6,294 research outputs found
Comment on "Measurement of time of arrival in quantum mechanics"
The analysis of the model quantum clocks proposed by Aharonov et al. [Phys.
Rev. A 57 (1998) 4130 - quant-ph/9709031] requires considering evanescent
components, previously ignored. We also clarify the meaning of the operational
time of arrival distribution which had been investigated.Comment: 3 inlined figures; comment on quant-ph/970903
Moller operators and Lippmann-Schwinger equations for step-like potentials
The Moller operators and the asociated Lippman-Schwinger equations obtained
from different partitionings of the Hamiltonian for a step-like potential
barrier are worked out, compared and related.Comment: 15 pages, 1 inlined figure, iopart.cl
Epistemic Complexity and the Journeyman-Expert Transition
Physics students can encounter difficulties in physics problem solving as a
result of failing to use knowledge that they have but do not perceive as
relevant or appropriate. In previous work the authors have demonstrated that
some of these difficulties may be epistemological. Students may limit the kinds
of knowledge that they use. For example, they may use formal manipulations and
ignore physical sense making or vice versa. Both beginning (novice) and
intermediate (journeymen) students demonstrate these difficulties. Learning
both to switch one's epistemological lens on a problem and to integrate
different kinds of knowledge is a critical component of learning to solve
problems in physics effectively. In this paper, we present two case studies in
which journeyman students (upper-division physics majors) demonstrate switching
between epistemological resources in approaching a complex problem. We
conjecture that mastering these epistemological skills is an essential
component of learning complex problem solving in physics.Comment: 12 page
The Gravitational Lens CFRS03.1077
An exquisite gravitational arc with a radius of 2.1" has been discovered
around the z = 0.938 field elliptical galaxy CFRS03.1077 during HST
observations of Canada-France Redshift Survey (CFRS) fields. Spectroscopic
observations of the arc show that the redshift of the resolved lensed galaxy is
z = 2.941. This gravitational lens-source system is well-fitted using the
position angle and ellipticity derived from the visible matter distribution and
an isothermal mass profile with a mass corresponding to sigma =387+-5 km/s.
Surprisingly, given the evidence for passive evolution of elliptical galaxies,
this is in good agreement with an estimate based on the fundamental plane for z
= 0 ellipticals. This, perhaps, indicates that this galaxy has not shared in
the significant evolution observed for average elliptical galaxies at z ~ 1. A
second elliptical galaxy with similar luminosity from the CFRS survey, CFRS
14.1311 at z=0.807, is also a lens but in this case the lens model gives a much
smaller mass-to-light ratio, i.e., it appears to confirm the expected
evolution. This suggests that this pair of field elliptical galaxies may have
very different evolutionary histories, a significant result if confirmed.
Clearly, CFRS03.1077 demonstrates that these "Einstein rings" are powerful
probes of high redshift galaxies.Comment: 11 pages, 5 figures, accepted by Ap.
Hydrodynamic lift on bound vesicles
Bound vesicles subject to lateral forces such as arising from shear flow are
investigated theoretically by combining a lubrication analysis of the bound
part with a scaling approach to the global motion. A minor inclination of the
bound part leads to significant lift due to the additive effects of lateral and
tank-treading motions. With increasing shear rate, the vesicle unbinds from the
substrate at a critical value. Estimates are in agreement with recent
experimental data.Comment: 9 pages, one figur
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
IMAGES I. Strong evolution of galaxy kinematics since z=1
(abbreviated) We present the first results of the ESO large program,
``IMAGES'' which aims at obtaining robust measurements of the kinematics of
distant galaxies using the multi-IFU mode of GIRAFFE on the VLT. 3D
spectroscopy is essential to robustly measure the often distorted kinematics of
distant galaxies (e.g., Flores et al. 2006). We derive the velocity fields and
-maps of 36 galaxies at 0.4<z<0.75 from the kinematics of the [OII]
emission line doublet, and generate a robust technique to identify the nature
of the velocity fields based on the pixels of the highest signal-to-noise
ratios (S/N). We have gathered a unique sample of 63 velocity fields of
emission line galaxies (W0([OII]) > 15 A) at z=0.4-0.75, which are a
representative subsample of the population of M_stellar>1.5x10^{10} M_sun
emission line galaxies in this redshift range, and are largely unaffected by
cosmic variance. Taking into account all galaxies -with or without emission
lines- in that redshift range, we find that at least 41+/-7% of them have
anomalous kinematics, i.e., they are not dynamically relaxed. This includes
26+/-7% of distant galaxies with complex kinematics, i.e., they are not simply
pressure or rotationally supported. Our result implies that galaxy kinematics
are among the most rapidly evolving properties, because locally, only a few
percent of the galaxies in this mass range have complex kinematics.Comment: 17 pages, 6 figures, Accepted by A&
Potential, core-level and d band shifts at transition metal surfaces
We have extended the validity of the correlation between the surface
3d-core-level shift (SCLS) and the surface d band shift (SDBS) to the entire 4d
transition metal series and to the neighboring elements Sr and Ag via accurate
first-principles calculations. We find that the correlation is quasilinear and
robust with respect to the differencies both between initial and final-state
calculations of the SCLS's and two distinct measures of the SDBS's. We show
that despite the complex spatial dependence of the surface potential shift
(SPS) and the location of the 3d and 4d orbitals in different regions of space,
the correlation exists because the sampling of the SPS by the 3d and 4d
orbitals remains similar. We show further that the sign change of the SCLS's
across the transition series does indeed arise from the d band-narrowing
mechanism previously proposed. However, while in the heavier transition metals
the predicted increase of d electrons in the surface layer relative to the bulk
arises primarily from transfers from s and p states to d states within the
surface layer, in the lighter transition metals the predicted decrease of
surface d electrons arises primarily from flow out into the vacuum.Comment: RevTex, 22 pages, 5 figures in uufiles form, to appear in Phys.Rev.
Instability of antiferromagnetic magnons in strong fields
We predict that spin-waves in an ordered quantum antiferromagnet (AFM) in a
strong magnetic field become unstable with respect to spontaneous two-magnon
decays. At zero temperature, the instability occurs between the threshold field
and the saturation field . As an example, we investigate the
high-field dynamics of a Heisenberg antiferromagnet on a square lattice and
show that the single-magnon branch of the spectrum disappears in the most part
of the Brillouin zone.Comment: RevTeX, 4 pages, 3 figures, accepted to PR
- …