191 research outputs found
An expressively complete linear time temporal logic for Mazurkiewicz traces
A basic result concerning LTL, the propositional temporal logic of linear time, is that it is expressively complete; it is equal in expressive power to the first order theory of sequences. We present here a smooth extension of this result to the class of partial orders known as Mazurkiewicz traces. These partial orders arise in a variety of contexts in concurrency theory and they provide the conceptual basis for many of the partial order reduction methods that have been developed in connection with LTL-specifications. We show that LTrL, our linear time temporal logic, is equal in expressive power to the first order theory of traces when interpreted over (finite and) infinite traces. This result fills a prominent gap in the existing logical theory of infinite traces. LTrL also provides a syntactic characterisation of the so-called trace consistent (robust) LTL-specifications. These are specifications expressed as LTL formulas that do not distinguish between different linearisations of the same trace and hence are amenable to partial order reduction methods
Reachability for dynamic parametric processes
In a dynamic parametric process every subprocess may spawn arbitrarily many,
identical child processes, that may communicate either over global variables,
or over local variables that are shared with their parent.
We show that reachability for dynamic parametric processes is decidable under
mild assumptions. These assumptions are e.g. met if individual processes are
realized by pushdown systems, or even higher-order pushdown systems. We also
provide algorithms for subclasses of pushdown dynamic parametric processes,
with complexity ranging between NP and DEXPTIME.Comment: 31 page
Formation of diluted III–V nitride thin films by N ion implantation
iluted III–Nₓ–V₁ˍₓ alloys were successfully synthesized by nitrogen implantation into GaAs,InP, and AlyGa1−yAs. In all three cases the fundamental band-gap energy for the ion beam synthesized III–Nₓ–V₁ˍₓ alloys was found to decrease with increasing N implantation dose in a manner similar to that observed in epitaxially grownGaNₓAs1−x and InNₓP₁ˍₓalloys. In GaNₓAs₁ˍₓ the highest value of x (fraction of “active” substitutional N on As sublattice) achieved was 0.006. It was observed that NAs is thermally unstable at temperatures higher than 850 °C. The highest value of x achieved in InNₓP₁ˍₓ was higher, 0.012, and the NP was found to be stable to at least 850 °C. In addition, the N activation efficiency in implantedInNₓP₁ˍₓ was at least a factor of 2 higher than that in GaNₓAs₁ˍₓ under similar processing conditions. AlyGa1−yNₓAs₁ˍₓ had not been made previously by epitaxial techniques. N implantation was successful in producing AlyGa1−yNₓAs₁ˍₓalloys. Notably, the band gap of these alloys remains direct, even above the value of y (y>0.44) where the band gap of the host material is indirect.This work was supported by the ‘‘Photovoltaic Materials
Focus Area’’ in the DOE Center of Excellence for the Synthesis
and Processing of Advanced Materials, Office of Science,
Office of Basic Energy Sciences, Division of Materials
Sciences under U.S. Department of Energy Contract No. DE-ACO3-76SF00098. The work at UCSD was partially supported
by Midwest Research Institute under subcontractor
No. AAD-9-18668-7 from NREL
Variation of elastic scattering across a quantum well
The Drude scattering times of electrons in two subbands of a parabolic
quantum well have been studied at constant electron sheet density and different
positions of the electron distribution along the growth direction. The
scattering times obtained by magnetotransport measurements decrease as the
electrons are displaced towards the well edges, although the lowest-subband
density increases. By comparing the measurements with calculations of the
scattering times of a two-subband system, new information on the location of
the relevant scatterers and the anisotropy of intersubband scattering is
obtained. It is found that the scattering time of electrons in the lower
subband depends sensitively on the position of the scatterers, which also
explains the measured dependence of the scattering on the carrier density. The
measurements indicate segregation of scatterers from the substrate side towards
the quantum well during growth.Comment: 4 pages, 4 figure
Completeness for Flat Modal Fixpoint Logics
This paper exhibits a general and uniform method to prove completeness for
certain modal fixpoint logics. Given a set \Gamma of modal formulas of the form
\gamma(x, p1, . . ., pn), where x occurs only positively in \gamma, the
language L\sharp (\Gamma) is obtained by adding to the language of polymodal
logic a connective \sharp\_\gamma for each \gamma \epsilon. The term
\sharp\_\gamma (\varphi1, . . ., \varphin) is meant to be interpreted as the
least fixed point of the functional interpretation of the term \gamma(x,
\varphi 1, . . ., \varphi n). We consider the following problem: given \Gamma,
construct an axiom system which is sound and complete with respect to the
concrete interpretation of the language L\sharp (\Gamma) on Kripke frames. We
prove two results that solve this problem. First, let K\sharp (\Gamma) be the
logic obtained from the basic polymodal K by adding a Kozen-Park style fixpoint
axiom and a least fixpoint rule, for each fixpoint connective \sharp\_\gamma.
Provided that each indexing formula \gamma satisfies the syntactic criterion of
being untied in x, we prove this axiom system to be complete. Second,
addressing the general case, we prove the soundness and completeness of an
extension K+ (\Gamma) of K\_\sharp (\Gamma). This extension is obtained via an
effective procedure that, given an indexing formula \gamma as input, returns a
finite set of axioms and derivation rules for \sharp\_\gamma, of size bounded
by the length of \gamma. Thus the axiom system K+ (\Gamma) is finite whenever
\Gamma is finite
Diluted II-VI Oxide Semiconductors with Multiple Band Gaps
We report the realization of a new multi-band-gap semiconductor. The highly
mismatched alloy Zn1-yMnyOxTe1-x has been synthesized using the combination of
oxygen ion implantation and pulsed laser melting. Incorporation of small
quantities of isovalent oxygen leads to the formation of a narrow,
oxygen-derived band of extended states located within the band gap of the
Zn1-yMnyTe host. When only 1.3% of Te atoms is replaced with oxygen in a
Zn0.88Mn0.12Te crystal (with band gap of 2.32 eV) the resulting band structure
consists of two direct band gaps with interband transitions at ~1.77 eV and 2.7
eV. This remarkable modification of the band structure is well described by the
band anticrossing model in which the interactions between the oxygen-derived
band and the conduction band are considered. With multiple band gaps that fall
within the solar energy spectrum, Zn1-yMnyOxTe1-x is a material perfectly
satisfying the conditions for single-junction photovoltaics with the potential
for power conversion efficiencies surpassing 50%.Comment: 12 pages, 4 figure
Efficient Emptiness Check for Timed B\"uchi Automata (Extended version)
The B\"uchi non-emptiness problem for timed automata refers to deciding if a
given automaton has an infinite non-Zeno run satisfying the B\"uchi accepting
condition. The standard solution to this problem involves adding an auxiliary
clock to take care of the non-Zenoness. In this paper, it is shown that this
simple transformation may sometimes result in an exponential blowup. A
construction avoiding this blowup is proposed. It is also shown that in many
cases, non-Zenoness can be ascertained without extra construction. An
on-the-fly algorithm for the non-emptiness problem, using non-Zenoness
construction only when required, is proposed. Experiments carried out with a
prototype implementation of the algorithm are reported.Comment: Published in the Special Issue on Computer Aided Verification - CAV
2010; Formal Methods in System Design, 201
Magnetotransport properties of a polarization-doped three-dimensional electron slab
We present evidence of strong Shubnikov-de-Haas magnetoresistance
oscillations in a polarization-doped degenerate three-dimensional electron slab
in an AlGaN semiconductor system. The degenerate free carriers
are generated by a novel technique by grading a polar alloy semiconductor with
spatially changing polarization. Analysis of the magnetotransport data enables
us to extract an effective mass of and a quantum
scattering time of . Analysis of scattering processes helps
us extract an alloy scattering parameter for the AlGaN material
system to be
Метод расчета теплообмена излучением в топке осесимметричной конфигурации на основе уравнений для компонент суммарного вектора потока лучистой энергии. Инженерная методика
На основе системы уравнений первой части статьи, получены разностные уравнения квазиодномерного метода расчета теплообмена излучением в топке котла осесимметричной конфигурации. Записаны уравнения и разностные формулы метода инженерного расчета теплообмена излучением при наличии экранной сетки около поверхности горелки. Методика пригодна для использования в инженерных расчетах при определении оптимальной конфигурации топки и определении оптимального расстояния экранной сетки до поверхности горелки
High resistivity and ultrafast carrier lifetime in argon implanted GaAs
We have investigated the optoelectronic and structural properties of GaAs that has been implanted with Ar ions and subsequently annealed. The material exhibits all the basic optical and electronic characteristics typically observed in nonstoichiometric, As implanted or low‐temperature‐grown GaAs. Annealing of Ar implanted GaAs at 600 °C produces a highly resistive material with a subpicosecond trapping lifetime for photoexcited carriers. Transmission electron microscopy shows that, instead of As precipitates, characteristic for the nonstoichiometeric GaAs, voids ranging in size from 3 to 5 nm are observed in Ar implanted and annealed GaAs. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69637/2/APPLAB-69-17-2569-1.pd
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