195 research outputs found
The Bernstein problem for intrinsic graphs in Heisenberg groups and calibrations
In this paper we deal with some problems concerning minimal hypersurfaces in
Carnot-Caratheodory (CC) structures. More precisely we will introduce a general
calibration method in this setting and we will study the Bernstein problem for
entire regular intrinsic minimal graphs in a meaningful and simpler class of CC
spaces, i.e. the Heisenberg group H^n. In particular we will positively answer
to the Bernstein problem in the case n=1 and we will provide counterexamples
when n>=5
Lorentzian compact manifolds: isometries and geodesics
In this work we investigate families of compact Lorentzian manifolds in
dimension four. We show that every lightlike geodesic on such spaces is
periodic, while there are closed and non-closed spacelike and timelike
geodesics. Their isometry groups are computed. We also show that there is a non
trivial action by isometries of \Heis_3(\RR) on the nilmanifold S^1\times
(\Gamma_k \bsh \Heis_3(\RR)) for a lattice of \Heis_3(\RR).Comment: 17 page
Ion and X-ray micro-beam induced charge collection and their applications in CVD diamond detector characterisation
We have used a micrometer size X-ray beam generated from a synchrotron light source at the European Synchrotron Radiation Facility (ESRF) in Grenoble and a 2 MeV proton micro-beam at the Italian National Laboratory (LNL) of Legnaro to image the electronic transport properties of a CVD diamond detector developed within the CERN RD42 collaboration. The focused X-rays or protons are scanned over the device surface, and the induced current or charge pulse is measured and plotted on two dimensional maps. Due to the polycrystalline nature of the material, the maps are not homogeneous and both the techniques show structures ascribable to diamond grains. It was found that the uniformity of the maps depends on the lateral scale (binning) and on the analytical depth of the micro-probes
Electric force microscopy characterization of buried graphitic channels microfabricated by MeV ion beam implantation
Measurement and modelling of anomalous polarity pulses in a multi-electrode diamond detector
In multi-electrode detectors, the motion of excess carriers generated by
ionizing radiation induces charge pulses at the electrodes, whose intensities
and polarities depend on the geometrical, electrostatic and carriers transport
properties of the device. The resulting charge sharing effects may lead to
bipolar currents, pulse height defects and anomalous polarity signals affecting
the response of the device to ionizing radiation. This latter effect has
recently attracted attention in commonly used detector materials, but different
interpretations have been suggested, depending on the material, the geometry of
the device and the nature of the ionizing radiation. In this letter, we report
on the investigation in the formation of anomalous polarity pulses in a
multi-electrode diamond detector with buried graphitic electrodes. In
particular, we propose a purely electrostatic model based on the
Shockley-Ramo-Gunn theory, providing a satisfactory description of anomalous
pulses observed in charge collection efficiency maps measured by means of Ion
Beam Induced Charge (IBIC) microscopy, and suitable for a general application
in multi-electrode devices and detectors.Comment: 8 pages, 4 figure
The symbiotic star CH Cygni – II. The ejecta from the 1998-2000 active phase
We present Hubble Space Telescope (HST) imaging, a Very Large Array (VLA) radio map (4.74 GHz), optical high-resolution (echelle) spectroscopy and UBV photoelectric photometry of the symbiotic star CH Cyg obtained during its 1998–2000 active phase. The HST imaging, taken during eclipse, shows the central stars are embedded in a nebula extending to 620 ± 150 au for a distance of 270 ± 66 pc. The inner nebula is strongly influenced by the onset of activity and associated outflow in 1998. The surface brightness contours of the contemporaneous radio VLA observation agree well with HST images. Photometric observations of the broad 1999 U-minimum suggest that it is due to the eclipse of the active hot component by the giant on the long-period (14.5 yr) outer orbit. We also find that the onset of the 1998 and the 1992 active periods occur at the same orbital phase of the inner binary. Spectroscopic observations reveal two types of outflow from the active star: a high-velocity (>1200 km s−1) hot star wind sporadically alternating with a more massive outflow indicated by P-Cygni-like profiles. We present evidence connecting the extended nebulosity with the high-velocity shocked outflow, and hence the activity in the central binary
- …