A survey of nulling pulsars using the Giant Meterwave Radio Telescope

Several pulsars show sudden cessation of pulsed emission, which is known as pulsar nulling. In this paper, the nulling behaviour of 15 pulsars is presented. The nulling fractions of these pulsars, along with the degree of reduction in the pulse energy during the null phase, are reported for these pulsars. A quasi-periodic null-burst pattern is reported for PSR J1738-2330. The distributions of lengths of the null and the burst phases as well as the typical nulling time scales are estimated for eight strong pulsars. The nulling pattern of four pulsars with similar nulling fraction are found to be different from each other, suggesting that the fraction of null pulses does not quantify the nulling behaviour of a pulsar in full detail. Analysis of these distributions also indicate that while the null and the burst pulses occur in groups, the underlying distribution of the interval between a transition from the null to the burst phase and vice verse appears to be similar to that of a stochastic Poisson point process.Comment: 10 pages, 7 figures, accepted for publication in MNRA

Crossover of Level Statistics between Strong and Weak Localization in Two Dimensions

We investigate numerically the statistical properties of spectra of two-dimensional disordered systems by using the exact diagonalization and decimation method applied to the Anderson model. Statistics of spacings calculated for system sizes up to 1024 $\times$ 1024 lattice sites exhibits a crossover between Wigner and Poisson distributions. We perform a self-contained finite-size scaling analysis to find a single-valued one-parameter function $\gamma (L/\xi)$ which governs the crossover. The scaling parameter $\xi(W)$ is deduced and compared with the localization length. $\gamma ( L/\xi)$ does {\em not} show critical behavior and has two asymptotic regimes corresponding to weakly and strongly localized states.Comment: 4 pages in revtex, 3 postscript figure

Large-p_T Photoproduction of D^*+- Mesons in ep Collisions

The cross section for the inclusive photoproduction of large-p_T D^*+- mesons is calculated at next-to-leading order, adopting different approaches to describe the fragmentation of charm quarks into D^*+- mesons. We treat the charm quark according to the massless factorization scheme, where it is assumed to be one of the active flavours inside the proton and the photon. We present inclusive single-particle distributions in transverse momentum and rapidity, including the contributions due to both direct and resolved photons. We compare and assess the various implementations of fragmentation. We argue that, in the high-p_T regime, a particularly realistic description can be obtained by convoluting the Altarelli-Parisi-evolved fragmentation functions of Peterson et al. with the hard-scattering cross sections of massless partons where the factorization of the collinear singularities associated with final-state charm quarks is converted to the massive-charm scheme. The predictions thus obtained agree well with recent experimental data by the H1 and ZEUS Collaborations at DESY HERA.Comment: 31 pages (Latex), 13 figures (Postscript). This version of the manuscript is identical with the one being printed in Z. Phys.

Cross Sections for Charm Production in epep Collisions: Massive versus Massless Scheme

The next--to--leading order inclusive cross section for large-$p_\perp$ photoproduction of charm quarks at HERA is calculated in two different approaches. In the first approach the charm quarks are treated as massive objects which are strictly external to the proton and the photon while in the second approach the charm mass is neglected and the $c$ quark is assumed to be one of the active flavours in the proton and photon structure functions. We present single-inclusive distributions in transverse momentum and rapidity including direct and resolved photons. The cross section in the massless approach is found to be significantly larger than in the massive scheme. The deviation originates from several contributions which are disentangled. We argue that large-$p_\perp$ photoproduction of charm quarks at HERA will be sensitive to the charm content of the photon structure function.Comment: 11 pages, Latex, epsfig, 6 figures appended as uuencoded file (hardcopy can be obtained upon request from [email protected]

Transport through a one-dimensional quantum dot

We examine the effects of long-range interactions in a quantum wire with two impurities. We employ the bosonization technique and derive an effective action for the system. The effect of the long-range interaction on the charging energy and spectral properties of the island formed by the impurities and the linear transport is discussed.Comment: 7 pages, 2 figure

Fractional charges in pyrochlore lattices

A pyrochlore lattice is considered where the average electron number of electrons per site is half--integer, concentrating on the case of exactly half an electron per site. Strong on-site repulsions are assumed, so that all sites are either empty or singly occupied. Where there are in addition strong nearest--neighbour repulsions, a tetrahedron rule comes into effect, as previously suggested for magnetite. We show that in this case, there exist excitations with fractional charge (+/-) e/2. These are intimately connected with the high degeneracy of the ground state in the absence of kinetic energy terms. When an additional electron is inserted into the system, it decays into two point like excitations with charge -e/2, connected by a Heisenberg spin chain which carries the electron's spin.Comment: 10 pages, 4 eps figures. To appear in Decemeber issue of Annalen der Physi

Theory of measuring the "Luttinger-g" of a one-dimensional quantum dot

We study electron transport through a quantum dot in a Tomonaga-Luttinger liquid with an inhomogeneity induced either by a non-uniform electron interaction or by the presence of tunnel resistances of contacts. The non-analytic temperature behavior of the conductance peaks show crossovers determined by the different energy scales associated with the dot and the inhomogeneity despite the Coulomb blockade remains intact. This suggests an explanation of recent findings in semiconductor quantum wires and carbon nanotubes.Comment: 4 pages, 3 colour figures, to be published with Phys. Rev.

Charge and spin addition energies of one dimensional quantumn dot

We derive the effective action for a one dimensional electron island formed between a double barrier in a single channel quantum wire including the electron spin. Current and energy addition terms corresponding to charge and spin are identified. The influence of the range and the strength of the electron interaction and other system parameters on the charge and spin addition energies, and on the excitation spectra of the modes confined within the island is studied. We find by comparison with experiment that spin excitations in addition to non-zero range of the interaction and inhomogeneity effects are important for understanding the electron transport through one dimensional quantum islands in cleaved-edge-overgrowth systems.Comment: 11 pages, 3 figures, to be published in Physical Review