Stability and photoemission characteristics for GaAs photocathodes in a demountable vacuum system

The stability and photoemission characteristics for reflection-mode GaAs photocathodes in a demountable vacuum system have been investigated by using spectral response and x-ray photoelectron spectroscopy measurements at room temperature. We find that the shape of the spectral response curve for the cathode changes with time in the vacuum system, but after applying fresh cesium to the degraded cathode, the spectral response can almost be restored. The change and restoration of curve shape are mainly attributed to the evolution of the surface barrier. We illustrate the evolution and analyze the influence of the barrier on the spectral response of the cathode. (C) 2008 American Institute of Physics.

Memory effect in a system of zincblende Mn-rich Mn(Ga)As nanoclusters embedded in GaAs

Zincblende Mn-rich Mn(Ga)As nanoclusters embedded in GaAs matrices are fabricated by in situ postgrowth annealing diluted magnetic semiconductor (Ga,Mn)As films with Mn concentration ranging from 2.6% to 8% at 650 degrees C. Magnetization measurements show that memory effect and slow magnetic relaxation, the typical characteristics of the spin-glass-like phase, occur below the blocking temperature of 45 K in samples with high Mn concentration, while for samples with low Mn concentration, ferromagnetic order remains up to 360 K. The behavior of low-temperature spin dynamics can be explained by the hierarchical model. (c) 2007 American Institute of Physics.

High-power electroabsorption modulator for radio over fibre system

An electroabsorption modulator using an intra-step quantum well (IQW) active region is fabricated for a radio over fibre system. The strain-compensated InGaAsP/InGaAsP IQW shows good material quality and improved modulation properties, high extinction ratio efficiency (10 dB V-1) and low capacitance (< 0.42 pF), with which high frequency (> 15 GHz) can be obtained. High-speed measurement under high-power excitation shows no power saturation up to an excitation power of 21 dBm. To our knowledge, the input optical power is the highest reported for a multi-quantum well EAM without a heat sink.

The wave function evolution of an exciton in a bilayer system: Turning on a static in-plane electric field

The time evolution of the ground state wave function of an exciton in an ideal bilayer system is investigated within the framework of the effective-mass approximation. All of the moduli squared of the ground state wave functions evolve with time as cosine functions after an in-plane electric field is applied to the bilayer system. The variation amplitude and period of the modulus squared of the ground state wave function increase with the in-plane electric field F-r for a fixed in-plane relative coordinate r and fixed separation d between the electron and hole layers. Moreover, the variation amplitude and period of the modulus squared of the ground state wave function increase with the separation d for a fixed r and fixed in-plane electric field. Additionally, the modulus squared of the ground state wave function decreases as r increases at a given time t for fixed values of d and F-r. (c) 2007 American Institute of Physics.

The Stark effect on excitons in a bilayer system

The Stark effect on excitons in a bilayer system is investigated theoretically within the framework of the effective-mass approximation. The calculations indicate that the energy of the excitons decreases as the value of the in-plane electric field F increases at a fixed value of the distance d between the layers. However, the energy of the excitons increases with d at a fixed value of F. In particular, it increases linearly at small values of d but increases as 1/d at large values. Therefore, it can be concluded that excitons in a bilayer system have a small binding energy equal to the absolute value of the excitonic energy at large d or small F. In addition, the radiative lifetime of heavy-hole excitons in this system is calculated and is found to be short at small values of both F and d. The radiative lifetime of heavy-hole excitons in a bilayer system can be increased by two orders by an in-plane electric field of 2 kV/cm when d is twice the excitonic Rydberg. (c) 2006 American Institute of Physics.

Photoluminescence properties of a GaN0.015As0.985/GaAs single quantum well under short pulse excitation

Under short pulse laser excitation, we have observed an extra high-energy photoluminescence (PL) emission from GaNAs/GaAs single quantum wells (QWs). It dominates the PL spectra under high excitation and/or at high temperature. By measuring the PL dependence on both temperature and excitation power and by analyzing the time-resolved PL results, we have attributed the PL peak to the recombination of delocalized excitons in QWs. Furthermore, a competition process between localized and delocalized excitons is observed in the temperature-dependent PL spectra under the short pulse excitation. This competition is believed to be responsible for the temperature-induced S-shaped PL shift often observed in the disordered alloy semiconductor system under continuous-wave excitation. (C) 2001 American Institute of Physics.

A comparison of photoluminescence properties of InGaAs GaAs quantum dots with a single quantum well

Variable temperature photoluminescence (PL) measurements for In0.3Ga0.7As(6 nm)/GaAs(34 nm) quantum dot superlattices with a period of 20 and an In0.3Ga0.7As(6 nm)/GaAs(34 nm) reference single quantum well have been conducted. It is found that the temperature dependence is different between the quantum dots and the reference single quantum well. The PL peak energy of the single quantum well decreases faster than that of the quantum dots with increasing temperature. The PL peak energy for the InGaAs/GaAs quantum dots closely follows the InAs band gap in the temperature range from 11 to 170 K, while the PL peak energy for the InGaAs/GaAs quantum well closely follows the GaAs band gap. In comparison with InAs/GaAs quantum dots, the InGaAs/GaAs quantum dots are more typical as a zero-dimensional system since the unusual PL results, which appear in the former, are not obvious for the latter. (C) 1999 American Institute of Physics. [S0021-8979(99)08615-6].

The development of long - Term ambulatory pH monitoring system and its clinical application

This paper introduced a long-term ambulatory intragastric pH monitoring system, which is designed for prolonged ambulatory studies of Gastroesophageal Reflux Diseases. The whole system is composed of the gastric catheter with two pH sensors, a small data logger (Microdatalog), and a notebook PC. In this paper, the design of monitoring system hardware and software are described in detail. Clinical applications reveal good results.

Observation of Nucleation and Growth of CdS Nanocrystals in a Two-phase System

The nucleation and growth kinetics of CdS nanocrystals in a two-phase synthesis system have been investigated. It was found that the nucleation process is quite lengthy and overlapped with the growth process; nevertheless, as formed nanocrystals show extremely narrow size distribution owing to the unique heterogeneous reacting environment and Ostwald ripening growth. The nucleation and growth kinetics of the nanocrystals were also influenced strongly by the monomer concentration, capping agent concentration, and solvent polarity. It was also found that a high monomer concentration, a low capping agent concentration, and low solvent polarity lead to a higher maximum nucleus concentration and nanocrystal concentration, while high polarity solvents are favorable for the formation of nanocrystals with narrower size distribution and higher photoluminescence quantum yield.

Investigation of the effect of branched structure on the performances of the copolymers synthesized from ethylene and alpha-olefin with rac-Et(Ind)(2)ZrCl2/MMAO catalyst system

The branched copolymers prepared from ethylene and alpha-olefins using rac-Et(Ind)(2)ZrCl2/MMAO catalyst system were studied. Both the absolute molecular weight ((M) over bar (W)) and the molecular size (radius of glyration, R-g) of the polymers eluting from gel permeation chromatography (GPC) columns were obtained simultaneously via a high temperature GPC coupled with a two-angle laser light scattering (TALLS) detector. The branched structures and performances of the copolymers display approximate molecular weight and molecular sizes were investigated. Wide angle X-ray diffraction analyses indicate that 16-carbon side branch could co-crystallize effectively with backbone chain at low alpha-olefin incorporation. The melt behaviors of the copolymers were studied by dynamic rheological measurements. Both branch length and comonomer content affect considerably the loss modulus, storage modulus and complex viscosity of the copolymers. The relationship between the dynamic-mechanical behavior and the comonomer content of the copolymers was also examined by dynamic-mechanical experiments.

Polymerization of aniline in an aqueous system containing organic solvents

Polyaniline (PAn) with different molecular weight was prepared by adding organic solvents such as acetone, ethanol or THF into the polymerization mixture. Open-circuit potential measurements showed that the polymerization rate was lowered by the addition of the organic solvent Spectral studies showed that PAn intermediate before the oxidant was consumed was pernigraniline and it was reduced to emeraldine base rapidly by aniline in the termination period. A mechanism of chain propagation was proposed. Chain propagation and autoacceleration period were almost independent of addition of pernigraniline, and the autoacceleration of aniline polymerization is due to more rapid initiation rate. (C) 1998 Published by Elsevier Science S.A. All rights reserved.

Design and analysis of kinetic energy recovery system for automobiles: a case study for commuters in Edinburgh.

Transport and its energetic and environmental impacts affect our daily lives. The transport sector is the backbone of the United Kingdom’s economy with 2.3 million people being employed in this sector. With a high dependency on transport for passengers and freight and with the knowledge that oil reserves are rapidly decreasing a solution has to be identified for conserving fuel. Passenger vehicles account for 61% of the transport fuel consumed in the U.K. and should be seen as a key area to tackle. Despite the introduction and development of electric powered cars, the widespread infrastructure that is required is not in place and has attributed to their slow uptake, as well as the fact that the electric car’s performance is not yet comparable with the conventional internal combustion engine. The benefits of the introduction of kinetic energy recovery systems to be used in conjunction with internal combustion engines and designed such that the system could easily be fitted into future passenger vehicles are examined. In this article, a review of automobile kinetic energy recovery system is presented. It has been argued that the ultracapacitor technology offers a sustainable solution. An optimum design for the urban driving cycle experienced in the city of Edinburgh has been introduced. The potential for fuel savings is also presented