635 research outputs found
Magnetization of Charge-ordered la(2-x)sr(x)nio(4+delta)
We report magnetization measurements on La(2-x)Sr(x)NiO(4+ delta) single
crystals, with 0 < x < 0.5. Glassy behaviour associated with the formation of
spin-charge stripes, and a separate spin-glass phase at low temperatures were
observed. We have also found a `memory effect' in the magnetic field --
temperature history, which is found to be suppressed in the low temperature
spin state of the x = 0.33 crystal.Comment: 2 pages, 2 figures. Presented at ICM2003 to appear in J. Magn. Magn.
Mat
Magnetic Order and Dynamics in Stripe-Ordered La2-xSrxNiO4
We have studied magnetic correlations in several compositions of
stripe-ordered La2-xSrxNiO4. In this paper we show how polarized-neutron
scattering has helped uncover important features of the magnetic ordering and
spin dynamics. In particular, polarization analysis has enabled us (1) to
characterize a spin reorientation transition, (2) to identify anisotropy gaps
in the spin excitation spectrum, and (3) to investigate an anomalous dip in the
spin-wave intensity suggestive of coupling between collective spin and charge
excitations.Comment: 4 pages, 3 figs. Presented at PNSXM, Venice. To appear in Physica
Validity of the activPAL3 activity monitor in people moderately affected by Multiple Sclerosis
Background: Walking is the primary form of physical activity performed by people with Multiple Sclerosis (MS), therefore it is important to ensure the validity of tools employed to measure walking activity. The aim of this study was to assess the criterion validity of the activPAL3 activity monitor during overground walking in people with MS.\ud
Methods: Validity of the activPAL3 accelerometer was compared to video observation in 20 people moderately affected by MS. Participants walked 20-30m twice along a straight quiet corridor at a comfortable speed.\ud
Results: Inter-rater reliability of video observations was excellent (all intraclass correlations > 0.99). The mean difference (activPAL3- mean of raters) was -4.70 ¹ 9.09, -4.55 s ¹ 10.76 and 1.11 s ¹ 1.11 for steps taken, walking duration and upright duration respectively. These differences represented 8.7, 10.0 and 1.8% of the mean for each measure respectively. The activPAL3 tended to underestimate steps taken and walking duration in those who walked at cadences of ⤠38 steps/minute by 60% and 47% respectively.\ud
Discussion: The activPAL3 is valid for measuring walking activity in people moderately affected by MS. It is accurate for upright duration regardless of cadence. In participants with slow walking cadences, outcomes of steps taken and walking duration should be interpreted with caution
A Prismatic Analyser concept for Neutron Spectrometers
A development in modern neutron spectroscopy is to avoid the need of large
samples. We demonstrate how small samples together with the right choice of
analyser and detector components makes distance collimation an important
concept in crystal analyser spectrometers. We further show that this opens new
possibilities where neutrons with different energies are reflected by the same
analyser but counted in different detectors, thus improving both energy
resolution and total count rate compared to conventional spectrometers. The
technique can be combined with advanced focusing geometries and with
multiplexing instrument designs. We present a combination of simulations and
data with 3 energies from one analyser. The data was taken on a prototype
installed at PSI, Switzerland, and shows excellent agreement with the
predictions. Typical improvements will be 2 times finer resolution and a factor
1.9 in flux gain compared to a Rowland geometry or 3 times finer resolution and
a factor 3.2 in flux gain compared to a single flat analyser slab
Efficient Recognition of Partially Visible Objects Using a Logarithmic Complexity Matching Technique
An important task in computer vision is the recognition of partially visible two-dimensional objects in a gray scale image. Recent works addressing this problem have attempted to match spatially local features from the image to features generated by models of the objects. However, many algo rithms are considerably less efficient than they might be, typ ically being O(IN) or worse, where I is the number offeatures in the image and N is the number of features in the model set. This is invariably due to the feature-matching portion of the algorithm. In this paper we discuss an algorithm that significantly improves the efficiency offeature matching. In addition, we show experimentally that our recognition algo rithm is accurate and robust. Our algorithm uses the local shape of contour segments near critical points, represented in slope angle-arclength space (θ-s space), as fundamental fea ture vectors. These feature vectors are further processed by projecting them onto a subspace in θ-s space that is obtained by applying the Karhunen-Loève expansion to all such fea tures in the set of models, yielding the final feature vectors. This allows the data needed to store the features to be re duced, while retaining nearly all information important for recognition. The heart of the algorithm is a technique for performing matching between the observed image features and the precomputed model features, which reduces the runtime complexity from O(IN) to O(I log I + I log N), where I and N are as above. The matching is performed using a tree data structure, called a kD tree, which enables multidi mensional searches to be performed in O(log) time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66975/2/10.1177_027836498900800608.pd
Encoded Recoupling and Decoupling: An Alternative to Quantum Error Correcting Codes, Applied to Trapped Ion Quantum Computation
A recently developed theory for eliminating decoherence and design
constraints in quantum computers, ``encoded recoupling and decoupling'', is
shown to be fully compatible with a promising proposal for an architecture
enabling scalable ion-trap quantum computation [D. Kielpinski et al., Nature
417, 709 (2002)]. Logical qubits are encoded into pairs of ions. Logic gates
are implemented using the Sorensen-Molmer (SM) scheme applied to pairs of ions
at a time. The encoding offers continuous protection against collective
dephasing. Decoupling pulses, that are also implemented using the SM scheme
directly to the encoded qubits, are capable of further reducing various other
sources of qubit decoherence, such as due to differential dephasing and due to
decohered vibrational modes. The feasibility of using the relatively slow SM
pulses in a decoupling scheme quenching the latter source of decoherence
follows from the observed 1/f spectrum of the vibrational bath.Comment: 12 pages, no figure
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
Shape transitions far from stability: The nucleus 58Cr
Abstract Excited states up to I Ď = 8 + in the neutron-rich nucleus 58Cr have been identified by using a new experimental setup composed of the large acceptance magnetic spectrometer PRISMA and the highly efficient Îł-detector array CLARA. Interestingly, the excitation energy sequence of the ground-state band follows the one expected by the E ( 5 ) dynamical symmetry for a nucleus at the critical point of the shape phase transition from a spherical vibrator ( U ( 5 ) ) to a Îł-soft rotor ( O ( 6 ) ). For the first time, in the same physical system, large scale shell-model calculations in the full fp shell are compared to the E ( 5 ) analytical model results and to the Interacting Boson Model. The theoretical results are in excellent agreement with the present data
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
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