed on a Cu(111) surface.Comment: 5 pages, 4 figure
Does long-range antiferromagnetism help or inhibit superconductivity?
We analyze the possible existence of a superconducting state in a background
with long-range antiferromagnetism. We consider a generalized Hubbard model
with nearest-neighbor correlated hopping in a square lattice. Near half
filling, the model exhibits a d-wave-Bardeen-Cooper-Schrieffer (BCS) solution
in the paramagnetic state. The superconducting solution would be enhanced by
the antiferromagnetic background if the contribution of triplet pairs with
d-wave symmetry and total momentum (pi, pi) could be neglected. However, we
find that due to their contribution, the coexistence of superconductivity and
long-range antiferromagnetism is ruled out for large values of the Coulomb
repulsion U. Spin-density wave fluctuations (SDWF) do not change this result.Comment: 8 pages, 1 figure. Accepted for publication in Physica
Phase diagram of the chains in and
We use a mapping of the multiband Hubbard model for chains in
(R=Y or a rare earth) onto a model and the
description of the charge dynamics of the latter in terms pf s spinless model,
to study the electronic structure of the chains. We briefly review results for
the optical conductivity and we calculate the quantum phase diagram of quarter
filled chains including Coulomb repulsion up to that between
next-nearest-neighbor atoms , using the resulting effective
Hamiltonian, mapped onto an XXZ chain, and the method of crossing of excitation
spectra. The method gives accurate results for the boundaries of the metallic
phase in this case. The inclusion of greatly enhances the region of
metallic behavior of the chains.Comment: 10 pages, 3 figures. Submited to Phys. Rev.
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