38,504 research outputs found
Competing Phases, Strong Electron-Phonon Interaction and Superconductivity in Elemental Calcium under High Pressure
The observed "simple cubic" (sc) phase of elemental Ca at room temperature in
the 32-109 GPa range is, from linear response calculations, dynamically
unstable. By comparing first principle calculations of the enthalpy for five
sc-related (non-close-packed) structures, we find that all five structures
compete energetically at room temperature in the 40-90 GPa range, and three do
so in the 100-130 GPa range. Some competing structures below 90 GPa are
dynamically stable, i.e., no imaginary frequency, suggesting that these
sc-derived short-range-order local structures exist locally and can account for
the observed (average) "sc" diffraction pattern. In the dynamically stable
phases below 90 GPa, some low frequency phonon modes are present, contributing
to strong electron-phonon (EP) coupling as well as arising from the strong
coupling. Linear response calculations for two of the structures over 120 GPa
lead to critical temperatures in the 20-25 K range as is observed, and do so
without unusually soft modes.Comment: 8 pages, 6 figures, 1 table, accepted for publication in Phys. Rev.
Carbon coating of the SPS dipole chambers
The Electron Multipacting (EM) phenomenon is a limiting factor for the
achievement of high luminosity in accelerators for positively charged particles
and for the performance of RF devices. At CERN, the Super Proton Synchrotron
(SPS) must be upgraded in order to feed the Large Hadron Collider (LHC) with 25
ns bunch spaced beams. At such small bunch spacing, EM may limit the
performance of the SPS and consequently that of the LHC. To mitigate this
phenomenon CERN is developing a carbon thin film coating with low Secondary
Electron Yield (SEY) to coat the internal walls of the SPS dipoles beam pipes.
This paper presents the progresses in the coating technology, the performance
of the carbon coatings and the strategy for a large scale production.Comment: 7 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop
on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba,
Italy; CERN Yellow Report CERN-2013-002, pp.141-14
Polarization Induced Switching Effect in Graphene Nanoribbon Edge-Defect Junction
With nonequilibrium Green's function approach combined with density
functional theory, we perform an ab initio calculation to investigate transport
properties of graphene nanoribbon junctions self-consistently. Tight-binding
approximation is applied to model the zigzag graphene nanoribbon (ZGNR)
electrodes, and its validity is confirmed by comparison with GAUSSIAN03 PBC
calculation of the same system. The origin of abnormal jump points usually
appearing in the transmission spectrum is explained with the detailed
tight-binding ZGNR band structure. Transport property of an edge defect ZGNR
junction is investigated, and the tunable tunneling current can be sensitively
controlled by transverse electric fields.Comment: 18 pages, 8 figure
Decoherence and the retrieval of lost information
We found that in contrast with the common premise, a measurement on the
environment of an open quantum system can {\em reduce} its decoherence rate. We
demonstrate it by studying an example of indirect qubit's measurement, where
the information on its state is hidden in the environment. This information is
extracted by a distant device, coupled with the environment. We also show that
the reduction of decoherence generated by this device, is accompanied with
diminution of the environmental noise in a vicinity of the qubit. An
interpretation of these results in terms of quantum interference on large
scales is presented.Comment: 9 pages, 8 figures, additional explanations added, Phys. Rev. B, in
pres
Energy-efficiency improvements for optical access
This article discusses novel approaches to improve energy efficiency of different optical access technologies, including time division multiplexing passive optical network (TDM-PON), time and wavelength division multiplexing PON (TWDM-PON), point-to-point (PTP) access network, wavelength division multiplexing PON (WDM-PON), and orthogonal frequency division multiple access PON (OFDMA-PON). These approaches include cyclic sleep mode, energy-efficient bit interleaving protocol, power reduction at component level, or frequency band selection. Depending on the target optical access technology, one or a combination of different approaches can be applied
A note on multi-dimensional Camassa-Holm type systems on the torus
We present a -component nonlinear evolutionary PDE which includes the
-dimensional versions of the Camassa-Holm and the Hunter-Saxton systems as
well as their partially averaged variations. Our goal is to apply Arnold's
[V.I. Arnold, Sur la g\'eom\'etrie diff\'erentielle des groupes de Lie de
dimension infinie et ses applications \`a l'hydrodynamique des fluides
parfaits. Ann. Inst. Fourier (Grenoble) 16 (1966) 319-361], [D.G. Ebin and J.E.
Marsden, Groups of diffeomorphisms and the motion of an incompressible fluid.
Ann. of Math. 92(2) (1970) 102-163] geometric formalism to this general
equation in order to obtain results on well-posedness, conservation laws or
stability of its solutions. Following the line of arguments of the paper [M.
Kohlmann, The two-dimensional periodic -equation on the diffeomorphism group
of the torus. J. Phys. A.: Math. Theor. 44 (2011) 465205 (17 pp.)] we present
geometric aspects of a two-dimensional periodic --equation on the
diffeomorphism group of the torus in this context.Comment: 14 page
Long range magnetic ordering in NaIrO
We report a combined experimental and theoretical investigation of the
magnetic structure of the honeycomb lattice magnet NaIrO, a strong
candidate for a realization of a gapless spin-liquid. Using resonant x-ray
magnetic scattering at the Ir L-edge, we find 3D long range
antiferromagnetic order below T=13.3 K. From the azimuthal dependence of
the magnetic Bragg peak, the ordered moment is determined to be predominantly
along the {\it a}-axis. Combining the experimental data with first principles
calculations, we propose that the most likely spin structure is a novel
"zig-zag" structure
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