2,572 research outputs found
Oxygen-stripes in La0.5Ca0.5MnO3 from ab initio calculations
We investigate the electronic, magnetic and orbital properties of
La0.5Ca0.5MnO3 perovskite by means of an ab initio electronic structure
calculation within the Hartree-Fock approximation. Using the experimental
crystal structure reported by Radaelli et al. [Phys. Rev B 55, 3015 (1997)], we
find a charge-ordering stripe-like ground state. The periodicity of the
stripes, and the insulating CE-type magnetic structure are in agreement with
neutron x-ray and electron diffraction experiments. However, the detailed
structure is more complex than that envisaged by simple models of charge and
orbital order on Mn d-levels alone, and is better described as a charge-density
wave of oxygen holes, coupled to the Mn spin/orbital order.Comment: 4 pages, 3 figures. Version accepted for publication in PR
Thermodynamics of symmetric spin--orbital model: One- and two-dimensional cases
The specific heat and susceptibilities for the two- and one-dimensional
spin--orbital models are calculated in the framework of a spherically symmetric
self-consistent approach at different temperatures and relations between the
parameters of the system. It is shown that even in the absence of the
long-range spin and orbital order, the system exhibits the features in the
behavior of thermodynamic characteristics, which are typical of those
manifesting themselves at phase transitions. Such features are attributed to
the quantum entanglement of the coupled spin and orbital degrees of freedom.Comment: 7 pages, 9 figures, submitted to JETP Letter
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
Elementary excitations of the symmetric spin-orbital model: The XY limit
The elementary excitations of the 1D, symmetric, spin-orbital model are
investigated by studying two anisotropic versions of the model, the pure XY and
the dimerized XXZ case, with analytical and numerical methods. While they
preserve the symmetry between spin and orbital degrees of freedom, these models
allow for a simple and transparent picture of the low--lying excitations: In
the pure XY case, a phase separation takes place between two phases with
free--fermion like, gapless excitations, while in the dimerized case, the
low-energy effective Hamiltonian reduces to the 1D Ising model with gapped
excitations. In both cases, all the elementary excitations involve simultaneous
flips of the spin and orbital degrees of freedom, a clear indication of the
breakdown of the traditional mean-field theory.Comment: Revtex, two figure
Orbital ordering in frustrated Jahn-Teller systems
We consider the superexchange in `frustrated' Jahn-Teller systems, such as
the transition metal oxides NaNiO_2, LiNiO_2, and ZnMn_2O_4, in which
transition metal ions with doubly degenerate orbitals form a triangular or
pyrochlore lattice and are connected by the 90-degree metal-oxygen-metal bonds.
We show that this interaction is much different from a more familiar exchange
in systems with the 180-degree bonds, e.g. perovskites. In contrast to the
strong interplay between the orbital and spin degrees of freedom in
perovskites, in the 90-degree exchange systems spins and orbitals are
decoupled: the spin exchange is much weaker than the orbital one and it is
ferromagnetic for all orbital states. Due to frustration, the mean-field
orbital ground state is strongly degenerate. Quantum orbital fluctuations
select particular ferro-orbital states, such as the one observed in NaNiO_2. We
also discuss why LiNiO_2 may still behave as an orbital liquid.Comment: 5 pages, 3 figure
Spin and orbital excitation spectrum in the Kugel-Khomskii model
We discuss spin and orbital ordering in the twofold orbital degenerate
superexchange model in three dimensions relevant to perovskite transition metal
oxides. We focus on the particular point on the classical phase diagram where
orbital degeneracy is lifted by quantum effects exclusively. Dispersion and
damping of the spin and orbital excitations are calculated at this point taking
into account their mutual interaction. Interaction corrections to the
mean-field order parameters are found to be small. We conclude that
quasi-one-dimensional Neel spin order accompanied by the uniform
d_{3z^2-r^2}-type orbital ordering is stable against quantum fluctuations.Comment: 4 pages with 3 PS figures, 1 table, RevTeX, accepted to Phys. Rev. B.
Rapid Communicatio
Thermodynamics of the one-dimensional SU(4) symmetric spin-orbital model
The ground state properties and the thermodynamics of the one-dimensional
SU(4) symmetric spin system with orbital degeneracy are investigated using the
quantum Monte Carlo loop algorithm. The spin-spin correlation functions exhibit
a 4-site periodicity, and their low temperature behavior is controlled by two
correlation lengths that diverge like the inverse temperature, while the
entropy is linear in temperature and its slope is consistent with three gapless
modes of velocity . The physical implications of these results are
discussed.Comment: 4 pages, 4 figures, RevTe
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