6,666 research outputs found
Reduced Density Matrix Approach to Phononic Dissipation in Friction
Understanding mechanisms for energy dissipation from nanoparticles in contact
with large samples is a central problem in describing friction microscopically.
Calculation of the reduced density matrix appears to be the most suitable metho
to study such systems that are coupled to a large environment. In this paper
the time evolution of the reduced density matrix has been evaluated for an
arbitrary system coupled to a heat reservoir. The formalism is then applied to
study the vibrational relaxation following the stick-slip motion of a small
adsorbate on a surface. The frequency dependence of the relaxation time is also
determined.Comment: 16 pages, 3 figures (included), revte
A Turbulent Origin for Flocculent Spiral Structure in Galaxies: II. Observations and Models of M33
Fourier transform power spectra of azimuthal scans of the optical structure
of M33 are evaluated for B, V, and R passbands and fit to fractal models of
continuum emission with superposed star formation. Power spectra are also
determined for Halpha. The best models have intrinsic power spectra with 1D
slopes of around -0.7pm0.7, significantly shallower than the Kolmogorov
spectrum (slope =-1.7) but steeper than pure noise (slope=0). A fit to the
power spectrum of the flocculent galaxy NGC 5055 gives a steeper slope of
around -1.5pm0.2, which could be from turbulence. Both cases model the optical
light as a superposition of continuous and point-like stellar sources that
follow an underlying fractal pattern. Foreground bright stars are clipped in
the images, but they are so prominent in M33 that even their residual affects
the power spectrum, making it shallower than what is intrinsic to the galaxy. A
model consisting of random foreground stars added to the best model of NGC 5055
fits the observed power spectrum of M33 as well as the shallower intrinsic
power spectrum that was made without foreground stars. Thus the optical
structure in M33 could result from turbulence too.Comment: accepted by ApJ, 13 pages, 10 figure
Massive Dirac fermions and the zero field quantum Hall effect
Through an explicit calculation for a Lagrangian in quantum electrodynamics
in (2+1)-space--time dimensions (QED), making use of the relativistic Kubo
formula, we demonstrate that the filling factor accompanying the quantized
electrical conductivity for massive Dirac fermions of a single species in two
spatial dimensions is a half (in natural units) when time reversal and parity
symmetries of the Lagrangian are explicitly broken by the fermion mass term. We
then discuss the most general form of the QED Lagrangian, both for
irreducible and reducible representations of the Dirac matrices in the plane,
with emphasis on the appearance of a Chern-Simons term. We also identify the
value of the filling factor with a zero field quantum Hall effect (QHE).Comment: 15 pages. Accepted in Jour. Phys.
PaperMaker: validation of biomedical scientific publications
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Fourth generation electron cyclotron resonance ion sources.
The concepts and technical challenges related to developing a fourth generation electron cyclotron resonance (ECR) ion source with a rf frequency greater than 40 GHz and magnetic confinement fields greater than twice B(ECR) will be explored in this article. Based on the semiempirical frequency scaling of ECR plasma density with the square of operating frequency, there should be significant gains in performance over current third generation ECR ion sources, which operate at rf frequencies between 20 and 30 GHz. While the third generation ECR ion sources use NbTi superconducting solenoid and sextupole coils, the new sources will need to use different superconducting materials, such as Nb(3)Sn, to reach the required magnetic confinement, which scales linearly with rf frequency. Additional technical challenges include increased bremsstrahlung production, which may increase faster than the plasma density, bremsstrahlung heating of the cold mass, and the availability of high power continuous wave microwave sources at these frequencies. With each generation of ECR ion sources, there are new challenges to be mastered, but the potential for higher performance and reduced cost of the associated accelerator continues to make this a promising avenue for development
Sustaining municipal parks in an era of neoliberal austerity: The contested commercialisation of Gunnersbury Park
This paper analyses a potentially path shaping moment for the UKâs public parks by analysing a pivotal case study of park neoliberalisation. Like many municipal parks, Gunnersbury Park in West London is experiencing the effects of local government budget cuts. Governance, policy and physical changes have been introduced to reduce dependence on public funding and the result is a more commercially oriented park. This case is used to better understand how the period of neoliberal austerity 2010-2020 reshaped municipal parks. The paper highlights concerns over the transparency and accountability of the social enterprise that now manages Gunnersbury Park. It also shows how neoliberalisation and commercialisation are manifested in the park landscape: free events are replaced with ticketed ones, spaces for sport are transformed into bookable facilities, cafes are taken over by corporate chains and playgrounds are supplemented with paid entry alternatives. One of the main consequences is the financial and symbolic exclusion of those unable or unwilling to pay. The paper explores who has contested the recent changes, and why. Opponents are dismissed as idealistic NIMBYs but, by refusing to accept the post-political inevitability of park neoliberalisation, they are helping to ensure Gunnersbury Parkâs remains a public and open space. The case is contextualised by situating it within a review of new park governance arrangements across London, and by comparing neoliberalisation processes here with those affecting New York parks. Ultimately, the research highlights the pitfalls of shifting away from the public funding and public management of municipal parks.
Progress report of the third Generation ECR ion source fabrication
Recent progress in the construction of the 3rd Generation ECR ion source at the 88" cyclotron in Berkeley is reported. Test results of a full scale prototype superconducting magnet structure, which has been described in the last ECR Ion Source Workshop, lead to an improved coil design for the 3rd Generation ECR ion source. Solenoids of the new design have been fabricated and exceeded the design field values without quench. The new sextupole coils are currently being wound and will be tested this summer. This magnet structure consists of three solenoids and six race track coils with iron poles forming the sextupole. It is described in the report along with the structural support and coil winding specifications. The coils are designed to generate a 4T axial mirror field at injection and 3T at extraction and a radial sextupole field of 2.4 T at the plasma chamber wall. The high axial magnetic field of the 3rd Generation ECR ion source influences ion beam extraction considerably and we have initiated simulations of the extraction and beam transport system in order to enhance transmission through the injection beam line of the 88" cyclotron
Optimal eigenvalue estimate for the Dirac-Witten operator on bounded domains with smooth boundary
Eigenvalue estimate for the Dirac-Witten operator is given on bounded domains
(with smooth boundary) of spacelike hypersurfaces satisfying the dominant
energy condition, under four natural boundary conditions (MIT, APS, modified
APS, and chiral conditions). This result is a generalisation of Friedrich's
inequality for the usual Dirac operator. The limiting cases are also
investigated.Comment: 2007, 18 pages, submitted 02 June 200
Design of the low energy astrophysics research facility CLAIRE
Abstract A novel nuclear astrophysics facility, CLAIRE (Center for Low Energy Astrophysics and Interdisciplinary REsearch), is being designed at Lawrence Berkeley National Laboratory to address the need for precise fusion cross section measurements at near-solar energies ($20 keV). At these low energies, fusion cross sections decrease exponentially with energy and are expected to approach femtobarn levels or less. In order to measure such small cross sections, the CLAIRE facility will incorporate a versatile accelerator capable of transporting high current (>100 mA), low energy (50-300 keV) ion beams with a tight focus (<1 cm) to a cooled, dense gas-jet target. The conceptual design for this accelerator is discussed, and simulations of both beam extraction and transport are presented
QuantUM: Quantitative Safety Analysis of UML Models
When developing a safety-critical system it is essential to obtain an
assessment of different design alternatives. In particular, an early safety
assessment of the architectural design of a system is desirable. In spite of
the plethora of available formal quantitative analysis methods it is still
difficult for software and system architects to integrate these techniques into
their every day work. This is mainly due to the lack of methods that can be
directly applied to architecture level models, for instance given as UML
diagrams. Also, it is necessary that the description methods used do not
require a profound knowledge of formal methods. Our approach bridges this gap
and improves the integration of quantitative safety analysis methods into the
development process. All inputs of the analysis are specified at the level of a
UML model. This model is then automatically translated into the analysis model,
and the results of the analysis are consequently represented on the level of
the UML model. Thus the analysis model and the formal methods used during the
analysis are hidden from the user. We illustrate the usefulness of our approach
using an industrial strength case study.Comment: In Proceedings QAPL 2011, arXiv:1107.074
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