16,894 research outputs found
Quantum Hall Ferromagnets: Induced Topological term and electromagnetic interactions
The quantum Hall ground state in materials like GaAs is well known
to be ferromagnetic in nature. The exchange part of the Coulomb interaction
provides the necessary attractive force to align the electron spins
spontaneously. The gapless Goldstone modes are the angular deviations of the
magnetisation vector from its fixed ground state orientation. Furthermore, the
system is known to support electrically charged spin skyrmion configurations.
It has been claimed in the literature that these skyrmions are fermionic owing
to an induced topological Hopf term in the effective action governing the
Goldstone modes. However, objections have been raised against the method by
which this term has been obtained from the microscopics of the system. In this
article, we use the technique of the derivative expansion to derive, in an
unambiguous manner, the effective action of the angular degrees of freedom,
including the Hopf term. Furthermore, we have coupled perturbative
electromagnetic fields to the microscopic fermionic system in order to study
their effect on the spin excitations. We have obtained an elegant expression
for the electromagnetic coupling of the angular variables describing these spin
excitations.Comment: 23 pages, Plain TeX, no figure
PseudoSkyrmion Effects on Tunneling Conductivity in Coherent Bilayer Quantum Hall States at
We present a mechamism why interlayer tunneling conductivity in coherent
bilayer quantum Hall states at is anomalously large, but finite in the
recent experiment. According to the mechanism, pseudoSkyrmions causes the
finite conductivity, although there exists an expectation that dissipationless
tunneling current arises in the state. PseudoSkyrmions have an intrinsic
polarization field perpendicular to the layers, which causes the dissipation.
Using the mechanism we show that the large peak in the conductivity remains for
weak parallel magnetic field, but decay rapidly after its strength is beyond a
critical one, Tesla.Comment: 6 pages, no figure
Subclinical Hypertrophic Cardiomyopathy in Elite Athletes: Knowledge Gaps Persist
Subclinical hypertrophic cardiomyopathy (HCM) is a phenotypic entity that has emerged from the increased use of cardiovascular magnetic resonance imaging in the evaluation and family screening of patients with HCM. We describe the case of a competitive athlete with a sarcomere gene mutation and family history of HCM who was found to exhibit the subclinical HCM phenotype on cardiovascular magnetic resonance imaging in the absence of left ventricular hypertrophy. We discuss the clinical uncertainties in her management. (Level of Difficulty: Advanced.
Spin-Pseudospin Coherence and CP Skyrmions in Bilayer Quantum Hall Ferromagnets
We analyze bilayer quantum Hall ferromagnets, whose underlying symmetry group
is SU(4). Spin-pseudospin coherence develops spontaneously when the total
electron density is low enough. Quasiparticles are CP^3 skyrmions. One skyrmion
induces charge modulations on both of the two layers. At the filling factor one elementary excitation consists of a pair of skyrmions and its charge
is . Recent experimental data due to Sawada et al. [Phys. Rev. Lett. {\bf
80}, 4534 (1998)] support this conclusion.Comment: 4 pages including 2 figures (published version
Transition from small to large world in growing networks
We examine the global organization of growing networks in which a new vertex
is attached to already existing ones with a probability depending on their age.
We find that the network is infinite- or finite-dimensional depending on
whether the attachment probability decays slower or faster than .
The network becomes one-dimensional when the attachment probability decays
faster than . We describe structural characteristics of these
phases and transitions between them.Comment: 5 page
Developing a gas rocket performance prediction technique
A simple, semi-empirical performance correlation/prediction technique applicable to gaseous and liquid propellant rocket engines is presented. Excellent correlations were attained for over 100 test firings by adjusting the computation of the gaseous mixing of an unreactive, coaxial jet using a correlation factor, F, which resulted in prediction of the experimental combustion efficiency for each firing. Static pressure, mean velocity and turbulence intensity in the developing region of non-reactive coaxial jets, typical of those of coaxial injector elements were determined. Detailed profiles were obtained at twelve axial locations (extending from the nozzle exit for a distance of five diameters) downstream from a single element of the Bell Aerospace H2/O2 19-element coaxial injector. These data are compared with analytical predictions made using both eddy viscosity and turbulence kinetic energy mixing models and available computer codes. Comparisons were disappointing, demonstrating the necessity of developing improved turbulence models and computational techniques before detailed predictions of practical coaxial free jet flows are attempted
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