55,700 research outputs found
Evaluation du risque des rejets urbains dans les rivières : comment évaluer les concentrations variant dans le temps?
Quantum dislocations: the fate of multiple vacancies in two dimensional solid 4He
Defects are believed to play a fundamental role in the supersolid state of
4He. We have studied solid 4He in two dimensions (2D) as function of the number
of vacancies n_v, up to 30, inserted in the initial configuration at rho =
0.0765 A^-2, close to the melting density, with the exact zero temperature
Shadow Path Integral Ground State method. The crystalline order is found to be
stable also in presence of many vacancies and we observe two completely
different regimes. For small n_v, up to about 6, vacancies form a bound state
and cause a decrease of the crystalline order. At larger n_v, the formation
energy of an extra vacancy at fixed density decreases by one order of magnitude
to about 0.6 K. In the equilibrated state it is no more possible to recognize
vacancies because they mainly transform into quantum dislocations and
crystalline order is found almost independent on how many vacancies have been
inserted in the initial configuration. The one--body density matrix in this
latter regime shows a non decaying large distance tail: dislocations, that in
2D are point defects, turn out to be mobile, their number is fluctuating, and
they are able to induce exchanges of particles across the system mainly
triggered by the dislocation cores. These results indicate that the notion of
incommensurate versus commensurate state loses meaning for solid 4He in 2D,
because the number of lattice sites becomes ill defined when the system is not
commensurate. Crystalline order is found to be stable also in 3D in presence of
up to 100 vacancies
Deformed W_N algebras from elliptic sl(N) algebras
We extend to the sl(N) case the results that we previously obtained on the
construction of W_{q,p} algebras from the elliptic algebra
A_{q,p}(\hat{sl}(2)_c). The elliptic algebra A_{q,p}(\hat{sl}(N)_c) at the
critical level c=-N has an extended center containing trace-like operators
t(z). Families of Poisson structures indexed by N(N-1)/2 integers, defining
q-deformations of the W_N algebra, are constructed. The operators t(z) also
close an exchange algebra when (-p^1/2)^{NM} = q^{-c-N} for M in Z. It becomes
Abelian when in addition p=q^{Nh} where h is a non-zero integer. The Poisson
structures obtained in these classical limits contain different q-deformed W_N
algebras depending on the parity of h, characterizing the exchange structures
at p \ne q^{Nh} as new W_{q,p}(sl(N)) algebras.Comment: LaTeX2e Document - packages subeqn,amsfonts,amssymb - 30 page
Central extensions of classical and quantum q-Viraroso algebras
We investigate the central extensions of the q-deformed (classical and
quantum) Virasoro algebras constructed from the elliptic quantum algebra
A_{q,p}[sl(N)_c]. After establishing the expressions of the cocycle conditions,
we solve them, both in the classical and in the quantum case (for sl(2)). We
find that the consistent central extensions are much more general that those
found previously in the literature.Comment: Latex2e, needs amsfonts and amssymb package
From quantum to elliptic algebras
It is shown that the elliptic algebra at the
critical level c=-2 has a multidimensional center containing some trace-like
operators t(z). A family of Poisson structures indexed by a non-negative
integer and containing the q-deformed Virasoro algebra is constructed on this
center. We show also that t(z) close an exchange algebra when p^m=q^{c+2} for m
integer, they commute when in addition p=q^{2k} for k integer non-zero, and
they belong to the center of when k is odd. The
Poisson structures obtained for t(z) in these classical limits contain the
q-deformed Virasoro algebra, characterizing the structures at generic values of
p, q and m as new algebras.Comment: LaTeX2e Document - packages subeqn,amsfont
Experimental and numerical studies of ferritic stainless steel tubular cross sections under combined compression and bending
An experimental and numerical study of ferritic stainless steel tubular cross sections under combined loading is presented in this paper. Two square hollow section (SHS) sizes—SHS 40×40×240×40×2 and SHS 50×50×250×50×2 made of Grade EN 1.4509 (AISI 441) stainless steel—were considered in the experimental program, which included 2 concentrically loaded stub column tests, 2 four-point bending tests, and 14 eccentrically loaded stub column tests. In parallel with the experimental investigation, a finite-element (FE) study was also conducted. Following validation of the FE models against the test results, parametric analyses were carried out to generate further structural performance data. The experimental and numerical results were analyzed and compared with the design strengths predicted by the current European stainless steel design code EN 1993-1-4 and American stainless steel design specification SEI/ASCE-8. The comparisons revealed that the codified capacity predictions for ferritic stainless steel cross sections under combined loading are unduly conservative. The deformation-based continuous strength method (CSM) has been extended to cover the case of combined loading. The applicability of CSM to the design of ferritic stainless steel cross sections under combined loading was also evaluated. The CSM was shown to offer substantial improvements in design efficiency over existing codified methods. Finally, the reliability of the proposals was confirmed by means of statistical analyses according to both the SEI/ASCE-8 requirements and those of EN 1990
Universal construction of W_{p,q} algebras
We present a direct construction of abstract generators for q-deformed W_N
algebras. This procedure hinges upon a twisted trace formula for the elliptic
algebra A_{q,p}(sl(N)_c) generalizing the previously known formulae for quantum
groups.Comment: packages amsfonts, amssym
Sampling-helper: A web-based tool to assess the reliability of sampling strategies in sewers and receiving waters
An ultra-compact low temperature scanning probe microscope for magnetic fields above 30 T
We present the design of a highly compact High Field Scanning Probe
Microscope (HF-SPM) for operation at cryogenic temperatures in an extremely
high magnetic field, provided by a water-cooled Bitter magnet able to reach 38
T. The HF-SPM is 14 mm in diameter: an Attocube nano-positioner controls the
coarse approach of a piezo resistive AFM cantilever to a scanned sample. The
Bitter magnet constitutes an extreme environment for SPM due to the high level
of vibrational noise; the Bitter magnet noise at frequencies up to 300 kHz is
characterized and noise mitigation methods are described. The performance of
the HF-SPM is demonstrated by topographic imaging and noise measurements at up
to 30 T. Additionally, the use of the SPM as a three-dimensional dilatometer
for magnetostriction measurements is demonstrated via measurements on a
magnetically frustrated spinel sample.Comment: 6 pages, 5 figure
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