7,472 research outputs found
Concurrent constraint programming with process mobility
We propose an extension of concurrent constraint programming with primitives for process migration within a hierarchical network, and we study its semantics. To this purpose, we first investigate a "pure " paradigm for process migration, namely a paradigm where the only actions are those dealing with transmissions of processes. Our goal is to give a structural definition of the semantics of migration; namely, we want to describe the behaviour of the system, during the transmission of a process, in terms of the behaviour of the components. We achieve this goal by using a labeled transition system where the effects of sending a process, and requesting a process, are modeled by symmetric rules (similar to handshaking-rules for synchronous communication) between the two partner nodes in the network. Next, we extend our paradigm with the primitives of concurrent constraint programming, and we show how to enrich the semantics to cope with the notions of environment and constraint store. Finally, we show how the operational semantics can be used to define an interpreter for the basic calculus.
Inelastic neutron and x-ray scattering as probes of the sign structure of the Fe-pnictide superconducting gap
Neutron spin-flip scattering observations of a resonance in the
superconducting state is often taken as evidence of an unconventional
superconducting state in which the gap changes sign
for momentum transfers which play an important role in the pairing.
Recently questions regarding this identification for the Fe-pnictide
superconductors have been raised and it has been suggested that
. Here we propose that inelastic neutron or x-ray
scattering measurements of the spectral weight of a phonon of momentum can
distinguish between these two pairing scenarios.Comment: 4 pages, 4 figure
Exciton binding energies in carbon nanotubes from two-photon photoluminescence
One- and two-photon luminescence excitation spectroscopy showed a series of
distinct excitonic states in single-walled carbon nanotubes. The energy
splitting between one- and two-photon-active exciton states of different
wavefunction symmetry is the fingerprint of excitonic interactions in carbon
nanotubes. We determine exciton binding energies of 0.3-0.4 eV for different
nanotubes with diameters between 0.7 and 0.9 nm. Our results, which are
supported by ab-initio calculations of the linear and non-linear optical
spectra, prove that the elementary optical excitations of carbon nanotubes are
strongly Coulomb-correlated, quasi-one dimensionally confined electron-hole
pairs, stable even at room temperature. This alters our microscopic
understanding of both the electronic structure and the Coulomb interactions in
carbon nanotubes, and has direct impact on the optical and transport properties
of novel nanotube devices.Comment: 5 pages, 4 figure
Rotation Rate of Saturn's Magnetosphere using CAPS Plasma Measurements
We present the present status of an investigation of the rotation rate of Saturn's magnetosphere using a 3D velocity moment technique being developed at Goddard which is similar to the 2D version used by Sittler et al. for SOI and similar to that used by Thomsen et al.. This technique allows one to nearly cover the full energy range of the Cassini Plasma Spectrometer (CAPS) IMS from 1 V . E/Q < 50 kV. Since our technique maps the observations into a local inertial frame, it does work during roll maneuvers. We make comparisons with the bi-Maxwellian fitting technique developed by Wilson et al. and the similar velocity moment technique by Thomsen et al. . We concentrate our analysis when ion composition data is available, which is used to weight the non-compositional data, referred to as singles data, to separate H+, H2+ and water group ions (W+) from each other. The chosen periods have high enough telemetry rates (4 kbps or higher) so that coincidence ion data, similar to that used by Sittler et al. for SOI is available. The ion data set is especially valuable for measuring flow velocities for protons, which are more difficult to derive using singles data within the inner magnetosphere, where the signal is dominated by heavy ions (i.e., proton peak merges with W+ peak as low energy shoulder). Our technique uses a flux function, which is zero in the proper plasma flow frame, to estimate fluid parameter uncertainties. The comparisons investigate the experimental errors and potential for systematic errors in the analyses, including ours. The rolls provide the best data set when it comes to getting 4PI coverage of the plasma but are more susceptible to time aliasing effects. In the future we will then make comparisons with magnetic field observations, Saturn ionosphere conductivities as presently known and the field aligned currents necessary for the planet to enforce corotation of the rotating plasma
Raman imaging and electronic properties of graphene
Graphite is a well-studied material with known electronic and optical
properties. Graphene, on the other hand, which is just one layer of carbon
atoms arranged in a hexagonal lattice, has been studied theoretically for quite
some time but has only recently become accessible for experiments. Here we
demonstrate how single- and multi-layer graphene can be unambiguously
identified using Raman scattering. Furthermore, we use a scanning Raman set-up
to image few-layer graphene flakes of various heights. In transport experiments
we measure weak localization and conductance fluctuations in a graphene flake
of about 7 monolayer thickness. We obtain a phase-coherence length of about 2
m at a temperature of 2 K. Furthermore we investigate the conductivity
through single-layer graphene flakes and the tuning of electron and hole
densities via a back gate
Correction to “Interchange instability in the inner magnetosphere associated with geosynchronous particle flux decreases”
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94678/1/grl17909.pd
Continuous quantum nondemolition feedback and unconditional atomic spin squeezing
We discuss the theory and experimental considerations of a quantum feedback
scheme for producing deterministically reproducible spin squeezing. Continuous
nondemolition atom number measurement from monitoring a probe field
conditionally squeezes the sample. Simultaneous feedback of the measurement
results controls the quantum state such that the squeezing becomes
unconditional. We find that for very strong cavity coupling and a limited
number of atoms, the theoretical squeezing approaches the Heisenberg limit.
Strong squeezing will still be produced at weaker coupling and even in free
space (thus presenting a simple experimental test for quantum feedback). The
measurement and feedback can be stopped at any time, thereby freezing the
sample with a desired amount of squeezing.Comment: 17 pages, 5 figures, submitted to JP
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