150 research outputs found
Disproportionation Transition at Critical Interaction Strength: NaCoO
Charge disproportionation (CD) and spin differentiation in NaCoO
are studied using the correlated band theory approach. The simultaneous CD and
gap opening seen previously is followed through a first order charge
disproportionation transition 2Co Co+Co, whose ionic
identities are connected more closely to spin (S=0, S=1/2 respectively) than to
real charge. Disproportionation in the Co orbital is compensated by
opposing charge rearrangement in other 3d orbitals. At the transition large and
opposing discontinuities in the (all-electron) kinetic and potential energies
are slightly more than balanced by a gain in correlation energy. The CD state
is compared to characteristics of the observed charge-ordered insulating phase
in NaCoO, suggesting the Coulomb repulsion value is
concentration-dependent, with 3.5 eV.Comment: 4 pages and 4 embedded figure
Disproportionation and Metallization at Low-Spin to High-Spin Transition in Multiorbital Mott Systems
We study the thermally driven spin state transition in a two-orbital Hubbard
model with crystal field splitting, which provides a minimal description of the
physics of LaCoO3. We employ the dynamical mean-field theory with quantum
Monte-Carlo impurity solver. At intermediate temperatures we find a spin
disproportionated phase characterized by checkerboard order of sites with small
and large spin moments. The high temperature transition from the
disproportionated to a homogeneous phase is accompanied by vanishing of the
charge gap. With the increasing crystal-field splitting the temperature range
of the disproportionated phase shrinks and eventually disappears completely.Comment: 4+ pages, 4 figure
Correlations in a band insulator
We study a model of a covalent band insulator with on-site Coulomb repulsion
at half-filling using dynamical mean-field theory. Upon increasing the
interaction strength the system undergoes a discontinuous transition from a
correlated band insulator to a Mott insulator with hysteretic behavior at low
temperatures. Increasing the temperature in the band insulator close to the
insulator-insulator transition we find a crossover to a Mott insulator at
elevated temperatures. Remarkably, correlations decrease the energy gap in the
correlated band insulator. The gap renormalization can be traced to the
low-frequency behavior of the self-energy, analogously to the quasiparticle
renormalization in a Fermi liquid. While the uncorrelated band insulator is
characterized by a single gap for both charge and spin excitations, the spin
gap is smaller than the charge gap in the correlated system.Comment: 7 pages, 7 figure
Magnetic groundstate and Fermi surface of bcc Eu
Using spin-spiral technique within the full potential linearized
augmented-plane-waves (LAPW) electronic structure method we investigate the
magnon spectrum and N\'eel temperature of bcc Eu. Ground state corresponding to
an incommensurate spin-spiral is obtained in agreement with experiment and
previous calculations. We demonstrate that the magnetic coupling is primarily
through the intra-atomic and exchange and
Ruderman-Kittel-Kasuya-Yosida mechanism. We show that the existence of this
spin-spiral is closely connected to a nesting feature of the Fermi surface
which was not noticed before.Comment: 6 pages 8 figure
Magnetism in systems with various dimensionality: A comparison between Fe and Co
A systematic ab initio study is performed for the spin and orbital moments
and for the validity of the sum rules for x-ray magnetic circular dichroism for
Fe systems with various dimensionality (bulk, Pt-supported monolayers and
monatomic wires, free-standing monolayers and monatomic wires). Qualitatively,
the results are similar to those for the respective Co systems, with the main
difference that for the monatomic Fe wires the term in the spin sum rule
is much larger than for the Co wires. The spin and orbital moments induced in
the Pt substrate are also discussed.Comment: 4 page
Spin state transition and covalent bonding in LaCoO3
We use the dynamical mean-field theory to study a p-d Hubbard Hamiltonian for
LaCoO3 derived from ab initio calculations in local density approximation
(LDA+DMFT scheme). We address the origin of local moments observed above 100 K
and discuss their attribution to a particular atomic multiplet in the presence
of covalent Co-O bonding. We show that in solids such attribution, based on the
single ion picture, is in general not possible. We explain when and how the
single ion picture can be generalized to provide a useful approximation in
solids. Our results demonstrate that the apparent magnitude of the local moment
is not necessarily indicative of the underlying atomic multiplet. We conclude
that the local moment behavior in LaCoO3 arises from the high-spin state of Co
and explain the precise meaning of this statement
Spin state of negative charge-transfer material SrCoO3
We employ the combination of the density functional and the dynamical
mean-field theory (LDA+DMFT) to investigate the electronic structure and
magnetic properties of SrCoO3, monocrystal of which were prepared recently. Our
calculations lead to a ferromagnetic metal in agreement with experiment. We
find that, contrary to some suggestions, the local moment in SrCoO3 does not
arise from intermediate spin state, but is a result of coherent superposition
of many different atomic states. We discuss how attribution of magnetic
response to different atomic states in solids with local moments can be
quantified.Comment: 5 pages, 5 figure
Correlation Effects and Structural Dynamics in the -Pyrochlore Superconductor KOsO
Electronic, magnetic, and dynamical properties of the new superconducting
-pyrochlore KOsO and related RbOsO and CsOsO
compounds are calculated and compared with experiment and contrasted with
structurally related spinel pyrochlores. The calculated susceptibility Stoner
enhancement (110%) and thermal mass enhancement = 2.5-3 reflect
moderate but perhaps important Coulomb correlations. The K ion optic mode
is found to be unstable, allowing large excursions of 0.5-0.6 \AA from its
ideal site of the K ion along directions. This dynamical mode is much
less anharmonic in the isostructural Rb and Cs compounds (with larger cations),
perhaps accounting for their progressively lower values of T. Electron
scattering from this very anharmonic mode may be the cause of the anomalous
concave-downward resistivity that is seen only in KOsO.Comment: 6 pages, 8 figure
- …