24,786 research outputs found
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 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
which governs the crossover. The scaling parameter is
deduced and compared with the localization length. 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 Collisions: Massive versus Massless Scheme
The next--to--leading order inclusive cross section for large-
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 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- 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
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