3,223 research outputs found
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
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
Mott insulating state in a quarter-filled two-orbital Hubbard chain with different bandwidths
We investigate the ground-state properties of the one-dimensional two-band
Hubbard model with different bandwidths. The density-matrix renormalization
group method is applied to calculate the averaged electron occupancies as a
function of the chemical potential . Both at quarter and half fillings,
"charge plateaux" appear in the - plot, where diverges and
the Mott insulating states are realized. To see how the orbital polarization in
the one-quarter charge plateau develops, we apply the second-order perturbation
theory from the strong-coupling limit at quarter filling. The resultant
Kugel-Khomskii spin-orbital model includes a field coupled to
orbital pseudo-spins. This field originates from the discrepancy between the
two bandwidths and leads to a finite orbital pseudo-spin magnetization.Comment: 4 pages, 2 figures, Proceedings of LT2
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
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
Effect of electron-lattice interaction on the phase separation in strongly correlated electron systems with two types of charge carriers
The effect of electron-lattice interaction is studied for a strongly
correlated electron system described by the two-band Hubbard model. A two-fold
effect of electron-lattice interaction is taken into account: in non-diagonal
terms, it changes the effective bandwidth, whereas in diagonal terms, it shifts
the positions of the bands and the chemical potential. It is shown that this
interaction significantly affects the doping range corresponding to the
electronic phase separation and can even lead to a jump-like transition between
states with different values of strains.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
- …