Generation of a train of ultrashort pulses using periodic waves in tapered photonic crystal fibres

Funding This work was supported by the Ministry of Education , Nigeria for financial support through the TETFUND scholarship 55 scheme; CSIR [grant number 03(1264)/12/EMR-II].Peer reviewedPostprin

Elevated plasma homocysteine is associated with ischaemic heart disease in Hong Kong Chinese

published_or_final_versio

Electrical determination of the valence-band discontinuity in HgTe-CdTe heterojunctions

Current-voltage behavior is studied experimentally in a Hg0.78Cd0.22Te-CdTe-Hg0.78Cd0.22Te heterostructure grown by molecular beam epitaxy. At temperatures above 160 K, energy-band diagrams suggest that the dominant low-bias current is thermionic hole emission across the CdTe barrier layer. This interpretation yields a direct determination of 390±75 meV for the HgTe-CdTe valence-band discontinuity at 300 K. Similar analyses of current-voltage data taken at 190–300 K suggest that the valence-band offset decreases at low temperatures in this heterojunction

Combustion characterization of beneficiated coal-based fuels

The objectives of this project include: (1) the development of an engineering data base which will provide detailed information on the properties of BCFs influencing combustion, ash deposition, ash erosion, particulate collection, and emissions; and (2) the application of this technical data base to predict the performance and economic impacts of firing the BCFs in various commercial boiler designs. The technical approach used to develop the technical data includes: bench-scale fuel property, conbustion, and ash deposition tests; pilot-scale combustion and ash effects tests; and full-scale combustion tests. Subcontractors to CE to perform parts of the test work are the Massachusetts Institute of Technology (MIT), Physical Sciences, Inc. Technology Company (PSIT) and the University of North Dakota Energy and Environmental Research Center (UNDEERC). Twenty fuels will be characterized during the three-year base program: three feed coals, fifteen BCFs, and two conventionally cleaned coals for the full-scale tests. Approximately nine BCFs will be in dry ultra-fine coal (DUC) form, and six BCFs will be in coal-water fuel (CWF) form. Additional BCFs would be characterized during optional project supplements

Killing Vector Fields in Three Dimensions: A Method to Solve Massive Gravity Field Equations

Killing vector fields in three dimensions play important role in the construction of the related spacetime geometry. In this work we show that when a three dimensional geometry admits a Killing vector field then the Ricci tensor of the geometry is determined in terms of the Killing vector field and its scalars. In this way we can generate all products and covariant derivatives at any order of the ricci tensor. Using this property we give ways of solving the field equations of Topologically Massive Gravity (TMG) and New Massive Gravity (NMG) introduced recently. In particular when the scalars of the Killing vector field (timelike, spacelike and null cases) are constants then all three dimensional symmetric tensors of the geometry, the ricci and einstein tensors, their covariant derivatives at all orders, their products of all orders are completely determined by the Killing vector field and the metric. Hence the corresponding three dimensional metrics are strong candidates of solving all higher derivative gravitational field equations in three dimensions.Comment: 25 pages, some changes made and some references added, to be published in Classical and Quantum Gravit

Low-Energy Charge-Density Excitations in MgB2_{2}: Striking Interplay between Single-Particle and Collective Behavior for Large Momenta

A sharp feature in the charge-density excitation spectra of single-crystal MgB$_{2}$, displaying a remarkable cosine-like, periodic energy dispersion with momentum transfer ($q$) along the $c^{*}$-axis, has been observed for the first time by high-resolution non-resonant inelastic x-ray scattering (NIXS). Time-dependent density-functional theory calculations show that the physics underlying the NIXS data is strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local-field effects. As a result, the small-$q$ collective mode residing in the single-particle excitation gap of the B $\pi$ bands reappears periodically in higher Brillouin zones. The NIXS data thus embody a novel signature of the layered electronic structure of MgB$_{2}$.Comment: 5 pages, 4 figures, submitted to PR

Robust Dropping Criteria for F-norm Minimization Based Sparse Approximate Inverse Preconditioning

Dropping tolerance criteria play a central role in Sparse Approximate Inverse preconditioning. Such criteria have received, however, little attention and have been treated heuristically in the following manner: If the size of an entry is below some empirically small positive quantity, then it is set to zero. The meaning of "small" is vague and has not been considered rigorously. It has not been clear how dropping tolerances affect the quality and effectiveness of a preconditioner $M$. In this paper, we focus on the adaptive Power Sparse Approximate Inverse algorithm and establish a mathematical theory on robust selection criteria for dropping tolerances. Using the theory, we derive an adaptive dropping criterion that is used to drop entries of small magnitude dynamically during the setup process of $M$. The proposed criterion enables us to make $M$ both as sparse as possible as well as to be of comparable quality to the potentially denser matrix which is obtained without dropping. As a byproduct, the theory applies to static F-norm minimization based preconditioning procedures, and a similar dropping criterion is given that can be used to sparsify a matrix after it has been computed by a static sparse approximate inverse procedure. In contrast to the adaptive procedure, dropping in the static procedure does not reduce the setup time of the matrix but makes the application of the sparser $M$ for Krylov iterations cheaper. Numerical experiments reported confirm the theory and illustrate the robustness and effectiveness of the dropping criteria.Comment: 27 pages, 2 figure

Microscopic theory of quantum dot interactions with quantum light: local field effect

A theory of both linear and nonlinear electromagnetic response of a single QD exposed to quantum light, accounting the depolarization induced local--field has been developed. Based on the microscopic Hamiltonian accounting for the electron--hole exchange interaction, an effective two--body Hamiltonian has been derived and expressed in terms of the incident electric field, with a separate term describing the QD depolarization. The quantum equations of motion have been formulated and solved with the Hamiltonian for various types of the QD excitation, such as Fock qubit, coherent fields, vacuum state of electromagnetic field and light with arbitrary photonic state distribution. For a QD exposed to coherent light, we predict the appearance of two oscillatory regimes in the Rabi effect separated by the bifurcation. In the first regime, the standard collapse--revivals phenomenon do not reveal itself and the QD population inversion is found to be negative, while in the second one, the collapse--revivals picture is found to be strongly distorted as compared with that predicted by the standard Jaynes-Cummings model. %The model developed can easily be extended to %%electromagnetic excitation. For the case of QD interaction with arbitrary quantum light state in the linear regime, it has been shown that the local field induce a fine structure of the absorbtion spectrum. Instead of a single line with frequency corresponding to which the exciton transition frequency, a duplet is appeared with one component shifted by the amount of the local field coupling parameter. It has been demonstrated the strong light--mater coupling regime arises in the weak-field limit. A physical interpretation of the predicted effects has been proposed.Comment: 14 pages, 7 figure

Hyperfine Fields in an Ag/Fe Multilayer Film Investigated with 8Li beta-Detected Nuclear Magnetic Resonance

Low energy $\beta$-detected nuclear magnetic resonance ($\beta$-NMR) was used to investigate the spatial dependence of the hyperfine magnetic fields induced by Fe in the nonmagnetic Ag of an Au(40 \AA)/Ag(200 \AA)/Fe(140 \AA) (001) magnetic multilayer (MML) grown on GaAs. The resonance lineshape in the Ag layer shows dramatic broadening compared to intrinsic Ag. This broadening is attributed to large induced magnetic fields in this layer by the magnetic Fe layer. We find that the induced hyperfine field in the Ag follows a power law decay away from the Ag/Fe interface with power $-1.93(8)$, and a field extrapolated to $0.23(5)$ T at the interface.Comment: 5 pages, 4 figure. To be published in Phys. Rev.

Gradient flows and instantons at a Lifshitz point

I provide a broad framework to embed gradient flow equations in non-relativistic field theory models that exhibit anisotropic scaling. The prime example is the heat equation arising from a Lifshitz scalar field theory; other examples include the Allen-Cahn equation that models the evolution of phase boundaries. Then, I review recent results reported in arXiv:1002.0062 describing instantons of Horava-Lifshitz gravity as eternal solutions of certain geometric flow equations on 3-manifolds. These instanton solutions are in general chiral when the anisotropic scaling exponent is z=3. Some general connections with the Onsager-Machlup theory of non-equilibrium processes are also briefly discussed in this context. Thus, theories of Lifshitz type in d+1 dimensions can be used as off-shell toy models for dynamical vacuum selection of relativistic field theories in d dimensions.Comment: 19 pages, 1 figure, contribution to conference proceedings (NEB14); minor typos corrected in v