167 research outputs found
Spontaneous symmetry breaking in a generalized orbital compass model
We introduce a generalized two-dimensional orbital compass model, which
interpolates continuously from the classical Ising model to the orbital compass
model with frustrated quantum interactions, and investigate it using the
multiscale entanglement renormalization ansatz (MERA). The results demonstrate
that increasing frustration of exchange interactions triggers a second order
quantum phase transition to a degenerate symmetry broken state which minimizes
one of the interactions in the orbital compass model. Using boson expansion
within the spin-wave theory we unravel the physical mechanism of the symmetry
breaking transition as promoted by weak quantum fluctuations and explain why
this transition occurs only surprisingly close to the maximally frustrated
interactions of the orbital compass model. The spin waves remain gapful at the
critical point, and both the boson expansion and MERA do not find any
algebraically decaying spin-spin correlations in the critical ground state.Comment: 9 pages, 6 figures, improved presentation, version to appear in Phys.
Rev.
On the Neglect of Local Coulomb Interaction on Oxygens in Perovskites Described by the Multi-band Model
On the example of TiO layer (such as realized in SrTiO) we study
electronic structure of multi-band models describing transition metal
perovskites. In agreement with the experiment, the studied system is predicted
to be a robust nonmagnetic insulator. A realistic treatment of electronic
structure requires one to introduce non-zero Coulomb local interactions at
oxygen orbitals. However, up till now majority of papers based upon multi-band
models made an approximation of neglecting such interactions. We show that this
simplification does not lead to serious problems in predictions of the
electronic structure provided the Coulomb interactions at titanium ions and
charge transfer gap are suitably renormalized (so they become entirely
different with respect to the true microscopic model parameters).Comment: 1 figure, Physics of Magnetism 2017, Pozna\'n, June 201
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
Compass model on a ladder and square clusters
We obtained exact heat capacities of the quantum compass model on the square
L x L clusters with L=2,3,4,5 using Kernel Polynomial Method and compare them
with heat capacity of a large compass ladder. Intersite correlations found in
the ground state for these systems demonstrate that the quantum compass model
differs from its classical version.Comment: 4 pages, 2 figures, submitted to J. Phys. Conf. Serie
Magnetic properties of nanoscale compass-Heisenberg planar clusters
We study a model of spins 1/2 on a square lattice, generalizing the quantum
compass model via the addition of perturbing Heisenberg interactions between
nearest neighbors, and investigate its phase diagram and magnetic excitations.
This model has motivations both from the field of strongly correlated systems
with orbital degeneracy and from that of solid-state based devices proposed for
quantum computing. We find that the high degeneracy of ground states of the
compass model is fragile and changes into twofold degenerate ground states for
any finite amplitude of Heisenberg coupling. By computing the spin structure
factors of finite clusters with Lanczos diagonalization, we evidence a rich
variety of phases characterized by Z2 symmetry, that are either ferromagnetic,
C-type antiferromagnetic, or of Neel type, and analyze the effects of quantum
fluctuations on phase boundaries. In the ordered phases the anisotropy of
compass interactions leads to a finite excitation gap to spin waves. We show
that for small nanoscale clusters with large anisotropy gap the lowest
excitations are column-flip excitations that emerge due to Heisenberg
perturbations from the manifold of degenerate ground states of the compass
model. We derive an effective one-dimensional XYZ model which faithfully
reproduces the exact structure of these excited states and elucidates their
microscopic origin. The low energy column-flip or compass-type excitations are
robust against decoherence processes and are therefore well designed for
storing information in quantum computing. We also point out that the dipolar
interactions between nitrogen-vacancy centers forming a rectangular lattice in
a diamond matrix may permit a solid-state realization of the anisotropic
compass-Heisenberg model.Comment: 24 pages, 18 figure
Charge density wave in the spin ladder of SrCaCuO
We consider a multiband charge transfer model for a single spin ladder
describing the holes in SrCaCuO. Using Hartree-Fock
approximation we show how the charge density wave, with its periodicity
dependent on doping as recently observed in the experiment, can be stabilized
by purely electronic many-body interactions.Comment: 4 pages, 2 figures, accepted for publication in Physica C as the
proceedings of the M2S-HTSC VIII Conference, Dresden 200
Valence Bond Crystal and possible orbital pinball liquid in a t2g model
We study a model for orbitally degenerate Mott insulators, where localized
electrons possess t_2g degrees of freedom coupled by several, competing,
exchange mechanisms. We provide evidence for two distinct strongly fluctuating
regimes, depending on whether superexchange or direct exchange mechanism
predominates. In the superexchange-dominated regime, the ground state is
dimerized, with nearest neighbor orbital singlets covering the lattice. By
deriving an effective quantum dimer model and analyzing it numerically, we
characterize this dimerized phase as a valence bond crystal stabilized by
singlet resonances within a large unit cell. In the opposite regime, with
predominant direct exchange, the combined analysis of the original model and
another effective model adapted to the local constraints, shows that subleading
perturbations select a highly resonating ground state, with coexisting diagonal
and off-diagonal long-range orbital orders.Comment: 14 pages, 13 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
t-J model of coupled CuO ladders in SrCaCuO
Starting from the proper charge transfer model for CuO coupled
ladders in SrCaCuO we derive the low energy
Hamiltonian for this system. It occurs that the widely used ladder t-J model is
not sufficient and has to be supplemented by the Coulomb repulsion term between
holes in the neighboring ladders. Furthermore, we show how a simple mean-field
solution of the derived t-J model may explain the onset of the charge density
wave with the odd period in SrCaCuO.Comment: 8 pages, 4 figures, 2 table
{\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
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