350 research outputs found
Coulomb Drag in the Exciton Regime in Electron-Hole Bilayers
We report electrical transport measurements on GaAs/AlGaAs based
electron-hole bilayers. These systems are expected to make a transition from a
pair of weakly coupled two-dimensional systems to a strongly coupled exciton
system as the barrier between the layers is reduced. Once excitons form,
phenomena such as Bose-Einstein condensation of excitons could be observed. In
our devices, electrons and holes are confined in double quantum wells, and
carriers in the devices are induced with top and bottom gates leading to
variable density in each layer. Separate contact to each layer allows Coulomb
drag transport measurements where current is driven in one layer while voltage
is measured in the other. Coulomb drag is sensitive to interlayer coupling and
has been predicted to provide a strong signature of exciton condensation. Drag
measurement on EHBLs with a 30 nm barrier are consistent with drag between two
weakly coupled 2D Fermi systems where the drag decreases as the temperature is
reduced. When the barrier is reduced to 20 nm, we observe a consistent increase
in the drag resistance as the temperature is reduced. These results indicate
the onset of a much stronger coupling between the electrons and holes which
leads to exciton formation and possibly phenomena related to exciton
condensation.Comment: 12 pages, 3 figure
Charmonium-hadron interactions from QCD
The heavy quark system is an excellent probe to learn about the QCD dynamics
at finite density. First, we discuss the properties of the and
meson at finite nucleon density. We discuss why their properties should change
at finite density and then introduce an exact QCD relation among these hadron
properties and the energy momentum tensor of the medium. Second, we discuss
attempts to calculate charmonium-hadron total cross section using effective
hadronic models and perturbative QCD. We emphasize a recent calculation, where
the cross section is derived using QCD factorization theorem. We conclude by
discussing some challenges for SIS 200.Comment: 8 pages, Presented at 6th International Conference on Strange Quarks
in Matter: 2001: A Flavorspace Odyssey (SQM2001), Frankfurt, Germany, 25-29
Sep 2001, submitted to J. Phys.
Mass modification of D-meson in hot hadronic matter
We evaluate the in-medium and -meson masses in hot hadronic
matter induced by interactions with the light hadron sector described in a
chiral SU(3) model. The effective Lagrangian approach is generalized to SU(4)
to include charmed mesons. We find that the D-mass drops substantially at
finite temperatures and densities, which open the channels of the decay of the
charmonium states (, , ) to pairs in
the thermal medium. The effects of vacuum polarisations from the baryon sector
on the medium modification of the -meson mass relative to those obtained in
the mean field approximation are investigated. The results of the present work
are compared to calculations based on the QCD sum-rule approach, the
quark-meson coupling model, chiral perturbation theory, as well as to studies
of quarkonium dissociation using heavy quark potential from lattice QCD.Comment: 18 pages including 7 figures, minor revision of the text, figure
styles modified, to appear in Phys. Rev.
Molecular and biochemical alterations in tubular epithelial cells of patients with isolated methylmalonic aciduria
Methylmalonic acidurias (MMAurias) are a group of inherited disorders in the catabolism of branched-chain amino acids, odd-chain fatty acids and cholesterol caused by complete or partial deficiency of methylmalonyl-CoA mutase (mut0 and mut- subtype respectively) and by defects in the metabolism of its cofactor 5âČ-deoxyadenosylcobalamin (cblA, cblB or cblD variant 2 type). A long-term complication found in patients with mut0 and cblB variant is chronic tubulointerstitial nephritis. The underlying pathomechanism has remained unknown. We established an in vitro model of tubular epithelial cells from patient urine (hTEC; 9 controls, 5 mut0, 1 cblB). In all human tubular epithelial cell (hTEC) lines we found specific tubular markers (AQP1, UMOD, AQP2). Patient cells showed disturbance of energy metabolism in glycolysis, mitochondrial respiratory chain and Krebs cycle in concert with increased reactive oxygen species (ROS) formation. Electron micrographs indicated increased autophagosome production and endoplasmic reticulum stress, which was supported by positive acridine orange staining and elevated levels of LC3 II, P62 and pIRE1. Screening mTOR signaling revealed a release of inhibition of autophagy. Patient hTEC produced and secreted elevated amounts of the pro-inflammatory cytokine IL8, which was highly correlated with the acridine orange staining. Summarizing, hTEC of MMAuria patients are characterized by disturbed energy metabolism and ROS production that lead to increased autophagy and IL8 secretio
Layer interdependence of transport in an undoped electron-hole bilayer
The layer interdependence of transport in an undoped electron-hole bilayer
(uEHBL) device was studied as a function of carrier density, interlayer
electric field, and temperature. The uEHBL device consisted of a density
tunable, independently contacted two-dimensional electron gas (2DEG) and
two-dimensional hole gas (2DHG) induced via field effect in distinct GaAs
quantum wells separated by a 30 nm AlGaAs barrier. Transport
measurements were made simultaneously on each layer using the van der Pauw
method. An increase in 2DHG mobility with increasing 2DEG density was observed,
while the 2DEG mobility showed negligible dependence on the 2DHG density.
Decreasing the interlayer electric-field and thereby increasing interlayer
separation also increased the 2DHG mobility with negligible effects on the 2DEG
mobility. The change in interlayer separation as interlayer electric-field
changed was estimated using 2DHG Coulomb drag measurements. The results were
consistent with mobility of each layer being only indirectly dependent on the
adjacent layer density and dominated by background impurity scattering.
Temperature dependencies were also determined for the resistivity of each
layer.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
Event-based camera refractory period characterization and initial clock drift evaluation
Event-based camera (EBC) technology provides high-dynamic range operation and shows promise for efficient capture of spatio-temporal information, producing a sparse data stream and enabling consideration of nontraditional data processing solutions (e.g., new algorithms, neuromorphic processors, etc.). Given the fundamental difference in camera architecture, the EBC response and noise behavior differ considerably compared to standard CCD/CMOS framing sensors. These differences necessitate the development of new characterization techniques and sensor models to evaluate hardware performance and elucidate the trade-space between the two camera architectures. Laboratory characterization techniques reported previously include noise level as a function of static scene light level (background activity) and contrast responses referred to as S-curves. Here we present further progress on development of basic characterization methods and test capabilities for commercial-off-the-shelf (COTS) visible EBCs, with a focus on measurement of pixel deadtime (refractory period) including results for the 4th-generation sensor from Prophesee and Sony. Refractory period is empirically determined from analysis of the interspike intervals (ISIs), and results visualized using log-histograms of the minimum per-pixel ISI values for a subset of pixels activated by a controlled dynamic scene. Our tests of the Prophesee gen4 EVKv2 yield refractory period estimates ranging from 6.1 msec to 6.8 ÎŒsec going from the slowest (20) to fastest (100) settings of the relevant bias parameter, bias_refr. We also introduce and demonstrate the concept of pixel bandwidth measurement from data captured while viewing a static scene â based on recording data at a range of refractory period setting and then analyzing noise-event statistics. Finally, we present initial results for estimating and correcting EBC clock drift using a GPS PPS signal to generate special timing events in the event-list data streams generated by the DAVIS346 and DVXplorer EBCs from iniVation
Inelastic light, neutron, and X-ray scatterings related to the heterogeneous elasticity of glasses
The effects of plasticization of poly(methyl methacrylate) glass on the boson
peaks observed by Raman and neutron scattering are compared. In plasticized
glass the cohesion heterogeneities are responsible for the neutron boson peak
and partially for the Raman one, which is enhanced by the composition
heterogeneities. Because the composition heterogeneities have a size similar to
that of the cohesion ones and form quasiperiodic clusters, as observed by small
angle X-ray scattering, it is inferred that the cohesion heterogeneities in a
normal glass form nearly periodic arrangements too. Such structure at the
nanometric scale explains the linear dispersion of the vibrational frequency
versus the transfer momentum observed by inelastic X-ray scattering.Comment: 9 pages, 2 figures, to be published in J. Non-Cryst. Solids
(Proceedings of the 4th IDMRCS
Neutron dose rate at the SwissFEL injector test facility: first measurements
At the Paul Scherrer Institute, the new SwissFEL Free Electron Laser facility is currently in the design phase. It is foreseen to accelerate electrons up to a maximum energy of 7 GeV with a pulsed time structure. An injector test facility is operated at a maximum energy of 300 MeV and serves as the principal test and demonstration plant for the SwissFEL project. Secondary radiation is created in unavoidable interactions of the primary beam with beamline components. The resulting ambient dose-equivalent rate due to neutrons was measured along the beamline with different commercially available survey instruments. The present study compares the readings of these neutron detectors (one of them is specifically designed for measurements in pulsed fields). The experiments were carried out in both, a normal and a diagnostic mode of operation of the injecto
Theory of sound attenuation in glasses: The role of thermal vibrations
Sound attenuation and internal friction coefficients are calculated for a
realistic model of amorphous silicon. It is found that, contrary to previous
views, thermal vibrations can induce sound attenuation at ultrasonic and
hypersonic frequencies that is of the same order or even larger than in
crystals. The reason is the internal-strain induced anomalously large
Gr\"uneisen parameters of the low-frequency resonant modes.Comment: 8 pages, 3 figures; to appear in PR
- âŠ