25,691 research outputs found
Lattice-dynamical calculation of phonon scattering at a disordered interface
For an fcc crystal with central force interactions and separately for a
scalar model on a square lattice, we compute exactly the phonon transmission
coefficient through a disordered planar interface between two
identical semi - infinite leads. At high frequencies exhibits a
strong frequency dependence which is determined by the correlation length of
the disorder.Comment: to appear in Physica B, proceedings of the 9th international
conference on phonon scatterin
Analysis of Possible Quantum Metastable States in Ballistic Graphene-based Josephson Junctions
Graphene is a relatively new material (2004) made of atomic layers of carbon
arranged in a honeycomb lattice. Josephson junction devices are made from
graphene by depositing two parallel superconducting leads on a graphene flake.
These devices have hysteretic current-voltage characteristics with a
supercurrent branch and Shapiro steps appear when irradiated with microwaves.
These properties motivate us to investigate the presence of quantum metastable
states similar to those found in conventional current-biased Josephson
junctions. We present work investigating the nature of these metastable states
for ballistic graphene Josephson junctions. We model the effective Washboard
potential for these devices and estimate parameters, such as energy level
spacing and critical currents, to deduce the design needed to observe
metastable states. We propose devices consisting of a parallel on-chip
capacitor and suspended graphene. The capacitor is needed to lower the energy
level spacing down to the experimentally accessible range of 1-20 GHz. The
suspended graphene helps reduce the noise that may otherwise come from
two-level states in the insulating oxide layer. Moreover, back-gate voltage
control of its critical current introduces another knob for quantum control. We
will also report on current experimental progress in the area of fabrication of
this proposed device.Comment: 4 pages, 5 figures, Accepted for publication in IEEE Transactions on
Applied Superconductivity from ASC 2010. Additional figures, additional
calculation
Lepton asymmetry and primordial nucleosynthesis in the era of precision cosmology
We calculate and display the primordial light-element abundances as a
function of a neutrino degeneracy parameter \xi common to all flavors. It is
the only unknown parameter characterizing the thermal medium at the primordial
nucleosynthesis epoch. The observed primordial helium abundance Y_p is the most
sensitive cosmic ``leptometer.'' Adopting the conservative Y_p error analysis
of Olive and Skillman implies -0.04 \alt \xi \alt 0.07 whereas the errors
stated by Izotov and Thuan imply \xi=0.0245+-0.0092 (1 sigma). Improved
determinations of the baryon abundance have no significant impact on this
situation. A determination of Y_p that reliably distinguishes between a
vanishing or nonvanishing \xi is a crucial test of the cosmological standard
assumption that sphaleron effects equilibrate the cosmic lepton and baryon
asymmetries.Comment: 5 pages, 2 figures; minor changes, references added, replaced to
match the published version in PRD (Brief Reports
Material selection for a new type of fire extinguisher
Nowadays safety is a hot topic, damage inflicted to human beings is intolerable. Fire safety is a big concern in industrial areas, but in residential areas a lot less precautions are in place. Therefore a new type of fire extinguisher should be developed that should encourage the installation of fire extinguishers in commercial environments and at home. The design of this fire extinguisher has to answer to a lot of demands. From a legal point of view, the extinguisher has to comply with the PED regulations and the EN 3 standard. Extra demands are, given the purpose, superb performance, great ergonomics and an attractive visual design. One of the steps in the design process is to make a material selection based on needed and desired properties of materials. Also the possible processes for manufacturing are an important parameter
Ab initio calculation of spin fluctuation spectra using time dependent density functional perturbation theory, planewaves, and pseudopotentials
We present an implementation of time-dependent density functional
perturbation theory for spin fluctuations, based on planewaves and
pseudopotentials. We compute the dynamic spin susceptibility self-consistently
by solving the time-dependent Sternheimer equation, within the adiabatic local
density approximation to the exchange and correlation kernel. We demonstrate
our implementation by calculating the spin susceptibility of representative
elemental transition metals, namely bcc Fe, fcc Ni and bcc Cr. The calculated
magnon dispersion relations of Fe and Ni are in agreement with previous work.
The calculated spin susceptibility of Cr exhibits a soft-paramagnon
instability, indicating the tendency of the Cr spins to condense in a
incommensurate spin density wave phase, in agreement with experiment
Ultraviolet spectroscopy of old novae and symbiotic stars
The IUE spectra are presented for two old novae and for two of the symbiotic variables. Prominent emission line spectra are revealed as a continuum whose appearance is effected by the system inclination. These data provide evidence for hot companions in the symbiotic stars, making plausible the binary model for these peculiar stars. Recent IUE spectra of dwarf novae provide additional support for the existence of optically thick accretion disks in active binary systems. The ultraviolet data of the eclipsing dwarf novae EX Hya and BV Cen appear flatter than for the noneclipsing systems, an effect which could be ascribed to the system inclination
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