34,301 research outputs found
Crossover from Fermi-Pasta-Ulam to normal diffusive behaviour in heat conduction through open anharmonic lattices
We study heat conduction in one, two and three dimensional anharmonic
lattices connected to stochastic Langevin heat baths. The inter-atomic
potential of the lattices is double-well type, i.e., with . We observe two different
temperature regimes of transport: a high-temperature regime where asymptotic
length dependence of nonequilibrium steady state heat current is similar to the
well-known Fermi-Pasta-Ulam lattices with an inter-atomic potential, with . A low temperature regime where
heat conduction is diffusive normal satisfying Fourier's law. We present our
simulation results at different temperature regimes in all dimensions.Comment: 5 pages, 7 figure
The cosmic dust analyzer: Experimental evaluation of an impact ionization model
A thermal equilibrium plasma model is used to process data from an impact ionization time-of-flight mass spectrometer in order to convert the raw ion data to relative abundances of the elemental constituents of cosmic dust particles
A continuous low star formation rate in IZw 18 ?
Deep long-slit spectroscopic observations of the blue compact galaxy IZw 18
obtained with the CFH 3.6 m Telescope are presented. The very low value of
oxygen abundance previously reported is confirmed and a very homogeneous
abundance distribution is found (no variation larger than 0.05 dex) over the
whole ionized region. We concur with Tenorio-Tagle (1996) and Devost et al.
(1997) that the observed abundance level cannot result from the material
ejected by the stars formed in the current burst, and propose that the observed
metals were formed in a previous star formation episode. Metals ejected in the
current burst of star formation remain most probably hidden in a hot phase and
are undetectable using optical spectroscopy. We discuss different scenarios of
star formation in IZw 18. Combining various observational facts, for instance
the faint star formation rate observed in low surface brightness galaxies van
Zee et al. (1997), it is proposed that a low and continuous rate of star
formation occurring during quiescent phases between bursts could be a
significant source of metal enrichment of the interstellar medium.Comment: 10 pages, 4 Postscript figures, to be published in Astronomy and
Astrophysics main journa
A Bell-Evans-Polanyi principle for molecular dynamics trajectories and its implications for global optimization
The Bell-Evans-Polanyi principle that is valid for a chemical reaction that
proceeds along the reaction coordinate over the transition state is extended to
molecular dynamics trajectories that in general do not cross the dividing
surface between the initial and the final local minima at the exact transition
state. Our molecular dynamics Bell-Evans-Polanyi principle states that low
energy molecular dynamics trajectories are more likely to lead into the basin
of attraction of a low energy local minimum than high energy trajectories. In
the context of global optimization schemes based on molecular dynamics our
molecular dynamics Bell-Evans-Polanyi principle implies that using low energy
trajectories one needs to visit a smaller number of distinguishable local
minima before finding the global minimum than when using high energy
trajectories
Physics Prospects at the Hadron Colliders
I start with a brief introduction to the elementary particles and their
interactions, Higgs mechanism and supersymmetry. The major physics objectives
of the Tevatron and LHC colliders are identified. The status and prospects of
the top quark, charged Higgs boson and superparticle searches are discussed in
detail, while those of the neutral Higgs boson(s) are covered in a parallel
talk by R.J.N. Phillips at this workshop.Comment: 16 pages Latex + 15 figures (available on request
A Hybrid model for the origin of photoluminescence from Ge nanocrystals in SiO matrix
In spite of several articles, the origin of visible luminescence from
germanium nanocrystals in SiO matrix is controversial even today. Some
authors attribute the luminescence to quantum confinement of charge carriers in
these nanocrystals. On the other hand, surface or defect states formed during
the growth process, have also been proposed as the source of luminescence in
this system. We have addressed this long standing query by simultaneous
photoluminescence and Raman measurements on germanium nanocrystals embedded in
SiO matrix, grown by two different techniques: (i) low energy
ion-implantation and (ii) atom beam sputtering. Along with our own experimental
observations, we have summarized relevant information available in the
literature and proposed a \emph{Hybrid Model} to explain the visible
photoluminescence from nanocrystalline germanium in SiO matrix.Comment: 23 pages, 8 figure
The Magnetic Field of the H~II Region NGC 6334A
We have studied the polarization characteristics and Faraday rotation of the
extragalactic radio source J172043554, that appears projected on the north
lobe of the galactic H II region NGC 6334A. From observations made with the
Very Large Array at 6.0 and 3.6 cm in three different epochs (1994, 1997, and
2006), we estimate a rotation measure of +5100900 rad m for the
extragalactic source. This large rotation measure implies a line-of-sight
average magnetic field of G, the largest
obtained by this method for an H II region. NGC 6334A is significantly denser
than other H II regions studied and this larger magnetic field is expected on
the grounds of magnetic flux conservation. The ratio of thermal to magnetic
pressure is 5, in the range of values determined for more diffuse H II
regions.Comment: 5 pages, 0 figure
Viscous corrections to the resistance of nano-junctions: a dispersion relation approach
It is well known that the viscosity of a homogeneous electron liquid diverges
in the limits of zero frequency and zero temperature. A nanojunction breaks
translational invariance and necessarily cuts off this divergence. However, the
estimate of the ensuing viscosity is far from trivial. Here, we propose an
approach based on a Kramers-Kr\"onig dispersion relation, which connects the
zero-frequency viscosity, , to the high-frequency shear modulus,
, of the electron liquid via , with
the junction-specific momentum relaxation time. By making use of a
simple formula derived from time-dependent current-density functional theory we
then estimate the many-body contributions to the resistance for an integrable
junction potential and find that these viscous effects may be much larger than
previously suggested for junctions of low conductance.Comment: 6 pages, 5 figures, Revised versio
FIBONACCI SUPERLATTICES OF NARROW-GAP III-V SEMICONDUCTORS
We report theoretical electronic structure of Fibonacci superlattices of
narrow-gap III-V semiconductors. Electron dynamics is accurately described
within the envelope-function approximation in a two-band model.
Quasiperiodicity is introduced by considering two different III-V semiconductor
layers and arranging them according to the Fibonacci series along the growth
direction. The resulting energy spectrum is then found by solving exactly the
corresponding effective-mass (Dirac-like) wave equation using tranfer-matrix
techniques. We find that a self-similar electronic spectrum can be seen in the
band structure. Electronic transport properties of samples are also studied and
related to the degree of spatial localization of electronic envelope-functions
via Landauer resistance and Lyapunov coefficient. As a working example, we
consider type II InAs/GaSb superlattices and discuss in detail our results in
this system.Comment: REVTeX 3.0, 16 pages, 8 figures available upon request. To appear in
Semiconductor Science and Technolog
- …