10,720 research outputs found
Capability of Cherenkov Telescopes to Observe Ultra-fast Optical Flares
The large optical reflector (~ 100 m^2) of a H.E.S.S. Cherenkov telescope was
used to search for very fast optical transients of astrophysical origin. 43
hours of observations targeting stellar-mass black holes and neutron stars were
obtained using a dedicated photometer with microsecond time resolution. The
photometer consists of seven photomultiplier tube pixels: a central one to
monitor the target and a surrounding ring of six pixels to veto background
events. The light curves of all pixels were recorded continuously and were
searched offline with a matched-filtering technique for flares with a duration
of 2 us to 100 ms. As expected, many unresolved (500 us)
background events originating in the earth's atmosphere were detected. In the
time range 3 to 500 us the measurement is essentially background-free, with
only eight events detected in 43 h; five from lightning and three presumably
from a piece of space debris. The detection of flashes of brightness ~ 0.1 Jy
and only 20 us duration from the space debris shows the potential of this setup
to find rare optical flares on timescales of tens of microseconds. This
timescale corresponds to the light crossing time of stellar-mass black holes
and neutron stars.Comment: Accepted for publication in Astroparticle Physics, 8 pages, 9
figures, 1 tabl
Performance of high resistivity n+pp+ silicon solar cells under 1 MeV electron irradiation
High resistivity (1250 and 84 ohm-cm) n(+)pp(+) silicon solar cells were irradiated and their performance evaluated as a function of fluence. The greatest degradation in power occurred for the higher resistivity cell. The data were analyzed under open circuit conditions, and the components of V sub oc determined as a function of fluence. It was found that the voltage contributions from the front and back junctions decreased while the base component (V sub B) increased with fluence. The anomalous behavior of V sub B was attributed to an increase in the base minority carrier gradient with fluence. An argument that the increased power degradation in the 1250 ohm-cm cells was attributable to an increased voltage drop in the base is presented. Diffusion lengths calculated under high injection conditions were significantly greater than those determined under low injection. This was attributed to a saturation of recombination centers under high injection conditions
Spatial rogue waves in photorefractive SBN crystals
We report on the excitation of large-amplitude waves, with a probability of
around 1% of total peaks, on a photorefractive SBN crystal by using a simple
experimental setup at room temperature. We excite the system using a narrow
Gaussian beam and observe different dynamical regimes tailored by the value and
time rate of an applied voltage. We identify two main dynamical regimes: a
caustic one for energy spreading and a speckling one for peak emergence. Our
observations are well described by a two-dimensional Schr\"odinger model with
saturable local nonlinearity.Comment: 4 pages, 4 figure
Optical signature of the pressure-induced dimerization in the honeycomb iridate -LiIrO
We studied the effect of external pressure on the electrodynamic properties
of -LiIrO single crystals in the frequency range of the phonon
modes and the Ir - transitions. The abrupt hardening of several phonon
modes under pressure supports the onset of the dimerized phase at the critical
pressure =3.8 GPa. With increasing pressure an overall decrease in
spectral weight of the Ir - transitions is found up to . Above
, the local (on-site) - excitations gain spectral weight with
increasing pressure, which hints at a pressure-induced increase in the
octahedral distortions. The non-local (intersite) Ir - transitions show a
monotonic blue-shift and decrease in spectral weight. The changes observed for
the non-local excitations are most prominent well above , namely for
pressures 12 GPa, and only small changes occur for pressures close to
. The profile of the optical conductivity at high pressures (20 GPa)
appears to be indicative for the dimerized state in iridates.Comment: 10 pages, 6 figures; accepted for publication in Phys. Rev.
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