24 research outputs found
Quantum Manifestations of Graphene Edge Stress and Edge Instability: A First-Principles Study
We have performed first-principles calculations of graphene edge stresses,
which display two interesting quantum manifestations absent from the classical
interpretation: the armchair edge stress oscillates with a nanoribbon width,
and the zigzag edge stress is noticeably reduced by spin polarization. Such
quantum stress effects in turn manifest in mechanical edge twisting and warping
instability, showing features not captured by empirical potentials or continuum
theory. Edge adsorption of H and Stone-Wales reconstruction are shown to
provide alternative mechanisms in relieving the edge compression and hence to
stabilize the planar edge structure.Comment: 5figure
Two component dark matter
We explain the PAMELA positron excess and the PPB-BETS/ATIC e+ + e- data
using a simple two component dark matter model (2DM). The two particle species
in the dark matter sector are assumed to be in thermal equilibrium in the early
universe. While one particle is stable and is the present day dark matter, the
second one is metastable and decays after the universe is 10^-8 s old. In this
model it is simple to accommodate the large boost factors required to explain
the PAMELA positron excess without the need for large spikes in the local dark
matter density. We provide the constraints on the parameters of the model and
comment on possible signals at future colliders.Comment: 6 pages, 2 figures, discussion clarified and extende
Absolute electron and positron fluxes from PAMELA/Fermi and Dark Matter
We extract the positron and electron fluxes in the energy range 10 - 100 GeV
by combining the recent data from PAMELA and Fermi LAT. The {\it absolute
positron and electron} fluxes thus obtained are found to obey the power laws:
and respectively, which can be confirmed by the
upcoming data from PAMELA. The positron flux appears to indicate an excess at
energies E\gsim 50 GeV even if the uncertainty in the secondary positron flux
is added to the Galactic positron background. This leaves enough motivation for
considering new physics, such as annihilation or decay of dark matter, as the
origin of positron excess in the cosmic rays.Comment: Accepted by JCA
Mechanically Robust Tri-Wing Graphene Nanoribbons with Tunable Electronic and Magnetic Properties
Inspired by strong mechanical stability of "Y"-shaped beams for building construction, we design a new class of quasione-dimensional graphene nanostructures, namely, tri-wing graphene (TWG) nanoribbons. TWG possesses significantly augmented mechanical stability against torsional and compression forces, and also each wing of the TWG can retain independent electronic properties of the constituent graphene nanoribbons. As such, by tailoring the wing structures, the TWGs can provide broader property tunability for nanoelectronic application. In addition, zigzag-edged TWG is a metallic ferromagnet with a large magnetic moment. When its edges are decorated with suitable chemical functional groups, a TWG can be converted to a half metal for potential spintronic applications