2,533 research outputs found
LDA+DMFT computation of the electronic spectrum of NiO
The electronic spectrum, energy gap and local magnetic moment of paramagnetic
NiO are computed by using the local density approximation plus dynamical
mean-field theory (LDA+DMFT). To this end the noninteracting Hamiltonian
obtained within the local density approximation (LDA) is expressed in Wannier
functions basis, with only the five anti-bonding bands with mainly Ni 3d
character taken into account. Complementing it by local Coulomb interactions
one arrives at a material-specific many-body Hamiltonian which is solved by
DMFT together with quantum Monte-Carlo (QMC) simulations. The large insulating
gap in NiO is found to be a result of the strong electronic correlations in the
paramagnetic state. In the vicinity of the gap region, the shape of the
electronic spectrum calculated in this way is in good agreement with the
experimental x-ray-photoemission and bremsstrahlung-isochromat-spectroscopy
results of Sawatzky and Allen. The value of the local magnetic moment computed
in the paramagnetic phase (PM) agrees well with that measured in the
antiferromagnetic (AFM) phase. Our results for the electronic spectrum and the
local magnetic moment in the PM phase are in accordance with the experimental
finding that AFM long-range order has no significant influence on the
electronic structure of NiO.Comment: 15 pages, 6 figures, 1 table; published versio
Diagrammatic routes to nonlocal correlations beyond dynamical mean field theory
Strong electronic correlations pose one of the biggest challenges to solid
state theory. We review recently developed methods that address this problem by
starting with the local, eminently important correlations of dynamical mean
field theory (DMFT). On top of this, non-local correlations on all length
scales are generated through Feynman diagrams, with a local two-particle vertex
instead of the bare Coulomb interaction as a building block. With these
diagrammatic extensions of DMFT long-range charge-, magnetic-, and
superconducting fluctuations as well as (quantum) criticality can be addressed
in strongly correlated electron systems. We provide an overview of the
successes and results achieved---hitherto mainly for model Hamiltonians---and
outline future prospects for realistic material calculations.Comment: 60 pages, 42 figures, replaced by the version to be published in Rev.
Mod. Phys. 201
Magnetic susceptibility, exchange interactions and spin-wave spectra in the local spin density approximation
Starting from exact expression for the dynamical spin susceptibility in the
time-dependent density functional theory a controversial issue about exchange
interaction parameters and spin-wave excitation spectra of itinerant electron
ferromagnets is reconsidered. It is shown that the original expressions for
exchange integrals based on the magnetic force theorem (J. Phys. F14 L125
(1984)) are optimal for the calculations of the magnon spectrum whereas static
response function is better described by the ``renormalized'' magnetic force
theorem by P. Bruno (Phys. Rev. Lett. 90, 087205 (2003)). This conclusion is
confirmed by the {\it ab initio} calculations for Fe and Ni.Comment: 12 pages, 2 figures, submitted to JPC
k-dependent spectrum and optical conductivity near metal-insulator transition in multi-orbital Hubbard bands
We apply the dynamical mean field theory (DMFT) in the iterative perturbation
theory(IPT) to doubly degenerate eg bands and triply degenerate tg bands on a
simple cubic lattice and calculate the spectrum and optical conductivity in
arbitrary electron occupation. The spectrum simultaneously shows the effects of
multiplet structure and DMFT together with the electron ionization and affinity
levels of different electron occupations, coherent peaks at the Fermi energy in
the metallic phase and a gap at an integer filling of electrons for
sufficiently large Coulomb U. We also calculate the critical value of the
Coulomb U for degenerate orbitals.Comment: 8 pages, 6 figure
Exchange interactions and magnetic properties of the layered vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9
We have performed ab-initio calculations of exchange couplings in the layered
vanadates CaV2O5, MgV2O5, CaV3O7 and CaV4O9. The uniform susceptibility of the
Heisenberg model with these exchange couplings is then calculated by quantum
Monte Carlo method; it agrees well with the experimental measurements. Based on
our results we naturally explain the unusual magnetic properties of these
materials, especially the huge difference in spin gap between CaV2O5 and
MgV2O5, the unusual long range order in CaV3O7 and the "plaquette resonating
valence bond (RVB)" spin gap in CaV4O9
Sustaining entrepreneurial business: a complexity perspective on processes that produce emergent practice
This article examines the management practices in an entrepreneurial small firm which sustain the business. Using a longitudinal qualitative case study, four general processes are identified (experimentation, reflexivity, organising and sensing), that together provide a mechanism to sustain the enterprise. The analysis draws on concepts from entrepreneurship and complexity science. We suggest that an entrepreneur’s awareness of the role of these parallel processes will facilitate their approaches to sustaining and developing enterprises. We also suggest that these processes operate in parallel at multiple levels, including the self, the business and inter-firm networks. This finding contributes to a general theory of entrepreneurship. A number of areas for further research are discussed arising from this result
Antiferromagnetism and d-wave superconductivity in cuprates: a uster DMFT study
We present a new approach to investigate the coexistence of
antiferromagnetism and d-wave superconductivity in the two dimensional extended
Hubbard model within a numerically exact cluster dynamical mean-field
approximation. Self-consistent solutions with two non-zero order parameters
exists in the wide range of doping level and temperatures. A linearized
equation for energy spectrum near the Fermi level have been solved. The
resulting d-wave gap has the correct magnitude and k-dependence but some
distortion compare to the pure d_{x^2-y^2} superconducting order parameter due
to the presence of underlying antiferromagnetic ordering.Comment: 4 pages, 3 figure
Comparative study of correlation effects in CaVO3 and SrVO3
We present parameter-free LDA+DMFT (local density approximation + dynamical
mean field theory) results for the many-body spectra of cubic SrVO3 and
orthorhombic CaVO3. Both systems are found to be strongly correlated metals,
but not on the verge of a metal-insulator transition. In spite of the
considerably smaller V-O-V bond angle in CaVO3 the LDA+DMFT spectra of the two
systems for energies E<E_F are very similar, their quasiparticle parts being
almost identical. The calculated spectrum for E>E_F shows more pronounced,
albeit still small, differences. This is in contrast to earlier theoretical and
experimental conclusions, but in good agreement with recent bulk-sensitive
photoemission and x-ray absorption experiments.Comment: 15 pages, 6 figure
Cluster Dynamical Mean-field calculations for TiOCl
Based on a combination of cluster dynamical mean field theory (DMFT) and
density functional calculations, we calculated the angle-integrated spectral
density in the layered quantum magnet TiOCl. The agreement with recent
photoemission and oxygen K-edge X-ray absorption spectroscopy experiments is
found to be good. Th e improvement achieved with this calculation with respect
to previous single-site DMFT calculations is an indication of the correlated
nature and low-dimensionality of TiOCl.Comment: 9 pages, 3 figures, improved version as publishe
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