Stabilisation of Seven Directions in an Early Universe M -- theory Model

Our model consists of intersecting 22'55' branes in M theory distributed uniformly in the common transverse space. Equations of state follow from U duality symmetries. In this model, three spatial directions expand, and seven directions stabilise to constant sizes. From string theory perspective, the dilaton is hence stabilised. The constant sizes depend on certain imbalance among initial values. One naturally obtains M_{11} \simeq M_s \simeq M_4 and g_s \simeq 1 within a few orders of magnitude. Smaller numbers, for example M_s \simeq 10^{- 16} M_4, are also possible but require fine tuning.Comment: 11 Pagesl. Latex file. Version 2: Minor changes and a reference added. Version 3: Title changed; many textual modifications, mainly to keep within journal's length constraints, and to explain the differences between brane gas models and intersecting brane configurations used here; references added. To appear in Physical Review D (Rapid communications

Anisotropy of the Stone-Wales Defect and Warping of Graphene Nano-ribbons: A First-principles Analysis

Stone-Wales (SW) defects, analogous to dislocations in crystals, play an important role in mechanical behavior of $sp^2$-bonded carbon based materials. Here, we show using first-principles calculations that a marked anisotropy in the interaction among the SW defects has interesting consequences when such defects are present near the edges of a graphene nano-ribbon: depending on their orientation with respect to edge, they result in compressive or tensile stress, and the former is responsible to depression or warping of the graphene nano-ribbon. Such warping results in delocalization of electrons in the defect states.Comment: 8 page

Quantum Isometries of the finite noncommutative geometry of the Standard Model

We compute the quantum isometry group of the finite noncommutative geometry F describing the internal degrees of freedom in the Standard Model of particle physics. We show that this provides genuine quantum symmetries of the spectral triple corresponding to M x F where M is a compact spin manifold. We also prove that the bosonic and fermionic part of the spectral action are preserved by these symmetries.Comment: 29 pages, no figures v3: minor change

10 + 1 to 3 + 1 in an Early Universe with mutually BPS Intersecting Branes

We assume that the early universe is homogeneous, anisotropic, and is dominated by the mutually BPS 22'55' intersecting branes of M theory. The spatial directions are all taken to be toroidal. Using analytical and numerical methods, we study the evolution of such an universe. We find that, asymptotically, three spatial directions expand to infinity and the remaining spatial directions reach stabilised values. Any stabilised values can be obtained by a fine tuning of initial brane densities. We give a physical description of the stabilisation mechanism. Also, from the perspective of four dimensional spacetime, the effective four dimensional Newton's constant G_4 is now time varying. Its time dependence will follow from explicit solutions. We find in the present case that, asymptotically, G_4 exhibits characteristic log periodic oscillations.Comment: Latex file, 59 pages, 7 figures. Version 2: A minor correction and a reference added. Version 3: Critical discussion of the main assumptions is added in sections I and VIII; two references added. To appear in Physical Review

Sensory organ like response determines the magnetism of zigzag-edged honeycomb nanoribbons

We present an analytical theory for the magnetic phase diagram for zigzag edge terminated honeycomb nanoribbons described by a Hubbard model with an interaction parameter U . We show that the edge magnetic moment varies as ln U and uncover its dependence on the width W of the ribbon. The physics of this owes its origin to the sensory organ like response of the nanoribbons, demonstrating that considerations beyond the usual Stoner-Landau theory are necessary to understand the magnetism of these systems. A first order magnetic transition from an anti-parallel orientation of the moments on opposite edges to a parallel orientation occurs upon doping with holes or electrons. The critical doping for this transition is shown to depend inversely on the width of the ribbon. Using variational Monte-Carlo calculations, we show that magnetism is robust to fluctuations. Additionally, we show that the magnetic phase diagram is generic to zigzag edge terminated nanostructures such as nanodots. Furthermore, we perform first principles modeling to show how such magnetic transitions can be realized in substituted graphene nanoribbons.Comment: 5 pages, 5 figure

Antibaryon yields in ultrahigh-energy collisions and the astroparticle implications

Some of the most recent results on antibaryon production in some ultrahigh-energy nuclear collisions are being analysed on the basis of a particle production model which has had some intrinsic features of non-standard nature. It is seen that the model could accommodate the data without any induction of the quark-gluon plasma (QGP) ideas which have, truly speaking, become a fad in the domain of the ultrahigh-energy nuclear collisions with some probable implications for or impact upon astroparticle physics