1,597 research outputs found
The matching problem between functional shapes via a BV-penalty term: a -convergence result
In this paper we study a variant of the matching model between functional
shapes introduced in \cite{ABN}. Such a model allows to compare surfaces
equipped with a signal and the matching energy is defined by the -norm of
the signal on the surface and a varifold-type attachment term.
In this work we study the problem with fixed geometry which means that we
optimize the initial signal (supported on the initial surface) with respect to
a target signal supported on a different surface. In particular, we consider a
or -penalty for the signal instead of its -norm. Several
numerical examples are shown in order to prove that the -penalty improves
the quality of the matching. Moreover, we prove a -convergence result
for the discrete matching energy towards the continuous-one
Room temperature Peierls distortion in small radius nanotubes
By means of {\it ab initio} simulations, we investigate the phonon band
structure and electron-phonon coupling in small 4-\AA diameter nanotubes. We
show that both the C(5,0) and C(3,3) tubes undergo above room temperature a
Peierls transition mediated by an acoustical long-wavelength and an optical
phonons respectively. In the armchair geometry, we verify that the
electron-phonon coupling parameter originates mainly from phonons at
and is strongly enhanced when the diameter decreases. These results
question the origin of superconductivity in small diameter nanotubes.Comment: submitted 21oct2004 accepted 6jan2005 (Phys.Rev.Lett.
First-Principle Description of Correlation Effects in Layered Materials
We present a first-principles description of anisotropic materials
characterized by having both weak (dispersion-like) and strong covalent bonds,
based on the Adiabatic--Connection Fluctuation--Dissipation Theorem within
Density Functional Theory. For hexagonal boron nitride the in-plane and out of
plane bonding as well as vibrational dynamics are well described both at
equilibrium and when the layers are pulled apart. Also bonding in covalent and
ionic solids is described. The formalism allows to ping-down the deficiencies
of common exchange-correlation functionals and provides insight towards the
inclusion of dispersion interactions into the correlation functional.Comment: Accepted for publication in Physical Review Letter
Anomalous Aharonov--Bohm gap oscillations in carbon nanotubes
The gap oscillations caused by a magnetic flux penetrating a carbon nanotube
represent one of the most spectacular observation of the Aharonov-Bohm effect
at the nano--scale. Our understanding of this effect is, however, based on the
assumption that the electrons are strictly confined on the tube surface, on
trajectories that are not modified by curvature effects. Using an ab-initio
approach based on Density Functional Theory we show that this assumption fails
at the nano-scale inducing important corrections to the physics of the
Aharonov-Bohm effect. Curvature effects and electronic density spilled out of
the nanotube surface are shown to break the periodicity of the gap
oscillations. We predict the key phenomenological features of this anomalous
Aharonov-Bohm effect in semi-conductive and metallic tubes and the existence of
a large metallic phase in the low flux regime of Multi-walled nanotubes, also
suggesting possible experiments to validate our results.Comment: 7 figure
A generic framework for context-sensitive analysis of modular programs
Context-sensitive analysis provides information which is potentially more accurate than that provided by context-free analysis. Such information can then be applied in order to validate/debug the program and/or to specialize the program obtaining important improvements. Unfortunately, context-sensitive analysis of modular programs poses important theoretical and practical problems. One solution, used in several proposals, is to resort to context-free analysis. Other proposals do address
context-sensitive analysis, but are only applicable when the description domain used satisfies rather restrictive properties. In this paper, we argĂĽe that a general framework for context-sensitive analysis of modular programs, Le., one that allows using all the domains which have proved useful in practice in the non-modular setting, is indeed feasible and very useful. Driven by our experience in the design and implementation of analysis and specialization techniques in the context of CiaoPP, the Ciao
system preprocessor, in this paper we discuss a number of design goals for context-sensitive analysis of modular programs as well as the problems which arise in trying to meet these goals. We also provide a high-level description of a framework for analysis of modular programs which does
substantially meet these objectives. This framework is generic in that it can be instantiated in different ways in order to adapt to different contexts. Finally, the behavior of the different instantiations w.r.t. the design goals that motivate our work is also discussed
Bending modes, elastic constants and mechanical stability of graphitic systems
The thermodynamic and mechanical properties of graphitic systems are strongly
dependent on the shear elastic constant C44. Using state-of-the-art density
functional calculations, we provide the first complete determination of their
elastic constants and exfoliation energies. We show that stacking
misorientations lead to a severe lowering of C44 of at least one order of
magnitude. The lower exfoliation energy and the lower C44 (more bending modes)
suggest that flakes with random stacking should be easier to exfoliate than the
ones with perfect or rhombohedral stacking. We also predict ultralow friction
behaviour in turbostratic graphitic systems.Comment: 7 pages, 6 figure
Determination of the Intershell Conductance in Multiwalled Carbon Nanotubes
We report on the intershell electron transport in multiwalled carbon
nanotubes (MWNT). To do this, local and nonlocal four-point measurements are
used to study the current path through the different shells of a MWNT. For
short electrode separations 1 m the current mainly flows
through the two outer shells, described by a resistive transmission line with
an intershell conductance per length of ~(10 k\Omega)^{-1}/m. The
intershell transport is tunnel-type and the transmission is consistent with the
estimate based on the overlap between -orbitals of neighboring shells.Comment: 5 pages, 4 figure
Issues for the Next Generation of Galaxy Surveys
I argue that the weight of the available evidence favours the conclusions
that galaxies are unbiased tracers of mass, the mean mass density (excluding a
cosmological constant or its equivalent) is less than the critical Einstein-de
Sitter value, and an isocurvature model for structure formation offers a viable
and arguably attractive model for the early assembly of galaxies. If valid
these conclusions complicate our work of adding structure formation to the
standard model for cosmology, but it seems sensible to pay attention to
evidence.Comment: 14 pages, 3 postscript figures, uses rspublic.st
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