Regis-Darwin specified in the p-Calculus

There now is a translator for DARWIN programs that automatically generates their π-calculus equivalents. A variety of errors in DARWIN programs can be detected at the π-calculus level. These include detection of recursive structures, unbound ports and ports that are bound in the wrong direction. It can also be used to confirm whether two REGIS-DARWIN programs are equivalent

Synthetic Chemotaxis and Collective Behavior in Active Matter

Conspectus: The ability to navigate in chemical gradients, called chemotaxis, is crucial for the survival of microorganisms. It allows them to find food and to escape from toxins. Many microorganisms can produce the chemicals to which they respond themselves and use chemotaxis for signalling which can be seen as a basic form of communication. Remarkably, the past decade has let to the development of synthetic microswimmers like e.g. autophoretic Janus colloids, which can self-propel through a solvent, analogously to bacteria and other microorganims. The mechanism underlying their self-propulsion involves the production of certain chemicals. The same chemicals involved in the self-propulsion mechanism also act on other microswimmers and bias their swimming direction towards (or away from) the producing microswimmer. Synthetic microswimmers therefore provide a synthetic analogue to chemotactic motile microorganisms. When these interactions are attractive, they commonly lead to clusters, even at low particle density. These clusters may either proceed towards macrophase separation, resembling Dictyostelium aggregation, or, as shown very recently, lead to dynamic clusters of self-limited size (dynamic clustering) as seen in experiments in autophoretic Janus colloids. Besides the classical case where chemical interactions are attractive, this Account discusses, as its main focus, repulsive chemical interactions, which can create a new and less known avenue to pattern formation in active systems leading to a variety of pattern, including clusters which are surrounded by shells of chemicals, travelling waves and more complex continously reshaping patterns. In all these cases `synthetic signalling' can crucially determine the collective behavior of synthetic microswimmer ensembles and can be used as a design principle to create patterns in motile active particles

System structuring: a convergence of theory and practice?

Darwin is a general purpose structuring tool of use in building complex distributed systems from diverse components and diverse component interaction mechanisms. It is in essence a declarative binding language which can be used to define hierarchic compositions of interconnected components. Distribution is dealt with orthogonally to system structuring. The language allows the specification of both static structures and dynamic structures which evolve during execution. The central abstractions managed by Darwin are components and services. Bindings are formed by manipulating references to services. The paper describes the operational semantics of Darwin in terms of the pi-calculus, MilnerÆs calculus of mobile processes. The correspondence between the treatment of names in the pi-calculus and the management of service references in Darwin leads to an elegant and concise pi-calculus model of DarwinÆs operational semantics. The model has proved useful in arguing the correctness of Darwin implementations and in designing extensions to Darwin and reasoning about their behaviour. The paper discusses the reasons why other formalisms fail to capture elegantly the system structuring concepts on which Darwin is based

Developing an undergraduate software engineering degree

As those who have done it can attest, developing an undergraduate degree in software engineering is a daunting and challenging task, and there have been instances where a department has tried, but failed to get its program approved. A strong desire to develop a program in software engineering together with interested faculty may not be enough to build a credible degree, let alone a curriculum that will be approved by all the administrative and State organizations who may have a say in it .This panel brings together a group whose experience in developing software engineering degrees at their respective institutions may be helpful to those thinking about doing so. Each member of the group will describe his/her experiences in developing an undergraduate program in software engineering and address key issues and problems that should be considered in any such effort. There will also be ample opportunity for interaction among the participants

Ulta-slow relaxation in discontinuous-film based electron glasses

We present field effect measurements on discontinuous 2D thin films which are composed of a sub monolayer of nano-grains of Au, Ni, Ag or Al. Like other electron glasses these systems exhibit slow conductance relaxation and memory effects. However, unlike other systems, the discontinuous films exhibit a dramatic slowing down of the dynamics below a characteristic temperature $T^*$. $T^*$ is typically between 10-50K and is sample dependent. For $T<T^*$ the sample exhibits a few other peculiar features such as repeatable conductance fluctuations in millimeter size samples. We suggest that the enhanced system sluggishness is related to the current carrying network becoming very dilute in discontinuous films so that the system contains many parts which are electrically very weakly connected and the transport is dominated by very few weak links. This enables studying the glassy properties of the sample as it transitions from a macroscopic sample to a mesocopic sample, hence, the results provide new insight on the underlying physics of electron glasses.Comment: 4 pages, 4 figure

Holistic specifications for robust programs

Functional specifications describe what program components can do: the sufficient conditions to invoke components' operations. They allow us to reason about the use of components in a closed world setting, where components interact with known client code, and where the client code must establish the appropriate pre-conditions before calling into a component. Sufficient conditions are not enough to reason about the use of components in an \emph{open world} setting, where components interact with external code, possibly of unknown provenance, and where components may evolve over time. In this open world setting, we must also consider the possible external code. \emph{necessary} conditions, i.e, what are the conditions without which an effect will not happen. In this paper we propose the Chainmail specification language for writing {holistic specifications that focus on necessary conditions (as well as sufficient conditions). We give a formal semantics for \Chainmail, and discuss several examples. The core of \Chainmail has been mechanised in the Coq proof assistant

Improved quality GaN by growth on compliant silicon-on-insulator substrates using metalorganic chemical vapor deposition

The use of compliant silicon-on-insulator (SOI) substrates instead of Si substrates is shown to improve the quality of epitaxial GaN layers by releasing the strain and absorbing the generated threading dislocations in the thin Si overlay of the SOI substrate. GaN layers have been grown on SOI substrates by low-pressure metalorganic chemical vapor deposition and various growth conditions and compared with GaN layers grown on Si substrates. Crystal uniformity, surface morphology, and number of threading dislocations of GaN layers grown on SOI substrates are improved compared to layers grown directly on Si substrates as evidenced by x-ray diffraction spectroscopy (XRD) and transmission electron microscopy. Full width at half maximum XRD values improved from 672 to 378 arcsec by growth on SOI instead of Si substrates. The GaN layers grown directly on Si substrates are highly resistive while all as-grown GaN layers on SOI substrates are unintentionally nn type. For a 1–2 μm thick GaN layer grown on SOI, the electron mobility is typically in the order of 102 cm2/V s102cm2/Vs with a background carrier concentration in the range of 1.5–5.0×1017 cm−3.1.5–5.0×1017cm−3. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70972/2/JAPIAU-83-7-3829-1.pd

Extending SHAPES for SIMD architectures

SIMD (Single Instruction, Multiple Data) instruction sets are ubiquitous on modern hardware, but rarely used in software projects. A major reason for this is that efficient SIMD code requires data to be laid out in memory in an unconventional manner, forcing developers to explicitly refactor their code and data structures in order to make use of SIMD. In previous work, we proposed SHAPES, an abstract layout specification for enabling memory optimisations for managed, object-oriented languages. In this paper, we explain how, by extending SHAPES with well-known constructs from the literature, which are not specific to SIMD, we can extend SHAPES to compile programs to use SIMD instructions. The resulting language (sketch) seems able to exploit SIMD capabilities without sacrificing ease of development