Broadband Epsilon-Near-Zero Metamaterials with Step-Like Metal-Dielectric Multilayer Structures

The concept of the broadband epsilon-near-zero meta-atom consisting of layered stacks with specified metallic filling ratio and thickness is proposed based on the Bergman spectral representation of the effective permittivity. The step-like metal-dielectric multilayer structures are designed to achieve realistic broadband epsilon-near-zero meta-atoms in optical frequency range. These meta-atoms can be integrated as building blocks for unconventional optical components with exotic electromagnetic properties over a wide frequency range, such as the demonstrated broadband directional emission and phase front shaping.Comment: 18 pages, 7 figure

Realizing broadband electromagnetic transparency with a graded-permittivity sphere

Broadband electromagnetic transparency phenomenon is realized with a well-designed graded-permittivity sphere, which has an extremely low scattering cross section over a wide frequency range, based on the generalized Mie scattering theory and numerical simulation in full-wave condition. The dynamic polarization cancellation is revealed by studying the variation of the polarization with respect to the frequency. Furthermore, a properly-designed multi-shell sphere is also proposed and examined in order to reduce the rigorous conditions for realizing the broadband transparency in experiments.Comment: 15 pages, 4 figure

Agent-based model with asymmetric trading and herding for complex financial systems

Background: For complex financial systems, the negative and positive return-volatility correlations, i.e., the so-called leverage and anti-leverage effects, are particularly important for the understanding of the price dynamics. However, the microscopic origination of the leverage and anti-leverage effects is still not understood, and how to produce these effects in agent-based modeling remains open. On the other hand, in constructing microscopic models, it is a promising conception to determine model parameters from empirical data rather than from statistical fitting of the results. Methods: To study the microscopic origination of the return-volatility correlation in financial systems, we take into account the individual and collective behaviors of investors in real markets, and construct an agent-based model. The agents are linked with each other and trade in groups, and particularly, two novel microscopic mechanisms, i.e., investors' asymmetric trading and herding in bull and bear markets, are introduced. Further, we propose effective methods to determine the key parameters in our model from historical market data. Results: With the model parameters determined for six representative stock-market indices in the world respectively, we obtain the corresponding leverage or anti-leverage effect from the simulation, and the effect is in agreement with the empirical one on amplitude and duration. At the same time, our model produces other features of the real markets, such as the fat-tail distribution of returns and the long-term correlation of volatilities. Conclusions: We reveal that for the leverage and anti-leverage effects, both the investors' asymmetric trading and herding are essential generation mechanisms. These two microscopic mechanisms and the methods for the determination of the key parameters can be applied to other complex systems with similar asymmetries.Comment: 17 pages, 6 figure

Octet-baryon masses in finite space

We report on a recent study of finite-volume effects on the lowest-lying octet baryon masses using the covariant baryon chiral perturbation theory up to next-to-leading order by analysing the latest $n_f = 2 + 1$ lattice QCD results from the NPLQCD Collaboration.Comment: 4 pages, 1 figure; parallel talk delivered by XLR at the 14th national conference on nuclear structure, April 12nd - 16th, 2012, Huzhou, Chin

Research trends in task-based language teaching: A bibliometric analysis from 1985 to 2020

This study offers a bibliometric analysis of research trends in task-based language teaching (TBLT) from 1985 to 2020. The analysis covers research questions related to the publication trends, venues for publication, productive authors, highly cited articles and references and, more importantly, the most frequently explored TBLT-related topics and their developmental patterns across the past 35 years. Results showed that TBLT was still mostly approached from the traditional cognitive-interactionist and psycholinguistic perspectives with a focus on tasks, individuals (i.e., learners and teachers), task-related variables (e.g., task complexity and task repetition), task performance, and the resultant linguistic forms. While this field of research has witnessed a growing interest in learners’ individual differences and computer-mediated, technologies-assisted learning, a decreasing trend has been observed in topics related to error and recast. Implications for task-based research, pedagogy, and research methodologies are discussed.This study offers a bibliometric analysis of research trends in task-based language teaching (TBLT) from 1985 to 2020. The analysis covers research questions related to the publication trends, venues for publication, productive authors, highly cited articles and references and, more importantly, the most frequently explored TBLT-related topics and their developmental patterns across the past 35 years. Results showed that TBLT was still mostly approached from the traditional cognitive-interactionist and psycholinguistic perspectives with a focus on tasks, individuals (i.e., learners and teachers), task-related variables (e.g., task complexity and task repetition), task performance, and the resultant linguistic forms. While this field of research has witnessed a growing interest in learners’ individual differences and computer-mediated, technologies-assisted learning, a decreasing trend has been observed in topics related to error and recast. Implications for task-based research, pedagogy, and research methodologies are discussed