Pseudospin induced chirality with Staggered Optical Graphene

Pseudospin plays a very important role in understanding various interesting physical phenomena associated with 2D materials such as graphene. It has been proposed that pseudospin is directly related to angular momentum, and it was recently experimentally demonstrated that orbit angular momentum is an intrinsic property of pseudospin in a photonic honeycomb lattice. However, in photonics, the interaction between spin and pseudospin for light has never been investigated. In this Letter, we propose that, in an optical analogue of staggered graphene, i.e. a photonic honeycomb lattice waveguide with in-plane inversion symmetry breaking, the pseudospin mode can strongly couple to the spin of an optical beam incident along certain directions. The spin-pseudospin coupling, caused by the spin-orbit conversion in the scattering process, induces a strong optical chiral effect for the transmitted optical beam. Spin-pseudospin coupling of light opens door to the design of pseudospin-mediated spin or valley selective photonic devices

Existence and Multiplicity of Solutions for Fractional pp-Laplacian Equation Involving Critical Concave-convex Nonlinearities

We investigate the following fractional $p$-Laplacian equation $$\begin{cases} \begin{aligned} (-\Delta)_p^s u&=\lambda |u|^{q-2}u+|u|^{p_s^*-2}u &&\text{in}~\Omega,\\ u &=0 &&\text{in}~ \mathbb{R}^n\setminus\Omega, \end{aligned} \end{cases}$$ where $s\in (0,1)$, $p>q>1$, $n>sp$, $\lambda>0$, $p_s^*=\frac{np}{n-sp}$ and $\Omega$ is a bounded domain (with $C^{1, 1}$ boundary). Firstly, we get a dichotomy result for the existence of positive solution with respect to $\lambda$. For $p\ge 2$, $p-1\frac{sp(q+1)}{q+1-p}$, we provide two positive solutions for small $\lambda$. Finally, without sign constraint, for $\lambda$ sufficiently small, we show the existence of infinitely many solutions

In Search of English as a Foreign Language (EFL) Teachers\u27 Knowledge of Vocabulary Instruction

Researchers have explored second language (L2) teachers\u27 knowledge focusing not only on their prior language learning experience, previous L2 teacher education, and teaching practices, but also on specific curricular areas, such as teaching L2 grammar, teaching L2 reading, and teaching L2 writing. This line of research has contributed to L2 teacher education, particularly how to develop an effective knowledge base for teacher candidates. This dissertation was conducted to investigate English as a foreign language (EFL) teachers\u27 knowledge of vocabulary instruction. Specifically, employing three qualitative techniques for data collection (i.e., interviews, classroom observations, and stimulated recall), the study examined seven Chinese EFL university teachers\u27knowledge of vocabulary instruction from four dimensions: their beliefs about vocabulary learning, their understandings about vocabulary teaching, the relationship between their knowledge of vocabulary instruction and vocabulary teaching practices, and the sources of their knowledge about vocabulary instruction. The findings of the study indicate that Chinese EFL teachers have well-developed content knowledge of EFL vocabulary. They also have well-established belief systems about how to learn and teach vocabulary. Moreover, their beliefs about vocabulary teaching tend to be consistent with their vocabulary teaching practices though some inconsistencies have been identified as well. It was also found that Chinese EFL teachers¡¯ knowledge of vocabulary instruction is derived from a variety of sources, of which formal EFL education and teaching practices are considered as the two most influential. EFL teachers\u27 individual differences were also identified to impact their beliefs about vocabulary instruction. This dissertation has at least three potential contributions. As one of the first attempts to investigate teacher knowledge of vocabulary instruction in the field of L2 teacher education, this research expands studies on L2 teachers\u27 knowledge base. It also provides information about L2 teachers\u27 knowledge in one less studied context, i.e., Chinese EFL vocabulary teaching. Finally, the use of observations, interviews, and stimulated recall to collect data in this study serves as an impetus for enriching techniques to examine Chinese EFL teacher knowledge

Energy-efficient and Power-dense DC-DC Converters in Data Center and Electric Vehicle Applications Using Wide Bandgap Devices

The ever increasing demands in the energy conversion market propel power converters towards high efficiency and high power density. With fast development of data processing capability in the data center, the server will include more processors, memories, chipsets and hard drives than ever, which requires more efficient and compact power converters. Meanwhile, the energy-efficient and power-dense converters for the electric vehicle also result in longer driving range as well as more passengers and cargo capacities. DC-DC converters are indispensable power stages for both applications. In order to address the efficiency and density requirements of the DC-DC converters in these applications, several related research topics are discussed in this dissertation. For the DC-DC converter in the data center application, a LLC resonant converter based on the newly emerged GaN devices is developed to improve the efficiency over the traditional Si-based converter. The relationship between the critical device parameters and converter loss is established. A new perspective of extra winding loss due to the asymmetrical primary and secondary side current in LLC resonant converter is proposed. The extra winding loss is related to the critical device parameters as well. The GaN device benefits on device loss and transformer winding loss is analyzed. An improved LLC resonant converter design method considering the device loss and transformer winding loss is proposed. For the DC-DC converter in the electric vehicle application, an integrated DC-DC converter that combines the on-board charger DC-DC converter and drivetrain DC-DC converter is developed. The integrated DC-DC converter is considered to operate in different modes. The existing dual active bridge (DAB) DC-DC converter originally designed for the charger is proposed to operate in the drivetrain mode to improve the efficiency at the light load and high voltage step-up ratio conditions of the traditional drivetrain DC-DC converter. Design method and loss model are proposed for the integrated converter in the drivetrain mode. A scaled-down integrated DC-DC converter prototype is developed to verify the design and loss model

A Fast Radio Burst Search Method for VLBI Observation

We introduce the cross spectrum based FRB (Fast Radio Burst) search method for VLBI observation. This method optimizes the fringe fitting scheme in geodetic VLBI data post processing, which fully utilizes the cross spectrum fringe phase information and therefore maximizes the power of single pulse signals. Working with cross spectrum greatly reduces the effect of radio frequency interference (RFI) compared with using auto spectrum. Single pulse detection confidence increases by cross identifying detections from multiple baselines. By combining the power of multiple baselines, we may improve the detection sensitivity. Our method is similar to that of coherent beam forming, but without the computational expense to form a great number of beams to cover the whole field of view of our telescopes. The data processing pipeline designed for this method is easy to implement and parallelize, which can be deployed in various kinds of VLBI observations. In particular, we point out that VGOS observations are very suitable for FRB search.Comment: Accepted for publication in A