5,153 research outputs found
Dielectrophoresis model for the colossal electroresistance of phase-separated manganites
We propose a dielectrophoresis model for phase-separated manganites. Without
increase of the fraction of metallic phase, an insulator-metal transition
occurs when a uniform electric field applied across the system exceeds a
threshold value. Driven by the dielectrophoretic force, the metallic clusters
reconfigure themselves into stripes along the direction of electric field,
leading to the filamentous percolation. This process, which is time-dependent,
irreversible and anisotropic, is a probable origin of the colossal
electroresistance in manganites.Comment: 4 pages, 5 figure
Quantum confinement effects on the ordering of the lowest-lying excited states in conjugated chains
The symmetrized density matrix renormalization group approach is applied
within the extended Hubbard-Peierls model (with parameters U/t, V/t, and bond
alternation \delta) to study the ordering of the lowest one-photon
(1^{1}B^{-}_u) and two-photon (2^{1}A^{+}_g) states in one- dimensional
conjugated systems with chain lengths, N, up to N=80 sites. Three different
types of crossovers are studied, as a function of U/t, \delta, and N. The
U-crossover emphasizes the larger ionic character of the 2A_g state compared to
the lowest triplet excitation. The \delta crossover shows strong dependence on
both N and U/t. The N-crossover illustrates the more localized nature of the
2A_g excitation relative to the 1B_u excitation at intermediate correlation
strengths.Comment: Latex file; figures available upon request. Submitted to PR
Spin-transfer torques in anti-ferromagnetic metals from first principles
In spite of the absence of a macroscopic magnetic moment, an anti-ferromagnet
is spin-polarized on an atomic scale. The electric current passing through a
conducting anti-ferromagnet is polarized as well, leading to spin-transfer
torques when the order parameter is textured, such as in anti-ferromagnetic
non-collinear spin valves and domain walls. We report a first principles study
on the electronic transport properties of anti-ferromagnetic systems. The
current-induced spin torques acting on the magnetic moments are comparable with
those in conventional ferromagnetic materials, leading to measurable angular
resistances and current-induced magnetization dynamics. In contrast to
ferromagnets, spin torques in anti-ferromagnets are very nonlocal. The torques
acting far away from the center of an anti-ferromagnetic domain wall should
facilitate current-induced domain wall motion.Comment: The paper has substantially been rewritten, 4 pages, 5 figure
Origin of the multiferroic spiral spin-order in the RMnO3 perovskites
The origin of the spiral spin-order in perovskite multiferroic manganites
MnO ( Tb or Dy) is here investigated using a two -orbitals double-exchange model. Our main result is that the experimentally
observed spiral phase can be stabilized by introducing a relatively weak
next-nearest-neighbor superexchange coupling ( of the nearest-neighbor
superexchange). Moreover, the Jahn-Teller lattice distortion is also shown to
be essential to obtain a realistic spiral period. Supporting our conclusions,
the generic phase diagram of undoped perovskite manganites is obtained using
Monte Carlo simulations, showing phase transitions from the A-type
antiferromagnet, to the spiral phase, and finally to the E-type
antiferromagnet, with decreasing size of the ions. These results are
qualitatively explained by the enhanced relative intensity of the
superexchanges.Comment: 6 pages, 4 figure
Non-magnetic B-site Impurities Induce Ferromagnetic Tendencies in CE Manganites
Using a two-orbital model and Monte Carlo simulations, we investigate the
effect of nonmagnetic B-site substitution on half-doped CE-type manganites. The
lattice defects induced by this substitution destabilize the CE phase, which
transforms into (1) the ferromagnetic (FM) metallic competing state, or (2) a
regime with short-range FM clusters, or (3) a spin-glass state, depending on
couplings and on the valence of the B-site substitution. While a C-type
antiferromagnetic state is usually associated with an average
charge density less than 0.5, the nonmagnetic B-site substitution that lowers
the charge density is still found to enhance the FM tendency in our
simulations. The present calculations are in qualitative agreement with
experiments and provide a rationalization for the complex role of nonmagnetic
B-site substitution in modulating the phase transitions in manganites.Comment: 8 pages, 5 figure
Low-Lying Electronic Excitations and Nonlinear Optic Properties of Polymers via Symmetrized Density Matrix Renormalization Group Method
A symmetrized Density Matrix Renormalization Group procedure together with
the correction vector approach is shown to be highly accurate for obtaining
dynamic linear and third order polarizabilities of one-dimensional Hubbard and
models. The model is seen to show characteristically different
third harmonic generation response in the CDW and SDW phases. This can be
rationalized from the excitation spectrum of the systems.Comment: 4 pages Latex; 3 eps figures available upon request; Proceedings of
ICSM '96, to appear in Synth. Metals, 199
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