2,480 research outputs found
Effective Hamiltonian of Three-orbital Hubbard Model on Pyrochlore Lattice: Application to LiV_2O_4
We investigate heavy fermion behaviors in the vanadium spinel LiV_2O_4. We
start from a three-orbital Hubbard model on the pyrochlore lattice and derive
its low-energy effective Hamiltonian by an approach of real-space
renormalization group type. One important tetrahedron configuration in the
rochlore lattice has a three-fold orbital degeneracy and spin S=1, and
correspondingly, the effective Hamiltonian has spin and orbital exchange
interactions of Kugel-Khomskii type as well as correlated electron hoppings.
Analyzing the effective Hamiltonian, we find that ferromagnetic double exchange
processes compete with antiferromagnetic superexchange processes and various
spin and orbital exchange processes are competing to each other. These results
suggest the absence of phase transition in spin and orbital spaces down to very
low temperatures and their large fluctuations in the low-energy sector, which
are key issues for understanding the heavy fermion behavior in LiV_2O_4.Comment: 26 pages, 26 figure
Orbitally-driven Peierls state in spinels
We consider the superstructures, which can be formed in spinels containing on
B-sites the transition-metal ions with partially filled t2g levels. We show
that, when such systems are close to itinerant state (e.g. have an
insulator-metal transition), there may appear in them an orbitally-driven
Peierls state. We explain by this mechanism the very unusual superstructures
observed in CuIr2S4 (octamers) and MgTi2O4 (chiral superstructures) and suggest
that similar phenomenon should be observed in NaTiO2 and possibly in some other
systems.Comment: 4 pages, 3 figure
Quantum Versus Jahn-Teller Orbital Physics in YVO and LaVO
We argue that the large Jahn-Teller (JT) distortions in YVO and LaVO
should suppress the quantum orbital fluctuation. The unusual magnetic
properties can be well explained based on LDA+ calculations using
experimental structures, in terms of the JT orbital. The observed splitting of
the spin-wave dispersions for YVO in C-type antiferromagnetic state is
attributed to the inequivalent VO layers in the crystal structure, instead
of the ``orbital Peierls state''. Alternative stacking of -plane exchange
couplings produces the c-axis spin-wave splitting, thus the spin system is
highly three dimensional rather than quasi-one-dimensional. Similar splitting
is also predicted for LaVO, although it is weak.Comment: 4 pages, 2 tables, 2 figures, (accepted by PRL
The spin-wave spectrum of the Jahn-Teller system LaTiO3
We present an analytical calculation of the spin-wave spectrum of the
Jahn-Teller system LaTiO3. The calculation includes all superexchange couplings
between nearest-neighbor Ti ions allowed by the space-group symmetries: The
isotropic Heisenberg couplings and the antisymmetric (Dzyaloshinskii-Moriya)
and symmetric anisotropies. The calculated spin-wave dispersion has four
branches, two nearly degenerate branches with small zone-center gaps and two
practically indistinguishable high-energy branches having large zone-center
gaps. The two lower-energy modes are found to be in satisfying agreement with
neutron-scattering experiments. In particular, the experimentally detected
approximate isotropy in the Brillouin zone and the small zone-center gap are
well reproduced by the calculations. The higher-energy branches have not been
detected yet by neutron scattering but their zone-center gaps are in satisfying
agreement with recent Raman data.Comment: 13 pages, 5 figure
Dimensional tuning of electronic states under strong and frustrated interactions
We study a model of strongly interacting spinless fermions on an anisotropic
triangular lattice. At half-filling and the limit of strong repulsive
nearest-neighbor interactions, the fermions align in stripes and form an
insulating state. When a particle is doped, it either follows a one-dimensional
free motion along the stripes or fractionalizes perpendicular to the stripes.
The two propagations yield a dimensional tuning of the electronic state. We
study the stability of this phase and derive an effective model to describe the
low-energy excitations. Spectral functions are presented which can be used to
experimentally detect signatures of the charge excitations.Comment: 4pages 4figures included. to appear in Phys. Rev. Lett. vol. 10
Anharmonic effect on lattice distortion, orbital ordering and magnetic properties in Cs2AgF4
We develop the cluster self-consistent field method incorporating both
electronic and lattice degrees of freedom to study the origin of ferromagnetism
in CsAgF. After self-consistently determining the harmonic and
anharmonic Jahn-Teller distortions, we show that the anharmonic distortion
stabilizes the staggered x-z/y-z orbital and
ferromagnetic ground state, rather than the antiferromagnetic one. The
amplitudes of lattice distortions, Q and Q, the magnetic coupling
strengthes, J, and the magnetic moment, are in good agreement with the
experimental observation.Comment: 13 pages, 5 figure
Structural, orbital, and magnetic order in vanadium spinels
Vanadium spinels (ZnV_2O_4, MgV_2O_4, and CdV_2O_4) exhibit a sequence of
structural and magnetic phase transitions, reflecting the interplay of lattice,
orbital, and spin degrees of freedom. We offer a theoretical model taking into
account the relativistic spin-orbit interaction, collective Jahn-Teller effect,
and spin frustration. Below the structural transition, vanadium ions exhibit
ferroorbital order and the magnet is best viewed as two sets of
antiferromagnetic chains with a single-ion Ising anisotropy. Magnetic order,
parametrized by two Ising variables, appears at a tetracritical point.Comment: v3: streamlined introductio
Spin-orbital gap of multiorbital antiferromagnet
In order to discuss the spin-gap formation in a multiorbital system, we
analyze an e_g-orbital Hubbard model on a geometrically frustrated zigzag chain
by using a density-matrix renormalization group method. Due to the appearance
of a ferro-orbital arrangement, the system is regarded as a one-orbital system,
while the degree of spin frustration is controlled by the spatial anisotropy of
the orbital. In the region of strong spin frustration, we observe a finite
energy gap between ground and first-excited states, which should be called a
spin-orbital gap. The physical meaning is clarified by an effective Heisenberg
spin model including correctly the effect of the orbital arrangement influenced
by the spin excitation.Comment: 8 pages, 6 figures, extended versio
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