21,828 research outputs found
Properties of optically selected BL Lac candidates from the SDSS
\textbf{Context.} Deep optical surveys open the avenue for find large numbers
of BL Lac objects that are hard to identify because they lack the unique
properties classifying them as such. While radio or X-ray surveys typically
reveal dozens of sources, recent compilations based on optical criteria alone
have increased the number of BL Lac candidates considerably. However, these
compilations are subject to biases and may contain a substantial number of
contaminating sources. \textbf{Aims.} In this paper we extend our analysis of
182 optically selected BL Lac object candidates from the SDSS with respect to
an earlier study. The main goal is to determine the number of bona fide BL Lac
objects in this sample. \textbf{Methods.} We examine their variability
characteristics, determine their broad-band radio-UV SEDs, and search for the
presence of a host galaxy. In addition we present new optical spectra for 27
targets with improved S/N with respect to the SDSS spectra. \textbf{Results.}
At least 59% of our targets have shown variability between SDSS DR2 and our
observations by more than 0.1-0.27 mag de- pending on the telescope used. A
host galaxy was detected in 36% of our targets. The host galaxy type and
luminosities are consistent with earlier studies of BL Lac host galaxies.
Simple fits to broad-band SEDS for 104 targets of our sample derived
synchrotron peak frequencies between with a peak at
. Our new optical spectra do not reveal any new
redshift for any of our objects. Thus the sample contains a large number of
bona fide BL Lac objects and seems to contain a substantial fraction of
intermediate-frequency peaked BL Lacs.Comment: Accepted for publication in A\&
Spectroscopic studies of individual plasmon resonant nanoparticles
We present a detailed description of the apparatus and techniques that we have utilized in our experimental study of individual plasmon resonant nanoparticles, along with a brief description of some major results. The apparatus consists of a spectroscopic system combined with a modified darkfield microscope, which enables the user to sequentially select individual resonant nanostructures in the microscopic field of view for spectroscopic study. Plasmon resonant nanostructures scatter light elastically, and typically have very large scattering cross-sections at their resonant optical wavelengths. In general, spectra can be obtained with acquisition times between .1 to 30 seconds, and color images can be captured using consumer digital color cameras. Spheres, tetrahedrons, and pentagonal platelets were fabricated using colloidal chemistry techniques. To produce highly anisotropic structures such as nanorods and "barbells", templates were used. Many of these nanostructures have been individually spectroscopically characterized, and their spectra correlated with their shape and size as determined by transmission electron microscope (TEM). The unique shape, size, composition, and dielectric surroundings of the individual plasmon resonant nanostructures determine their plasmon resonant behavior. We will show how the composition of the substrate on which the particles are immobilized and the dielectric of the surrounding medium have a significant effect on the plasmon resonance of the individual particles
Quantitative assessment of pinning forces and the superconducting gap in NbN thin films from complementary magnetic force microscopy and transport measurements
Epitaxial niobium-nitride thin films with a critical temperature of Tc=16K
and a thickness of 100nm were fabricated on MgO(100) substrates by pulsed laser
deposition. Low-temperature magnetic force microscopy (MFM) images of the
supercurrent vortices were measured after field cooling in a magnetic field of
3mT at various temperatures. Temperature dependence of the penetration depth
has been evaluated by a two-dimensional fitting of the vortex profiles in the
monopole-monopole model. Its subsequent fit to a single s-wave gap function
results in the superconducting gap amplitude Delta(0) = 2.9 meV = 2.1*kB*Tc, in
perfect agreement with previous reports. The pinning force has been
independently estimated from local depinning of individual vortices by lateral
forces exerted by the MFM tip and from transport measurements. A good
quantitative agreement between the two techniques shows that for low fields, B
<< Hc2, MFM is a powerful and reliable technique to probe the local variations
of the pinning landscape. We also demonstrate that the monopole model can be
successfully applied even for thin films with a thickness comparable to the
penetration depth.Comment: 6 pages, 6 figures, 2 table
A quasi classical approach to fully differential ionization cross sections
A classical approximation to time dependent quantum mechanical scattering in
the M\o{}ller formalism is presented. Numerically, our approach is similar to a
standard Classical-Trajectory-Monte-Carlo calculation. Conceptually, however,
our formulation allows one to release the restriction to stationary initial
distributions. This is achieved by a classical forward-backward propagation
technique. As a first application and for comparison with experiment we present
fully differential cross sections for electron impact ionization of atomic
hydrogen in the Erhardt geometry.Comment: 6 pages, 2 figure
Spin injection from perpendicular magnetized ferromagnetic -MnGa into (Al,Ga)As heterostructures
Electrical spin injection from ferromagnetic -MnGa into an (Al,Ga)As
p-i-n light emitting diode (LED) is demonstrated. The -MnGa layers show
strong perpendicular magnetocrystalline anisotropy, enabling detection of spin
injection at remanence without an applied magnetic field. The bias and
temperature dependence of the spin injection are found to be qualitatively
similar to Fe-based spin LED devices. A Hanle effect is observed and
demonstrates complete depolarization of spins in the semiconductor in a
transverse magnetic field.Comment: 4 pages, 3 figure
Dissipative Taylor-Couette flows under the influence of helical magnetic fields
The linear stability of MHD Taylor-Couette flows in axially unbounded
cylinders is considered, for magnetic Prandtl number unity. Magnetic fields
varying from purely axial to purely azimuthal are imposed, with a general
helical field parameterized by \beta=B_\phi/B_z. We map out the transition from
the standard MRI for \beta=0 to the nonaxisymmetric Azimuthal MagnetoRotational
Instability (AMRI) for \beta\to \infty. For finite \beta, positive and negative
wave numbers m, corresponding to right and left spirals, are no longer
identical. The transition from \beta=0 to \beta\to\infty includes all the
possible forms of MRI with axisymmetric and nonaxisymmetric modes. For the
nonaxisymmetric modes, the most unstable mode spirals in the opposite direction
to the background field. The standard (\beta=0) MRI is axisymmetric for weak
fields (including the instability with the lowest Reynolds number) but is
nonaxisymmetric for stronger fields. If the azimuthal field is due in part to
an axial current flowing through the fluid itself (and not just along the
central axis), then it is also unstable to the nonaxisymmetric Tayler
instability, which is most effective without rotation. For large \beta this
instability has wavenumber m=1, whereas for \beta\simeq 1 m=2 is most unstable.
The most unstable mode spirals in the same direction as the background field.Comment: 9 pages, 11 figure
Atmospheric circulation patterns, cloud-to-ground lightning, and locally intense convective rainfall associated with debris flow initiation in the Dolomite Alps of northeastern Italy
The Dolomite Alps of northeastern Italy experience debris flows with great
frequency during the summer months. An ample supply of unconsolidated
material on steep slopes and a summer season climate regime characterized by
recurrent thunderstorms combine to produce an abundance of these destructive
hydro-geologic events. In the past, debris flow events have been studied
primarily in the context of their geologic and geomorphic characteristics.
The atmospheric contribution to these mass-wasting events has been limited
to recording rainfall and developing intensity thresholds for debris
mobilization. This study aims to expand the examination of atmospheric
processes that preceded both locally intense convective rainfall (LICR) and
debris flows in the Dolomite region. 500 hPa pressure level plots of
geopotential heights were constructed for a period of 3Â days prior to
debris flow events to gain insight into the synoptic-scale processes which
provide an environment conducive to LICR in the Dolomites. Cloud-to-ground (CG)
lightning flash data recorded at the meso-scale were incorporated to
assess the convective environment proximal to debris flow source regions.
Twelve events were analyzed and from this analysis three common synoptic-scale circulation patterns were identified. Evaluation of CG flashes at
smaller spatial and temporal scales illustrated that convective processes
vary in their production of CF flashes (total number) and the spatial
distribution of flashes can also be quite different between events over
longer periods. During the 60 min interval immediately preceding debris
flow a majority of cases exhibited spatial and temporal colocation of LICR
and CG flashes. Also a number of CG flash parameters were found to be
significantly correlated to rainfall intensity prior to debris flow initiation
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