Magnification relations of quad lenses and applications on Einstein crosses

In this work, we mainly study the magnification relations of quad lens models for cusp, fold and cross configurations. By dividing and ray-tracing in different image regions, we numerically derive the positions and magnifications of the four images for a point source lying inside of the astroid caustic. Then, based on the magnifications, we calculate the signed cusp and fold relations for the singular isothermal elliptical lenses. The signed fold relation map has positive and negative regions, and the positive region is usually larger than the negative region as has been confirmed before. It can also explain that for many observed fold image pairs, the fluxes of the Fermat minimum images are apt to be larger than those of the saddle images. We define a new quantity cross relation which describes the magnification discrepancy between two minimum images and two saddle images. Distance ratio is also defined as the ratio of the distance of two saddle images to that of two minimum images. We calculate the cross relations and distance ratios for nine observed Einstein crosses. In theory, for most of the quad lens models, the cross relations decrease as the distance ratios increase. In observation, the cross relations of the nine samples do not agree with the quad lens models very well, nevertheless, the cross relations of the nine samples do not give obvious evidence for anomalous flux ratio as the cusp and fold types do. Then, we discuss several reasons for the disagreement, and expect good consistencies for more precise observations and better lens models in the future.Comment: 12 pages, 11 figures, accepted for publication in MNRA

Structure of CdTe/ZnTe superlattices

The structure of CdTe/ZnTe superlattices has been analyzed through θ/2θ x‐ray diffraction, photoluminescence, and in situ reflection high‐energy electron diffraction (RHEED) measurements. Samples are found to break away from Cd_(x)Zn_(1−x)Te buffer layers as a consequence of the 6% lattice mismatch in this system. However, defect densities in these superlattices are seen to drop dramatically away from the buffer layer interface, accounting for the intense photoluminescence and high‐average strain fields seen in each of our samples. Observed variations in residual strains suggest that growth conditions play a role in forming misfit defects. This could explain discrepancies with calculated values of critical thickness based on models which neglect growth conditions. Photoluminescence spectra reveal that layer‐to‐layer growth proceeded with single monolayer uniformity, suggesting highly reproducible growth. Our results give hope for relatively defect‐free Cd_(x)Zn_(1−x)Te/Cd_(y)Zn_(1−y)Te superlattices with the potential for applications to optoelectronics offered by intense visible light emitters

An authentication scheme to defend against UDP DrDoS attacks in 5G networks

5th generation wireless systems are coming. While we are excited about the delay-free high speeds 5G will bring, security problems are becoming more and more serious. Increasingly rampant Distributed Denial of service (DDoS) attacks, particularly Distributed Reflection Denial of Service (DrDoS) attacks with User Datagram Protocols (UDPs) have developed into a global problem. This article presents a design, implementation, analysis, and experimental evaluation of an authentication scheme, a defense against UDP DrDoS attacks, by which attackers cleverly use rebound server farms to bounce a flood of packets to a target host. We call our solution IEWA because it combines the concepts of increasing expenses and weak authentication. In this paper, we apply IEWA to Network Time Protocol (NTP). First, we simulate and compare the original and improved protocols. Next, we verify the effectiveness of our proposed scheme. We show that our improved scheme is safer than the original scheme. Finally, we compare our solution with existing state-of-the-art schemes, using indicators such as communication overhead, server storage costs, client storage costs, computation costs of server and computation costs of client. We find that our scheme improves system stability and security, reduces communication overhead, server storage cost and computational costs. Our solution not only improves the NTP protocol to mitigate DrDoS attacks, but also strengthens other UDP protocols that are vulnerable to DrDoS attacks. Therefore, our solution can be used as a solution to UDP DrDoS attacks in 5G Network

Testing of a novel web browser interface for the Chinese market

This paper compares the perspicacity, appropriateness and preference of web browser icons from leading software providers with those of a culture-specific design. This online study was conducted in Taiwan and involved 103 participants, who were given three sets of web browser icons to review, namely Microsoft Internet Explorer, Macintosh Safari, and culturally specific icons created using the Culture-Centred Design methodology. The findings of the study show that all three sets have generally high recognition rates, but that some icon functions (e.g. Go/Visit and Favourite) in all three sets have poor recognition rates and are considered inappropriate

Observation of Fermi-energy dependent unitary impurity resonances in a strong topological insulator Bi_2Se_3 with scanning tunneling spectroscopy

Scanning tunneling spectroscopic studies of Bi_2Se_3 epitaxial films on Si (111) substrates reveal highly localized unitary impurity resonances associated with non-magnetic quantum impurities. The strength of the resonances depends on the energy difference between the Fermi level (E_F) and the Dirac point (E_D) and diverges as E_F approaches E_D. The Dirac-cone surface state of the host recovers within ~ 2Å spatial distance from impurities, suggesting robust topological protection of the surface state of topological insulators against high-density impurities that preserve time reversal symmetry

A Signaling-based Incentive Mechanism for Device-to-Device Content Sharing in Cellular Networks

In this letter, we model the Device-to-device (D2D) content sharing problem as a labor market where the base station (BS) acts as the principal and content providers serve as agents. A signaling-based content-sharing incentive (SCSI) mechanism is designed to encourage candidate content providers to participate in content sharing, and the optimal strategy for each content provider is derived to maximize their utility (monetary profit) while guaranteeing a non-negative utility for the BS. Simulation results show that the proposed SCSI mechanism can increase the content provider’s utility and participating enthusiasm in D2D content sharing

Coexistence of Wi-Fi and Heterogeneous Small Cell Networks Sharing Unlicensed Spectrum

As two major players in terrestrial wireless communications, Wi-Fi systems and cellular networks have different origins and have largely evolved separately. Motivated by the exponentially increasing wireless data demand, cellular networks are evolving towards a heterogeneous and small cell network architecture, wherein small cells are expected to provide very high capacity. However, due to the limited licensed spectrum for cellular networks, any effort to achieve capacity growth through network densification will face the challenge of severe inter-cell interference. In view of this, recent standardization developments have started to consider the opportunities for cellular networks to use the unlicensed spectrum bands, including the 2.4 GHz and 5 GHz bands that are currently used by Wi-Fi, Zigbee and some other communication systems. In this article, we look into the coexistence of Wi-Fi and 4G cellular networks sharing the unlicensed spectrum. We introduce a network architecture where small cells use the same unlicensed spectrum that Wi-Fi systems operate in without affecting the performance of Wi-Fi systems. We present an almost blank subframe (ABS) scheme without priority to mitigate the co-channel interference from small cells to Wi-Fi systems, and propose an interference avoidance scheme based on small cells estimating the density of nearby Wi-Fi access points to facilitate their coexistence while sharing the same unlicensed spectrum. Simulation results show that the proposed network architecture and interference avoidance schemes can significantly increase the capacity of 4G heterogeneous cellular networks while maintaining the service quality of Wi-Fi systems