3,538 research outputs found
Characterisation of spin-incoherent transport in one dimension
Spin-incoherent transport in quantum wires, whereby exchange coupling between neighbouring electrons is overcome by thermal energy, leading to the suppression of spin modes of transport expected in a Luttinger liquid, has been observed in the form of a conductance plateau at e(2)/h in the absence of a magnetic field. We present here further characterisation of this spin-incoherent plateau in a source-drain bias, which causes it to evolves to 0.85 x 2e(2)/h. Laterally shifting the channel and illuminating the sample allows us to verify its origin
Multiplicative random walk Metropolis-Hastings on the real line
In this article we propose multiplication based random walk Metropolis
Hastings (MH) algorithm on the real line. We call it the random dive MH (RDMH)
algorithm. This algorithm, even if simple to apply, was not studied earlier in
Markov chain Monte Carlo literature. The associated kernel is shown to have
standard properties like irreducibility, aperiodicity and Harris recurrence
under some mild assumptions. These ensure basic convergence (ergodicity) of the
kernel. Further the kernel is shown to be geometric ergodic for a large class
of target densities on . This class even contains realistic target
densities for which random walk or Langevin MH are not geometrically ergodic.
Three simulation studies are given to demonstrate the mixing property and
superiority of RDMH to standard MH algorithms on real line. A share-price
return data is also analyzed and the results are compared with those available
in the literature
Headwater Influences on Downstream Water Quality
We investigated the influence of riparian and whole watershed land use as a function of stream size on surface water chemistry and assessed regional variation in these relationships. Sixty-eight watersheds in four level III U.S. EPA ecoregions in eastern Kansas were selected as study sites. Riparian land cover and watershed land use were quantified for the entire watershed, and by Strahler order. Multiple regression analyses using riparian land cover classifications as independent variables explained among-site variation in water chemistry parameters, particularly total nitrogen (41%), nitrate (61%), and total phosphorus (63%) concentrations. Whole watershed land use explained slightly less variance, but riparian and whole watershed land use were so tightly correlated that it was difficult to separate their effects. Water chemistry parameters sampled in downstream reaches were most closely correlated with riparian land cover adjacent to the smallest (first-order) streams of watersheds or land use in the entire watershed, with riparian zones immediately upstream of sampling sites offering less explanatory power as stream size increased. Interestingly, headwater effects were evident even at times when these small streams were unlikely to be flowing. Relationships were similar among ecoregions, indicating that land use characteristics were most responsible for water quality variation among watersheds. These findings suggest that nonpoint pollution control strategies should consider the influence of small upland streams and protection of downstream riparian zones alone is not sufficient to protect water quality
Nature of the many-body excitations in a quantum wire: Theory and experiment
The natural excitations of an interacting one-dimensional system at low
energy are hydrodynamic modes of Luttinger liquid, protected by the Lorentz
invariance of the linear dispersion. We show that beyond low energies, where
quadratic dispersion reduces the symmetry to Galilean, the main character of
the many-body excitations changes into a hierarchy: calculations of dynamic
correlation functions for fermions (without spin) show that the spectral
weights of the excitations are proportional to powers of
, where is a length-scale related to
interactions and is the system length. Thus only small numbers of
excitations carry the principal spectral power in representative regions on the
energy-momentum planes. We have analysed the spectral function in detail and
have shown that the first-level (strongest) excitations form a mode with
parabolic dispersion, like that of a renormalised single particle. The
second-level excitations produce a singular power-law line shape to the
first-level mode and multiple power-laws at the spectral edge. We have
illustrated crossover to Luttinger liquid at low energy by calculating the
local density of state through all energy scales: from linear to non-linear,
and to above the chemical potential energies. In order to test this model, we
have carried out experiments to measure momentum-resolved tunnelling of
electrons (fermions with spin) from/to a wire formed within a GaAs
heterostructure. We observe well-resolved spin-charge separation at low energy
with appreciable interaction strength and only a parabolic dispersion of the
first-level mode at higher energies. We find structure resembling the
second-level excitations, which dies away rapidly at high momentum in line with
the theoretical predictions here.We acknowledge financial support from the UK EPSRC through Grants No. EP/J01690X/1 and No. EP/J016888/1 and from the DFG through SFB/TRR 49. This research was supported in part by the National Science Foundation under Grant No. NSF PHY11-25915.This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevB.93.07514
Hierarchy of modes in an interacting one-dimensional system.
Studying interacting fermions in one dimension at high energy, we find a hierarchy in the spectral weights of the excitations theoretically, and we observe evidence for second-level excitations experimentally. Diagonalizing a model of fermions (without spin), we show that levels of the hierarchy are separated by powers of R^{2}/L^{2}, where R is a length scale related to interactions and L is the system length. The first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalized single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power laws at the spectral edge. We measure momentum-resolved tunneling of electrons (fermions with spin) from or to a wire formed within a GaAs heterostructure, which shows parabolic dispersion of the first-level mode and well-resolved spin-charge separation at low energy with appreciable interaction strength. We find structure resembling the second-level excitations, which dies away quite rapidly at high momentum.We acknowledge financial support from the UK
EPSRC through Grant No. EP/J01690X/1 and
EP/J016888/1.This is the accepted manuscript. The final version is available at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.196401
Characterisation and tracking of membrane surfaces at NASA Langley Research Centre
Aerospace engineers require measurements of the shape of aerodynamic surfaces and the six degree of freedom (6DoF) position and
orientation of aerospace models to analyse structural dynamics and aerodynamic forces. The measurement technique must be noncontact,
accurate, reliable, have a high sample rate and preferably be non-intrusive. Close range photogrammetry based on multiple,
synchronised, commercial-off-the-shelf digital cameras can supply surface shape and 6DoF data at 5-15Hz with customisable
accuracies. This paper describes data acquisition systems designed and implemented at NASA Langley Research Center to capture
surface shapes and 6DoF data. System calibration and data processing techniques are discussed. Examples of experiments and data
outputs are described
Three-dimensional jamming and flows of soft glassy materials
Various disordered dense systems such as foams, gels, emulsions and colloidal
suspensions, exhibit a jamming transition from a liquid state (they flow) to a
solid state below a yield stress. Their structure, thoroughly studied with
powerful means of 3D characterization, exhibits some analogy with that of
glasses which led to call them soft glassy materials. However, despite its
importance for geophysical and industrial applications, their rheological
behavior, and its microscopic origin, is still poorly known, in particular
because of its nonlinear nature. Here we show from two original experiments
that a simple 3D continuum description of the behaviour of soft glassy
materials can be built. We first show that when a flow is imposed in some
direction there is no yield resistance to a secondary flow: these systems are
always unjammed simultaneously in all directions of space. The 3D jamming
criterion appears to be the plasticity criterion encountered in most solids. We
also find that they behave as simple liquids in the direction orthogonal to
that of the main flow; their viscosity is inversely proportional to the main
flow shear rate, as a signature of shear-induced structural relaxation, in
close similarity with the structural relaxations driven by temperature and
density in other glassy systems.Comment: http://www.nature.com/nmat/journal/v9/n2/abs/nmat2615.htm
Male mating biology
Before sterile mass-reared mosquitoes are released in an attempt to control local populations, many facets of male mating biology need to be elucidated. Large knowledge gaps exist in how both sexes meet in space and time, the correlation of male size and mating success and in which arenas matings are successful. Previous failures in mosquito sterile insect technique (SIT) projects have been linked to poor knowledge of local mating behaviours or the selection of deleterious phenotypes during colonisation and long-term mass rearing. Careful selection of mating characteristics must be combined with intensive field trials to ensure phenotypic characters are not antagonistic to longevity, dispersal, or mating behaviours in released males. Success has been achieved, even when colonised vectors were less competitive, due in part to extensive field trials to ensure mating compatibility and effective dispersal. The study of male mating biology in other dipterans has improved the success of operational SIT programmes. Contributing factors include inter-sexual selection, pheromone based attraction, the ability to detect alterations in local mating behaviours, and the effects of long-term colonisation on mating competitiveness. Although great strides have been made in other SIT programmes, this knowledge may not be germane to anophelines, and this has led to a recent increase in research in this area
Serotonin tranporter methylation and response to cognitive behaviour therapy in children with anxiety disorders
Anxiety disorders that are the most commonly occurring psychiatric disorders in childhood, are associated with a range of social and educational impairments and often continue into adulthood. Cognitive behaviour therapy (CBT) is an effective treatment option for the majority of cases, although up to 35-45% of children do not achieve remission. Recent research suggests that some genetic variants may be associated with a more beneficial response to psychological therapy. Epigenetic mechanisms such as DNA methylation work at the interface between genetic and environmental influences. Furthermore, epigenetic alterations at the serotonin transporter (SERT) promoter region have been associated with environmental influences such as stressful life experiences. In this study, we measured DNA methylation upstream of SERT in 116 children with an anxiety disorder, before and after receiving CBT. Change during treatment in percentage DNA methylation was significantly different in treatment responders vs nonresponders. This effect was driven by one CpG site in particular, at which responders increased in methylation, whereas nonresponders showed a decrease in DNA methylation. This is the first study to demonstrate differences in SERT methylation change in association with response to a purely psychological therapy. These findings confirm that biological changes occur alongside changes in symptomatology following a psychological therapy such as CBT
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