Massive Fields and the 2D String

The first massive level of closed bosonic string theory is studied. Free-field equations are derived by imposing Weyl invariance on the world sheet. A two-parameter solution to the equation of motion and constraints is found in two dimensions with a flat linear-dilaton background. One-to-one tachyon scattering is studied in this background. The results support Dhar, Mandal and Wadia's proposal that 2D critical string theory corresponds to the c=1 matrix model in which both sides of the Fermi sea are excited.Comment: 17 pages, Latex. V2: One ref added, minor rephrasing of the first paragraph in Sec.3.1, typos in (56) and (57) correcte

Effective anisotropies and energy barriers of magnetic nanoparticles with Néel surface anisotropy

Magnetic nanoparticles with Néel surface anisotropy, different internal structures, surface arrangements, and elongation are modeled as many-spin systems. The results suggest that the energy of many-spin nanoparticles cut from cubic lattices can be represented by an effective one-spin potential containing uniaxial and cubic anisotropies. It is shown that the values and signs of the corresponding constants depend strongly on the particle's surface arrangement, internal structure, and shape. Particles cut from a simple cubic lattice have the opposite sign of the effective cubic term, as compared to particles cut from the face-centered cubic lattice. Furthermore, other remarkable phenomena are observed in nanoparticles with relatively strong surface effects. (i) In elongated particles surface effects can change the sign of the uniaxial anisotropy. (ii) In symmetric particles (spherical and truncated octahedral) with cubic core anisotropy surface effects can change the sing of the latter. We also show that the competition between the core and surface anisotropies leads to a new energy that contributes to both the second- and fourth-order effective anisotropies. We evaluate energy barriers ΔE as functions of the strength of the surface anisotropy and the particle size. The results are analyzed with the help of the effective one-spin potential, which allows us to assess the consistency of the widely used formula ΔE/V= K∞ +6 Ks /D, where K∞ is the core anisotropy constant, Ks is a phenomenological constant related to surface anisotropy, and D is the particle's diameter. We show that the energy barriers are consistent with this formula only for elongated particles for which the surface contribution to the effective uniaxial anisotropy scales with the surface and is linear in the constant of the Néel surface anisotropy. © 2007 The American Physical Society

Forensic Accounting and the Combating of Economic and Financial Crimes in Ghana

Economic and financial crimes have plagued every corner of the economies of the world. These crimes affect all firms and the economies of nations (developed, developing and under-developed). Continuous research confirmed a strong demand for the need of the prevention and detection of these crimes by institutions and nations. However, economic and financial crimes are difficult to detect and most of those detected are not reported for the fear of bad publicity and loss of investor confidence. Forensic accountants are perceived to have the training and skills needed to combat economic and financial crimes. Therefore, the research examined the relevance of forensic accounting technique application in the combating of these crimes in Ghana. The research employed survey research design by sampling all the technical officers (66) of Economic and Organized Crime Office of Ghana and data was analyzed through a regression model. It was found that, the application of forensic accounting technique has significant impact on the combating of economic and financial crimes in Ghana. Therefore, all institutions (anti-corruption agencies and companies) should establish forensic accounting unit to help strengthen internal controls and ensure thorough investigation in order to prevent, deter and detect financial and economic crimes. Moreover, Institute of Chartered Accountants-Ghana and National Accreditation Board of Ghana should ensure that forensic accounting courses are included in the academic and professional curricula of Ghana Universities and professional institutions to ensure the training and increase the awareness of forensic accounting in Ghana

Condensation Transitions in a One-Dimensional Zero-Range Process with a Single Defect Site

Condensation occurs in nonequilibrium steady states when a finite fraction of particles in the system occupies a single lattice site. We study condensation transitions in a one-dimensional zero-range process with a single defect site. The system is analysed in the grand canonical and canonical ensembles and the two are contrasted. Two distinct condensation mechanisms are found in the grand canonical ensemble. Discrepancies between the infinite and large but finite systems' particle current versus particle density diagrams are investigated and an explanation for how the finite current goes above a maximum value predicted for infinite systems is found in the canonical ensemble.Comment: 18 pages, 4 figures, revtex

Multiscale modeling of magnetic materials: Temperature dependence of the exchange stiffness

For finite-temperature micromagnetic simulations the knowledge of the temperature dependence of the exchange stiffness plays a central role. We use two approaches for the calculation of the thermodynamic exchange parameter from spin models: (i) based on the domain-wall energy and (ii) based on the spin-wave dispersion. The corresponding analytical and numerical approaches are introduced and compared. A general theory for the temperature dependence and scaling of the exchange stiffness is developed using the classical spectral density method. The low-temperature exchange stiffness A is found to scale with magnetization as m(1.66) for systems on a simple cubic lattice and as m(1.76) for an FePt Hamiltonian parametrized through ab initio calculations. The additional reduction in the scaling exponent, as compared to the mean-field theory (A similar to m(2)), comes from the nonlinear spin-wave effects

Phase Transition in Two Species Zero-Range Process

We study a zero-range process with two species of interacting particles. We show that the steady state assumes a simple factorised form, provided the dynamics satisfy certain conditions, which we derive. The steady state exhibits a new mechanism of condensation transition wherein one species induces the condensation of the other. We study this mechanism for a specific choice of dynamics.Comment: 8 pages, 3 figure

Control and modelling of capillary flow of epoxy resin in aligned carbon nanotube forests

This paper examines the mechanism of infiltration by capillary flow of epoxy resin into vertically-aligned carbon nanotube forests. The resin viscosity during curing was characterized by rheometry. Carbon nanotube forests were brought into contact with resin at a range of times during curing, therefore at a range of viscosities. The penetration of the resin into the forests was measured using electron microscopy, X-ray micro-computed tomography and energy-dispersive X-ray spectroscopy, the latter relying on a chromium-complex dye additive which acts as a marker for the presence of resin. Experimental results were compared to a simulation based on the Implicit Lucas–Washburn equation for capillary flow. It was found that prior to the resin gel point, the resin penetrates through the full height of the forest. Close to the gel point, the flow into the forest ceases, leaving unwetted regions of nanotubes. Understanding the relationship between resin flow in nanotube structures and the resin viscosity and curing has important application in the fabrication of nanocomposite materials. This “partial wetting” effect is a key requirement for a previously proposed method for the fabrication of carbon nanotube composites by additive manufacture (AM) which would provide strong interlayer reinforcement combined with the versatility of AM.Airbus Corp. Ltd. (Airbus Group)University of Exete

Fabrication of Three Dimensional Layered Vertically Aligned Carbon Nanotube Structures and their Potential Applications

This paper proposes a new technique for fabrication of vertically aligned carbon nanotube (VACNT) structures, controlled in shape, height and functionality, through continuous successive growth of VACNT layers by chemical vapour deposition (CVD) combined with patterning strategies. This was achieved by vacuum deposition of additional catalyst material onto the original VACNT “forest” layer. A second forest layer is then observed to grow underneath the first by CVD. It is proposed that the new catalyst material diffuses through the porous nanotube forest to coat the growth substrate underneath. The enhanced height, coating, and vertical alignment of the nanotube forests were verified by electron microscope observation. By repeating this process, aligned nanotube bi-layers and tri-layers were grown, producing a “stack” of nanotube layers. By using a “shadow mask” patterning technique to screen areas of the original forest from catalyst deposition, the growth can be confined to specific areas of the substrate. Potentially, these multilayer nanotube structures would have diverse applications as long composite reinforcements, p–n junctions for electronic devices, or to allow the production of near net shape complex multilayer nanotube structures

Collective traffic-like movement of ants on a trail: dynamical phases and phase transitions

The traffic-like collective movement of ants on a trail can be described by a stochastic cellular automaton model. We have earlier investigated its unusual flow-density relation by using various mean field approximations and computer simulations. In this paper, we study the model following an alternative approach based on the analogy with the zero range process, which is one of the few known exactly solvable stochastic dynamical models. We show that our theory can quantitatively account for the unusual non-monotonic dependence of the average speed of the ants on their density for finite lattices with periodic boundary conditions. Moreover, we argue that the model exhibits a continuous phase transition at the critial density only in a limiting case. Furthermore, we investigate the phase diagram of the model by replacing the periodic boundary conditions by open boundary conditions.Comment: 8 pages, 6 figure