940 research outputs found
Na Induced Correlations in NaCoO
Increasing experimental evidence is building which indicates that signatures
of strong correlations are present in the Na rich region of NaCoO (ie.
) and absent in the Na poor region (ie. ). This is
unexpected given that NaCoO is a band insulator and CoO 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 sites (ie. Na
sites above/below Co sites)
Multi-patch model for transport properties of cuprate superconductors
A number of normal state transport properties of cuprate superconductors are
analyzed in detail using the Boltzmann equation. The momentum dependence of the
electronic structure and the strong momentum anisotropy of the electronic
scattering are included in a phenomenological way via a multi-patch model. The
Brillouin zone and the Fermi surface are divided in regions where scattering
between the electrons is strong and the Fermi velocity is low (hot patches) and
in regions where the scattering is weak and the Fermi velocity is large (cold
patches). We present several motivations for this phenomenology starting from
various microscopic approaches. A solution of the Boltzmann equation in the
case of N patches is obtained and an expression for the distribution function
away from equilibrium is given. Within this framework, and limiting our
analysis to the two patches case, the temperature dependence of resistivity,
thermoelectric power, Hall angle, magnetoresistance and thermal Hall
conductivity are studied in a systematic way analyzing the role of the patch
geometry and the temperature dependence of the scattering rates. In the case of
Bi-based cuprates, using ARPES data for the electronic structure, and assuming
an inter-patch scattering between hot and cold states with a linear temperature
dependence, a reasonable agreement with the available experiments is obtained.Comment: 18 pages, 18 figures, to be published on Eur. Phys. J.
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
Fermi arcs and the hidden zeros of the Green's function in the pseudogap state
We investigate the low energy properties of a correlated metal in the
proximity of a Mott insulator within the Hubbard model in two dimensions. We
introduce a new version of the Cellular Dynamical Mean Field Theory using
cumulants as the basic irreducible objects. These are used for re-constructing
the lattice quantities from their cluster counterparts. The zero temperature
one particle Green's function is characterized by the appearance of lines of
zeros, in addition to a Fermi surface which changes topology as a function of
doping. We show that these features are intimately connected to the opening of
a pseudogap in the one particle spectrum and provide a simple picture for the
appearance of Fermi arcs.Comment: revised version; 5 pages, 3 figure
Non-local Coulomb interactions and metal-insulator transition in TiO: a cluster LDA+DMFT approach
We present an ab initio quantum theory of the metal-insulator transition in
TiO. The recently developed cluster LDA+DMFT scheme is applied to
describe the many-body features of this compound. The conventional single site
DMFT cannot reproduce a low temperature insulating phase for any reasonable
values of the Coulomb interaction. We show that the non-local Coulomb
interactions and the strong chemical bonding within Ti-Ti pair is the origin of
the small gap insulating ground state of TiO
Correlation-enhanced electron-phonon coupling: Applications of GW and screened hybrid functional to bismuthates, chloronitrides, and other high Tc superconductors
We show that the electron-phonon coupling (EPC) in many materials can be
significantly underestimated by the standard density functional theory (DFT) in
the local density approximation (LDA) due to large non-local correlation
effects. We present a simple yet efficient methodology to evaluate the
realistic EPC going beyond LDA by using more advanced and accurate GW and
screened hybrid functional DFT approaches. The corrections we propose explain
the extraordinarily high superconducting temperatures that are observed in two
distinct classes of compounds-the bismuthates and the transition metal
chloronitrides, thus solving a thirty-year-old puzzle. Our work calls for the
critically reevaluation of the EPC of certain phonon modes in many other
materials such as cuprates and iron-based superconductors. The proposed
methodology can be used to design new correlation-enhanced high temperature
superconductors and other functional materials involving electron-phonon
interaction.Comment: Substantilly extended version of the previous manuscript, 19 pages,
10 figures, accepted for publication in Phys. Rev.
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