1,459 research outputs found
Neutron-induced background in the CONUS experiment
CONUS is a novel experiment aiming at detecting elastic neutrino nucleus
scattering in the fully coherent regime using high-purity Germanium (Ge)
detectors and a reactor as antineutrino () source. The detector setup
is installed at the commercial nuclear power plant in Brokdorf, Germany, at a
very small distance to the reactor core in order to guarantee a high flux of
more than 10/(scm). For the experiment, a good
understanding of neutron-induced background events is required, as the neutron
recoil signals can mimic the predicted neutrino interactions. Especially
neutron-induced events correlated with the thermal power generation are
troublesome for CONUS. On-site measurements revealed the presence of a thermal
power correlated, highly thermalized neutron field with a fluence rate of
(74530)cmd. These neutrons that are produced by nuclear
fission inside the reactor core, are reduced by a factor of 10 on
their way to the CONUS shield. With a high-purity Ge detector without shield
the -ray background was examined including highly thermal power
correlated N decay products as well as -lines from neutron
capture. Using the measured neutron spectrum as input, it was shown, with the
help of Monte Carlo simulations, that the thermal power correlated field is
successfully mitigated by the installed CONUS shield. The reactor-induced
background contribution in the region of interest is exceeded by the expected
signal by at least one order of magnitude assuming a realistic ionization
quenching factor of 0.2.Comment: 28 pages, 28 figure
Measurement of miniband parameters of a doped superlattice by photoluminescence in high magnetic fields
We have studied a 50/50\AA superlattice of GaAs/AlGaAs
composition, modulation-doped with Si, to produce
cm electrons per superlattice period. The modulation-doping was tailored
to avoid the formation of Tamm states, and photoluminescence due to interband
transitions from extended superlattice states was detected. By studying the
effects of a quantizing magnetic field on the superlattice photoluminescence,
the miniband energy width, the reduced effective mass of the electron-hole
pair, and the band gap renormalization could be deduced.Comment: minor typing errors (minus sign in eq. (5)
Effects of prostaglandin I2 and forskolin on the secretion from platelets evoked at basal concentrations of cytoplasmic free calcium by thrombin, collagen, phorbol ester and exogenous diacylglycerol
Many-body theory of pump-probe spectra for highly excited semiconductors
We present a unified theory for pump-probe spectra in highly excited
semiconductors, which is applicable throughout the whole density regime
including the high-density electron-hole BCS state and the low-density
excitonic Bose-Einstein condensate (BEC). The analysis is based on the BCS-like
pairing theory combined with the Bethe-Salpeter (BS) equation, which first
enables us to incorporate the state-filling effect, the band-gap
renormalization and the strong/weak electron-hole pair correlations in a
unified manner. We show that the electron-hole BCS state is distinctly
stabilized by the intense pump-light, and this result strongly suggests that
the macroscopic quantum state can be observed under the strong photoexcitation.
The calculated spectra considerably deviate from results given by the BCS-like
mean field theory and the simple BS equation without electron-hole pair
correlation especially in the intermediate density states between the
electron-hole BCS state and the excitonic BEC state. In particular, we find the
sharp stimulated emission and absorption lines which originate from the optical
transition accompanied by the collective phase fluctuation mode in the
electron-hole BCS state. From the pump-probe spectral viewpoint, we show that
this fluctuation mode changes to the exciton mode with decreasing carrier
densityComment: RevTeX 11 pages, 10 figures. To appear in Phys.Rev.B1
Vortices on Hyperbolic Surfaces
It is shown that abelian Higgs vortices on a hyperbolic surface can be
constructed geometrically from holomorphic maps , where is also
a hyperbolic surface. The fields depend on and on the metrics of and
. The vortex centres are the ramification points, where the derivative of
vanishes. The magnitude of the Higgs field measures the extent to which
is locally an isometry.
Witten's construction of vortices on the hyperbolic plane is rederived, and
new examples of vortices on compact surfaces and on hyperbolic surfaces of
revolution are obtained. The interpretation of these solutions as
SO(3)-invariant, self-dual SU(2) Yang--Mills fields on is also given.Comment: Revised version: new section on four-dimensional interpretation of
hyperbolic vortices added
Calcium homeostasis in intact lymphocytes: cytoplasmic free calcium monitored with a new, intracellularly trapped fluorescent indicator.
Quasiparticle density of states in dirty high-T_c superconductors
We study the density of quasiparticle states of dirty d-wave superconductors.
We show the existence of singular corrections to the density of states due to
quantum interference effects. We then argue that the density of states actually
vanishes in the localized phase as or depending on whether time
reversal is a good symmetry or not. We verify this result for systems without
time reversal symmetry in one dimension using supersymmetry techniques. This
simple, instructive calculation also provides the exact universal scaling
function for the density of states for the crossover from ballistic to
localized behaviour in one dimension. Above two dimensions, we argue that in
contrast to the conventional Anderson localization transition, the density of
states has critical singularities which we calculate in a
expansion. We discuss consequences of our results for various experiments on
dirty high- materials
Acoustically driven storage of light in a quantum well
The strong piezoelectric fields accompanying a surface acoustic wave on a
semiconductor quantum well structure are employed to dissociate optically
generated excitons and efficiently trap the created electron hole pairs in the
moving lateral potential superlattice of the sound wave. The resulting spatial
separation of the photogenerated ambipolar charges leads to an increase of the
radiative lifetime by orders of magnitude as compared to the unperturbed
excitons. External and deliberate screening of the lateral piezoelectric fields
triggers radiative recombination after very long storage times at a remote
location on the sample.Comment: 4 PostScript figures included, Physical Review Letters, in pres
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