893 research outputs found
Potential Energy Surface for H_2 Dissociation over Pd(100)
The potential energy surface (PES) of dissociative adsorption of H_2 on
Pd(100) is investigated using density functional theory and the full-potential
linear augmented plane wave (FP-LAPW) method. Several dissociation pathways are
identified which have a vanishing energy barrier. A pronounced dependence of
the potential energy on ``cartwheel'' rotations of the molecular axis is found.
The calculated PES shows no indication of the presence of a precursor state in
front of the surface. Both results indicate that steering effects determine the
observed decrease of the sticking coefficient at low energies of the H_2
molecules. We show that the topology of the PES is related to the dependence of
the covalent H(s)-Pd(d) interactions on the orientation of the H_2 molecule.Comment: RevTeX, 8 pages, 5 figures in uufiles forma
Phase transition in a static granular system
We find that a column of glass beads exhibits a well-defined transition
between two phases that differ in their resistance to shear. Pulses of
fluidization are used to prepare static states with well-defined particle
volume fractions in the range 0.57-0.63. The resistance to shear is
determined by slowly inserting a rod into the column of beads. The transition
occurs at for a range of speeds of the rod.Comment: 4 pages, 4 figures. The paper is significantly extended, including
new dat
Subthreshold antiproton production in proton-carbon reactions
Data from KEK on subthreshold antiproton as well as on pi(+-) and K(+-)
production in proton-nucleus reactions are described at projectile energies
between 3.5 and 12.0 GeV. We use a model which considers a hadron-nucleus
reaction as an incoherent sum over collisions of the projectile with a varying
number of target nucleons. It samples complete events and allows thus for the
simultaneous consideration of all particle species measured. The overall
reproduction of the data is quite satisfactory. It is shown that the
contributions from the interaction of the projectile with groups of several
target nucleons are decisive for the description of subthreshold production.
Since the collective features of subthreshold production become especially
significant far below the threshold, the results are extrapolated down to COSY
energies. It is concluded that an antiproton measurement at ANKE-COSY should be
feasible, if the high background of other particles can be efficiently
suppressed.Comment: 15 pages, 5 figures, gzipped tar file, submitted to J. Phys. G v2:
Modification of text due to demands of referee
High-sensitivity monitoring of micromechanical vibration using optical whispering gallery mode resonators
The inherent coupling of optical and mechanical modes in high finesse optical
microresonators provide a natural, highly sensitive transduction mechanism for
micromechanical vibrations. Using homodyne and polarization spectroscopy
techniques, we achieve shot-noise limited displacement sensitivities of
10^(-19) m Hz^(-1/2). In an unprecedented manner, this enables the detection
and study of a variety of mechanical modes, which are identified as radial
breathing, flexural and torsional modes using 3-dimensional finite element
modelling. Furthermore, a broadband equivalent displacement noise is measured
and found to agree well with models for thermorefractive noise in silica
dielectric cavities. Implications for ground-state cooling, displacement
sensing and Kerr squeezing are discussed.Comment: 25 pages, 8 figure
Attosecond electronic recollision as field detector
We demonstrate the complete reconstruction of the electric field of
visible-infrared pulses with energy as low as a few tens of nanojoules. The
technique allows for the reconstruction of the instantaneous electric field
vector direction and magnitude, thus giving access to the characterisation of
pulses with an arbitrary time-dependent polarisation state. The technique
combines extreme ultraviolet interferometry with the generation of isolated
attosecond pulses.Comment: 5 figure
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80,000,000 hooligans. Discourses of resistance to racism and xenophobia in German punk lyrics 1991-1994
The late eighties and early nineties in Germany were not only marked by the fall of the Wall and German unification, but also by the dramatization of the political issue of asylum, resulting in outbreaks of xenophobic violence. In the context of the asylum debate of the early nineties, a number of punk bands produced songs between 1991 and 1994 which criticise the xenophobic climate created by the asylum debate and undermine an exculpatory official discourse about the violent attacks. The lyrics of these songs will be analysed as instances of counter-discourse emerging from a subcultural sphere that nurtures a critical distance towards hegemonic public and political discourse, arguing that Critical Discourse Analysis should pay more attention to defiance of hegemonic discourse
Time-Resolved Measurement of Interatomic Coulombic Decay in Ne_2
The lifetime of interatomic Coulombic decay (ICD) [L. S. Cederbaum et al.,
Phys. Rev. Lett. 79, 4778 (1997)] in Ne_2 is determined via an extreme
ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg. The
pump pulse creates a 2s inner-shell vacancy in one of the two Ne atoms,
whereupon the ionized dimer undergoes ICD resulting in a repulsive
Ne^{+}(2p^{-1}) - Ne^{+}(2p^{-1}) state, which is probed with a second pulse,
removing a further electron. The yield of coincident Ne^{+} - Ne^{2+} pairs is
recorded as a function of the pump-probe delay, allowing us to deduce the ICD
lifetime of the Ne_{2}^{+}(2s^{-1}) state to be (150 +/- 50) fs in agreement
with quantum calculations.Comment: 5 pages, 3 figures, accepted by PRL on July 11th, 201
Kaon Zero-Point Fluctuations in Neutron Star Matter
We investigate the contribution of zero-point motion, arising from
fluctuations in kaon modes, to the ground state properties of neutron star
matter containing a Bose condensate of kaons. The zero-point energy is derived
via the thermodynamic partition function, by integrating out fluctuations for
an arbitrary value of the condensate field. It is shown that the vacuum
counterterms of the chiral Lagrangian ensure the cancellation of divergences
dependent on , the charge chemical potential, which may be regarded as an
external vector potential. The total grand potential, consisting of the
tree-level potential, the zero-point contribution, and the counterterm
potential, is extremized to yield a locally charge neutral, beta-equilibrated
and minimum energy ground state. In some regions of parameter space we
encounter the well-known problem of a complex effective potential. Where the
potential is real and solutions can be obtained, the contributions from
fluctuations are found to be small in comparison with tree-level contributions.Comment: 40 pages RevTeX, 3 epsf figure
Dilatancy transition in a granular model
We introduce a model of granular matter and use a stress ensemble to analyze
shearing. Monte Carlo simulation shows the model to exhibit a second order
phase transition, associated with the onset of dilatancy.Comment: Future versions can be obtained from:
http://www.ma.utexas.edu/users/radin/papers/shear2.pd
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