157 research outputs found
BKT universality class in the presence of correlated disorder
The correct detection of the Berezinskii-Kosterlitz-Thouless (BKT) transition
in quasi-two-dimensional superconductors still remains a controversial issue.
Its main signatures, indeed, are often at odds with the theoretical
expectations. In a recent work\cite{me} we have shown that the presence of
spatially correlated disorder plays a key role in this sense being it the
reason underlying the experimentally-observed smearing of the universal
superfluid-density jump. In the present paper, we closely investigate the
effects of correlated disorder on the BKT transition, addressing specifically
the issue whether it changes or not the BKT universality class.Comment: The present paper has already been submitted as Special Issue
"Proceedings of the conference SuperFluctuations 2017
Broadening of the Berezinskii-Kosterlitz-Thouless transition by correlated disorder
The Berezinskii-Kosterlitz-Thouless (BKT) transition in two-dimensional superconductors is usually expected to be protected against disorder. However, its fingerprints in a real system, such as, e.g., the universal superfluid- density jump, are often at odds with this expectation. Here, we show that the disorder-induced granularity of the superconducting state modifies the nucleation mechanism for vortex-antivortex pairs. This leads to a considerable smearing of the universal superfluid-density jump as compared to the paradigmatic clean case, in agreement with experimental observations
Secure Multiparty Sessions with Topics
Multiparty session calculi have been recently equipped with security
requirements, in order to guarantee properties such as access control and leak
freedom. However, the proposed security requirements seem to be overly
restrictive in some cases. In particular, a party is not allowed to communicate
any kind of public information after receiving a secret information. This does
not seem justified in case the two pieces of information are totally unrelated.
The aim of the present paper is to overcome this restriction, by designing a
type discipline for a simple multiparty session calculus, which classifies
messages according to their topics and allows unrestricted sequencing of
messages on independent topics.Comment: In Proceedings PLACES 2016, arXiv:1606.0540
Self-Adaptation and Secure Information Flow in Multiparty Structured Communications: A Unified Perspective
We present initial results on a comprehensive model of structured
communications, in which self- adaptation and security concerns are jointly
addressed. More specifically, we propose a model of self-adaptive, multiparty
communications with secure information flow guarantees. In this model, security
violations occur when processes attempt to read or write messages of
inappropriate security levels within directed exchanges. Such violations
trigger adaptation mechanisms that prevent the violations to occur and/or to
propagate their effect in the choreography. Our model is equipped with local
and global mechanisms for reacting to security violations; type soundness
results ensure that global protocols are still correctly executed, while the
system adapts itself to preserve security.Comment: In Proceedings BEAT 2014, arXiv:1408.556
Reactive concurrent programming revisited
In this note we revisit the so-called reactive programming style, which
evolves from the synchronous programming model of the Esterel language by
weakening the assumption that the absence of an event can be detected
instantaneously. We review some research directions that have been explored
since the emergence of the reactive model ten years ago. We shall also outline
some questions that remain to be investigated
An algebraic characterization of observational equivalence
AbstractWe show that observational equivalence can be characterized by saturating homomorphisms (with respect to Hennessy-Milner logic), thus bringing together results developed independently by Castellani and by Arnold and Dicky on characterizations of transition system equivalences. We take this opportunity to compare Castellani's abstraction homomorphisms and Arnold-Dicky's saturating homomorphisms. It turns out that they are very similar notions: their difference in formulation is partly due to the fact that abstraction homomorphisms were defined on a restricted class of transition systems
State-oriented noninterference for CCS
We address the question of typing noninterference (NI) in Milner's Calculus of Communicating Systems (CCS), in such a way that Milner's translation of a standard parallel imperative language into CCS preserves both an existing NI property and the associated type system. Recently, Focardi, Rossi and Sabelfeld have shown that a variant of Milner's translation, restricted to the sequential fragment of the language, maps a time-sensitive NI property to that of Persistent Bisimulation-based Non Deducibility on Compositions (PBNDC) on CCS. However, since CCS was not equipped with a security type system, the question of whether the translation preserves types could not be addressed. We extend Focardi, Rossi and Sabelfeld's result by showing that a slightly different variant of Milner's translation preserves a time-insensitive NI property on the full parallel language, by mapping it again to PBNDC. As a by-product, we formalise a folklore result, namely that Milner's translation preserves a natural behavioural equivalence on programs. We present a type system ensuring the PBNDC-property on CCS, inspired from type systems for the pi-calculus. Unfortunately, this type system as it stands is too restrictive to grant the expected type preservation result. We sketch a solution to overcome this problem
Disordered XY model: effective medium theory and beyond
We study the effect of uncorrelated random disorder on the temperature
dependence of the superfluid stiffness in the two-dimensional classical XY
model. By means of a perturbative expansion in the disorder potential,
equivalent to the T-matrix approximation, we provide an extension of the
effective-medium-theory result able to describe the low-temperature stiffness,
and its separate diamagnetic and paramagnetic contributions. These analytical
results provide an excellent description of the Monte Carlo simulations for two
prototype examples of uncorrelated disorder. Our findings offer an interesting
perspective on the effects of quenched disorder on longitudinal phase
fluctuations in two-dimensional superfluid systems.Comment: 11 pages, 5 figure
Report on WS25CCC Workshop "25 Years of Combining Compositionality and Concurrency"
International audienceIn the peak of the summer 2013, between the 7th and the 9th of August, the workshop "25 Years of Combining Compositionality and Concurrency" took place in Königswinter, a picturesque little town overlooking the Rhine river, in the outskirts of Bonn. The event, organised by Ursula Goltz, Rob van Glabbeek and Ernst-Rüdiger Olderog, was meant to celebrate and revisit, a quarter of a century later, the workshop "Combining Compositionality and Concurrency" (CCC88) that had been held in March 1988 in the same hotel, the Loreley, on the initiative of the same trio of researchers (the first two of which were still PhD students at the time). Both workshops were by invitation only, and each attracted 34 participants. Because of its timely character and its deliberate focus on bridging the gap be-tween process calculi and "true-concurrency" models, the original CCC88 workshop, targeting a group of active researchers from both fields, had generated much enthusiasm and discussion. It had therefore gradually acquired, at least in the memories of its participants, the mythical status of a "foundational event". It was then quite natural for the organisers, 25 years later, to envisage a kind of jubilee event, which could bring together a number of participants from the original workshop, as well as younger researchers who had joined in more recent years the field of concurrency theory, now much broader and well-established
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