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: $E^{-2.65}$ and $E^{-3.06}$ 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