Peak effect in the critical current of type II superconductors with strong magnetic vortex pinning

We perform 2D Langevin simulations studying the peak effect (PE) of the critical current taking into account the temperature dependence of the competing forces. We observe and report that the PE results from the competition of vortex-vortex interactions and vortex-pin interactions which have different temperature dependencies. The simulations reveal that the PE can take place only for certain pinning strengths, densities of pinning centres, and driving forces, which is in good agreement with experiments. No apparent vortex order-disorder transition is observed across the PE regime. In addition, the PE is a dynamical phenomenon, and thermal fluctuations can speed up the process for the formation of the PE

The roles of climate, geography and natural selection as drivers of genetic and phenotypic differentiation in a widespread amphibian Hyla annectans (Anura: Hylidae)

The role of geological events and Pleistocene climatic fluctuations as drivers of current patterns of genetic variation in extant species has been a topic of continued interest among evolutionary biologists. Nevertheless, comprehensive studies of widely distributed species are still rare, especially from Asia. Using geographically extensive sampling of many individuals and a large number of nuclear single nucleotide polymorphisms (SNPs), we studied the phylogeography and historical demography ofHyla annectanspopulations in southern China. Thirty-five sampled populations were grouped into seven clearly defined genetic clusters that closely match phenotype-based subspecies classification. These lineages diverged 2.32-5.23 million years ago (Ma), a timing that closely aligns with the rapid and drastic uplifting of the Qinghai-Tibet Plateau and adjacent southwest China. Demographic analyses and species distribution models indicate that different populations of this species have responded differently to past climatic changes. In the Hengduan Mountains, most populations experienced a bottleneck, whereas the populations located outside of the Hengduan Mountains have gradually declined in size since the end of the last glaciation. In addition, the levels of phenotypic and genetic divergence were strongly correlated across major clades. These results highlight the combined effects of geological events and past climatic fluctuations, as well as natural selection, as drivers of contemporary patterns of genetic and phenotypic variation in a widely distributed anuran in Asia.Peer reviewe

Measuring the uncertainty of differences for contrasting groups

In this paper, we propose an empirical likelihood (EL) based strategy for building confidence intervals for differences between two contrasting groups. The proposed method can deal with the situations when we know little prior knowledge about the two groups, which are referred to as non-parametric situations. We experimentally evaluate our method on UCI datasets and observe that proposed EL based method outperforms other methods. Copyright © 2007, Association for the Advancement of Artificial Intelligence(www.aaai.org). All rights reserved

Dip effect in ac susceptibility due to surface barrier with flux creep

A model is proposed to describe the effect of surface barrier (SB) on ac susceptibility (ACS) and a different kind of dip effect ~DE! in ACS is observed. Simulation based on this model with flux creep reveals two dips in ACS curve, one at temperature Td8 in real part x8 and the other at temperature Td9 in imaginary part x9. These two dips are different from the ones resulting from the peak effect in critical current density jc , where the dips in x8 and x9 occur at the same temperature Tp . The DE is also characterized by a large x9 and a large ux8(Td8)u. The ACS curves for single crystals Bi2Sr2CaCu2O8 have been observed and compared with the DE in YBa2Cu3O7 samples, confirming the numerical results. It is also shown that when flux creep is absent only kinks appear in x8 and x9 for a sample with SB

Stochastic Stability Analysis and Control of Secondary Frequency Regulation for Islanded Microgrids under Random Denial of Service Attacks

As communication networks are increasingly implemented to support the information exchange between microgrid control centers and/or local controllers, they expose microgrids to cyber-attack threats. This paper aims to analyze the stochastic stability of islanded microgrids in the presence of random DoS attack and propose a mode-dependent resilient controller to mitigate the influence of DoS attacks. Specifically, the small-signal model of the microgrid under the DoS attack is integrated as a stochastic jump system with state continuity disruptions. A new vulnerability metric is defined by using observability Gramians of the stochastic jump system, to measure the vulnerability of the system regarding DoS attack choices. The Lyapunov function analysis is conducted to find conditions sustaining the stochastic stability of the islanded microgrid in the form of linear matrix inequalities (LMIs). A mode-dependent control approach is proposed for microgrids to mitigate the influence of random DoS attacks. In case studies, the vulnerability analysis and time-domain simulation results show the performance of the investigated microgrid can be degraded when the random DoS attacks exist. When the proposed mode-based secondary frequency controllers are installed, the islanded microgrid can sustain its stability during the attacking period and system dynamics rapidly converge when the DoS attack is over

Distributed fuzzy filtering for load frequency control of non-linear interconnected power systems under cyber-physical attacks

In this study, a new distributed filtering approach is proposed for the load frequency control of uncertain non-linear power systems with cyber-physical attacks. Specifically, the non-linear power system is firstly modelled under interval type-2 Takagi-Sugeno fuzzy framework and the uncertainty therein is captured by designing corresponding membership functions. Both denial of service cyber attack and physical sensor attack are considered and modelled as independent Bernoulli process. Based on the Lyapunov stability theory, less conservative sufficient conditions have been derived to guarantee the robustly mean-square asymptotic stability with an average ℋ∞ performance standard γ for the dynamic filtering error system. Moreover, artful matrix transformation techniques have been adopted to decouple the intertwined matrix variables in designing the distributed filter gains. In simulations, a three-area non-linear power system with internal uncertainties is used to validate the robustness of the proposed distributed filtering strategy to system parametric uncertainties and cyber-physical attacks