257 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
Carrier dynamics and coherent acoustic phonons in nitride heterostructures
We model generation and propagation of coherent acoustic phonons in
piezoelectric InGaN/GaN multi-quantum wells embedded in a \textit{pin} diode
structure and compute the time resolved reflectivity signal in simulated
pump-probe experiments. Carriers are created in the InGaN wells by ultrafast
pumping below the GaN band gap and the dynamics of the photoexcited carriers is
treated in a Boltzmann equation framework. Coherent acoustic phonons are
generated in the quantum well via both deformation potential electron-phonon
and piezoelectric electron-phonon interaction with photogenerated carriers,
with the latter mechanism being the dominant one. Coherent longitudinal
acoustic phonons propagate into the structure at the sound speed modifying the
optical properties and giving rise to a giant oscillatory differential
reflectivity signal. We demonstrate that coherent optical control of the
differential reflectivity can be achieved using a delayed control pulse.Comment: 14 pages, 11 figure
10252 Abstracts Collection -- Game Semantics and Program Verification
From 20th to 25th June 2010, the Dagstuhl Seminar
"Game Semantics and Program Verification\u27\u27 was held
in Schloss Dagstuhl - Leibniz Center for Informatics.
During the seminar, several participants presented their current
research, and ongoing work and open problems were discussed.
Abstracts of the presentations given during the seminar
as well as abstracts of seminar results and ideas are put
together in this paper. The first section
describes the seminar topics and goals in general.
Links to extended abstracts or full papers are provided, if available
10252 Executive Summary -- Game Semantics and Program Verification
The seminar took place from 20th until 25th June 2010.
Its primary aim was to foster interaction between researchers
working on modelling programs/proofs using games and
the verification community. The meeting brought together
28 researchers from eight different countries,
both junior and senior, for a systematic assessment of what
the two areas have to offer to one another,
critical evaluation of what has been achieved so far,
with a view to establishing common research goals for the future
Effect of Native Defects on Optical Properties of InxGa1-xN Alloys
The energy position of the optical absorption edge and the free carrier
populations in InxGa1-xN ternary alloys can be controlled using high energy
4He+ irradiation. The blue shift of the absorption edge after irradiation in
In-rich material (x > 0.34) is attributed to the band-filling effect
(Burstein-Moss shift) due to the native donors introduced by the irradiation.
In Ga-rich material, optical absorption measurements show that the
irradiation-introduced native defects are inside the bandgap, where they are
incorporated as acceptors. The observed irradiation-produced changes in the
optical absorption edge and the carrier populations in InxGa1-xN are in
excellent agreement with the predictions of the amphoteric defect model
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
Verifying higher-order concurrency with data automata
Using a combination of automata-theoretic and game-semantic techniques, we propose a method for analysing higher-order concurrent programs. Our language of choice is Finitary Idealised Concurrent Algol (FICA) due to its relatively simple fully abstract game model.Our first contribution is an automata model over a tree-structured infinite data alphabet, called split automata, whose distinctive feature is the separation of control and memory. We show that every FICA term can be translated into such an automaton. Thanks to the structure of split automata, we are able to observe subtle aspects of the underlying game semantics.This enables us to identify a fragment of FICA with iteration and limited synchronisation (but without recursion), for which, in contrast to the whole FICA, a variety of verification problems turn out to be decidable
Fundamental Curie temperature limit in ferromagnetic GaMnAs
We provide experimental evidence that the upper limit of ~110 K commonly
observed for the Curie temperature T_C of Ga(1-x)Mn(x)As is caused by the
Fermi-level-induced hole saturation. Ion channeling, electrical and
magnetization measurements on a series of Ga(1-x-y)Mn(x)Be(y)As layers show a
dramatic increase of the concentration of Mn interstitials accompanied by a
reduction of T_C with increasing Be concentration, while the free hole
concentration remains relatively constant at ~5x10^20 cm^-3. These results
indicate that the concentrations of free holes and ferromagnetically active Mn
spins are governed by the position of the Fermi level, which controls the
formation energy of compensating interstitial Mn donors.Comment: 14 pages, 3 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
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