1,864 research outputs found
Design and development of a low temperature, inductance based high frequency ac susceptometer
We report on the development of an induction based low temperature high
frequency ac susceptometer capable of measuring at frequencies up to 3.5 MHz
and at temperatures between 2 K and 300 K. Careful balancing of the detection
coils and calibration have allowed a sample magnetic moment resolution of
at 1 MHz. We will discuss the design and
characterization of the susceptometer, and explain the calibration process. We
also include some example measurements on the spin ice material CdErS
and iron oxide based nanoparticles to illustrate functionality
The structure of quotients of the Onsager algebra by closed ideals
We study the Onsager algebra from the ideal theoretic point of view. A
complete classification of closed ideals and the structure of quotient algebras
are obtained. We also discuss the solvable algebra aspect of the Onsager
algebra through the use of formal Lie algebras.Comment: 33 pages, Latex, small topos corrected-Journal versio
Distribution and abundance of fish and crayfish in a Waikato stream in relation to basin area
The aim of this study was to relate the longitudinal distribution of fish and crayfish to increasing basin area and physical site characteristics in the Mangaotama Stream, Waikato region, North Island, New Zealand. Fish and crayfish were captured with two-pass removal electroshocking at 11 sites located in hill-country with pasture, native forest, and mixed land uses within the 21.6 km2 basin. Number of fish species and lineal biomass of fish increased with increasing basin area, but barriers to upstream fish migration also influenced fish distribution; only climbing and non-migratory species were present above a series of small waterfalls. Fish biomass increased in direct proportion to stream width, suggesting that fish used much of the available channel, and stream width was closely related to basin area. Conversely, the abundance of crayfish was related to the amount of edge habitat, and therefore crayfish did not increase in abundance as basin area increased. Densities of all fish species combined ranged from 17 to 459 fish 100 m-2, and biomass ranged from 14 to 206 g m-2. Eels dominated the fish assemblages, comprising 85-100% of the total biomass; longfinned eels the majority of the biomass at most sites. Despite the open access of the lower sites to introduced brown trout, native species dominated all the fish communities sampled
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Investigation of a cloud-cover modification to SPCTRAL2, SERI's simple model for cloudless-sky, spectral solar irradiance
This report summarizes the investigation of a cloud-cover modification to SPCTRAL2, SERI's simple model for cloudless-sky, spectral solar irradiance. Our approach was to develop a modifier that relies on commonly acquired meteorological and broadband-irradiance data rather than detailed cloud properties that are generally not available. The method was to normalize modeled, cloudless-sky spectral irradiance to a measured broadband-irradiance value under cloudy skies, and then to compare the normalized, modeled data with measured spectral-irradiance data to empirically derive spectral modifiers that improve the agreement between modeled and measured data. Results indicate the possible form of the spectral corrections; however, we must analyze additional data to develop a spectral transmission function for cloudy-sky conditions
Exact eigenspectrum of the symmetric simple exclusion process on the complete, complete bipartite, and related graphs
We show that the infinitesimal generator of the symmetric simple exclusion
process, recast as a quantum spin-1/2 ferromagnetic Heisenberg model, can be
solved by elementary techniques on the complete, complete bipartite, and
related multipartite graphs. Some of the resulting infinitesimal generators are
formally identical to homogeneous as well as mixed higher spins models. The
degeneracies of the eigenspectra are described in detail, and the
Clebsch-Gordan machinery needed to deal with arbitrary spin-s representations
of the SU(2) is briefly developed. We mention in passing how our results fit
within the related questions of a ferromagnetic ordering of energy levels and a
conjecture according to which the spectral gaps of the random walk and the
interchange process on finite simple graphs must be equal.Comment: Final version as published, 19 pages, 4 figures, 40 references given
in full forma
Risk, precaution and science: towards a more constructive policy debate. Talking point on the precautionary principle
Few issues in contemporary risk policy are as momentous or contentious as the precautionary principle. Since it first emerged in German environmental policy, it has been championed by environmentalists and consumer protection groups, and resisted by the industries they oppose (Raffensperger & Tickner, 1999). Various versions of the principle now proliferate across different national and international jurisdictions and policy areas (Fisher, 2002). From a guiding theme in European Commission (EC) environmental policy, it has become a general principle of EC law (CEC, 2000; Vos & Wendler, 2006). Its influence has extended from the regulation of environmental, technological and health risks to the wider governance of science, innovation and trade (O'Riordan & Cameron, 1994)
Weak charge form factor and radius of 208Pb through parity violation in electron scattering
We use distorted wave electron scattering calculations to extract the weak
charge form factor F_W(q), the weak charge radius R_W, and the point neutron
radius R_n, of 208Pb from the PREX parity violating asymmetry measurement. The
form factor is the Fourier transform of the weak charge density at the average
momentum transfer q=0.475 fm. We find F_W(q) =0.204 \pm 0.028 (exp) \pm
0.001 (model). We use the Helm model to infer the weak radius from F_W(q). We
find R_W= 5.826 \pm 0.181 (exp) \pm 0.027 (model) fm. Here the exp error
includes PREX statistical and systematic errors, while the model error
describes the uncertainty in R_W from uncertainties in the surface thickness
\sigma of the weak charge density. The weak radius is larger than the charge
radius, implying a "weak charge skin" where the surface region is relatively
enriched in weak charges compared to (electromagnetic) charges. We extract the
point neutron radius R_n=5.751 \pm 0.175 (exp) \pm 0.026 (model) \pm 0.005
(strange) fm$, from R_W. Here there is only a very small error (strange) from
possible strange quark contributions. We find R_n to be slightly smaller than
R_W because of the nucleon's size. Finally, we find a neutron skin thickness of
R_n-R_p=0.302\pm 0.175 (exp) \pm 0.026 (model) \pm 0.005 (strange) fm, where
R_p is the point proton radius.Comment: 5 pages, 1 figure, published in Phys Rev. C. Only one change in this
version: we have added one author, also to metadat
Experimental measurement of the isolated magnetic susceptibility
The isolated susceptibility may be defined as a
(non-thermodynamic) average over the canonical ensemble, but while it has often
been discussed in the literature, it has not been clearly measured. Here, we
demonstrate an unambiguous measurement of at avoided
nuclear-electronic level crossings in a dilute spin ice system, containing
well-separated holmium ions. We show that quantifies the
superposition of quasi-classical spin states at these points, and is a direct
measure of state concurrence and populations.Comment: 9 pages, & figure
Breakdown of Lindstedt Expansion for Chaotic Maps
In a previous paper of one of us [Europhys. Lett. 59 (2002), 330--336] the
validity of Greene's method for determining the critical constant of the
standard map (SM) was questioned on the basis of some numerical findings. Here
we come back to that analysis and we provide an interpretation of the numerical
results by showing that no contradiction is found with respect to Greene's
method. We show that the previous results based on the expansion in Lindstedt
series do correspond to the transition value but for a different map: the
semi-standard map (SSM). Moreover, we study the expansion obtained from the SM
and SSM by suppressing the small divisors. The first case turns out to be
related to Kepler's equation after a proper transformation of variables. In
both cases we give an analytical solution for the radius of convergence, that
represents the singularity in the complex plane closest to the origin. Also
here, the radius of convergence of the SM's analogue turns out to be lower than
the one of the SSM. However, despite the absence of small denominators these
two radii are lower than the ones of the true maps for golden mean winding
numbers. Finally, the analyticity domain and, in particular, the critical
constant for the two maps without small divisors are studied analytically and
numerically. The analyticity domain appears to be an perfect circle for the SSM
analogue, while it is stretched along the real axis for the SM analogue
yielding a critical constant that is larger than its radius of convergence.Comment: 12 pages, 3 figure
Phase Transitions and Spatio-Temporal Fluctuations in Stochastic Lattice Lotka-Volterra Models
We study the general properties of stochastic two-species models for
predator-prey competition and coexistence with Lotka-Volterra type interactions
defined on a -dimensional lattice. Introducing spatial degrees of freedom
and allowing for stochastic fluctuations generically invalidates the classical,
deterministic mean-field picture. Already within mean-field theory, however,
spatial constraints, modeling locally limited resources, lead to the emergence
of a continuous active-to-absorbing state phase transition. Field-theoretic
arguments, supported by Monte Carlo simulation results, indicate that this
transition, which represents an extinction threshold for the predator
population, is governed by the directed percolation universality class. In the
active state, where predators and prey coexist, the classical center
singularities with associated population cycles are replaced by either nodes or
foci. In the vicinity of the stable nodes, the system is characterized by
essentially stationary localized clusters of predators in a sea of prey. Near
the stable foci, however, the stochastic lattice Lotka-Volterra system displays
complex, correlated spatio-temporal patterns of competing activity fronts.
Correspondingly, the population densities in our numerical simulations turn out
to oscillate irregularly in time, with amplitudes that tend to zero in the
thermodynamic limit. Yet in finite systems these oscillatory fluctuations are
quite persistent, and their features are determined by the intrinsic
interaction rates rather than the initial conditions. We emphasize the
robustness of this scenario with respect to various model perturbations.Comment: 19 pages, 11 figures, 2-column revtex4 format. Minor modifications.
Accepted in the Journal of Statistical Physics. Movies corresponding to
Figures 2 and 3 are available at
http://www.phys.vt.edu/~tauber/PredatorPrey/movies
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