150 research outputs found
Spin-Orbit Scattering and Time-Reversal Symmetry: Detection of a Spin by Tunneling
We consider the possibility of detecting spin precession in a magnetic field
by nonequilibrium transport processes. We find that time reversal symmetry
imposes strong constraints on the problem. Suppose the tunneling occurs
directly between systems at two different chemical potentials, rather than
sequentially via a third system at an intermediate chemical potential. Then,
unless the magnetic fields are extremely strong or spin polarized electrons are
used, the periodic signal in the current results from beating together two
different precession frequencies, so that observing a signal near the Larmor
frequency in this case requires having some cluster with a factor close to
zero.Comment: 4 pages, 1 figur
The motion of superconducting vortices in thin films of varying thickness
The interaction of superconducting vortices with superconductor/vacuum interfaces is considered. A vortex is first shown to intersect such an interface normally. Various thin-film models are then formulated, corresponding to different parameter regimes. A local analysis of a vortex is performed, and a law of motion for each vortex deduced. This law of motion implies that the vortex will move to the locally thinnest part of the film, and is consistent with the vortex moving under the curvature induced by being forced to intersect the boundaries of the film normall
Superconducting states in the tetrahedral compound PrOs4Sb12
We find possible superconducting states for tetrahedral (Th) symmetry
crystals with strong spin-orbit coupling using Landau theory. Additional
symmetry breaking within the superconducting state is considered. We discuss
nodes of the gap functions for the different states, secondary superconducting
order parameters and coupling to the elastic strain. By comparing our results
to experiments, we find that superconductivity in PrOs4Sb12 is best described
by the three-dimensional representations of point group Th.Comment: 9 pages, 2 figures. Expanded version submitted to Physical Review
Influence of a humidor on the aerodynamics of baseballs
We investigate whether storing baseballs in a controlled humidity environment
significantly affects their aerodynamic properties. To do this, we measure the
change in diameter and mass of baseballs as a function of relative humidity
(RH) in which the balls are stored. We then model trajectories for pitched and
batted baseballs to assess the difference between those stored at 30% RH versus
50% RH. The results show that a drier baseball may be expected to curve
slightly more than a humidified one for a given pitch velocity. We also find
that the aerodynamics alone would add ~2 feet to the distance a moister ball is
hit. However, this is compensated by a ~6 foot reduction in batted distance due
to the well known change in coefficient of restitution of the ball. We discuss
consequences of these results for baseball played at Coors Field in Denver,
where baseballs have been stored in a humidor at 50% RH since 2002.Comment: 21 pages, 7 figures, modified and re-posted 2/2
Local shell-to-shell energy transfer via nonlocal Interactions in fluid turbulence
In this paper we analytically compute the strength of nonlinear interactions
in a triad, and the energy exchanges between wavenumber shells in
incompressible fluid turbulence. The computation has been done using
first-order perturbative field theory. In three dimension, magnitude of triad
interactions is large for nonlocal triads, and small for local triads. However,
the shell-to-shell energy transfer rate is found to be local and forward. This
result is due to the fact that the nonlocal triads occupy much less Fourier
space volume than the local ones. The analytical results on three-dimensional
shell-to-shell energy transfer match with their numerical counterparts. In
two-dimensional turbulence, the energy transfer rates to the near-by shells are
forward, but to the distant shells are backward; the cumulative effect is an
inverse cascade of energy.Comment: 10 pages, Revtex
Bulk and Boundary Critical Behavior at Lifshitz Points
Lifshitz points are multicritical points at which a disordered phase, a
homogeneous ordered phase, and a modulated ordered phase meet. Their bulk
universality classes are described by natural generalizations of the standard
model. Analyzing these models systematically via modern
field-theoretic renormalization group methods has been a long-standing
challenge ever since their introduction in the middle of the 1970s. We survey
the recent progress made in this direction, discussing results obtained via
dimensionality expansions, how they compare with Monte Carlo results, and open
problems. These advances opened the way towards systematic studies of boundary
critical behavior at -axial Lifshitz points. The possible boundary critical
behavior depends on whether the surface plane is perpendicular to one of the
modulation axes or parallel to all of them. We show that the semi-infinite
field theories representing the corresponding surface universality classes in
these two cases of perpendicular and parallel surface orientation differ
crucially in their Hamiltonian's boundary terms and the implied boundary
conditions, and explain recent results along with our current understanding of
this matter.Comment: Invited contribution to STATPHYS 22, to be published in the
Proceedings of the 22nd International Conference on Statistical Physics
(STATPHYS 22) of the International Union of Pure and Applied Physics (IUPAP),
4--9 July 2004, Bangalore, Indi
Hierarchical Cluster Analysis: Comparison of Three Linkage Measures and Application to Psychological Data
How a Diverse Research Ecosystem Has Generated New Rehabilitation Technologies: Review of NIDILRR’s Rehabilitation Engineering Research Centers
Over 50 million United States citizens (1 in 6 people in the US) have a developmental, acquired, or degenerative disability. The average US citizen can expect to live 20% of his or her life with a disability. Rehabilitation technologies play a major role in improving the quality of life for people with a disability, yet widespread and highly challenging needs remain. Within the US, a major effort aimed at the creation and evaluation of rehabilitation technology has been the Rehabilitation Engineering Research Centers (RERCs) sponsored by the National Institute on Disability, Independent Living, and Rehabilitation Research. As envisioned at their conception by a panel of the National Academy of Science in 1970, these centers were intended to take a “total approach to rehabilitation”, combining medicine, engineering, and related science, to improve the quality of life of individuals with a disability. Here, we review the scope, achievements, and ongoing projects of an unbiased sample of 19 currently active or recently terminated RERCs. Specifically, for each center, we briefly explain the needs it targets, summarize key historical advances, identify emerging innovations, and consider future directions. Our assessment from this review is that the RERC program indeed involves a multidisciplinary approach, with 36 professional fields involved, although 70% of research and development staff are in engineering fields, 23% in clinical fields, and only 7% in basic science fields; significantly, 11% of the professional staff have a disability related to their research. We observe that the RERC program has substantially diversified the scope of its work since the 1970’s, addressing more types of disabilities using more technologies, and, in particular, often now focusing on information technologies. RERC work also now often views users as integrated into an interdependent society through technologies that both people with and without disabilities co-use (such as the internet, wireless communication, and architecture). In addition, RERC research has evolved to view users as able at improving outcomes through learning, exercise, and plasticity (rather than being static), which can be optimally timed. We provide examples of rehabilitation technology innovation produced by the RERCs that illustrate this increasingly diversifying scope and evolving perspective. We conclude by discussing growth opportunities and possible future directions of the RERC program
- …