5,311 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
- …