111 research outputs found
Comparative study of the electronic structures of Fe3O4 and Fe2SiO4
The electronic properties of two spinels FeO and FeSiO are
studied by the density functional theory. The local Coulomb repulsion and
the Hund's exchange between the electrons on iron are included. For
, both spinels are half-metals, with the minority states at the
Fermi level. Magnetite remains a metal in a cubic phase even at large values of
. The metal-insulator transition is induced by the phonon, which
lowers the total energy and stabilizes the charge-orbital ordering.
FeSiO transforms to a Mott insulating state for eV with a gap
. The antiferromagnetic interactions induce the tetragonal
distortion, which releases the geometrical frustration and stabilizes the
long-range order. The differences of electronic structures in the high-symmetry
cubic phases and the distorted low-symmetry phases of both spinels are
discussed.Comment: 6 pages, 6 figure
{\it Ab initio} calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers
The magnetization distribution, its energetic characterization by the
interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt
multilayers are investigated using density functional theory combined with the
direct method to determine phonon frequencies. It is found that ferromagnetic
order between consecutive Fe layers is favoured, with the enhanced magnetic
moments at the interface. The bilinear and biquadratic coupling coefficients
between Fe layers are shown to saturate fast with increasing thickness of
nonmagnetic Pt layers which separate them. The phonon calculations demonstrate
a rather strong dependence of partial iron phonon densities of states on the
actual position of Fe monolayer in the multilayer structure.Comment: 7 pages, 8 figure
Orbital liquid in ferromagnetic manganites: The orbital Hubbard model for electrons
We have analyzed the symmetry properties and the ground state of an orbital
Hubbard model with two orbital flavors, describing a partly filled
spin-polarized band on a cubic lattice, as in ferromagnetic manganites.
We demonstrate that the off-diagonal hopping responsible for transitions
between and orbitals, and the absence of SU(2) invariance
in orbital space, have important implications. One finds that superexchange
contributes in all orbital ordered states, the Nagaoka theorem does not apply,
and the kinetic energy is much enhanced as compared with the spin case.
Therefore, orbital ordered states are harder to stabilize in the Hartree-Fock
approximation (HFA), and the onset of a uniform ferro-orbital polarization and
antiferro-orbital instability are similar to each other, unlike in spin case.
Next we formulate a cubic (gauge) invariant slave boson approach using the
orbitals with complex coefficients. In the mean-field approximation it leads to
the renormalization of the kinetic energy, and provides a reliable estimate for
the ground state energy of the disordered state. Using this approach one finds
that the HFA fails qualitatively in the regime of large Coulomb repulsion
-- the orbital order is unstable, and instead a strongly
correlated orbital liquid with disordered orbitals is realized at any electron
filling.Comment: 25 pages, 9 figure
Orbital degeneracy as a source of frustration in LiNiO
Motivated by the absence of cooperative Jahn-Teller effect and of magnetic
ordering in LiNiO, a layered oxide with triangular planes, we study a
general spin-orbital model on the triangular lattice. A mean-field approach
reveals the presence of several singlet phases between the SU(4) symmetric
point and a ferromagnetic phase, a conclusion supported by exact
diagonalizations of finite clusters. We argue that one of the phases,
characterized by a large number of low-lying singlets associated to dimer
coverings of the triangular lattice, could explain the properties of LiNiO,
while a ferro-orbital phase that lies nearby in parameter space leads to a new
prediction for the magnetic properties of NaNiO.Comment: 18 pages, 17 figure
Second Josephson excitations beyond mean field as a toy model for thermal pressure: exact quantum dynamics and the quantum phase model
A simple four-mode Bose-Hubbard model with intrinsic time scale separation
can be considered as a paradigm for mesoscopic quantum systems in thermal
contact. In our previous work we showed that in addition to coherent particle
exchange, a novel slow collective excitation can be identified by a series of
Holstein-Primakoff transformations. This resonant energy exchange mode is not
predicted by linear Bogoliubov theory, and its frequency is sensitive to
interactions among Bogoliubov quasi-particles; it may be referred to as a
second Josephson oscillation, in analogy to the second sound mode of liquid
Helium II. In this paper we will explore this system beyond the
Gross-Pitaevskii mean field regime. We directly compare the classical mean
field dynamics to the exact full quantum many-particle dynamics and show good
agreement over a large range of the system parameters. The second Josephson
frequency becomes imaginary for stronger interactions, however, indicating
dynamical instability of the symmetric state. By means of a generalized quantum
phase model for the full four-mode system, we then show that, in this regime,
high-energy Bogoliubov quasiparticles tend to accumulate in one pair of sites,
while the actual particles preferentially occupy the opposite pair. We
interpret this as a simple model for thermal pressure
Long-range disassortative correlations in generic random trees
We explicitly calculate the distance dependent correlation functions in a
maximal entropy ensemble of random trees. We show that correlations remain
disassortative at all distances and vanish only as a second inverse power of
the distance. We discuss in detail the example of scale-free trees where the
diverging second moment of the degree distribution leads to some interesting
phenomena.Comment: Revised according to referee comments; 7 pages and 7 figures
Spin-Orbital Entanglement and Violation of the Goodenough-Kanamori Rules
We point out that large composite spin-orbital fluctuations in Mott
insulators with orbital degeneracy are a manifestation of quantum
entanglement of spin and orbital variables. This results in a dynamical nature
of the spin superexchange interactions, which fluctuate over positive and
negative values, and leads to an apparent violation of the Goodenough-Kanamori
rules. [{\it Published in Phys. Rev. Lett. {\bf 96}, 147205 (2006).}]Comment: 4 pages, 2 figure
Compass-Heisenberg Model on the Square Lattice : Spin Order and Excitations
We explore the physics of the anisotropic compass model under the influence
of perturbing Heisenberg interactions and present the phase diagram with
multiple quantum phase transitions. The macroscopic ground state degeneracy of
the compass model is lifted in the thermodynamic limit already by infinitesimal
Heisenberg coupling, which selects different ground states with Z_2 symmetry
depending on the sign and size of the coupling constants --- then low energy
excitations are spin waves, while the compass states reflecting columnar order
are separated from them by a macroscopic gap. Nevertheless, nanoscale
structures relevant for quantum computation purposes may be tuned such that the
compass states are the lowest energy excitations, thereby avoiding decoherence,
if a size criterion derived by us is fulfilled.Comment: 6 pages, 5 figure
Magnetic phases near the Van Hove singularity in s- and d-band Hubbard model
We investigate the magnetic instabilities of the nondegenerate (s-band) and a
degenerate (d-band) Hubbard model in two dimensions using many-body effects due
to the particle-particle diagrams and Hund's rule local correlations. The
density of states and the position of Van Hove singularity change depending on
the value of next-nearest neighbor hopping t'. The Stoner parameter is strongly
reduced in the s-band case, and ferromagnetism survives only if electron
density is small, and the band is almost flat at small momenta due to
next-nearest neighbor hopping. In contrast, for the d-band case the reduction
of the Stoner parameter which follows from particle-particle correlations is
much smaller and ferromagnetism survives to a large extent. Inclusion of local
spin-spin correlations has a limited destabilizing effect on the magnetic
states.Comment: 8 pages, 7 figure
Fingerprints of spin-orbital physics in cubic Mott insulators: Magnetic exchange interactions and optical spectral weights
The temperature dependence and anisotropy of optical spectral weights
associated with different multiplet transitions is determined by the spin and
orbital correlations. To provide a systematic basis to exploit this close
relationship between magnetism and optical spectra, we present and analyze the
spin-orbital superexchange models for a series of representative
orbital-degenerate transition metal oxides with different multiplet structure.
For each case we derive the magnetic exchange constants, which determine the
spin wave dispersions, as well as the partial optical sum rules. The magnetic
and optical properties of early transition metal oxides with degenerate
orbitals (titanates and vanadates with perovskite structure) are shown
to depend only on two parameters, viz. the superexchange energy and the
ratio of Hund's exchange to the intraorbital Coulomb interaction, and on
the actual orbital state. In systems important corrections follow from
charge transfer excitations, and we show that KCuF can be classified as a
charge transfer insulator, while LaMnO is a Mott insulator with moderate
charge transfer contributions. In some cases orbital fluctuations are quenched
and decoupling of spin and orbital degrees of freedom with static orbital order
gives satisfactory results for the optical weights. On the example of cubic
vanadates we describe a case where the full quantum spin-orbital physics must
be considered. Thus information on optical excitations, their energies,
temperature dependence and anisotropy, combined with the results of magnetic
neutron scattering experiments, provides an important consistency test of the
spin-orbital models, and indicates whether orbital and/or spin fluctuations are
important in a given compound.Comment: 34 pages, 16 figure
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