413,829 research outputs found
Transport properties of a two-dimensional electron liquid at high magnetic field
The chiral Luttinger liquid model for the edge dynamics of a two-dimensional
electron gas in a strong magnetic field is derived from coarse-graining and a
lowest Landau level projection procedure at arbitrary filling factors
-- without reference to the quantum Hall effect. Based on this model, we
develop a formalism to calculate the Landauer-B\"uttiker conductances in
generic experimental set-ups including multiple leads and voltage probes. In
the absence of tunneling between the edges the "ideal" Hall conductances
( if lead is immediately upstream of lead ,
and otherwise) are recovered. Tunneling of quasiparticles of
fractional charge between different edges is then included as an
additional term in the Hamiltonian. In the limit of weak tunneling we obtain
explicit expressions for the corrections to the ideal conductances. As an
illustration of the formalism we compute the current- and temperature-dependent
resistance of a quantum point contact localized at the center of
a gate-induced constriction in a quantum Hall bar. The exponent in the
low-current relation shows a nontrivial
dependence on the strength of the inter-edge interaction, and its value changes
as , where is the Hall voltage, falls below
a characteristic crossover energy , where is the edge
wave velocity and is the length of the constriction. The consequences of
this crossover are discussed vis-a-vis recent experiments in the weak tunneling
regime.Comment: 20 pages, 8 figures, Revtex4, adjourned with referee's comments,
added references and typos correcte
Improved spectral descriptions of planetary nebulae central stars
Context. At least 492 central stars of Galactic planetary nebulae (CSPNs)
have been assigned spectral types. Since many CSPNs are faint, these
classification efforts are frequently made at low spectral resolution. However,
the stellar Balmer absorption lines are contaminated with nebular emission;
therefore in many cases a low-resolution spectrum does not enable the
determination of the H abundance in the CSPN photosphere. Whether or not the
photosphere is H deficient is arguably the most important fact we should expect
to extract from the CSPN spectrum, and should be the basis for an adequate
spectral classification system.
Aims. Our purpose is to provide accurate spectral classifications and
contribute to the knowledge of central stars of planetary nebulae and stellar
evolution.
Methods. We have obtained and studied higher quality spectra of CSPNs
described in the literature as weak emission-line star (WELS). We provide
descriptions of 19 CSPN spectra. These stars had been previously classified at
low spectral resolution. We used medium-resolution spectra taken with the
Gemini Multi-Object Spectrograph (GMOS). We provide spectral types in the
Morgan-Keenan (MK) system whenever possible.
Results. Twelve stars in our sample appear to have normal H rich photospheric
abundances, and five stars remain unclassified. The rest (two) are most
probably H deficient. Of all central stars described by other authors as WELS,
we find that at least 26% of them are, in fact, H rich O stars, and at least 3%
are H deficient. This supports the suggestion that the denomination WELS should
not be taken as a spectral type, because, as a WELS based on low-resolution
spectra, it cannot provide enough information about the photospheric H
abundance.Comment: Accepted to be published in the A&
"May I borrow Your Filter?" Exchanging Filters to Combat Spam in a Community
Leveraging social networks in computer systems can be effective in dealing with a number of trust and security issues. Spam is one such issue where the "wisdom of crowds" can be harnessed by mining the collective knowledge of ordinary individuals. In this paper, we present a mechanism through which members of a virtual community can exchange information to combat spam. Previous attempts at collaborative spam filtering have concentrated on digest-based indexing techniques to share digests or fingerprints of emails that are known to be spam. We take a different approach and allow users to share their spam filters instead, thus dramatically reducing the amount of traffic generated in the network. The resultant diversity in the filters and cooperation in a community allows it to respond to spam in an autonomic fashion. As a test case for exchanging filters we use the popular SpamAssassin spam filtering software and show that exchanging spam filters provides an alternative method to improve spam filtering performance
Covariant Sectors with Infinite Dimension and Positivity of the Energy
We consider a Moebius covariant sector, possibly with infinite dimension, of
a local conformal net of von Neumann algebras on the circle. If the sector has
finite index, it has automatically positive energy. In the infinite index case,
we show the spectrum of the energy always to contain the positive real line,
but, as seen by an example, it may contain negative values. We then consider
nets with Haag duality on the real line, or equivalently sectors with
non-solitonic extension to the dual net; we give a criterion for irreducible
sectors to have positive energy, namely this is the case iff there exists an
unbounded Moebius covariant left inverse. As a consequence the class of sectors
with positive energy is stable under composition, conjugation and direct
integral decomposition.Comment: 25 pages, Latex2
A physically-based approach for evaluating the hydraulic invariance in urban transformations
Transformation of urban areas satisfies hydraulic invariance (HI) if the maximum flow rate outgoing
the area stays unchanged. The HI can be respected by dimensioning appropriate water storage
volumes or low impact developments (LID) to balance the soil sealing and ground levelling effects.
In order to comply with HI, some Italian regional legislation and river basin authority provide for
the creation of storage tanks whose volume must be estimated through simple conceptual rainfallrunoff
models. In this work a physically based approach for evaluating HI is proposed. It is based on
interpolating the results from a large number of hydraulic simulations conducted using FullSWOF,
which is an open source code developed by the University of Orléans. In this software the shallow
water equations are solved using a finite volume scheme and friction laws and infiltration models
are included. Simulations have been carried out considering the effect of three properties of the
area, that is: the saturated hydraulic conductivity of soil, the slope of ground surface and the standard
deviation of ground elevation around the mean level. Using the results, interpolating laws for
the peak discharge and the critical rainfall duration as function of the three basin parameters have
been derived. A parametric hydrograph as a function of the basin parameters and rainfall duration
is defined and a HI evaluation method based on routing the parametric hydrograph is proposed.
The results from this approach have been compared with those from non-physically based methods
currently used, such as the direct rainfall approach and the linear reservoir approach. The comparison
shows that the difference between these conceptual methods with that one proposed here
is strongly dependent on the runoff coefficient value. It is also not possible to predict whether they
are conservative or not
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