Response of Glass and Liquid Phases in the Vortex lattice to an external AC magnetic field at different frequencies

We individuated a method to distinguish a glass phase from a highly viscous liquid phase in a lattice of vortices, established in type-two superconductors. Our analysis is based on the study of the temperature dependence of numerically obtained 1st and 3rd harmonics curves of the AC magnetic susceptibility, by changing the frequency of the applied AC magnetic field. The harmonics are obtained by integrating the non-linear diffusion equation for the magnetic field, with different voltage-current characteristics, corresponding to the two different phases. This method could be applied to the analysis of experimental curves in order to determine if the detected magnetic response of superconducting samples may be interpreted in terms of vortex glass or Kim-Anderson model.Comment: 6 pages, 5 figures, to be published on Physica

Competition between intrinsic and extrinsic effects in the quenching of the superconducting state in FeSeTe thin films

We report the first experimental observation of the quenching of the superconducting state in current-voltage characteristics of an iron-based superconductor, namely, in FeSeTe thin films. Based on available theoretical models, our analysis suggests the presence of an intrinsic flux-flow electronic instability along with non-negligible extrinsic thermal effects. The coexistence and competition of these two mechanisms classify the observed instability as halfway between those of low-temperature and of high-temperature superconductors, where thermal effects are respectively largely negligible or predominant.Comment: 7 pages, 5 figures, fixed typo

Nonuniversal temperature dependencies of the low-frequency ac magnetic susceptibility in high-T c superconductors

The complex ac magnetic susceptibilities ({\ensuremath{\chi}}_{n}={\ensuremath{\chi}}_{n}^{\ensuremath{'}}+i{\ensuremath{\chi}}_{n}^{\ensuremath{''}}) of high-${T}_{c}$ superconductors in absence of dc fields have been studied by numerically solving the nonlinear diffusion equation for the magnetic flux, where the diffusivity is determined by the resistivity. In our approach the parallel resistor model between the creep and flux flow resistivities is used, so that the crossover between different flux dynamic processes (thermally activated flux flow, flux creep, flux flow) can naturally arise. For this reason we remark that, as the frequency increases, the presence of a different nonlinearity in different regions of the I\ensuremath{-}V characteristic determines nonuniversal temperature dependencies of the {\ensuremath{\chi}}_{n}, i.e., the {\ensuremath{\chi}}_{n} are found to be not universal functions of a frequency- and temperature-dependent single parameter. Moreover, the actual frequency-dependent behavior is also shown to be strictly related to the particular pinning model chosen for the simulations. Indeed, for large values of the reduced pinning potential $(U/KTg~220)$ and for increasing frequency, a transition has been observed between dynamic regimes dominated by creep and flux flow processes. On the other hand, for smaller reduced pinning potentials, a transition from the thermally activated flux flow (Taff) to the flow regime occurs. In qualitative agreement with available experimental data but in contrast with previously used simpler models, the amplitude of the peak of the imaginary part of the first harmonic is shown to be frequency dependent. Moreover the frequency dependence of its peak temperature shows large discrepancies with approximated analytical predictions. Finally, the shapes of the temperature dependencies of the higher harmonics are found to be strongly affected by the frequency

Stability mechanisms of high current transport in iron-chalcogenides superconducting films

The improvement in the fabrication techniques of iron-based superconductors have made these materials real competitors of high temperature superconductors and MgB$_2$. In particular, iron-chalcogenides have proved to be the most promising for the realization of high current carrying tapes. But their use on a large scale cannot be achieved without the understanding of the current stability mechanisms in these compounds. Indeed, we have recently observed the presence of flux flow instabilities features in Fe(Se,Te) thin films grown on CaF$_2$. Here we present the results of current-voltage characterizations at different temperatures and applied magnetic fields on Fe(Se,Te) microbridges grown on CaF$_2$. These results will be analyzed from the point of view of the most validated models with the aim to identify the nature of the flux flow instabilities features (i.e., thermal or electronic), in order to give a further advance to the high current carrying capability of iron-chalcogenide superconductors.Comment: 4 pages, 3 figure

Automatic interface generation for enumerative model checking

Explicit state model checking techniques suffer from the state explosion problem [7]. Interfaces [6, 2] can provide a partial solution to this problem by means of compositional state space reduction and can thus be applied when verifying interestingly large examples. Interface generation has till now been largely a manual process, were experts in the system or protocol to be verified describe the interface. This can lead to errors appearing in the verification process unless overheads to check the correctness of the interface are carried out. We address this issue by looking at automatic generation of interfaces, which by the very nature of their construction can be guaranteed to be correct. This report outlines preliminary experiments carried out on automatic techniques for interface generation together with their proofs of correctness.peer-reviewe

Automatic interface generation for compositional verification

Compositional verification, the incremental generation and composition of the state graphs of individual processes to produce the global state graph, tries to address the state explosion problem for systems of communicating processes. The main problem with this approach is that intermediate state graphs are sometimes larger than the overall global system. To overcome this problem, interfaces, and refined interfaces, which take into account a system’s environment have been developed. The number of states of these interfaces plays a vital role in their applicability in terms of computational complexity, which is proportional to the number of states in the interface. The direct use of complete subcomponents of the global system as interfaces, thus usually fails, and it is up to the system designer to describe smaller interfaces to be used in the reduction. To avoid having to verify the correctness of such manually generated interfaces, we propose automatic techniques to generate correct interfaces. The challenge is to produce interfaces small in size, yet effective for reduction. In this paper, we present techniques to structurally produce language over-approximations of labelled transition systems which can be used as correct interfaces, and combine them with refined interfaces. The techniques are applied to a number of case-studies, analysing the trade-off between interface size and effectiveness.peer-reviewe

Model checking user interfaces

User interfaces are crucial for the success of most software projects. As software grows in complexity there is a similar growth in the user interface com- plexity which leads to bugs which may be difficult to find by means of testing. In this paper we use the method of automated model checking to verify user interfaces with respect to a formal specification. We present an algorithm for the automated abstraction of the user interface model of a given system, which uses asynchronous and interleaving composition of a number of programs. This technique was successful at verifying the user interface of case study and brings us one step forward towards push button verification.peer-reviewe

Politiche e pratiche per l’educazione linguistica, il multilinguismo e la comunicazione interculturale

Uno dei compiti fondamentali dell’educazione linguistica è valorizzare la diversità, non solamente perché ci si trova spesso dinanzi ad apprendenti di nazionalità diverse, ma anche perché vari sono i motivi per cui si apprendono le lingue oggi, sovente determinati da esigenze di integrazione sociale e di opportunità lavorative. Tramite il multilinguismo si valorizzano competenze linguistiche e si creano opportunità di comunicazione interculturale. Nel contempo le politiche linguistiche vanno valutate e rinnovate in continuazione. Questi temi vengono affrontati in questo volume grazie a contributi che si diversificano sia sul piano delle lingue oggetto di studio sia quello teorico-concettuale, pur avendo in comune l’interesse per la linguistica applicata e per l’educazione linguistica.peer-reviewe