54,956 research outputs found
Crosstalk Correction in Atomic Force Microscopy
Commercial atomic force microscopes usually use a four-segmented photodiode
to detect the motion of the cantilever via laser beam deflection. This read-out
technique enables to measure bending and torsion of the cantilever separately.
A slight angle between the orientation of the photodiode and the plane of the
readout beam, however, causes false signals in both readout channels, so-called
crosstalk, that may lead to misinterpretation of the acquired data. We
demonstrate this fault with images recorded in contact mode on ferroelectric
crystals and present an electronic circuit to compensate for it, thereby
enabling crosstalk-free imaging
Rubber friction on wet and dry road surfaces: the sealing effect
Rubber friction on wet rough substrates at low velocities is typically 20-30%
smaller than for the corresponding dry surfaces. We show that this cannot be
due to hydrodynamics and propose a novel explanation based on a sealing effect
exerted by rubber on substrate "pools" filled with water. Water effectively
smoothens the substrate, reducing the major friction contribution due to
induced viscoelastic deformations of the rubber by surface asperities. The
theory is illustrated with applications related to tire-road friction.Comment: Format Revtex 4; 8 pages, 11 figures (no color); Published on Phys.
Rev. B (http://link.aps.org/abstract/PRB/v71/e035428); previous work on the
same topic: cond-mat/041204
Nucleation and cluster formation in low-density nucleonic matter: A mechanism for ternary fission
Ternary fission yields in the reaction 241Pu(nth,f) are calculated using a
new model which assumes a nucleation-time moderated chemical equilibrium in the
low density matter which constitutes the neck region of the scissioning system.
The temperature, density, proton fraction and fission time required to fit the
experimental data are derived and discussed. A reasonably good fit to the
experimental data is obtained. This model provides a natural explanation for
the observed yields of heavier isotopes relative to those of the lighter
isotopes, the observation of low proton yields relative to 2H and 3H yields and
the non-observation of 3He, all features which are shared by similar thermal
neutron induced and spontaneous fissioning systems.Comment: 6 pages, 3 figure
Electron-hole pairs during the adsorption dynamics of O2 on Pd(100) - Exciting or not?
During the exothermic adsorption of molecules at solid surfaces dissipation
of the released energy occurs via the excitation of electronic and phononic
degrees of freedom. For metallic substrates the role of the nonadiabatic
electronic excitation channel has been controversially discussed, as the
absence of a band gap could favour an easy coupling to a manifold of
electronhole pairs of arbitrarily low energies. We analyse this situation for
the highly exothermic showcase system of molecular oxygen dissociating at
Pd(100), using time-dependent perturbation theory applied to first-principles
electronic-structure calculations. For a range of different trajectories of
impinging O2 molecules we compute largely varying electron-hole pair spectra,
which underlines the necessity to consider the high-dimensionality of the
surface dynamical process when assessing the total energy loss into this
dissipation channel. Despite the high Pd density of states at the Fermi level,
the concomitant non-adiabatic energy losses nevertheless never exceed about 5%
of the available chemisorption energy. While this supports an electronically
adiabatic description of the predominant heat dissipation into the phononic
system, we critically discuss the non-adiabatic excitations in the context of
the O2 spin transition during the dissociation process.Comment: 20 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.html [added two references, changed
V_{fsa} to V_{6D}, modified a few formulations in interpretation of spin
asymmetry of eh-spectra, added missing equals sign in Eg.(2.10)
Bat Quickness and Bat Velocity for Left- and Right-Handed Softball Swings
Please view abstract in the attached PDF fil
Spatial correlators in strongly coupled plasmas
We numerically calculate the spatial correlators of the scalar and
pseudoscalar operators and , in SU(3) Yang-Mills theory at
zero and finite-temperature on the lattice. We compare the results over the
distances to the free-field prediction, to the
operator-product expansion as well as to the strongly coupled large-
\sN=4 super-Yang-Mills theory, where results are obtained by AdS/CFT methods.
For , both channels exhibit stronger spatial correlations than
in the vacuum, and we give an explanation for this, using sum-rules and the
operator-product expansion. The AdS/CFT calculation provides a
semi-quantitatively successful description of the vacuum-subtracted
correlator, renormalized in the 3-loop scheme, in the
interval of temperatures , while the free-field prediction has
the wrong sign. The and correlators are predicted to have the
same functional form both at weak coupling and in the strongly coupled SYM
theory. The Yang-Mills plasma does not meet that expectation below .
Instead we find that strong fluctuations of are present at least up
to that temperature. We discuss the impact of our results on our understanding
of the quark-gluon plasma.Comment: 32 pages, 9 figures, 4 tables; added some references, more detailed
captions, conclusions unchange
Radioactive Probes of the Supernova-Contaminated Solar Nebula: Evidence that the Sun was Born in a Cluster
We construct a simple model for radioisotopic enrichment of the protosolar
nebula by injection from a nearby supernova, based on the inverse square law
for ejecta dispersion. We find that the presolar radioisotopes abundances
(i.e., in solar masses) demand a nearby supernova: its distance can be no
larger than 66 times the size of the protosolar nebula, at a 90% confidence
level, assuming 1 solar mass of protosolar material. The relevant size of the
nebula depends on its state of evolution at the time of radioactivity
injection. In one scenario, a collection of low-mass stars, including our sun,
formed in a group or cluster with an intermediate- to high-mass star that ended
its life as a supernova while our sun was still a protostar, a starless core,
or perhaps a diffuse cloud. Using recent observations of protostars to estimate
the size of the protosolar nebula constrains the distance of the supernova at
0.02 to 1.6 pc. The supernova distance limit is consistent with the scales of
low-mass stars formation around one or more massive stars, but it is closer
than expected were the sun formed in an isolated, solitary state. Consequently,
if any presolar radioactivities originated via supernova injection, we must
conclude that our sun was a member of such a group or cluster that has since
dispersed, and thus that solar system formation should be understood in this
context. In addition, we show that the timescale from explosion to the creation
of small bodies was on the order of 1.8 Myr (formal 90% confidence range of 0
to 2.2 Myr), and thus the temporal choreography from supernova ejecta to
meteorites is important. Finally, we can not distinguish between progenitor
masses from 15 to 25 solar masses in the nucleosynthesis models; however, the
20 solar mass model is somewhat preferred.Comment: ApJ accepted, 19 pages, 3 figure
A Note on Bimodal Accretion Disks
The existence of bimodal disks is investigated. Following a simple argument
based on energetic considerations we show that stationary, bimodal accretion
disk models in which a Shakura--Sunyaev disk (SSD) at large radii matches an
advection dominated accretion flow (ADAF) at smaller radii are never possible
using the standard slim disk approach, unless some extra energy flux is
present. The same argument, however, predicts the possibility of a transition
from an outer Shapiro--Lightman--Eardley (SLE) disk to an ADAF, and from a SLE
disk to a SSD. Both types of solutions have been found.Comment: 9 pages including 9 figures, accepted for publication in The
Astrophysical Journa
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