3,109 research outputs found
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
Exact spin-orbital separation in a solvable model in one dimension
A one-dimensional model of coupled spin-1/2 spins and pseudospin-1/2 orbitals
with nearest-neighbor interaction is rigorously shown to exhibit spin-orbital
separation by means of a non-local unitary transformation. On an open chain,
this transformation completely decouples the spins from the orbitals in such a
way that the spins become paramagnetic while the orbitals form the soluble XXZ
Heisenberg model. The nature of various correlations is discussed. The more
general cases, which allow spin-orbital separation by the same method, are
pointed out. A generalization for the orbital pseudospin greater than 1/2 is
also discussed. Some qualitative connections are drawn with the recently
observed spin-orbital separation in Sr2CuO3.Comment: 5 page
Ground-state phase diagrams of the generalized Falicov-Kimball model with Hund coupling
Charge and spin orderings are studied on the simplest 1D and the 2D square
lattice within the generalized Falicov-Kimball model with Hund coupling between
localized and itinerant electrons. Using the restricted phase diagrams method
(RPDM) a number of simple rules of formation of various sorts of ground state
phases have been detected. In particular, relationships between density of
current carriers (electrons or holes) and type of charge and magnetic
arrangement has been determined. In 2D in the mixed valence regime only axial
stripes (vertical or horizontal) have been found for intermediate values of the
coupling constants. They are composed of ferromagnetic or antiferromagnetic
chains interchanged with non-magnetic ones. For band fillings close to the half
filling stripe phases oriented along one of the main diagonal direction are
formed. The results suggest a possibility of tuning modulations of charge and
magnetic superstructures with a change of doping.Comment: 10 pages, 6 figures; Fig. 2 slightly modified and the text adjusted
accordingly, references adde
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
Ferromagnetism and orbital order in the two-orbital Hubbard model
We investigate spin and orbital states of the two-orbital Hubbard model on a
square lattice by using a variational Monte Carlo method at quarter-filling,
i.e., the electron number per site is one. As a variational wave function, we
consider a Gutzwiller projected wave function of a mean-field type wave
function for a staggered spin and/or orbital ordered state. Then, we evaluate
expectation value of energy for the variational wave functions by using the
Monte Carlo method and determine the ground state. In the strong Coulomb
interaction region, the ground state is the perfect ferromagnetic state with
antiferro-orbital (AF-orbital) order. By decreasing the interaction, we find
that the disordered state becomes the ground state. Although we have also
considered the paramagnetic state with AF-orbital order, i.e., purely orbital
ordered state, and partial ferromagnetic states with and without AF-orbital
order, they do not become the ground state.Comment: 4 pages, 1 figure, accepted for publication in Journal of Physics:
Conference Serie
Jahn-Teller distortions and phase separation in doped manganites
A "minimal model" of the Kondo-lattice type is used to describe a competition
between the localization and metallicity in doped manganites and related
magnetic oxides with Jahn-Teller ions. It is shown that the number of itinerant
charge carriers can be significantly lower than that implied by the doping
level x. A strong tendency to the phase separation is demonstrated for a wide
range of intermediate doping concentrations vanishing at low and high doping.
The phase diagram of the model in the x-T plane is constructed. At low
temperatures, the system is in a state with a long-range magnetic order:
antiferromagnetic (AF), ferromagnetic (FM), or AF-FM phase separated (PS)
state. At high temperatures, there can exist two types of the paramagnetic (PM)
state with zero and nonzero density of the itinerant electrons. In the
intermediate temperature range, the phase diagram includes different kinds of
the PS states: AF-FM, FM-PM, and PM with different content of itinerant
electrons. The applied magnetic field changes the phase diagram favoring the FM
ordering. It is shown that the variation of temperature or magnetic field can
induce the metal-insulator transition in a certain range of doping levels.Comment: 14 pages, 7 figures, submitted to Phys. Rev. B.; v.2 contains the
changes introduced according to comments of the PRB Referees; in v. 3, some
misprints are correcte
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
Orbital ordering in charge transfer insulators
We discuss a new mechanism of orbital ordering, which in charge transfer
insulators is more important than the usual exchange interactions and which can
make the very type of the ground state of a charge transfer insulator, i.e. its
orbital and magnetic ordering, different from that of a Mott-Hubbard insulator.
This purely electronic mechanism allows us to explain why orbitals in
Jahn-Teller materials typically order at higher temperatures than spins, and to
understand the type of orbital ordering in a number of materials, e.g.
K_2CuF_4, without invoking the electron-lattice interaction.Comment: 4 pages, 2 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
Cooperative Jahn-Teller Effect and Electron-Phonon Coupling in
A classical model for the lattice distortions of \lax is derived and, in a
mean field approximation, solved. The model is based on previous work by
Kanamori and involves localized Mn d-electrons (which induce tetragonal
distortions of the oxygen octahedra surrounding the Mn) and localized holes
(which induce breathing distortions). Parameters are determined by fitting to
the room temperature structure of . The energy gained by formation of
a local lattice distortion is found to be large, most likely eV
per site, implying a strong electorn-phonon coupling and supporting polaronic
models of transport in the doped materials. The structural transition is shown
to be of the order-disorder type; the rapid x-dependence of the transition
temperature is argued to occur because added holes produce a "random" field
which misaligns the nearby sites.Comment: 24 pages. No figures. One Table. Late
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