8,057 research outputs found
Effects of Defects on Friction for a Xe Film Sliding on Ag(111)
The effects of a step defect and a random array of point defects (such as
vacancies or substitutional impurities) on the force of friction acting on a
xenon monolayer film as it slides on a silver (111) substrate are studied by
molecular dynamic simulations and compared with the results of lowest order
perturbation theory in the substrate corrugation potential. For the case of a
step, the magnitude and velocity dependence of the friction force are strongly
dependent on the direction of sliding respect to the step and the corrugation
strength. When the applied force F is perpendicular to the step, the film is
pinned forF less than a critical force Fc. Motion of the film along the step,
however, is not pinned. Fluctuations in the sliding velocity in time provide
evidence of both stick-slip motion and thermally activated creep. Simulations
done with a substrate containing a 5 percent concentration of random point
defects for various directions of the applied force show that the film is
pinned for the force below a critical value. The critical force, however, is
still much lower than the effective inertial force exerted on the film by the
oscillations of the substrate in experiments done with a quartz crystal
microbalance (QCM). Lowest order perturbation theory in the substrate potential
is shown to give results consistent with the simulations, and it is used to
give a physical picture of what could be expected for real surfaces which
contain many defects.Comment: 13 pages, 17 figures, latex plus postscript files for figure
PDS 144: the first confirmed Herbig Ae-Herbig Ae wide binary
PDS 144 is a pair of Herbig Ae stars that are separated by 5.'' 35 on the sky. It has previously been shown to have an A2Ve Herbig Ae star viewed at 83 degrees inclination as its northern member and an A5Ve Herbig Ae star as its southern member. Direct imagery revealed a disk occulting PDS 144 N-the first edge-on disk observed around a Herbig Ae star. The lack of an obvious disk in direct imagery suggested PDS 144 S might be viewed face-on or not physically associated with PDS 144 N. Multi-epoch Hubble Space Telescope imagery of PDS 144 with a 5 year baseline demonstrates PDS 144 N & S are comoving and have a common proper motion with TYC 6782-878-1. TYC 6782-878-1 has previously been identified as a member of Upper Sco sub-association A at d = 145 +/- 2 pc with an age of 5-10 Myr. Ground-based imagery reveals jets and a string of Herbig-Haro knots extending 13' (possibly further) which are aligned to within 7 degrees +/- 6 degrees on the sky. By combining proper motion data and the absence of a dark mid-plane with radial velocity data, we measure the inclination of PDS 144 S to be i = 73 degrees +/- 7 degrees. The radial velocity of the jets from PDS 144 N & S indicates they, and therefore their disks, are misaligned by 25 degrees +/- 9 degrees. This degree of misalignment is similar to that seen in T Tauri wide binaries.Peer reviewe
Metal-Insulator oscillations in a Two-dimensional Electron-Hole system
The electrical transport properties of a bipolar InAs/GaSb system have been
studied in magnetic field. The resistivity oscillates between insulating and
metallic behaviour while the quantum Hall effect shows a digital character
oscillating from 0 to 1 conducatance quantum e^2/h. The insulating behaviour is
attributed to the formation of a total energy gap in the system. A novel looped
edge state picture is proposed associated with the appearance of a voltage
between Hall probes which is symmetric on magnetic field reversal.Comment: 4 pages, 5 Postscript figures: revised versio
Primordial black hole constraints in cosmologies with early matter domination
Moduli fields, a natural prediction of any supergravity and
superstring-inspired supersymmetry theory, may lead to a prolonged period of
matter domination in the early Universe. This can be observationally viable
provided the moduli decay early enough to avoid harming nucleosynthesis. If
primordial black holes form, they would be expected to do so before or during
this matter dominated era. We examine the extent to which the standard
primordial black hole constraints are weakened in such a cosmology. Permitted
mass fractions of black holes at formation are of order , rather than
the usual or so. If the black holes form from density perturbations
with a power-law spectrum, its spectral index is limited to ,
rather than the obtained in the standard cosmology.Comment: 7 pages RevTeX file with four figures incorporated (uses RevTeX and
epsf). Also available by e-mailing ARL, or by WWW at
http://star-www.maps.susx.ac.uk/papers/infcos_papers.htm
High-spin states with seniority v=4,4,6 in 119-126Sn
The 119-126Sn nuclei have been produced as fission fragments in two reactions
induced by heavy ions: 12C+238U at 90 MeV bombarding energy, 18O+208Pb at 85
MeV. Their level schemes have been built from gamma rays detected using the
Euroball array. High-spin states located above the long-lived isomeric states
of the even- and odd-A 120-126Sn nuclei have been identified. Moreover isomeric
states lying around 4.5 MeV have been established in 120,122,124,126Sn from the
delayed coincidences between the fission fragment detector SAPhIR and the
Euroball array. The states located above 3-MeV excitation energy are ascribed
to several broken pairs of neutrons occupying the nu h11/2 orbit. The maximum
value of angular momentum available in such a high-j shell, i.e. for
mid-occupation and the breaking of the three neutron pairs, has been
identified. This process is observed for the first time in spherical nuclei.Comment: 20 pages, 22 figures, 12 tables, accepted for publication in Physical
Review
Uniform random generation of large acyclic digraphs
Directed acyclic graphs are the basic representation of the structure
underlying Bayesian networks, which represent multivariate probability
distributions. In many practical applications, such as the reverse engineering
of gene regulatory networks, not only the estimation of model parameters but
the reconstruction of the structure itself is of great interest. As well as for
the assessment of different structure learning algorithms in simulation
studies, a uniform sample from the space of directed acyclic graphs is required
to evaluate the prevalence of certain structural features. Here we analyse how
to sample acyclic digraphs uniformly at random through recursive enumeration,
an approach previously thought too computationally involved. Based on
complexity considerations, we discuss in particular how the enumeration
directly provides an exact method, which avoids the convergence issues of the
alternative Markov chain methods and is actually computationally much faster.
The limiting behaviour of the distribution of acyclic digraphs then allows us
to sample arbitrarily large graphs. Building on the ideas of recursive
enumeration based sampling we also introduce a novel hybrid Markov chain with
much faster convergence than current alternatives while still being easy to
adapt to various restrictions. Finally we discuss how to include such
restrictions in the combinatorial enumeration and the new hybrid Markov chain
method for efficient uniform sampling of the corresponding graphs.Comment: 15 pages, 2 figures. To appear in Statistics and Computin
On right conjugacy closed loops of twice prime order
The right conjugacy closed loops of order 2p, where p is an odd prime, are
classified up to isomorphism.Comment: Clarified definitions, added some remarks and a tabl
Fv antibodies to aflatoxin B1 derived from a pre-immunized antibody phage display library system
The production and characterization of recombinant antibodies to aflatoxin B[SUB1] (AFB[SUB1]), a potent mycotoxin and carcinogen is described. The antibody fragments produced were then applied for use in a surface plasmon resonance-based biosensor (BIAcore), which measures biomolecular interactions in 'real-time'. Single chain Fv (scFv) antibodies were generated to aflatoxin B1 from an established phage display system, which incorporated a range of different plasmids for efficient scFv expression. The scFv's were used in the development of a competitive ELISA, and also for the development of surface plasmon resonance (SPR)-based inhibition immunoassays. They were found to be suitable for the detection of AFB[SUB1], in this format, with the assays being sensitive and reproducible
Integronlike Structures in Campylobacter spp. of Human and Animal Origin
Resistance to antimicrobial agents used to treat severe Campylobacter spp. gastroenteritis is increasing worldwide. We assessed the antimicrobial resistance patterns of Campylobacter spp. isolates of human and animal origin. More than half (n = 32) were resistant to sulphonamide, a feature known to be associated with the presence of integrons. Analysis of these integrons will further our understanding of Campylobacter spp. epidemiology
Time-Resolved Studies of Stick-Slip Friction in Sheared Granular Layers
Sensitive and fast force measurements are performed on sheared granular
layers undergoing stick-slip motion, along with simultaneous imaging. A full
study has been done for spherical particles with a +-20% size distribution.
Stick-slip motion due to repetitive fluidization of the layer occurs for low
driving velocities. Between major slip events, slight creep occurs that is
variable from one event to the next. The effects of changing the stiffness k
and velocity V of the driving system are studied in detail. The stick-slip
motion is almost periodic for spherical particles over a wide range of
parameters, but becomes irregular when k is large and V is relatively small. At
larger V, the motion becomes smoother and is affected by the inertia of the
upper plate bounding the layer. Measurements of the period T and amplitude A of
the relative motion are presented as a function of V. At a critical value Vc, a
transition to continuous sliding motion occurs that is discontinuous for k not
too large. The time dependence of the instantaneous velocity of the upper plate
and the frictional force produced by the granular layer are determined within
individual slipping events. The force is a multi-valued function of the
instantaneous velocity, with pronounced hysteresis and a sudden drop prior to
resticking. Measurements of vertical displacement reveal a small dilation of
the material (about one tenth of the mean particle size in a layer 20 particles
deep) associated with each slip event. Finally, optical imaging reveals that
localized microscopic rearrangements precede (and follow) each slip event. The
behavior of smooth particles is contrasted with that of rough particles.Comment: 20, pages, 17 figures, to appear in Phys. Rev.
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