486 research outputs found
Detection of Coulomb Charging around an Antidot
We have detected oscillations of the charge around a potential hill (antidot)
in a two-dimensional electron gas as a function of a perpendicular magnetic
field B. The field confines electrons around the antidot in closed orbits, the
areas of which are quantised through the Aharonov-Bohm effect. Increasing B
reduces each state's area, pushing electrons closer to the centre, until enough
charge builds up for an electron to tunnel out. This is a new form of the
Coulomb blockade seen in electrostatically confined dots. We have also studied
h/2e oscillations and found evidence for coupling of opposite spin states of
the lowest Landau level.Comment: 3 pages, 3 Postscript figures, submitted to the proceedings of
EP2DS-1
Sticky stuff : redefining bedform prediction in modern and ancient environments
This work was funded by the UK Natural Environment Research Council (NERC) under the COHBED project (NE/1027223/1). Paterson was funded by the Marine Alliance for Science and Technology for Scotland (MASTS).The dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments. However, the effect of physical cohesion imparted by cohesive clay within mixed sand-mud substrates has not been examined, despite its recognized influence on sediment stability. Here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow. The results show that bedform dimensions and steepness decrease linearly with clay content, and comparison with existing predictors of bedform dimensions, established within cohesionless sediments, reveals significant over-prediction of bedform size for all but the lowermost clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross-sets in turbidites. The results therefore offer a step change in our understanding of bedform formation and dynamics in these, and many other, sedimentary environments.Publisher PDFPeer reviewe
Laboratory study on the effects of hydro kinetic turbines on hydrodynamics and sediment dynamics
© 2018 The Authors The need for hydrokinetic turbine wake characterisation and their environmental impact has led to a number of studies. However, a small number of them have taken into account mobile sediment bed effects. The aim of the present work is to study the impact of the presence of a horizontal-axis three-bladed turbine with the flow and a mobile sediment bed. We use a series of laboratory experiments with a scaled modelled turbine installed in a flume with a mobile sandy bed at the bottom. Acoustic instruments were used to monitor flow, suspended sediment and bed behaviour. Results show a velocity decrease of about 50% throughout the water column and no flow recovery after a distance of 15 rotor diameters. Clearly visible ripples in the absence of the model turbine were replaced by horseshoe-shaped scour pit in the near wake region, and a depositional heap in the far wake. Suspended sediment differences were recorded in the streamwise direction with a possible effect of the wake as far as 15 rotor diameters. These results imply potentially important effects on the efficiency of turbine arrays, if the flow were to be lower than expected, on turbine foundations and modify coastal sediment transport
STM characterization of the Si-P heterodimer
We use scanning tunneling microscopy (STM) and Auger electron spectroscopy to
study the behavior of adsorbed phosphine (PH) on Si(001), as a function
of annealing temperature, paying particular attention to the formation of the
Si-P heterodimer. Dosing the Si(001) surface with 0.002 Langmuirs of
PH results in the adsorption of PH (x=2,3) onto the surface and
some etching of Si to form individual Si ad-dimers. Annealing to 350C
results in the incorporation of P into the surface layer to form Si-P
heterodimers and the formation of short 1-dimensional Si dimer chains and
monohydrides. In filled state STM images, isolated Si-P heterodimers appear as
zig-zag features on the surface due to the static dimer buckling induced by the
heterodimer. In the presence of a moderate coverage of monohydrides this static
buckling is lifted, rending the Si-P heterodimers invisible in filled state
images. However, we find that we can image the heterodimer at all H coverages
using empty state imaging. The ability to identify single P atoms incorporated
into Si(001) will be invaluable in the development of nanoscale electronic
devices based on controlled atomic-scale doping of Si.Comment: 6 pages, 4 figures (only 72dpi
Ideal hierarchical secret sharing schemes
Hierarchical secret sharing is among the most natural generalizations of threshold secret sharing, and it has attracted a lot of attention from the invention of secret sharing until nowadays. Several constructions of ideal hierarchical secret sharing schemes have been proposed, but it was not known what access structures admit such a scheme. We solve this problem by providing a natural definition for the family of the hierarchical access structures and, more importantly, by presenting a complete characterization of the ideal hierarchical access structures, that is, the ones admitting an ideal secret sharing scheme. Our characterization deals with the properties of the hierarchically minimal sets of the access structure, which are the minimal qualified sets whose participants are in the lowest possible levels in the hierarchy. By using our characterization, it can be efficiently checked whether any given hierarchical access structure that is defined by its hierarchically minimal sets is ideal. We use the well known connection between ideal secret sharing and matroids and, in particular, the fact that every ideal access structure is a matroid port. In addition, we use recent results on ideal multipartite access structures and the connection between multipartite matroids and integer polymatroids. We prove that every ideal hierarchical access structure is the port of a representable matroid and, more specifically, we prove that every ideal structure in this family admits ideal linear secret sharing schemes over fields of all characteristics. In addition, methods to construct such ideal schemes can be derived from the results in this paper and the aforementioned ones on ideal multipartite secret sharing. Finally, we use our results to find a new proof for the characterization of the ideal weighted threshold access structures that is simpler than the existing one.Peer ReviewedPostprint (author's final draft
Application of Bayesian statistics to estimate nitrous oxide emission factors of the nitrogen fertilisers in UK grasslands
Trapezoidal integration by linear interpolation of data points is by far the most commonly used method of
cumulative flux calculations of nitrous oxide (N2O) in studies that use flux chambers; however, this method is
incapable of providing accurate uncertainty estimates. A Bayesian approach was used to calculate N2O emission
factors (EFs) and their associated uncertainties from flux chamber measurements made after the application of
nitrogen fertilisers, in the form of ammonium nitrate (AN), urea (Ur) and urea treated with Agrotain® urease
inhibitor (UI) at four grassland sites in the UK. The comparison between the cumulative fluxes estimated using
the Bayesian and linear interpolation methods were broadly similar (R2=0.79); however, the Bayesian method
was capable of providing realistic uncertainties when a limited number of data points is available. The study
reports mean EF values (and 95% confidence intervals) of 0.60 ± 0.63, 0.29 ± 0.22 and 0.26 ± 0.17% of
applied N emitted as N2O for the AN, Ur and UI treatments, respectively. There was no significant difference
between N2O emissions from the Ur and UI treatments. In the case of the automatic chamber data collected at
one site in this study, the data did not fit the log-normal model, implying that more complex models may be
needed, particularly for measurement data with high temporal resolutioninfo:eu-repo/semantics/publishedVersio
Temperature dependence of the breakdown of the quantum Hall effect studied by induced currents
Copyright © 2004 The American Physical SocietyWe have developed a model of the high-current breakdown of the integer quantum Hall effect, as measured in contactless experiments using a highly-sensitive torsion balance magnetometer. The model predicts that, for empirically “low-mobility” samples (μ<75 m2 V−1 s−1), the critical current for breakdown should decrease with, and have a linear dependence on, temperature. This prediction is verified experimentally with the addition of a low-temperature saturation of the critical current at a temperature that depends on both sample number density and filling factor. It is shown that this saturation is consistent with quasielastic inter-Landau-level scattering when the maximum electric field in the sample reaches a large enough value. In addition we show how this model can be extended to give qualitative agreement with experiments on high-mobility samples
On the optimization of bipartite secret sharing schemes
Optimizing the ratio between the maximum length of the shares and the length of the secret value in secret sharing schemes for general access structures is an extremely difficult and long-standing open problem. In this paper, we study it for bipartite access structures, in which the set of participants
is divided in two parts, and all participants in each part play an equivalent role. We focus on the search of lower bounds by using a special class of polymatroids that is introduced here, the bipartite ones. We present a method based on linear programming to compute, for every given bipartite access structure, the best lower bound that can be obtained by this combinatorial method. In addition, we obtain some general lower bounds that improve the previously known ones, and we construct optimal secret sharing schemes for a family of bipartite access structures.Postprint (author’s final draft
Perfect Secrecy Systems Immune to Spoofing Attacks
We present novel perfect secrecy systems that provide immunity to spoofing
attacks under equiprobable source probability distributions. On the theoretical
side, relying on an existence result for -designs by Teirlinck, our
construction method constructively generates systems that can reach an
arbitrary high level of security. On the practical side, we obtain, via cyclic
difference families, very efficient constructions of new optimal systems that
are onefold secure against spoofing. Moreover, we construct, by means of
-designs for large values of , the first near-optimal systems that are 5-
and 6-fold secure as well as further systems with a feasible number of keys
that are 7-fold secure against spoofing. We apply our results furthermore to a
recently extended authentication model, where the opponent has access to a
verification oracle. We obtain this way novel perfect secrecy systems with
immunity to spoofing in the verification oracle model.Comment: 10 pages (double-column); to appear in "International Journal of
Information Security
Air fluorescence measurements in the spectral range 300-420 nm using a 28.5 GeV electron beam
Measurements are reported of the yield and spectrum of fluorescence, excited
by a 28.5 GeV electron beam, in air at a range of pressures of interest to
ultra-high energy cosmic ray detectors. The wavelength range was 300 - 420 nm.
System calibration has been performed using Rayleigh scattering of a nitrogen
laser beam. In atmospheric pressure dry air at 304 K the yield is 20.8 +/- 1.6
photons per MeV.Comment: 29 pages, 10 figures. Submitted to Astroparticle Physic
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