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 G$_0$W 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

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

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 $GW$ 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 $r_s \simeq 3$ 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