1,112 research outputs found
Global well-posedness of the KP-I initial-value problem in the energy space
We prove that the KP-I initial value problem is globally well-posed in the
natural energy space of the equation
The inner kiloparsec of the jet in 3C264
We present new multi-frequency EVN, MERLIN and VLA observations of the radio
source 3C264, sensitive to linear scales ranging from the parsec to several
kiloparsecs. The observations confirm the existence of regions with different
properties in the first kiloparsec of the jet. The most remarkable feature is
the transition between a well collimated narrow jet at distances from the core
below 80 pc, to a conical-shaped wide jet, with an opening angle of 20 degrees.
Another change of properties, consisting of an apparent deflection of the jet
ridge line and a diminution of the surface brightness, occurs at a distance of
300 pc from the core, coincident with the radius of a ring observed at optical
wavelengths. Our observations add new pieces of information on the spectrum of
the radio-optical jet of 3C264, with results consistent with a synchrotron
emission mechanism and a spectrum break frequency in the infrared. Brightness
profiles taken perpendicularly to the jet of 3C264 are consistent with a spine
brightened jet at distances below 100 pc from the core, and an edge-brightened
jet beyond, which can be interpreted as evidence of a transverse jet velocity
structure. Our observations do not allow us to distinguish between the presence
of a face--on dust and gas disk at the center of the host galaxy of 3C264, or
rather an evacuated bubble. However, the properties of the jet structure, the
changes in the polarization angle, and the plausible jet orientation can be
naturally brought into agreement in the bubble scenario.Comment: 10 pages, 9 figures, accepted in A&
Finite size effects near the onset of the oscillatory instability
A system of two complex Ginzburg - Landau equations is considered that applies at the onset of the oscillatory instability in spatial domains whose size is large (but finite) in one direction; the dependent variables are the slowly modulated complex amplitudes of two counterpropagating wavetrains. In order to obtain a well posed problem, four boundary conditions must be imposed at the boundaries. Two of them were already known, and the other two are first derived in this paper. In the generic case when the group velocity is of order unity, the resulting problem has terms that are not of the same order of magnitude. This fact allows us to consider two distinguished limits and to derive two associated (simpler) sub-models, that are briefly discussed. Our results predict quite a rich variety of complex dynamics that is due to both the modulational instability and finite size effects
Moduli potentials in string compactifications with fluxes: mapping the Discretuum
We find de Sitter and flat space solutions with all moduli stabilized in four
dimensional supergravity theories derived from the heterotic and type II string
theories, and explain how all the previously known obstacles to finding such
solutions can be removed. Further, we argue that if the compact manifold allows
a large enough space of discrete topological choices then it is possible to
tune the parameters of the four dimensional supergravity such that a hierarchy
is created and the solutions lie in the outer region of moduli space in which
the compact volume is large in string units, the string coupling is weak, and
string perturbation theory is valid. We show that at least two light chiral
superfields are required for this scenario to work, however, one field is
sufficient to obtain a minimum with an acceptably small and negative
cosmological constant. We discuss cosmological issues of the scenario and the
possible role of anthropic considerations in choosing the vacuum of the theory.
We conclude that the most likely stable vacuua are in or near the central
region of moduli space where string perturbation theory is not strictly valid,
and that anthropic considerations cannot help much in choosing a vacuum.Comment: 34 pages, no figure
Coherent Single Charge Transport in Molecular-Scale Silicon Nanowire Transistors
We report low-temperature electrical transport studies of molecule-scale
silicon nanowires. Individual nanowires exhibit well-defined Coulomb blockade
oscillations characteristic of charge addition to a single nanostructure with
length scales up to at least 400 nm. Further studies demonstrate coherent
charge transport through discrete single particle quantum levels extending the
whole device, and show that the ground state spin configuration follows the
Lieb-Mattis theorem. In addition, depletion of the nanowires suggests that
phase coherent single-dot characteristics are accessible in a regime where
correlations are strong.Comment: 4 pages and 4 figure
Experimental study of a proximity focusing Cherenkov counter prototype for the AMS experiment
A study prototype of Proximity Focussing Ring Imaging Cherenkov counter has
been built and tested with several radiators using separately cosmic-ray
particles and 12C beam fragmentation products at several energies. Counter
prototype and experimental setup are described, and the results of measurements
reported and compared with simulation results.The performances are discussed in
the perspective of the final counter design.Comment: 27 pages, 16 figures, submitted to NIM
The stellar content of low redshift BL Lac host galaxies from multicolour imaging
We present B-band imaging of 18 low redshift (z<0.3) BL Lac objects for which
their host galaxies were previously resolved in the R-band and the
near-infrared H-band. For a subset of the objects, also U- and V-band imaging
is presented. These data are used to investigate the blue-red-near-infrared
colours and the colour gradients of the host galaxies of BL Lacs in comparison
with other elliptical galaxies with and without nuclear activity. In all cases
galaxies are well represented by an elliptical model, with average absolute
magnitude M_B=-21.6+-0.7 and average scale length R_e=7.6+-3.2 kpc. The
best-fit B-band Kormendy relation is in reasonable agreement with that obtained
for normal ellipticals and radio galaxies. This structural and dynamical
similarity indicates that all massive elliptical galaxies can experience
nuclear activity without significant perturbation of their global structure.
The distributions of the integrated blue/near-infrared colour (with average
B-H=3.5+-0.5) and colour gradient (with average Delta(B-R)/Delta(log
r)=-0.14+-0.75) of the BL Lac hosts are much wider than those for normal
ellipticals, and most BL Lac objects have bluer hosts and/or steeper colour
gradients than those in normal ellipticals. The blue colours are likely caused
by a young stellar population component, and indicates a link between star
formation caused by an interaction/merging event and the onset of the nuclear
activity. This result is corroborated by stellar population modelling,
indicating a presence of young/intermediate age populations in the majority of
the sample, in agreement with low redshift quasar hosts. The lack of strong
signs of interaction may require a significant time delay between the event
with associated star formation episodes and the start of the nuclear activity.Comment: 16 pages, 10 figure
Scanned Probe Microscopy of Electronic Transport in Carbon Nanotubes
We use electrostatic force microscopy and scanned gate microscopy to probe
the conducting properties of carbon nanotubes at room temperature. Multi-walled
carbon nanotubes are shown to be diffusive conductors, while metallic
single-walled carbon nanotubes are ballistic conductors over micron lengths.
Semiconducting single-walled carbon nanotubes are shown to have a series of
large barriers to conduction along their length. These measurements are also
used to probe the contact resistance and locate breaks in carbon nanotube
circuits.Comment: 4 page
Fermi-level alignment at metal-carbon nanotube interfaces: application to scanning tunneling spectroscopy
At any metal-carbon nanotube interface there is charge transfer and the
induced interfacial field determines the position of the carbon nanotube band
structure relative to the metal Fermi-level. In the case of a single-wall
carbon nanotube (SWNT) supported on a gold substrate, we show that the charge
transfers induce a local electrostatic potential perturbation which gives rise
to the observed Fermi-level shift in scanning tunneling spectroscopy (STS)
measurements. We also discuss the relevance of this study to recent experiments
on carbon nanotube transistors and argue that the Fermi-level alignment will be
different for carbon nanotube transistors with low resistance and high
resistance contacts.Comment: 4 pages, 3 ps figures, minor corrections, accepted by Phys. Rev. Let
Observations of HI Absorbing Gas in Compact Radio Sources at Cosmological Redshifts
We present an overview of the occurrence and properties of atomic gas
associated with compact radio sources at redshifts up to z=0.85. Searches for
HI 21cm absorption were made with the Westerbork Synthesis Radio Telescope at
UHF-high frequencies (725-1200 MHz). Detections were obtained for 19 of the 57
sources with usable spectra (33%). We have found a large range in line depths,
from tau=0.16 to tau<=0.001. There is a substantial variety of line profiles,
including Gaussians of less than 10km/s, to more typically 150km/s, as well as
irregular and multi-peaked absorption profiles, sometimes spanning several
hundred km/s. Assuming uniform coverage of the entire radio source, we obtain
column depths of atomic gas between 1e19 and 3.3e21(Tsp/100K)(1/f)cm^(-2).
There is evidence for significant gas motions, but in contrast to earlier
results at low redshift, there are many sources in which the HI velocity is
substantially negative (up to v=-1420km/s) with respect to the optical
redshift, suggesting that in these sources the atomic gas, rather than falling
into the centre, may be be flowing out, interacting with the jets, or rotating
around the nucleus.Comment: 10 pages, accepted for publication in A&
- …