945,935 research outputs found
Spin orientation by electric current in (110) quantum wells
We develop a theory of spin orientation by electric current in (110)-grown
semiconductor quantum wells. The controversy in the factor of two from two
existed approaches is resolved by pointing out the importance of energy
relaxation in this problem. The limiting cases of fast and slow energy
relaxation relative to spin relaxation are considered for asymmetric (110)
quantum wells. For symmetricly-doped structures the effect of spin orientation
is shown to exist due to spatial fluctuations of the Rashba spin-orbit
splitting. We demonstrate that the spin orientation depends strongly on the
correlation length of these fluctuations as well as on the ratio of the energy
and spin relaxation rates. The time-resolved kinetics of spin polarization by
electric current is also governed by the correlation length being not purely
exponential at slow energy relaxation. Electrical spin orientation in
two-dimensional topological insulators is calculated and compared with the spin
polarization induced by the magnetic field.Comment: 8 pages, 2 figure
Neutrino Masses and Mixings from String Theory Instantons
We study possible patterns of neutrino masses and mixings in string models in
which Majorana neutrino masses are generated by a certain class of string
theory instantons recently considered in the literature. These instantons may
generate either directly the dim=5 Weinberg operator or right-handed neutrino
Majorana masses, both with a certain flavour-factorised form. A hierarchy of
neutrino masses naturally appears from the exponentially suppressed
contributions of different instantons. The flavour structure is controlled by
string amplitudes involving neutrino fields and charged instanton zero modes.
For some simple choices for these amplitudes one finds neutrino mixing patterns
consistent with experimental results. In particular, we find that a
tri-bimaximal mixing pattern is obtained for simple symmetric values of the
string correlators.Comment: 24 pages, 2 figure
The effect of tip vortex structure on helicopter noise due to blade/vortex interaction
A potential cause of helicopter impulsive noise, commonly called blade slap, is the unsteady lift fluctuation on a rotor blade due to interaction with the vortex trailed from another blade. The relationship between vortex structure and the intensity of the acoustic signal is investigated. The analysis is based on a theoretical model for blade/vortex interaction. Unsteady lift on the blades due to blade/vortex interaction is calculated using linear unsteady aerodynamic theory, and expressions are derived for the directivity, frequency spectrum, and transient signal of the radiated noise. An inviscid rollup model is used to calculate the velocity profile in the trailing vortex from the spanwise distribution of blade tip loading. A few cases of tip loading are investigated, and numerical results are presented for the unsteady lift and acoustic signal due to blade/vortex interaction. The intensity of the acoustic signal is shown to be quite sensitive to changes in tip vortex structure
Poisson-Lie T-duality and N=2 superconformal WZNW models on compact groups
The supersymmetric generalization of Pisson-Lie T-duality in N=2
superconformal WZNW models on the compact groups is considered. It is shown
that the role of Drinfeld's doubles play the complexifications of the
corresponding compact groups. These complex doubles are used to define the
natural actions of the isotropic subgroups forming the doubles on the group
manifolds of the N=2 superconformal WZNW models. The Poisson- Lie T-duality in
N=2 superconformal U(2)-WZNW model considered in details. It is shown that this
model admits Poisson-Lie symmetries with respect to the isotropic subgroups
forming Drinfeld's double Gl(2,C). Poisson-Lie T-duality transformation maps
this model into itself but acts nontrivialy on the space of classical
solutions. Supersymmetric generalization of Poisson-Lie T-duality in N=2
superconformal WZNW models on the compact groups of higher dimensions is
proposed.Comment: 12 pages, latex, misprints correcte
Thermodynamics of the 3D Hubbard model on approach to the Neel transition
We study the thermodynamic properties of the 3D Hubbard model for
temperatures down to the Neel temperature using cluster dynamical mean-field
theory. In particular we calculate the energy, entropy, density, double
occupancy and nearest-neighbor spin correlations as a function of chemical
potential, temperature and repulsion strength. To make contact with cold-gas
experiments, we also compute properties of the system subject to an external
trap in the local density approximation. We find that an entropy per particle
at is sufficient to achieve a Neel state in the
center of the trap, substantially higher than the entropy required in a
homogeneous system. Precursors to antiferromagnetism can clearly be observed in
nearest-neighbor spin correlators.Comment: 4 pages, 6 figure
Entanglement and dynamics of spin-chains in periodically-pulsed magnetic fields: accelerator modes
We study the dynamics of a single excitation in a Heisenberg spin-chain
subjected to a sequence of periodic pulses from an external, parabolic,
magnetic field. We show that, for experimentally reasonable parameters, a pair
of counter-propagating coherent states are ejected from the centre of the
chain. We find an illuminating correspondence with the quantum time evolution
of the well-known paradigm of quantum chaos, the Quantum Kicked Rotor (QKR).
From this we can analyse the entanglement production and interpret the
ejected coherent states as a manifestation of so-called `accelerator modes' of
a classically chaotic system.Comment: 5 pages, 2 figures; minor corrections, tidied presentatio
Field-induced decay dynamics in square-lattice antiferromagnet
Dynamical properties of the square-lattice Heisenberg antiferromagnet in
applied magnetic field are studied for arbitrary value S of the spin. Above the
threshold field for two-particle decays, the standard spin-wave theory yields
singular corrections to the excitation spectrum with logarithmic divergences
for certain momenta. We develop a self-consistent approximation applicable for
S >= 1, which avoids such singularities and provides regularized magnon decay
rates. Results for the dynamical structure factor obtained in this approach are
presented for S = 1 and S = 5/2.Comment: 12 pages, 11 figures, final versio
Information Theory based on Non-additive Information Content
We generalize the Shannon's information theory in a nonadditive way by
focusing on the source coding theorem. The nonadditive information content we
adopted is consistent with the concept of the form invariance structure of the
nonextensive entropy. Some general properties of the nonadditive information
entropy are studied, in addition, the relation between the nonadditivity
and the codeword length is pointed out.Comment: 9 pages, no figures, RevTex, accepted for publication in Phys. Rev.
E(an error in proof of theorem 1 was corrected, typos corrected
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