1,167 research outputs found
The impact of two-dimensional elastic disk
The impact of a two-dimensional elastic disk with a wall is numerically
studied. It is clarified that the coefficient of restitution (COR) decreases
with the impact velocity. The result is not consistent with the recent
quasi-static theory of inelastic collisions even for very slow impact. The
abrupt drop of COR is found due to the plastic deformation of the disk, which
is assisted by the initial internal motion.(to be published in J. Phys. Soc.
Jpn.)Comment: 6 Pages,2 figure
Performance of HPGe Detectors in High Magnetic Fields
A new generation of high-resolution hypernuclear gamma$-spectroscopy
experiments with high-purity germanium detectors (HPGe) are presently designed
at the FINUDA spectrometer at DAPhiNE, the Frascati phi-factory, and at PANDA,
the antiproton proton hadron spectrometer at the future FAIR facility. Both,
the FINUDA and PANDA spectrometers are built around the target region covering
a large solid angle. To maximise the detection efficiency the HPGe detectors
have to be located near the target, and therefore they have to be operated in
strong magnetic fields B ~ 1 T. The performance of HPGe detectors in such an
environment has not been well investigated so far. In the present work VEGA and
EUROBALL Cluster HPGe detectors were tested in the field provided by the ALADiN
magnet at GSI. No significant degradation of the energy resolution was found,
and a change in the rise time distribution of the pulses from preamplifiers was
observed. A correlation between rise time and pulse height was observed and is
used to correct the measured energy, recovering the energy resolution almost
completely. Moreover, no problems in the electronics due to the magnetic field
were observed.Comment: submitted to Nucl. Instrum. Meth. Phys. Res. A, LaTeX, 19 pages, 9
figure
Gate control of low-temperature spin dynamics in two-dimensional hole systems
We have investigated spin and carrier dynamics of resident holes in
high-mobility two-dimensional hole systems in GaAs/AlGaAs
single quantum wells at temperatures down to 400 mK. Time-resolved Faraday and
Kerr rotation, as well as time-resolved photoluminescence spectroscopy are
utilized in our study. We observe long-lived hole spin dynamics that are
strongly temperature dependent, indicating that in-plane localization is
crucial for hole spin coherence. By applying a gate voltage, we are able to
tune the observed hole g factor by more than 50 percent. Calculations of the
hole g tensor as a function of the applied bias show excellent agreement with
our experimental findings.Comment: 8 pages, 7 figure
Engineering ultralong spin coherence in two-dimensional hole systems at low temperatures
For the realisation of scalable solid-state quantum-bit systems, spins in
semiconductor quantum dots are promising candidates. A key requirement for
quantum logic operations is a sufficiently long coherence time of the spin
system. Recently, hole spins in III-V-based quantum dots were discussed as
alternatives to electron spins, since the hole spin, in contrast to the
electron spin, is not affected by contact hyperfine interaction with the
nuclear spins. Here, we report a breakthrough in the spin coherence times of
hole ensembles, confined in so called natural quantum dots, in narrow
GaAs/AlGaAs quantum wells at temperatures below 500 mK. Consistently,
time-resolved Faraday rotation and resonant spin amplification techniques
deliver hole-spin coherence times, which approach in the low magnetic field
limit values above 70 ns. The optical initialisation of the hole spin
polarisation, as well as the interconnected electron and hole spin dynamics in
our samples are well reproduced using a rate equation model.Comment: 16 pages, 6 figure
Propagating front in an excited granular layer
A partial monolayer of ~ 20000 uniform spherical steel beads, vibrated
vertically on a flat plate, shows remarkable ordering transitions and
cooperative behavior just below 1g maximum acceleration. We study the stability
of a quiescent disordered or ``amorphous'' state formed when the acceleration
is switched off in the excited ``gaseous'' state. The transition from the
amorphous state back to the gaseous state upon increasing the plate's
acceleration is generally subcritical: An external perturbation applied to one
bead initiates a propagating front that produces a rapid transition. We measure
the front velocity as a function of the applied acceleration. This phenomenon
is explained by a model based on a single vibrated particle with multiple
attractors that is perturbed by collisions. A simulation shows that a
sufficiently high rate of interparticle collisions can prevent trapping in the
attractor corresponding to the nonmoving ground state.Comment: 16 pages, 9 figures, revised version, to appear in Phys. Rev. E, May
199
Hydrodynamics of driven granular gases
Hydrodynamic equations for granular gases driven by the Fokker-Planck
operator are derived. Transport coefficients appeared in Navier-Stokes order
change from the values of a free cooling state to those of a steady state.Comment: 5 pages, 3 figure
On the ÎČ-detection efficiency of a combined Si and plastic stack detector for DESPEC
A Geant4 simulation has been carried out in order to determine the B-detection efficiency of a rare isotope
beam implantation setup, for decay spectroscopy experiments, comprising a number of Double Sided Silicon
Strip Detectors (DSSSDs) and two plastic scintillation detectors placed upstream and downstream. The
absolute efficiency for the emitted B-particle detection from radioactive fragments implanted in the DSSSDs
using fast-timing plastic-scintillator detector, is calculated. The detection efficiency of the setup has been
studied with two different distances between the Si layers and plastics. The requirement for the thickness of
the Si detector layers and its implication on the B-detection effciency has been investigated for 1 mm and
300 um thickness of Si layers. The combined efficiency of DSSSD and plastic detectors were also simulated
for two different thicknesses of the DSSSD
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