Zero bias anomalies of transport characteristics of single-barrier GaAs/AlAs/GaAs heterostructures as a result of resonance tunneling between parallel two-dimensional electron gases and suppression of resonance tunneling in a magnetic field as a manifestation of the Coulomb gap in the tunnel density of states

Electron tunnel transport across single-barrier GaAs/AlAs/GaAs heterostructures is investigated. It is shown that “zero bias anomalies”—extrema in differential conductivity close to the zero bias—in the structures investigated are caused by resonance tunneling between parallel two-dimensional electron gases in enriched layers, which are formed from both sides of the barriers due to the presence of Si donor impurities in barriers. The suppression of resonance tunneling between parallel two-dimensional electron gases is found in narrow ranges close to the zero bias (tunnel gap) in a strong magnetic field parallel to the current direction only when a single Landau level is occupied for each of the two-dimensional electron gases. Suppression is induced by a Coulomb gap at the Fermi level in the tunnel density of states. This experiment originally revealed the manifestation of a Coulomb gap in tunneling between parallel two-dimensional electron gases with relatively low mobilities. For these electron gases, the influence of disorder or random potential fluctuations on the mechanism of formation of the tunnel gap is noticeable.     

Surveillance of stored grain from the 1997 harvest in the United Kingdom for ochratoxin A

This survey examined 306 samples of farm-stored wheat, barley and oats as received at, or tested by, central grain depots in the UK. Samples were taken from lorries or from stored grain using the existing inhouse procedures used for quality checking and examined for ochratoxin A using a fully validated analytical HPLC method with a detection limit of 0.1 μg/kg. Ochratoxin A was detected in 21 % of the samples examined, with barley more frequently contaminated than wheat. Mean concentrations of ochratoxin A found for all samples were 0.69 μg/kg in barley, 0.29 μg/kg in wheat and 0.15 μg/kg in oats. The highest concentration found was 17.8 μg/kg in a barley feed although concentrations of 81 and 30 μg/kg were found in 'reject-grade' wheat samples whose results were excluded from the main survey. In summary, 2.7 and 0.3 % of samples exceeded concentrations of 5 and 10 μg/kg respectively. There appeared to be significant relationships between ochratoxin A concentrations and moisture content, storage time and geographical area. Although conditions at harvest in 1997 were quite variable countrywide and often wet, results were similar to those found in earlier surveys carried out in the UK.     

MLAS: Multiple level authentication scheme for VANETs

The vehicular ad hoc network (VANET) is an emerging type of network which enables vehicles on roads to inter-communicate for driving safety. The basic idea is to allow arbitrary vehicles to broadcast ad hoc messages (e.g. traffic accidents) to other vehicles. However, this raises the concern of security and privacy. Messages should be signed and verified before they are trusted while the real identity of vehicles should not be revealed, but traceable by authorized party. Existing solutions either rely too heavily on a tamper-proof hardware device, or do not have an effective message verification scheme. In this paper, we propose a multiple level authentication scheme which still makes use of tamper-proof devices but the strong assumption that a long-term system master secret is preloaded into all tamper-proof devices is removed. Instead the master secret can be updated if needed to increase the security level. On the other hand, messages sent by vehicles are classified into two types – regular messages and urgent messages. Regular messages can be verified by neighboring vehicles by means of Hash-based Message Authentication Code (HMAC) while urgent messages can only be verified with the aid of RSUs nearby by means of a conditional privacy-preserving authentication scheme. Through extensive simulation, we show that our multiple level authentication scheme is much more efficient that those RSU-aided authentication scheme as long as the proportion of urgent messages is less than 100%. The verification delay required can be up to 110 times smaller than other protocols. Our implementation shows that batch verification may not be as efficient as expected. In case without batch verification, the verification delay required by our scheme can even be up to 173 times smaller.     

Integrated Services over ATM/MAN-Based Personal Communication Networks

IEEE 802.6 metropolitan area networks (MANs) providean efficient architecture for supporting wirelesspersonal communication services (PCSs) overmetropolitan areas by interconnecting wireless basestations into personal communication networks (PCNs).They also serve as peripheral gathering networks totransport PCS traffic over a wide area broadbandintegrated services digital network employingasynchronous transfer mode (ATM), enabling integrationof PCNs in different cities into a global PCN. Bycombining the various access methods and employing adynamic bandwidth integration scheme, a MAN canfunction as a powerful distributed switch forintegrating different types of local traffic.Simulation results are presented to illustrate theadvantages of the integration scheme. To provideubiquitous PCS, we also propose a simplesuffix-dialing method for integrating telephone, fax,paging and email services over the ATM/MAN-based PCNsusing a single personal telecommunication number. Withthis method, the current practice of printing a longlist of phone numbers on one's business card will nolonger be necessary.     

Identifying FMRI model violations with lagrange multiplier tests

The standard modeling framework in functional magnetic resonance imaging (fMRI) is predicated on assumptions of linearity, time invariance and stationarity. These assumptions are rarely checked because doing so requires specialized software, although failure to do so can lead to bias and mistaken inference. Identifying model violations is an essential but largely neglected step in standard fMRI data analysis. Using Lagrange multiplier testing methods we have developed simple and efficient procedures for detecting model violations such as nonlinearity, nonstationarity and validity of the common double gamma specification for hemodynamic response. These procedures are computationally cheap and can easily be added to a conventional analysis. The test statistic is calculated at each voxel and displayed as a spatial anomaly map which shows regions where a model is violated. The methodology is illustrated with a large number of real data examples.     

Influence of the orientation of the silicon substrate on the properties of avalanche Si:Er:O light-emitting structures

The influence of the orientation of silicon on the structural and luminescence properties of avalanche light-emitting diodes fabricated by the coimplantation of erbium and oxygen followed by solid-phase epitaxial (SPE) crystallization of the amorphized layer is considered. The luminescence properties are a consequence of the formation of various structural defects during the SPE crystallization: V-shaped dislocations and erbium precipitates form in (100) Si:Er:O layers, and larger structural defects, i.e, twins, are observed in (111) Si:Er:O layers along with an increase in the dislocation density by more than four orders of magnitude in comparison with the (100) orientation. The luminescence properties of avalanche and tunnel light-emitting diodes are also compared. In contrast to tunnel diodes, in avalanche diodes erbium ions are excited in the entire space-charge layer, and the effective excitation cross section of the Er³⁺ ions and their lifetime in the excited state are 3–4 times larger. Fiz. Tekh. Poluprovodn. 33, 660–663 (June 1999)     

PFC控制器芯片内部剖析

获得接近1的功率因数（powerfactor）是目前电源供应商的目标,这需要在AC/DC前端采用PFC电路,将输出电压提高至高于最大交流线峰值,通常达385VDC.本文将阐释PFC控制器的内部模块示意图、结构模块之间的同步和链接,以及电流修整和保护电路功能.此外,还会给出实际波形和参数选择的参考。     

Securing Threshold Cryptosystems against Chosen Ciphertext Attack

For the most compelling applications of threshold cryptosystems, security against chosen cipher text attack is a requirement. However, prior to the results presented here, there appeared to be no practical threshold cryptosystems in the literature that were provably chosen ciphertext secure, even in the idealized random oracle model. The contribution of this paper is to present two very practical threshold cryptosystems, and to prove that they are secure against chosen ciphertext attack in the random oracle model. Not only are these protocols computationally very efficient, but they are also non-interactive, which means they can be easily run over an asynchronous communication network. Received November 2000 and revised September 2001 Online publication 11 March 2002     

Hierarchical Asymptotic Methods in the Theory of Cluster Free Electron Lasers

We describe an algorithm for the hierarchic method of averaged characteristics and discuss a practical application of this concept. The application of this method opens new prospects for the solutions of many difficult electrodynamic problems, which, until recently, were considered “unsolved”. We illustrate this method with the example of the effect of plural multi-harmonic parametric resonance of electron-beam waves. This effect can take place within high-current electron beams in the case of the realization of two-stream instability. The considered relativistic beam-model is treated as a model for the transit section of relevant klystron Cluster Two-stream Free Electron Laser (CTSFEL).