3,148 research outputs found
On Automorphisms and Universal R-Matrices at Roots of Unity
Invertible universal R-matrices of quantum Lie algebras do not exist at roots
of unity. There exist however quotients for which intertwiners of tensor
products of representations always exist, i.e. R-matrices exist in the
representations. One of these quotients, which is finite dimensional, has a
universal R-matrix. In this paper, we answer the following question: on which
condition are the different quotients of U_q(sl(2)) (Hopf)-equivalent? In the
case when they are equivalent, the universal R-matrix of one can be transformed
into a universal R-matrix of the other. We prove that this happens only when
q^4=1, and we explicitly give the expressions for the automorphisms and for the
transformed universal R-matrices in this case.Comment: 11 pages, minor TeXnical revision to allow automatic TeXin
Full counting statistics of nano-electromechanical systems
We develop a theory for the full counting statistics (FCS) for a class of
nanoelectromechanical systems (NEMS), describable by a Markovian generalized
master equation. The theory is applied to two specific examples of current
interest: vibrating C60 molecules and quantum shuttles. We report a numerical
evaluation of the first three cumulants for the C60-setup; for the quantum
shuttle we use the third cumulant to substantiate that the giant enhancement in
noise observed at the shuttling transition is due to a slow switching between
two competing conduction channels. Especially the last example illustrates the
power of the FCS.Comment: 7 pages, 3 figures; minor changes - final version as published in
Europhys. Let
Re-parameterization Invariance in Fractional Flux Periodicity
We analyze a common feature of a nontrivial fractional flux periodicity in
two-dimensional systems. We demonstrate that an addition of fractional flux can
be absorbed into re-parameterization of quantum numbers. For an exact
fractional periodicity, all the electronic states undergo the
re-parameterization, whereas for an approximate periodicity valid in a large
system, only the states near the Fermi level are involved in the
re-parameterization.Comment: 4 pages, 1 figure, minor changes, final version to appear in J. Phys.
Soc. Jp
Single-dopant resonance in a single-electron transistor
Single dopants in semiconductor nanostructures have been studied in great
details recently as they are good candidates for quantum bits, provided they
are coupled to a detector. Here we report coupling of a single As donor atom to
a single-electron transistor (SET) in a silicon nanowire field-effect
transistor. Both capacitive and tunnel coupling are achieved, the latter
resulting in a dramatic increase of the conductance through the SET, by up to
one order of magnitude. The experimental results are well explained by the rate
equations theory developed in parallel with the experiment.Comment: 16 pages, 8 figure
Representations of the quantum matrix algebra
It is shown that the finite dimensional irreducible representaions of the
quantum matrix algebra ( the coordinate ring of ) exist only when both q and p are roots of unity. In this case th e space of
states has either the topology of a torus or a cylinder which may be thought of
as generalizations of cyclic representations.Comment: 20 page
Seafloor change detection using multibeam echosounder backscatter: case study on the Belgian part of the North Sea
To characterize seafloor substrate type, seabed mapping and particularly multibeam echosounding are increasingly used. Yet, the utilisation of repetitive MBES-borne backscatter surveys to monitor the environmental status of the seafloor remains limited. Often methodological frameworks are missing, and should comprise of a suite of change detection procedures, similarly to those developed in the terrestrial sciences. In this study, pre-, ensemble and post-classification approaches were tested on an eight km2 study site within a Habitat Directive Area in the Belgian part of the North Sea. In this area, gravel beds with epifaunal assemblages were observed. Flourishing of the fauna is constrained by overtopping with sand or increased turbidity levels, which could result from anthropogenic activities. Monitoring of the gravel to sand ratio was hence put forward as an indicator of good environmental status. Seven acoustic surveys were undertaken from 2004 to 2015. The methods allowed quantifying temporal trends and patterns of change of the main substrate classes identified in the study area; namely fine to medium homogenous sand, medium sand with bioclastic detritus and medium to coarse sand with gravel. Results indicated that by considering the entire study area and the entire time series, the gravel to sand ratio fluctuated, but was overall stable. Nonetheless, when only the biodiversity hotspots were considered, net losses and a gradual trend, indicative of potential smothering, was captured by ensemble and post-classification approaches respectively. Additionally, a two-dimensional morphological analysis, based on the bathymetric data, suggested a loss of profile complexity from 2004 to 2015. Causal relationships with natural and anthropogenic stressors are yet to be established. The methodologies presented and discussed are repeatable and can be applied to broad-scale geographical extents given that broad-scale time series datasets become available
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Photometric observations of the radio bright B[e]/X-ray binary CI Cam
We present multiwavelength (optical, IR, radio) observations of CI Cam, the optical counterpart to the transient X-ray source XTE J0421+560. Pre-outburst quiescent observations reveal the presence of a dusty envelope around the system. Pronounced short term variability is observed at all wavebands from U-K, but no indication of prior flaring of a similar magnitude to the 1998 April outburst is found in these data.
Data obtained during the 1998 April X-ray flare reveal pronounced optical-radio flaring. The optical flux was observed to quickly return to quiescent levels, while the radio flare was of much longer duration. The optical component is likely to result from a combination of free-free/free-bound emission, emission line and thermal dust emission, caused by re-radiation of the X-ray flux, while the behaviour of the multiwavelength radio data is consistent with emission from expanding ejecta emitting via the synchrotron mechanism.
Post-outburst (1998 August-1999 March) U-M broadband photometric observations reveal that while the optical (UBV) flux remains at pre-outburst quiescent levels, near IR (JHKLM) fluxes exceed the pre-outburst fluxes by ~0.5 magnitudes. Modelling the pre- and post-outburst spectral energy distribution of CI Cam reveals that the structure and/or composition of the dusty component of the circumstellar envelope appears to have changed. Due to a lack of information on the precise chemical composition of the dust within the system several explanations for this behaviour are possible, such as the production of new dust at the inner edge of the envelope, or modification of the composition of the dust due to X-ray irradiation
Repetitive transcranial magnetic stimulation and transcranial direct current stimulation in motor rehabilitation after stroke: An update
AbstractStroke is a leading cause of adult motor disability. The number of stroke survivors is increasing in industrialized countries, and despite available treatments used in rehabilitation, the recovery of motor functions after stroke is often incomplete. Studies in the 1980s showed that non-invasive brain stimulation (mainly repetitive transcranial magnetic stimulation [rTMS] and transcranial direct current stimulation [tDCS]) could modulate cortical excitability and induce plasticity in healthy humans. These findings have opened the way to the therapeutic use of the 2 techniques for stroke. The mechanisms underlying the cortical effect of rTMS and tDCS differ. This paper summarizes data obtained in healthy subjects and gives a general review of the use of rTMS and tDCS in stroke patients with altered motor functions. From 1988 to 2012, approximately 1400 publications were devoted to the study of non-invasive brain stimulation in humans. However, for stroke patients with limb motor deficit, only 141 publications have been devoted to the effects of rTMS and 132 to those of tDCS. The Cochrane review devoted to the effects of rTMS found 19 randomized controlled trials involving 588 patients, and that devoted to tDCS found 18 randomized controlled trials involving 450 patients. Without doubt, rTMS and tDCS contribute to physiological and pathophysiological studies in motor control. However, despite the increasing number of studies devoted to the possible therapeutic use of non-invasive brain stimulation to improve motor recovery after stroke, further studies will be necessary to specify their use in rehabilitation
Dust Properties and Star-Formation Rates in Star-Forming Dwarf Galaxies
We have used the Spitzer Space Telescope to study the dust properties of a
sample of star-forming dwarf galaxies. The differences in the mid-infrared
spectral energy distributions for these galaxies which, in general, are low
metallicity systems, indicate differences in the physical properties, heating,
and/or distribution of the dust. Specifically, these galaxies have more hot
dust and/or very small grains and less PAH emission than either spiral or
higher luminosity starburst galaxies. As has been shown in previous studies,
there is a gradual decrease in PAH emission as a function of metallicity.
Because much of the energy from star formation in galaxies is re-radiated in
the mid-infrared, star-formation rate indicators based on both line and
continuum measurements in this wavelength range are coming into more common
usage. We show that the variations in the interstellar medium properties of
galaxies in our sample, as measured in the mid-infrared, result in over an
order of magnitude spread in the computed star-formation rates.Comment: 25 pages, 11 figures, 4 tables, ApJ accepte
Persistent currents in carbon nanotubes based rings
Persistent currents in rings constructed from carbon nanotubes are
investigated theoretically. After studying the contribution of finite
temperature or quenched disorder on covalent rings, the complexity due to the
bundle packing is addressed. The case of interacting nanotori and
self-interacting coiled nanotubes are analyzed in details in relation with
experiments.Comment: 7 sections, 9 figure
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