2,439 research outputs found
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
N\'eel and Spin-Peierls ground states of two-dimensional SU(N) quantum antiferromagnets
The two-dimensional SU(N) quantum antiferromagnet, a generalization of the
quantum Heisenberg model, is investigated by quantum Monte Carlo simulations.
The ground state for is found to be of the N\'eel type with broken
SU(N) symmetry, whereas it is of the Spin-Peierls type for with broken
lattice translational invariance. No intermediate spin-liquid phase was
observed in contrast to previous numerical simulations on smaller lattices
[Santoro et al., Phys. Rev. Lett. {\bf 83} 3065 (1999)].Comment: 4 pages, 4 figure
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
Double-exchange via degenerate orbitals
We consider the double-exchange for systems in which doped electrons occupy
degenerate orbitals, treating the realistic situation with double degenerate
orbitals. We show that the orbital degeneracy leads in general to
formation of anisotropic magnetic structures and that in particular, depending
on the doping concentration, the layered magnetic structures of the A-type and
chain-like structures of the C-type are stabilized. The phase-diagram that we
obtain provides an explanation for the experimentally observed magnetic
structures of some over-doped (electron-doped) manganites of the type
NdSrMnO, PrSrMnO and SmCaMnO
with .Comment: 4 pages, 1 figur
Magnetic Phase Transition of the Perovskite-type Ti Oxides
Properties and mechanism of the magnetic phase transition of the
perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle
distortion, are studied theoretically by using the effective spin and
pseudo-spin Hamiltonian with strong Coulomb repulsion. It is shown that the
A-type antiferromagnetic(AFM(A)) to ferromagnetic(FM) phase transition occurs
as the Ti-O-Ti bond angle is decreased. Through this phase transition, the
orbital state is hardly changed so that the spin-exchange coupling along the
c-axis changes nearly continuously from positive to negative and takes
approximately zero at the phase boundary. The resultant strong
two-dimensionality in the spin coupling causes a rapid suppression of the
critical temperature as is observed experimentally.Comment: 9 pages, 5 figure
Two-dimensional gapless spin liquids in frustrated SU(N) quantum magnets
A class of the symmetrically frustrated SU(N) models is constructed for
quantum magnets based on the generators of SU(N) group. The total Hamiltonian
lacks SU(N) symmtry. A mean field theory in the quasi-particle representation
is developed for spin liquid states. Numerical solutions in two dimension
indicate that the ground states are gapless and the quasi-particles are Dirac
particles. The mechanism may be helpful in exploring the spin liquid phases in
the spin-1 bilinear-biquadratic model and the spin-orbital model in higher
dimensions.Comment: 9 pages, 3 figures, to appear in New Journal of Physic
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Neutral Beam Injection Requirements and Design Issues for the National Compact Stellarator Experiment
The National Compact Stellarator Experiment (NCSX) will require 6 MW of 50 keV neutral beam injection (NBI) with initial pulse lengths of 500 msec and upgradeable to pulse lengths of 1.5 sec. This paper discusses the NCSX NBI requirements and design issues, and shows how these are provided by the candidate PBX-M [Princeton Beta Experiment-Modification] NBI system
Charge and orbital ordering in underdoped La1-xSrxMnO3
We have explored spin, charge and orbitally ordered states in La1-xSrxMnO3 (0
< x < 1/2) using model Hartree-Fock calculations on d-p-type lattice models. At
x=1/8, several charge and orbitally modulated states are found to be stable and
almost degenerate in energy with a homogeneous ferromagnetic state. The present
calculation indicates that a ferromagnetic state with a charge modulation along
the c-axis which is consistent with the experiment by Yamada et al. might be
responsible for the anomalous behavior around x = 1/8.Comment: 5 pages, 5 figure
Magnetic and Orbital States and Their Phase Transition of the Perovskite-Type Ti Oxides: Strong Coupling Approach
The properties and mechanism of the magnetic phase transition of the
perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle
distortion, are studied theoretically by using the effective spin and
pseudospin Hamiltonian with strong Coulomb repulsion. It is shown that the
A-type antiferromagnetic (AFM(A)) to ferromagnetic (FM) phase transition occurs
as the Ti-O-Ti bond angle is decreased. Through this phase transition, the
orbital state changes only little whereas the spin-exchange coupling along the
c-axis is expected to change from positive to negative nearly continuously and
approaches zero at the phase boundary. The resultant strong two-dimensionality
in the spin coupling causes rapid suppression of the critical temperature, as
observed experimentally. It may induce large quantum fluctuations in this
region.Comment: 13 pages, 15 figure
Origin of G-type Antiferromagnetism and Orbital-Spin Structures in
The possibility of the distortion of octahedra is
examined theoretically in order to understand the origin of the G-type
antiferromagnetism (AFM(G)) and experimentally observed puzzling properties of
. By utilizing an effective spin and pseudospin Hamiltonian with
the strong Coulomb repulsion, it is shown that AFM(G) state is stabilized
through the lift of the -orbital degeneracy accompanied by a tiny
-distortion . The estimated spin-exchange interaction is in agreement
with that obtained by the neutron scattering. Moreover, the level-splitting
energy due to the distortion can be considerably larger than the spin-orbit
interaction even when the distortion becomes smaller than the detectable limit
under the available experimental resolution. This suggests that the orbital
momentum is fully quenched and the relativistic spin-orbit interaction is not
effective in this system, in agreement with recent neutron-scattering
experiment.Comment: 9 pages, 6 figure
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