24,131 research outputs found
Coulomb Interactions and Ferromagnetism in Pure and Doped Graphene
We study the presence of ferromagnetism in the phase diagram of the
two-dimensional honeycomb lattice close to half-filling (graphene) as a
function of the strength of the Coulomb interaction and doping. We show that
exchange interactions between Dirac fermions can stabilize a ferromagnetic
phase at low doping when the coupling is sufficiently large. In clean systems,
the zero temperature phase diagram shows both first order and second order
transition lines and two distinct ferromagnetic phases: one phase with only one
type of carriers (either electrons or holes) and another with two types of
carriers (electrons and holes). Using the coherent phase approximation (CPA) we
argue that disorder further stabilizes the ferromagnetic phase.Comment: 10 pages; published versio
Conductance quantization in mesoscopic graphene
Using a generalized Landauer approach we study the non-linear transport in
mesoscopic graphene with zig-zag and armchair edges. We find that for clean
systems, the low-bias low-temperature conductance, G, of an armchair edge
system in quantized as G/t=4 n e^2/h, whereas for a zig-zag edge the
quantization changes to G/t t=4(n+1/2)e^2/h, where t is the transmission
probability and n is an integer. We also study the effects of a non-zero bias,
temperature, and magnetic field on the conductance. The magnetic field
dependence of the quantization plateaus in these systems is somewhat different
from the one found in the two-dimensional electron gas due to a different
Landau level quantization.Comment: 6 pages, 9 figures. Final version published in Physical Review
Localized states at zigzag edges of bilayer graphene
We report the existence of zero energy surface states localized at zigzag
edges of bilayer graphene. Working within the tight-binding approximation we
derive the analytic solution for the wavefunctions of these peculiar surface
states. It is shown that zero energy edge states in bilayer graphene can be
divided into two families: (i) states living only on a single plane, equivalent
to surface states in monolayer graphene; (ii) states with finite amplitude over
the two layers, with an enhanced penetration into the bulk. The bulk and
surface (edge) electronic structure of bilayer graphene nanoribbons is also
studied, both in the absence and in the presence of a bias voltage between
planes.Comment: 4 pages, 5 figure
Astrophysical parameters and orbital solution of the peculiar X-ray transient IGR J00370+6122
BD+6073 is the optical counterpart of the X-ray source IGR J00370+6122, a
probable accretion-powered X-ray pulsar. The X-ray light curve of this binary
system shows clear periodicity at 15.7 d, which has been interpreted as
repeated outbursts around the periastron of an eccentric orbit. We obtained
high-resolution spectra of BD+6073 at different epochs. We used the FASTWind
code to generate a stellar atmosphere model to fit the observed spectrum and
obtain physical magnitudes. The synthetic spectrum was used as a template for
cross-correlation with the observed spectra to measure radial velocities. The
radial velocity curve provided an orbital solution for the system. We have also
analysed the RXTE/ASM and Swift/BAT light curves to confirm the stability of
the periodicity. BD +6073 is a BN0.7 Ib low-luminosity supergiant located at an
approximate distance of 3.1 kpc, in the CasOB4 association. We derive
Teff=24000 K and log gc=3.0, and chemical abundances consistent with a
moderately high level of evolution. The spectroscopic and evolutionary masses
are consistent at the 1 sigma level with a mass of 15 solar masses. The
recurrence time of the X-ray flares is the orbital period of the system. The NS
is in a high eccentricity (e=0.56) orbit, and the X-ray emission is strongly
peaked around orbital phase 0.2, though the observations are consistent with
some level of X-ray activity happening at all orbital phases. The X-ray
behaviour of IGR J00370+6122 is reminiscent of intermediate SFXTs, though its
peak luminosity is rather low. The orbit is somewhat wider than those of
classical persistent supergiant X-ray binaries, which, combined with the low
luminosity of the mass donor, explains the low X-ray luminosity. IGR
J00370+6122 will likely evolve towards a persistent supergiant system,
highlighting the evolutionary connection between different classes of
wind-accreting X-ray sources.Comment: Accepted for publication in A&
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