11,353 research outputs found
A molecular-dynamics algorithm for mixed hard-core/continuous potentials
We present a new molecular-dynamics algorithm for integrating the equations
of motion for a system of particles interacting with mixed continuous/impulsive
forces. This method, which we call Impulsive Verlet, is constructed using
operator splitting techniques similar to those that have been used successfully
to generate a variety molecular-dynamics integrators. In numerical experiments,
the Impulsive Verlet method is shown to be superior to previous methods with
respect to stability and energy conservation in long simulations.Comment: 18 pages, 6 postscript figures, uses rotate.st
Fresh-Register Automata
What is a basic automata-theoretic model of computation with names and fresh-name generation? We introduce Fresh-Register Automata (FRA), a new class of automata which operate on an infinite alphabet of names and use a finite number of registers to store fresh names, and to compare incoming names with previously stored ones. These finite machines extend Kaminski and Francez’s Finite-Memory Automata by being able to recognise globally fresh inputs, that is, names fresh in the whole current run. We exam-ine the expressivity of FRA’s both from the aspect of accepted languages and of bisimulation equivalence. We establish primary properties and connections between automata of this kind, and an-swer key decidability questions. As a demonstrating example, we express the theory of the pi-calculus in FRA’s and characterise bisimulation equivalence by an appropriate, and decidable in the finitary case, notion in these automata
Induced radioactivity in LDEF components
A systematic study of the induced radioactivity of the Long Duration Exposure Facility (LDEF) is being carried out in order to gather information about the low earth orbit radiation environment and its effects on materials. The large mass of the LDEF spacecraft, its stabilized configuration, and long mission duration have presented an opportunity to determine space radiation-induced radioactivities with a precision not possible before. Data presented include preliminary activities for steel and aluminum structural samples, and activation subexperiment foils. Effects seen in the data show a clear indication of the trapped proton anisotropy in the South Atlantic Anomaly and suggest contributions from different sources of external radiation fluxes
Induced activation study of LDEF
Analysis of the induced radioactivity of the Long Duration Exposure Facility (LDEF) is continuing with extraction of specific activities for various spacecraft materials. Data and results of activation measurements from eight facilities are being collected for interpretation at Eastern Kentucky University and NASA/Marshall Space Flight Center. The major activation mechanism in LDEF components is the proton flux in the South Atlantic Anomaly (SAA). This flux is highly anisotropic, and could be sampled by taking advantage of the gravity-gradient stabilization of the LDEF. The directionally-dependent activation due to these protons was clearly observed in the data from aluminum experiment tray clamps (reaction product Na-22), steel trunnions (reaction product Mn-54 and others) and is also indicated by the presence of a variety of nuclides in other materials. A secondary production mechanism, thermal neutron capture, was observed in cobalt, indium, and tantalum, which are known to have large capture cross sections. Experiments containing samples of these metals and significant amounts of thermalizing low atomic number (Z) material showed clear evidence of enhanced activation of Co-60, In-114m, and Ta-182. Other mechanisms which activate spacecraft material that are not as easily separable from SAA proton activation, such as galactic proton bombardment and secondary production by fast neutrons, are being investigated by comparison to radiation environmental calculations. Deviations from one-dimensional radiation models indicate that these mechanisms are more important at greater shielding depths. The current status of the induced radioactivity measurements as of mid-year 1992 are reviewed. Specific activities for a number of materials which show SAA effects and thermal neutron capture are presented. The results for consistency by combining data from the participating institutions is also examined
Collection, analysis, and archival of LDEF activation data
The study of the induced radioactivity of samples intentionally placed aboard the Long Duration Exposure Facility (LDEF) and samples obtained from the LDEF structure is reviewed. The eight laboratories involved in the gamma-ray counting are listed and the scientists and the associated counting facilities are described. Presently, most of the gamma-ray counting has been completed and the spectra are being analyzed and corrected for efficiency and self absorption. The acquired spectra are being collected at Eastern Kentucky University for future reference. The results of these analyses are being compiled and reviewed for possible inconsistencies as well as for comparison with model calculations. These model calculations are being revised to include the changes in trapped-proton flux caused by the onset of the period of maximum solar activity and the rapidly decreasing spacecraft orbit. Tentative plans are given for the storage of the approximately 1000 gamma-ray spectra acquired in this study and the related experimental data
Carbon nanotube quantum dots on hexagonal boron nitride
We report the fabrication details and low-temperature characteristics of the
first carbon nanotube (CNT) quantum dots on flakes of hexagonal boron nitride
(hBN) as substrate. We demonstrate that CNTs can be grown on hBN by standard
chemical vapor deposition and that standard scanning electron microscopy
imaging and lithography can be employed to fabricate nanoelectronic structures
when using optimized parameters. This proof of concept paves the way to more
complex devices on hBN, with more predictable and reproducible characteristics
and electronic stability.Comment: 4 pages, 4 figure
Measurement of Temporal Correlations of the Overhauser Field in a Double Quantum Dot
In quantum dots made from materials with nonzero nuclear spins, hyperfine
coupling creates a fluctuating effective Zeeman field (Overhauser field) felt
by electrons, which can be a dominant source of spin qubit decoherence. We
characterize the spectral properties of the fluctuating Overhauser field in a
GaAs double quantum dot by measuring correlation functions and power spectra of
the rate of singlet-triplet mixing of two separated electrons. Away from zero
field, spectral weight is concentrated below 10 Hz, with 1/f^2 dependence on
frequency, f. This is consistent with a model of nuclear spin diffusion, and
indicates that decoherence can be largely suppressed by echo techniques.Comment: related papers available at http://marcuslab.harvard.ed
Effect of Exchange Interaction on Spin Dephasing in a Double Quantum Dot
We measure singlet-triplet dephasing in a two-electron double quantum dot in
the presence of an exchange interaction which can be electrically tuned from
much smaller to much larger than the hyperfine energy. Saturation of dephasing
and damped oscillations of the spin correlator as a function of time are
observed when the two interaction strengths are comparable. Both features of
the data are compared with predictions from a quasistatic model of the
hyperfine field.Comment: see related papers at http://marcuslab.harvard.ed
Coherent spin manipulation in an exchange-only qubit
Initialization, manipulation, and measurement of a three-spin qubit are
demonstrated using a few-electron triple quantum dot, where all operations can
be driven by tuning the nearest-neighbor exchange interaction. Multiplexed
reflectometry, applied to two nearby charge sensors, allows for qubit readout.
Decoherence is found to be consistent with predictions based on gate voltage
noise with a uniform power spectrum. The theory of the exchange-only qubit is
developed and it is shown that initialization of only two spins suffices for
operation. Requirements for full multi-qubit control using only exchange and
electrostatic interactions are outlined.Comment: related work at http://marcuslab.harvard.ed
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