362 research outputs found
Space capsule Patent
Manned space capsule configuration for orbital flight and atmospheric reentr
The retarding ion mass spectrometer on dynamics Explorer-A
An instrument designed to measure the details of the thermal plasma distribution combines the ion temperature-determining capability of the retarding potential analyzer with the compositional capabilities of the mass spectrometer and adds multiple sensor heads to sample all directions relative to the spacecraft ram directions. The retarding ion mass spectrometer, its operational modes and calibration are described as well as the data reduction plan, and the anticipated results
Effect of spatial resolution on the estimates of the coherence length of excitons in quantum wells
We evaluate the effect of diffraction-limited resolution of the optical
system on the estimates of the coherence length of 2D excitons deduced from the
interferometric study of the exciton emission. The results are applied for
refining our earlier estimates of the coherence length of a cold gas of
indirect excitons in coupled quantum wells [S. Yang et al., Phys. Rev. Lett.
97, 187402(2006)]. We show that the apparent coherence length is well
approximated by the quadratic sum of the actual exciton coherence length and
the diffraction correction given by the conventional Abbe limit divided by
3.14. In practice, accounting for diffraction is necessary only when the
coherence length is smaller than about one wavelength. The earlier conclusions
regarding the strong enhancement of the exciton coherence length at low
temperatures remain intact.Comment: 6 pages, 5 figure
The generalized 3-edge-connectivity of lexicographic product graphs
The generalized -edge-connectivity of a graph is a
generalization of the concept of edge-connectivity. The lexicographic product
of two graphs and , denoted by , is an important graph
product. In this paper, we mainly study the generalized 3-edge-connectivity of
, and get upper and lower bounds of .
Moreover, all bounds are sharp.Comment: 14 page
Bucolic Complexes
We introduce and investigate bucolic complexes, a common generalization of
systolic complexes and of CAT(0) cubical complexes. They are defined as simply
connected prism complexes satisfying some local combinatorial conditions. We
study various approaches to bucolic complexes: from graph-theoretic and
topological perspective, as well as from the point of view of geometric group
theory. In particular, we characterize bucolic complexes by some properties of
their 2-skeleta and 1-skeleta (that we call bucolic graphs), by which several
known results are generalized. We also show that locally-finite bucolic
complexes are contractible, and satisfy some nonpositive-curvature-like
properties.Comment: 45 pages, 4 figure
Reconciling social norms with personal interests: indigenous styles of identity formation among Pakistani youth
Research on identity formation has been conducted mostly in Western contexts. We extend and complement such research by exploring qualitatively the strategies and styles of identity formation employed by emerging adults in Pakistan. Whereas Western theories of identity formation often provide a negative view of normative orientation as “blind obedience” without exploring alternatives, our thematic analysis of semi-structured interviews with 12 Pakistani emerging adults suggests a much more complex interplay between personal interests and normative influences on identity formation. Participants described various ways of reconciling normative expectations (parental, religious, and cultural) with their personal interests, preferences, and explorations, when deciding about their careers, relationships, and values. In Pakistani culture, normative influences seem to play a more positive and flexible role in identity formation than is suggested by previous Western research
Parametric generation of second sound in superfluid helium: linear stability and nonlinear dynamics
We report the experimental studies of a parametric excitation of a second
sound (SS) by a first sound (FS) in a superfluid helium in a resonance cavity.
The results on several topics in this system are presented: (i) The linear
properties of the instability, namely, the threshold, its temperature and
geometrical dependencies, and the spectra of SS just above the onset were
measured. They were found to be in a good quantitative agreement with the
theory. (ii) It was shown that the mechanism of SS amplitude saturation is due
to the nonlinear attenuation of SS via three wave interactions between the SS
waves. Strong low frequency amplitude fluctuations of SS above the threshold
were observed. The spectra of these fluctuations had a universal shape with
exponentially decaying tails. Furthermore, the spectral width grew continuously
with the FS amplitude. The role of three and four wave interactions are
discussed with respect to the nonlinear SS behavior. The first evidence of
Gaussian statistics of the wave amplitudes for the parametrically generated
wave ensemble was obtained. (iii) The experiments on simultaneous pumping of
the FS and independent SS waves revealed new effects. Below the instability
threshold, the SS phase conjugation as a result of three-wave interactions
between the FS and SS waves was observed. Above the threshold two new effects
were found: a giant amplification of the SS wave intensity and strong resonance
oscillations of the SS wave amplitude as a function of the FS amplitude.
Qualitative explanations of these effects are suggested.Comment: 73 pages, 23 figures. to appear in Phys. Rev. B, July 1 st (2001
Modulational and Parametric Instabilities of the Discrete Nonlinear Schr\"odinger Equation
We examine the modulational and parametric instabilities arising in a
non-autonomous, discrete nonlinear Schr{\"o}dinger equation setting. The
principal motivation for our study stems from the dynamics of Bose-Einstein
condensates trapped in a deep optical lattice. We find that under periodic
variations of the heights of the interwell barriers (or equivalently of the
scattering length), additionally to the modulational instability, a window of
parametric instability becomes available to the system. We explore this
instability through multiple-scale analysis and identify it numerically. Its
principal dynamical characteristic is that, typically, it develops over much
larger times than the modulational instability, a feature that is qualitatively
justified by comparison of the corresponding instability growth rates
Pattern formation in 2-frequency forced parametric waves
We present an experimental investigation of superlattice patterns generated
on the surface of a fluid via parametric forcing with 2 commensurate
frequencies. The spatio-temporal behavior of 4 qualitatively different types of
superlattice patterns is described in detail. These states are generated via a
number of different 3--wave resonant interactions. They occur either as
symmetry--breaking bifurcations of hexagonal patterns composed of a single
unstable mode or via nonlinear interactions between the two primary unstable
modes generated by the two forcing frequencies. A coherent picture of these
states together with the phase space in which they appear is presented. In
addition, we describe a number of new superlattice states generated by 4--wave
interactions that arise when symmetry constraints rule out 3--wave resonances.Comment: The paper contains 34 pages and 53 figures and provides an extensive
review of both the theoretical and experimental work peformed in this syste
Keldysh Green's function approach to coherence in a non-equilibrium steady state: connecting Bose-Einstein condensation and lasing
Solid state quantum condensates often differ from previous examples of
condensates (such as Helium, ultra-cold atomic gases, and superconductors) in
that the quasiparticles condensing have relatively short lifetimes, and so as
for lasers, external pumping is required to maintain a steady state. On the
other hand, compared to lasers, the quasiparticles are generally more strongly
interacting, and therefore better able to thermalise. This leads to questions
of how to describe such non-equilibrium condensates, and their relation to
equilibrium condensates and lasers. This chapter discusses in detail how the
non-equilibrium Green's function approach can be applied to the description of
such a non-equilibrium condensate, in particular, a system of microcavity
polaritons, driven out of equilibrium by coupling to multiple baths. By
considering the steady states, and fluctuations about them, it is possible to
provide a description that relates both to equilibrium condensation and to
lasing, while at the same time, making clear the differences from simple
lasers
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