1,878 research outputs found
Applications of topology in magnetic fields
This thesis concerns applications of topology in magnetic fields. First, we examine
the influence of writhe in the stretch-twist-fold dynamo. We consider a thin flux
tube distorted by simple stretch, twist, and fold motions and calculate the helicity
and energy spectra. The writhe number assists in the calculations, as it tells us how
much the internal twist changes as the tube is distorted. In addition it provides
a valuable diagnostic for the degree of distortion. Non mirror-symmetric dynamos
typically generate magnetic helicity of one sign on large-scales and of the opposite
sign on small-scales. The calculations presented here confirm the hypothesis that
the large-scale helicity corresponds to writhe and the small-scale corresponds to
twist. In addition, the writhe helicity spectrum exhibits an interesting oscillatory
behaviour.
Second, we examine the effect of reconnection on the structure of a braided magnetic
field. A prominent model for both heating of the solar corona and the source
of small flares involves reconnection of braided magnetic flux elements. Much of
this braiding is thought to occur at as yet unresolved scales, for example braiding of
threads within an EUV or X-ray loop. However, some braiding may be still visible
at scales accessible to Trace or the EIS imager on Hinode. We suggest that attempts
to estimate the amount of braiding at these scales must take into account the degree
of coherence of the braid structure. We demonstrate that simple models of
braided magnetic fields which balance input of topological structure with reconnection
evolve to a self-organized critical state. An initially random braid can become
highly ordered, with coherence lengths obeying power law distributions. The energy
released during reconnection also obeys a power law
The effect of sublattice symmetry breaking on the electronic properties of a doped graphene
Motivated by a number of recent experimental studies, we have carried out the
microscopic calculation of the quasiparticle self-energy and spectral function
in a doped graphene when a symmetry breaking of the sublattices is occurred.
Our systematic study is based on the many-body GW approach that is
established on the random phase approximation and on graphene's massive Dirac
equation continuum model. We report extensive calculations of both the real and
imaginary parts of the quasiparticle self-energy in the presence of a gap
opening. We also present results for spectral function, renormalized Fermi
velocity and band gap renormalization of massive Dirac Fermions over a broad
range of electron densities. We further show that the mass generating in
graphene washes out the plasmaron peak in spectral weight.Comment: 22 Pages, 10 Figure
Many-body effective mass enhancement in a two-dimensional electron liquid
Motivated by a large number of recent magnetotransport studies we have
revisited the problem of the microscopic calculation of the quasiparticle
effective mass in a paramagnetic two-dimensional (2D) electron liquid (EL). Our
systematic study is based on a generalized approximation which makes use
of the many-body local fields and takes advantage of the results of the most
recent QMC calculations of the static charge- and spin-response of the 2D EL.
We report extensive calculations for the many-body effective mass enhancement
over a broad range of electron densities. In this respect we critically examine
the relative merits of the on-shell approximation, commonly used in
weak-coupling situations, {\it versus} the actual self-consistent solution of
the Dyson equation. We show that already for and higher, a
solution of the Dyson equation proves here necessary in order to obtain a well
behaved effective mass. Finally we also show that our theoretical results for a
quasi-2D EL, free of any adjustable fitting parameters, are in good qualitative
agreement with some recent measurements in a GaAs/AlGaAs heterostructure.Comment: 12 pages, 3 figures, CMT28 Conference Proceedings, work related to
cond-mat/041226
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