Physics

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We illustrate how the application of autonomous Hamiltonian dynamical system can definitively fix a sign for an otherwise indefinite sign of a certain astrophysical parameter. To illustrate it, we shall consider the Mannheim-Kazanasde Sitter solution of Weyl conformal gravity containing the parameter γ, which is believed to be significant in the galactic halo gravity. However, the sign of γ has so far been ambiguous. The strategy we adopt to fix it is to look for a finite maximum radius up to which the halo supports stable material circular orbits inside the de Sitter radius. The maximum radius for several observed lenses are calculated for both signs of γ, and with the observed value of cosmological constant Λ. These lenses (all having approximately the Einstein radius R E ≈ 10 23 cm) consistently yield a maximum radius (R max stable ≃ 4.25 × 10 27cm) inside the de Sitter radius of the universe only when γ is negative, while a positive γ yields R max stable always exceeding the de Sitter radius. © 2012 IACS.

World University root node

(SoNS) School of Natural Sciences

Indian Journal of Physics

Modeling by autonomous Hamiltonian system: Fixing the sign of a parameter

In the framework of the minimal cosmological standard model, the $\Lambda$CDM model, the dark matter density is now known within an error of a few percent; this error is expected to shrink even further once PLANCK data are analyzed. Matching this precision by theoretical calculations implies that at least leading radiative corrections to the annihilation cross section of the dark matter particles have to be included. Here we compute one kind of large corrections in the context of the minimal supersymmetric extension of the Standard Model: corrections associated with two-point function corrections on chargino and neutralino (collectively denoted by $\chi$) lines. These can be described by effective $\chi$-fermion-sfermion and $\chi$-$\chi$-Higgs couplings. We also employ one-loop corrected $\chi$ masses, using a recently developed version of the on-shell renormalization scheme. The resulting correction to the predicted dark matter density depends strongly on parameter space, but it can easily reach 3%. {\textcopyright} 2012 American Physical Society.

Fulltext

Physical Review D - Particles, Fields, Gravitation and Cosmology

Radiative corrections to the neutralino dark matter relic density: An effective coupling approach

Molybdenum disulphide, a diamagnetic layered dichalcogenide solid, is found to show magnetic ordering at room temperature when exposed to a 2 MeV proton beam. The temperature dependence of magnetization displays ferrimagnetic behavior with a Curie temperature of 895 K. A disorder mode corresponding to a zone-edge phonon and a Mo valence higher than +4 has been detected in the irradiated samples using Raman and x-ray photoelectron spectroscopy, respectively. The possible origins of long-range magnetic ordering in irradiated MoS 2 samples are discussed. © 2012 American Institute of Physics.

Applied Physics Letters

Magnetism in MoS 2 induced by proton irradiation

Selectively creating regions of spatially varying thickness may enable the utilization of the electronic properties of N-layer (N=1 or more) graphene and other similar layered materials (e.g., topological insulators or layered superconductors) for novel devices and functionalities on a single chip. The ablation threshold energy density increases dramatically for decreasing layer numbers of graphene originating from the dimensional crossover of the specific heat. For the 2D regime of graphite (up to N≈7) the dominant flexural mode specific heat (due to its N-1 dependence) gives rise to a strong layer number-dependence on the pulsed laser ablation threshold energy density, while for 3D regime (N&gt;7) the ablation threshold saturates due to dominant acoustic mode specific heat. As a result, several energy density windows exist between the minimum energy densities that are required for ablating single, bi, or more layers of graphene, allowing layer number selectivity. Copyright © 2011 Author(s).

AIP Advances

A new route to graphene layers by selective laser ablation

We report optical, electrical and magnetotransport properties of oxygen deficient SrTiO3 (SrTiO3-x) thin films fabricated by pulsed laser deposition technique. The oxygen vacancies (Ovac) in the thin film are expected to be uniform. By comparing its electrical properties to those of bulk SrTiO3-x, it was found that Ovac in bulk SrTiO3-x is far from uniform over the whole material. The metal-insulator transition (MIT) observed in the SrTiO3-x film was found to be induced by the carrier freeze-out effect. The low temperature frozen state can be reexcited by Joule heating, electric and intriguingly magnetic field. © 2011 American Physical Society.

Physical Review Letters

Metal-insulator transition in SrTiO3-x thin films induced by frozen-out carriers

We study the importance of inter-site coherences and quantum interference effects in the X-ray absorption spectroscopy (XAS) of mixed-valent systems. We demonstrate its importance, first for a simple diatomic mixed-valent molecule to elucidate the physics involved and finally for mixed-valent Ti oxides, using a model calculation including full spin and orbital multiplicities. Our calculations demonstrate the inefficacy of conventional approaches of describing the XAS from mixed-valent systems as incoherent combinations of XAS from the pure-valent end members and that multiplets forbidden in the single-site approximation can be effectively reached within a multi-site description. These conclusions play an important role in the interpretation of XAS and resonant X-ray scattering (RXS) in terms of the electronic structure of strongly correlated systems. Copyright {\textcopyright} EPLA, 2011.

The importance of inter-site coherences in the X-ray absorption spectra of mixed-valent systems

In a recent paper (S. Kumar, A. Pandey, Phys. Rev. E, 79, 2009, p. 026211) we considered Jacobi family (including Laguerre and Gaussian cases) of random matrix ensembles and reported exact solutions of crossover problems involving time-reversal symmetry breaking. In the present paper we give details of the work. We start with Dyson's Brownian motion description of random matrix ensembles and obtain universal hierarchic relations among the unfolded correlation functions. For arbitrary dimensions we derive the joint probability density (jpd) of eigenvalues for all transitions leading to unitary ensembles as equilibrium ensembles. We focus on the orthogonal-unitary and symplectic-unitary crossovers and give generic expressions for jpd of eigenvalues, two-point kernels and n-level correlation functions. This involves generalization of the theory of skew-orthogonal polynomials to crossover ensembles. We also consider crossovers in the circular ensembles to show the generality of our method. In the large dimensionality limit, correlations in spectra with arbitrary initial density are shown to be universal when expressed in terms of a rescaled symmetry breaking parameter. Applications of our crossover results to communication theory and quantum conductance problems are also briefly discussed. © 2011 Elsevier Inc.

Annals of Physics

Crossover ensembles of random matrices and skew-orthogonal polynomials

Two particles that are just shy of binding may develop an infinite number of shallow bound states when a third particle is added. This counter intuitive quantum mechanical result was first predicted by V. Efimov for identical bosons interacting with a short-range pair-wise potential. The so-called Efimov effect persists even for non-identical particles, provided at least two of the three bonds are almost bound. The Efimov effect has recently been verified experimentally using ultra-cold atoms. In this article, we explain the origin of this effect using only elementary quantum mechanics and summarize the experimental evidence for the Efimov effect. A new, simple derivation for the number of Efimov states is given in the Appendix.

American Journal of Physics

An elementary exposition of the Efimov effect

Pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione or diketopyrrolopyrrole (DPP) is a useful electron-withdrawing fused aromatic moiety for the preparation of donor-acceptor polymers as active semiconductors for organic electronics. This study uses a DPP-furan-containing building block, 3,6-di(furan-2-yl)pyrrolo[3,4- c]pyrrole-1,4(2H,5H)-dione (DBF), to couple with a 2,2′-bithiophene unit, forming a new donor-acceptor copolymer, PDBFBT. Compared to its structural analogue, 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DBT), DBF is found to cause blue shifts of the absorption spectra both in solution and in thin films and a slight reduction of the highest occupied molecular orbital (HOMO) energy level of the resulting PDBFBT. Despite the fact that its thin films are less crystalline and have a rather disordered chain orientation in the crystalline domains, PDBFBT shows very high hole mobility up to 1.54 cm 2 V-1 s-1 in bottom-gate, top-contact organic thin film transistors. © 2011 The Royal Society of Chemistry.

Journal of Materials Chemistry

3,6-Di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione and bithiophene copolymer with rather disordered chain orientation showing high mobility in organic thin film transistors

Graphene samples with 1, 2, and 4 layers and 1 + 1 folded bi-layers and graphite have been irradiated with 2 MeV protons at fluences ranging from 1 × 1015 to 6 × 1018 ions/cm2. The samples were characterized using visible and UV Raman spectroscopy and Raman microscopy. The ion-induced defects were found to decrease with increasing number of layers. Graphene samples suspended over etched holes in SiO 2 have been fabricated and used to investigate the influence of the substrate SiO2 for defect creation in graphene. While Raman vibrational modes at 1460 cm-1 and 1555 cm-1 have been observed in the visible Raman spectra of substantially damaged graphene samples, these modes were absent in the irradiated-suspended monolayer graphene. © 2011 American Institute of Physics.