16,965 research outputs found
Benchmarking electron-cloud simulations and pressure measurements at the LHC
During the beam commissioning of the Large Hadron Collider (LHC) with 150,
75, 50 and 25-ns bunch spacing, important electron-cloud effects, like pressure
rise, cryogenic heat load, beam instabilities or emittance growth, were
observed. A method has been developed to infer different key beam-pipe surface
parameters by benchmarking simulations and pressure rise observed in the
machine. This method allows us to monitor the scrubbing process (i.e. the
reduction of the secondary emission yield as a function of time) in the regions
where the vacuum-pressure gauges are located, in order to decide on the most
appropriate strategies for machine operation. In this paper we present the
methodology and first results from applying this technique to the LHC.Comment: 5 pages, 7 figures, presented at ECLOUD 12: Joint
INFN-CERN-EuCARD-AccNet Workshop on Electron-Cloud Effects, La Biodola, Isola
d Elba, Italy, 5-9 June 201
Critical behavior of spin and chiral degrees of freedom in three-dimensional disordered XY models studied by the nonequilibrium aging method
The critical behavior of the gauge-glass and the XY spin-glass models in
three dimensions is studied by analyzing their nonequilibrium aging dynamics. A
new numerical method, which relies on the calculation of the two-time
correlation and integrated response functions, is used to determine both the
critical temperature and the nonequilibrium scaling exponents, both for spin
and chiral degrees of freedom. First, the ferromagnetic XY model is studied to
validate this nonequilibirum aging method (NAM), since for this nondisordered
system we can compare with known results obtained with standard equilibrium and
nonequilibrium techniques. When applied to the case of the gauge-glass model,
we show that the NAM allows us to obtain precise and reliable values of its
critical quantities, improving previous estimates. The XY spin-glass model with
both Gaussian and bimodal bond distributions, is analyzed in more detail. The
spin and the chiral two-time correlation and integrated response functions are
calculated in our simulations. The results obtained mainly for Gaussian and, to
a lesser extent, for bimodal interactions, support the existence of a
spin-chiral decoupling scenario, where the chiral order occurs at a finite
temperature while the spin degrees of freedom order at very low or zero
temperature.Comment: 15 pages, 15 figures. Phys. Rev. B 89, 024408 (2014
Latin America and the European Left
Throughout the 30 years of neoliberal hegemony in Latin America, the stubborn and persistent resistance by all manner of social movements gave rise to a novel form of politics and political strategy that involved the thorough transformation of the state, constitutional principles and guiding societal principles aimed at putting the social, economic and political rights of human beings over and above the diktats of the market
Understanding delocalization in the Continuous Random Dimer model
We propose an explanation of the bands of extended states appearing in random
one dimensional models with correlated disorder, focusing on the Continuous
Random Dimer model [A.\ S\'{a}nchez, E.\ Maci\'a, and F.\ Dom\'\i nguez-Adame,
Phys.\ Rev.\ B {\bf 49}, 147 (1994)]. We show exactly that the transmission
coefficient at the resonant energy is independent of the number of host sites
between two consecutive dimers. This allows us to understand why are there
bands of extended states for every realization of the model as well as the
dependence of the bandwidths on the concentration. We carry out a perturbative
calculation that sheds more light on the above results. In the conclusion we
discuss generalizations of our results to other models and possible
applications which arise from our new insight of this problem.Comment: REVTeX 3.0, 4 pages, 4 figures (hard copy on request from
[email protected]), Submitted to Phys Rev
Intentionally disordered superlattices with high dc conductance
We study disordered quantum-well-based semiconductor superlattices where the
disorder is intentional and short-range correlated. Such systems consist of
quantum-wells of two different thicknesses randomly distributed along the
growth direction, with the additional constraint that wells of one kind always
appears in pairs. Imperfections due to interface roughness are considered by
allowing the quantum-well thicknesses to fluctuate around their {\em ideal}
values. As particular examples, we consider wide-gap
(GaAs-GaAlAs) and narrow-gap (InAs-GaSb) superlattices. We show
the existence of a band of extended states in perfect correlated disordered
superlattices, giving rise to a strong enhancement of their finite-temperature
dc conductance as compared to usual random ones whenever the Fermi level
matches this band. This feature is seen to survive even if interface roughness
is taken into account. Our predictions can be used to demonstrate
experimentally that structural correlations inhibit the localization effects of
disorder, even in the presence of imperfections. This effect might be the basis
of new, filter-like or other specific-purpose electronic devices.Comment: REVTeX 3.0, 20 pages, 7 uuencoded compressed PostScript figures as a
separate file. Submitted to IEEE J Quantum Elec
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