8,201 research outputs found
Relaxation of curvature induced elastic stress by the Asaro-Tiller-Grinfeld instability
A two-dimensional crystal on the surface of a sphere experiences elastic
stress due to the incompatibility of the crystal axes and the curvature. A
common mechanism to relax elastic stress is the Asaro-Tiller-Grinfeld (ATG)
instability. With a combined numerical and analytical approach we demonstrate,
that also curvature induced stress in surface crystals can be relaxed by the
long wave length ATG instability. The numerical results are obtained using a
surface phase-field crystal (PFC) model, from which we determine the
characteristic wave numbers of the ATG instability for various surface
coverages corresponding to different curvature induced compressions. The
results are compared with an analytic expression for the characteristic wave
number, obtained from a continuum approach which accounts for hexagonal
crystals and intrinsic PFC symmetries. We find our numerical results in
accordance with the analytical predictions.Comment: 6 pages, 5 figure
Triggering of Imaging Air Cherenkov Telescopes: PMT trigger rates due to night-sky photons
Imaging air Cherenkov telescopes are usually triggered on a coincidence of
two or sometimes more pixels, with discriminator thresholds in excess of 20
photoelectrons applied for each pixel. These thresholds required to suppress
night-sky background are significantly higher than expected on the basis of a
Poisson distribution in the number of night-sky photoelectrons generated during
the characteristic signal integration time.
We studied noise trigger rates under controlled conditions using an
artificial background light source. Large tails in the PMT amplitude response
to single photoelectrons are identified as a dominant contribution to noise
triggers. The rate of such events is very sensitive to PMT operating
parameters.Comment: 19 pages, latex,epsf, 7 figures appended as uuencoded file, submitted
to Journal of Physics
Spontaneous Dissociation of 85Rb Feshbach Molecules
The spontaneous dissociation of 85Rb dimers in the highest lying vibrational
level has been observed in the vicinity of the Feshbach resonance which was
used to produce them. The molecular lifetime shows a strong dependence on
magnetic field, varying by three orders of magnitude between 155.5 G and 162.2
G. Our measurements are in good agreement with theoretical predictions in which
molecular dissociation is driven by inelastic spin relaxation. Molecule
lifetimes of tens of milliseconds can be achieved close to resonance.Comment: 4 pages, 3 figure
Recommended from our members
Dust from collisions in circumstellar disks: similarities to meteoritic materials?
Si-induced superconductivity and structural transformations in DyRh4B4
DyRh4B4 has been known to crystallize in the primitive tetragonal
(pt)-structure and to exhibit a ferromagnetic transition at 12 K, the highest
magnetic transition temperature in the entire series of the RRh4B4 materials
[1]. We show here that our silicon-added samples of the nominal composition
DyRh4B4Si0.2 exhibit superconductivity below Tc ~ 4.5 K and an
antiferromagnetic transition below TN ~ 2.7 K. The 12 K transition observed in
the pt-DyRh4B4 is completely suppressed. Our annealed samples mainly consist of
domains of the chemical composition DyRh3.9B4.2Si0.08. These domains contain
two crystallographic phases belonging to the body-centred tetragonal
(bct)-structure and the orthorhombic (o)-structure. We have reasons to suggest
that superconductivity and antiferromagnetic ordering arise from bct- DyRh4B4
phase and, therefore, coexist below TN ~ 2.7 K.Comment: 11 pages, 6 figures, Accepted for publication in Journal of Alloys
and Compound
Design and Simulation of THz Quantum Cascade Lasers
Strategies and concepts for the design of THz emitters based on the quantum
cascade scheme are analyzed and modeled in terms of a fully three-dimensional
Monte Carlo approach; this allows for a proper inclusion of both
carrier-carrier and carrier-phonon scattering mechanisms. Starting from the
simulation of previously published far-infrared emitters, where no population
inversion is achieved, two innovative designs are proposed. The first one
follows the well-established chirped-superlattice scheme whereas the second one
employs a double-quantum well superlattice to allow energy relaxation through
optical phonon emission. For both cases a significant population inversion is
predicted at temperatures up to 80 K.Comment: 4 pages, 2 figures, 2 table
Finite-Temperature Dynamics and Thermal Intraband Magnon Scattering in Haldane Spin-One Chains
The antiferromagnetic spin-one chain is considerably one of the most
fundamental quantum many-body systems, with symmetry protected topological
order in the ground state. Here, we present results for its dynamical spin
structure factor at finite temperatures, based on a combination of exact
numerical diagonalization, matrix-product-state calculations and quantum Monte
Carlo simulations. Open finite chains exhibit a sub-gap band in the thermal
spectral functions, indicative of localized edge-states. Moreover, we observe
the thermal activation of a distinct low-energy continuum contribution to the
spin spectral function with an enhanced spectral weight at low momenta and its
upper threshold. This emerging thermal spectral feature of the Haldane spin-one
chain is shown to result from intra-band magnon scattering due to the thermal
population of the single-magnon branch, which features a large bandwidth-to-gap
ratio. These findings are discussed with respect to possible future studies on
spin-one chain compounds based on inelastic neutron scattering.Comment: 10 pages with 11 figures total (including Supplemental Material);
changes in v2: new Figs. S1 and S5, Fig. S3 expanded + related discussion +
many smaller modifications to match published versio
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