1,598 research outputs found
Quasi-separatrix layers and three-dimensional reconnection diagnostics for line-tied tearing modes
In three-dimensional magnetic configurations for a plasma in which no closed
field line or magnetic null exists, no magnetic reconnection can occur, by the
strictest definition of reconnection. A finitely long pinch with line-tied
boundary conditions, in which all the magnetic field lines start at one end of
the system and proceed to the opposite end, is an example of such a system.
Nevertheless, for a long system of this type, the physical behavior in
resistive magnetohydrodynamics (MHD) essentially involves reconnection. This
has been explained in terms comparing the geometric and tearing widths [1, 2].
The concept of a quasi-separatrix layer[3, 4] was developed for such systems.
In this paper we study a model for a line-tied system in which the
corresponding periodic system has an unstable tearing mode. We analyze this
system in terms of two magnetic field line diagnostics, the squashing
factor[3-5] and the electrostatic potential difference used in kinematic
reconnection studies[6, 7]. We discuss the physical and geometric significance
of these two diagnostics and compare them in the context of discerning
tearing-like behavior in line-tied modes. [1] G. L. Delzanno and J. M. Finn.
Physics of Plasmas, 15(3):032904, 2008. [2] Y.-M. Huang and E. G. Zweibel.
Physics of Plasmas, 16(4):042102, 2009. [3] E. R. Priest and P. D\'emoulin. J.
Geophys. Res., 100(A12):23443-23463, 1995. [4] P. D\'emoulin, J. C. Henoux, E.
R. Priest, and C. H. Mandrini. Astron. Astrophys., 308:643-655, Apr. 1996. [5]
V. S. Titov and G. Hornig. Advances in Space Research, 29(7):1087-1092, 2002.
[6] Y. Lau and J. M. Finn. The Astrophysical Journal, 350:672-691, Feb. 1990.
[7] Y. Lau and J. M. Finn. The Astrophysical Journal, 366:577-591, 1991.Comment: 13 pages, 9 figures, Submitted to Commun Nonlinear Sci Numer Simula
Electron Spin Decoherence in Bulk and Quantum Well Zincblende Semiconductors
A theory for longitudinal (T1) and transverse (T2) electron spin coherence
times in zincblende semiconductor quantum wells is developed based on a
non-perturbative nanostructure model solved in a fourteen-band restricted basis
set. Distinctly different dependences of coherence times on mobility,
quantization energy, and temperature are found from previous calculations.
Quantitative agreement between our calculations and measurements is found for
GaAs/AlGaAs, InGaAs/InP, and GaSb/AlSb quantum wells.Comment: 11 pages, 3 figure
Magnetic-field dependence of electron spin relaxation in n-type semiconductors
We present a theoretical investigation of the magnetic field dependence of
the longitudinal () and transverse () spin relaxation times of
conduction band electrons in n-type III-V semiconductors. In particular, we
find that the interplay between the Dyakonov-Perel process and an additional
spin relaxation channel, which originates from the electron wave vector
dependence of the electron -factor, yields a maximal at a finite
magnetic field. We compare our results with existing experimental data on
n-type GaAs and make specific additional predictions for the magnetic field
dependence of electron spin lifetimes.Comment: accepted for publication in PRB, minor changes to previous manuscrip
On the Derivation of Vector Radiative Transfer Equation for Polarized Radiative Transport in Graded Index Media
Light transport in graded index media follows a curved trajectory determined
by the Fermat's principle. Besides the effect of variation of the refractive
index on the transport of radiative intensity, the curved ray trajectory will
induce geometrical effects on the transport of polarization ellipse. This paper
presents a complete derivation of vector radiative transfer equation for
polarized radiation transport in absorption, emission and scattering graded
index media. The derivation is based on the analysis of the conserved
quantities for polarized light transport along curved trajectory and a novel
approach. The obtained transfer equation can be considered as a generalization
of the classic vector radiative transfer equation that is only valid for
uniform refractive index media. Several variant forms of the transport equation
are also presented, which include the form for Stokes parameters defined with a
fixed reference and the Eulerian forms in the ray coordinate and in several
common orthogonal coordinate systems.Comment: This paper has been submitted to JQSR
Measurement of adsorption of a single component from the liquid phase : modelling investigation and sensitivity analysis
In this work, we consider an alternative approach for the measurement of adsorption from the liquid phase. Consider a mixture consisting of a non-adsorbed component (B) and an adsorbed component (A) present at some low concentration. Initially, a feed of component B only flows through a column packed with an adsorbent. Then, the feed is switched to the mixture of A and B. As soon as the mixture enters the column, there will be a reduction in the outlet flow rate as component A leaves the liquid phase and passes into the adsorbed phase. There are three stages to this work. The first is to develop overall and component balances to show how the amount adsorbed of component A can be determined from the variation in the column outlet flow rate. The second is to determine the actual variation in the column outlet flow rate for both plug flow and axial-dispersed plug flow. The final stage is to consider the suitability of a gravity-fed system to deliver the feed to the column. An analysis of the results shows that the experimental arrangement should be able to accurately monitor adsorption from the liquid phase where the mass fraction of the solute is of the order of 1%: the limiting experimental factor is how constant the volumetric flow rate of the liquid feed can be maintained
Multifractal characterisation of length sequences of coding and noncoding segments in a complete genome
The coding and noncoding length sequences constructed from a complete genome
are characterised by multifractal analysis. The dimension spectrum and
its derivative, the 'analogous' specific heat , are calculated for the
coding and noncoding length sequences of bacteria, where is the moment
order of the partition sum of the sequences. From the shape of the
and curves, it is seen that there exists a clear difference between the
coding/noncoding length sequences of all organisms considered and a completely
random sequence. The complexity of noncoding length sequences is higher than
that of coding length sequences for bacteria. Almost all curves for
coding length sequences are flat, so their multifractality is small whereas
almost all curves for noncoding length sequences are multifractal-like.
We propose to characterise the bacteria according to the types of the
curves of their noncoding length sequences.Comment: 15 pages with 5 figures, Latex, Accepted for publication in Physica
Intersubband spin-density excitations in quantum wells with Rashba spin splitting
In inversion-asymmetric semiconductors, spin-orbit coupling induces a
k-dependent spin splitting of valence and conduction bands, which is a
well-known cause for spin decoherence in bulk and heterostructures.
Manipulating nonequilibrium spin coherence in device applications thus requires
understanding how valence and conduction band spin splitting affects carrier
spin dynamics. This paper studies the relevance of this decoherence mechanism
for collective intersubband spin-density excitations (SDEs) in quantum wells. A
density-functional formalism for the linear spin-density matrix response is
presented that describes SDEs in the conduction band of quantum wells with
subbands that may be non-parabolic and spin-split due to bulk or structural
inversion asymmetry (Rashba effect). As an example, we consider a 40 nm
GaAs/AlGaAs quantum well, including Rashba spin splitting of the conduction
subbands. We find a coupling and wavevector-dependent splitting of the
longitudinal and transverse SDEs. However, decoherence of the SDEs is not
determined by subband spin splitting, due to collective effects arising from
dynamical exchange and correlation.Comment: 10 pages, 4 figure
Spin relaxation in (110) and (001) InAs/GaSb superlattices
We report an enhancement of the electron spin relaxation time (T1) in a (110)
InAs/GaSb superlattice by more than an order of magnitude (25 times) relative
to the corresponding (001) structure. The spin dynamics were measured using
polarization sensitive pump probe techniques and a mid-infrared, subpicosecond
PPLN OPO. Longer T1 times in (110) superlattices are attributed to the
suppression of the native interface asymmetry and bulk inversion asymmetry
contributions to the precessional D'yakonov Perel spin relaxation process.
Calculations using a nonperturbative 14-band nanostructure model give good
agreement with experiment and indicate that possible structural inversion
asymmetry contributions to T1 associated with compositional mixing at the
superlattice interfaces may limit the observed spin lifetime in (110)
superlattices. Our findings have implications for potential spintronics
applications using InAs/GaSb heterostructures.Comment: 4 pages, 2 figure
The role of point-like topological excitations at criticality: from vortices to global monopoles
We determine the detailed thermodynamic behavior of vortices in the O(2)
scalar model in 2D and of global monopoles in the O(3) model in 3D. We
construct new numerical techniques, based on cluster decomposition algorithms,
to analyze the point defect configurations. We find that these criteria produce
results for the Kosterlitz-Thouless temperature in agreement with a topological
transition between a polarizable insulator and a conductor, at which free
topological charges appear in the system. For global monopoles we find no pair
unbinding transition. Instead a transition to a dense state where pairs are no
longer distinguishable occurs at T<Tc, without leading to long range disorder.
We produce both extensive numerical evidence of this behavior as well as a
semi-analytic treatment of the partition function for defects. General
expectations for N=D>3 are drawn, based on the observed behavior.Comment: 14 pages, REVTEX, 13 eps figure
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