1,271 research outputs found
Radiation effects on the electronic structure of bilayer graphene
We report on the effects of laser illumination on the electronic properties
of bilayer graphene. By using Floquet theory combined with Green's functions we
unveil the appeareance of laser-induced gaps not only at integer multiples of
but also at the Dirac point with features which are shown to
depend strongly on the laser polarization. Trigonal warping corrections are
shown to lead to important corrections for radiation in the THz range, reducing
the size of the dynamical gaps. Furthermore, our analysis of the topological
properties at low energies reveals that when irradiated with linearly polarized
light, ideal bilayer graphene behaves as a trivial insulator, whereas circular
polarization leads to a non-trivial insulator per valley.Comment: 5 pages 3 figure
Crafting zero-bias one-way transport of charge and spin
We explore the electronic structure and transport properties of a metal on
top of a (weakly coupled) two-dimensional topological insulator. Unlike the
widely studied junctions between topological non-trivial materials, the systems
studied here allow for a unique bandstructure and transport steering. First,
states on the topological insulator layer may coexist with the gapless bulk
and, second, the edge states on one edge can be selectively switched-off,
thereby leading to nearly perfect directional transport of charge and spin even
in the zero bias limit. We illustrate these phenomena for Bernal stacked
bilayer graphene with Haldane or intrinsic spin-orbit terms and a perpendicular
bias voltage. This opens a path for realizing directed transport in materials
such as van der Waals heterostructures, monolayer and ultrathin topological
insulators.Comment: 7 pages, 7 figure
Joint DAMA-TCP protocol optimization through multiple cross layer interactions in DVB RCS scenario
Two aspects of DVB-RCS standard can worsen performance of TCP data connections: DAMA access scheme, since it introduces additional and variable delay to the already significant propagation delay and the adoption of Adaptive Coding on the return link to maximize bandwidth efficiency to face variable weather conditions, because it results in variable bandwidth allocation. Both aspects can severely impact TCP performance, especially for what concerns now adaptation to varying channel conditions and channel usage efficiency. To optimize performance, in this paper cross-layer signaling among transport, MAC and physical layers of a DVB-RCS system is addressed. In particular MAC-TCP cross-layer is analyzed through the use of NS2 network simulator, showing the possible benefit in a DVB-RCS scenario
Th Ages for Metal-Poor Stars
With a sample of 22 metal-poor stars, we demonstrate that the heavy element
abundance pattern (Z > 55) is the same as the r-process contributions to the
solar nebula. This bolsters the results of previous studies that there is a
universal r-process production pattern. We use the abundance of thorium in five
metal-poor stars, along with an estimate of the initial Th abundance based on
the abundances of stable r-process elements, to measure their ages. We have
four field red giants with errors of 4.2 Gyr in their ages and one M92 giant
with an error of 5.6 Gyr, based on considering the sources of observational
error only. We obtain an average age of 11.4 Gyr, which depends critically on
the assumption of an initial production ratio of Th/Eu of 0.496. If the
Universe is 15 Gyr old, then the initial Th/Eu value should be 0.590, in
agreement with some theoretical models of the r-process.Comment: 26 pages, to be published in Ap
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