1,616 research outputs found
Coherent switching of semiconductor resonator solitons
We demonstrate switching on and off of spatial solitons in a semiconductor
microresonator by injection of light coherent with the background illumination.
Evidence results that the formation of the solitons and their switching does
not involve thermal processes.Comment: 3 pages, 5 figure
Spiral Defect Chaos in Large Aspect Ratio Rayleigh-Benard Convection
We report experiments on convection patterns in a cylindrical cell with a
large aspect ratio. The fluid had a Prandtl number of approximately 1. We
observed a chaotic pattern consisting of many rotating spirals and other
defects in the parameter range where theory predicts that steady straight rolls
should be stable. The correlation length of the pattern decreased rapidly with
increasing control parameter so that the size of a correlated area became much
smaller than the area of the cell. This suggests that the chaotic behavior is
intrinsic to large aspect ratio geometries.Comment: Preprint of experimental paper submitted to Phys. Rev. Lett. May 12
1993. Text is preceeded by many TeX macros. Figures 1 and 2 are rather lon
Singularity in the boundary resistance between superfluid He and a solid surface
We report new measurements in four cells of the thermal boundary resistance
between copper and He below but near the superfluid-transition
temperature . For fits of to the data yielded ,
whereas a fit to theoretical values based on the renormalization-group theory
yielded . Alternatively, a good fit of the theory to the data could
be obtained if the {\it amplitude} of the prediction was reduced by a factor
close to two. The results raise the question whether the boundary conditions
used in the theory should be modified.Comment: 4 pages, 4 figures, revte
Minimal Cosmogenic Neutrinos
The observed flux of ultra-high energy (UHE) cosmic rays (CRs) guarantees the
presence of high-energy cosmogenic neutrinos that are produced via
photo-hadronic interactions of CRs propagating through intergalactic space.
This flux of neutrinos doesn't share the many uncertainties associated with the
environment of the yet unknown CR sources. Cosmogenic neutrinos have
nevertheless a strong model dependence associated with the chemical
composition, source distribution or evolution and maximal injection energy of
UHE CRs. We discuss a lower limit on the cosmogenic neutrino spectrum which
depends on the observed UHE CR spectrum and composition and relates directly to
experimentally observable and model-independent quantities. We show explicit
limits for conservative assumptions about the source evolution.Comment: 6 pages, 3 figure
Power-Law Behavior of Power Spectra in Low Prandtl Number Rayleigh-Benard Convection
The origin of the power-law decay measured in the power spectra of low
Prandtl number Rayleigh-Benard convection near the onset of chaos is addressed
using long time numerical simulations of the three-dimensional Boussinesq
equations in cylindrical domains. The power-law is found to arise from
quasi-discontinuous changes in the slope of the time series of the heat
transport associated with the nucleation of dislocation pairs and roll
pinch-off events. For larger frequencies, the power spectra decay exponentially
as expected for time continuous deterministic dynamics.Comment: (10 pages, 6 figures
Analytic calculations of the spectra of ultra high energy cosmic ray nuclei. II. The general case of background radiation
We discuss the problem of ultra high energy nuclei propagation in
extragalactic background radiations. The present paper is the continuation of
the accompanying paper I where we have presented three new analytic methods to
calculate the fluxes and spectra of ultra high energy cosmic ray nuclei, both
primary and secondary, and secondary protons. The computation scheme in this
paper is based on the analytic solution of coupled kinetic equations, which
takes into account the continuous energy losses due to the expansion of the
universe and pair-production, together with photo-disintegration of nuclei.
This method includes in the most natural way the production of secondary nuclei
in the process of photo-disintegration of the primary nuclei during their
propagation through extragalactic background radiations. In paper I, in order
to present the suggested analytical schemes of calculations, we have considered
only the case of the cosmic microwave background radiation, in the present
paper we generalize this computation to all relevant background radiations,
including infra-red and visible/ultra-violet radiations, collectively referred
to as extragalactic background light. The analytic solutions allow transparent
physical interpretation of the obtained spectra. Extragalactic background light
plays an important role at intermediate energies of ultra high energy cosmic
ray nuclei. The most noticeable effect of the extragalactic background light is
the low-energy tail in the spectrum of secondary nuclei.Comment: The paper is the second part of a two papers series, it is composed
by 25 pages and 16 eps figures, version accepted for publication on
Astroparticle Physic
Wave-number Selection by Target Patterns and Side Walls in Rayleigh-Benard Convection
We present experimental results for Rayleigh-Benard convection patterns in a
cylindrical container with static side-wall forcing induced by a heater. This
forcing stabilized a pattern of concentric rolls (a target pattern) with the
central roll (the umbilicus) at the center of the cell after a jump from the
conduction to the convection state. A quasi-static increase of the control
parameter (epsilon) beyond 0.8 caused the umbilicus of the pattern to move off
center. As observed by others, a further quasi-static increase of epsilon up to
15.6 caused a sequence of transitions. Each transition began with the
displacement of the umbilicus and then proceeded with the loss of one
convection roll at the umbilicus and the return of the umbilicus to a location
near the center of the cell. Alternatively, with decreasing epsilon new rolls
formed at the umbilicus but large umbilicus displacements did not occur. In
addition to quantitative measurements of the umbilicus displacement, we
determined and analyzed the entire wave-director field of each image. The wave
numbers varied in the axial direction, with minima at the umbilicus and at the
cell wall and a maximum at a radial position close to 2/3 Gamma. The wave
numbers at the maximum showed hysteretic jumps at the transitions, but on
average agreed well with the theoretical predictions for the wave numbers
selected in the far field of an infinitely extended target pattern.Comment: ReVTeX, 11 pages, 16 eps figures include
Scaling of thermal conductivity of helium confined in pores
We have studied the thermal conductivity of confined superfluids on a
bar-like geometry. We use the planar magnet lattice model on a lattice with . We have applied open boundary conditions on the bar
sides (the confined directions of length ) and periodic along the long
direction. We have adopted a hybrid Monte Carlo algorithm to efficiently deal
with the critical slowing down and in order to solve the dynamical equations of
motion we use a discretization technique which introduces errors only
in the time step . Our results demonstrate the
validity of scaling using known values of the critical exponents and we
obtained the scaling function of the thermal resistivity. We find that our
results for the thermal resistivity scaling function are in very good agreement
with the available experimental results for pores using the tempComment: 5 two-column pages, 3 figures, Revtex
Framing hydropower as green energy: Assessing drivers, risks and tensions in the Eastern Himalayas
The culturally and ecologically diverse region of the Eastern Himalayas is the target of ambitious hydropower development plans. Policy discourses at national and international levels position this development as synergistically positive: it combines the production of clean energy to fuel economic growth at regional and national levels with initiatives to lift poor mountain communities out of poverty. Different from hydropower development in the 20th century in which development agencies and banks were important players, contemporary initiatives importantly rely on the involvement of private actors, with a prominent role of the private finance sector. This implies that hydropower development is not only financially viable but also understood as highly profitable. This paper examines the new development of hydropower in the Eastern Himalayas of Nepal and India. It questions its framing as green energy, interrogates its links with climate change, and examines its potential for investment and capital accumulation. To do this, we also review the evidence on the extent to which its construction and operation may modify existing hydrogeological processes and ecosystems, as well as its impacts on the livelihoods of diverse groups of people that depend on these. The paper concludes that hydropower development in the region is characterized by inherent contentions and uncertainties, refuting the idea that dams constitute development projects whose impacts can be simply predicted, controlled and mitigated. Indeed, in a highly complex geological, ecological, cultural and political context that is widely regarded to be especially vulnerable to the effects of climate change, hydropower as a development strategy makes for a toxic cocktail
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