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Thin brittle films on compliant substrates are used in many applications, as soft electronics and solar cells. When submitted to large tensile strains, those systems undergo multi-cracking. A saturation of the cracks pattern is observed, i.e. no new crack is formed above a given nominal applied strain. Moreover, a characteristic distance between the cracks is observed at saturation. A mechanical analysis is carried out in this paper in order to quantitatively predict the saturation phenomenon. Fracture in the brittle layer, delamination at the interface and plasticity in the substrate are taken into account. The results of finite elements simulations show that both the plastic deformation pattern inside the substrate and the strength of the brittle layer are key elements for predicting the cracks pattern at saturation.

International audience; In order to reduce the weight in automotive or aeronautic components, aluminum and its alloys are being used more and more. However, their mechanical properties are not as good as those of steel and can sometimes be below the necessary requirement. To overcome this issue, reinforcement of some parts is required. In this case, the mechanical reliability of the interface between the aluminum alloy matrix and its reinforcement has to be insured. This can be done by controlling the process parameters involved in developing a strong metallurgical bond between the two metals. In this work an Fe/Al-7 wt% Si alloy interface was investigated, aimed at linking the chemistry of the reaction layers with the toughness of the bond. First, the chemistry of the interface was characterized after immersion of Fe plates in an aluminum alloy melt. Then, the fracture energy of these assemblies was measured using a modified 4-point bending test for the adhesion measurement. In the case of pure elastic deformation during the loading, an analytical approach based on beam bending theory can be used to quantify the fracture energy. Nevertheless, in bonding between metallic partners, plastic dissipation can be expected. This is why a numerical calculation, based on a cohesive zone mode was proposed. The use of cohesive elements to describe the interface behavior allows for modeling both the threshold of de-cohesion and the progression of damage at each point of the interface. (C) Koninklijke Brill NV, Leiden, 2012

Sixteen shear capacity tests were performed on eight decommissioned AASHTO prestressed concrete girders that had been in service for over 42 years. These bridge members presented a unique opportunity to investigate carbon fiber-reinforced polymer (CFRP) retrofit schemes to enhance the shear capacity of underreinforced girders that were nonrectangular. Four destructive tests were performed to quantify the in-service strength of the girders and the remaining 12 tests were performed on CFRP retrofitted girders. In all, five configurations of the CFRP reinforcement were evaluated. Two anchoring techniques were investigated that either involved epoxying a horizontal CFRP strip over the vertical strips or a new methodology of epoxying a CFRP laminate into a groove over the vertical strips that was cut at the web-to-flange interface. Two methodologies that predicted the shear contribution of the carbon fiber reinforcement were compared with the test results. A carbon fiber-reinforcing scheme of vertical strips a...

The article presents results of the study of wetting of iron, nickel, and platinum substrates by molten CaO-SiO2-Al2O3 slag of near-eutectic composition by the sessile drop method in reducing atmosphere (oxygen partial pressure 10−20 to 10−18 atm). Terminal contact angles (after 240 minutes) for the CaO-SiO2-Al2O3 slag were as follows: for iron substrate, 55 ± 2 deg (1350 °C), 60 ± 2 deg (1390 °C), 44 ± 2 deg (1450 °C); for nickel, 59 ± 2 deg (1350 °C), 60 ± 2 deg (1390 °C); and for platinum, 15 ± 2 deg (1350 °C, 1390 °C, and 1450 °C). Under strongly reducing conditions in the graphite furnace, silica was reduced with dissolution of silicon into iron and nickel substrates, although the degree of SiO2 reduction from the slag was low. No silicon was detected in the platinum substrate. Wetting exhibited dynamic behavior. In nonisothermal continuous ramping and stepwise experiments, the contact angle decreased irrespective of changing temperature.

An article presents results of a study of wetting of iron, nickel, and platinum substrates by molten MnO-SiO2 slag of near-eutectic composition by the sessile-drop method in reducing atmosphere (oxygen partial pressure of 10−20 to 10−18 atm). Significant MnO reduction and manganese dissolution into substrate was observed in reactions with all three substrates. Silica also was reduced in reaction with Fe and Ni, although the extent of reduction was much smaller in comparison with MnO and was not observed in reaction with Pt. The dissolution of manganese led to modification of the interface with formation of molten phase for nickel and platinum substrates. Terminal contact angles (after 240 minutes) for MnO-SiO2 slag were as follows: for iron substrates—5 ± 2 deg at 1350 °C, 9 ± 2 deg at 1390 °C, and 6 ± 2 deg at 1450 °C; and for platinum—15 ± 2 deg at 1350 °C and 1390 °C, and 12 ± 2 deg at 1450 °C. The contact angle for the Ni- (MnO-SiO2) system was close to zero—3 ± 2 deg at 1350 °C and 1390 °C. Modification of the metal-oxide interface as a result of reduction of oxides and adsorption of oxygen made wetting dynamic and affected interfacial properties.

Ultra-high-resolution infra-red local vibrational mode (IR LVM) spectroscopy measurements together with density-functional calculations have been used to identify the signatures of close substituti ...

A high-Q cavity containing three layers of self-assembled InGaAs/GaAs quantum dots has been prepared. Emission occurs in the wavelength range 1275–1285 nm over the wafer surface. We have observed lasing thresholds in the power-in-versus-power-out characteristics, with associated changes in the angular emission profile, when the structure is optically pumped CW at 300 K. At high pump powers spectrally resolved lateral modes are seen in the emission spectra of a planar cavity and this is discussed in terms of the index change induced by the pump laser.

As the development of stretchable devices advances, engineering components with rigid films on soft substrates are becoming more numerous. We propose to analyse the buckle delamination of a film on a soft substrate, under a biaxial compressive stress state. This problem has already been investigated by an Euler column buckling analysis. In this paper, experiments on soft substrates are presented, which demonstrate that the buckle shape is, in some cases, better approximated by a “Mexican hat” shape. A model using a non-linear plate bonded to an elastic medium by a cohesive interaction is used to describe the delamination process. It is demonstrated that the “Mexican hat” shape modifies the crack propagation behavior for a soft substrate.

The potential of GaAs-based dilute nitride alloys, such as GaNAs and GaInNAs, for use in long-wavelength telecommunication applications has led to intensive research into their growth and physical properties. In order to produce high quality GaNAs-based devices it is essential that the material and growth source is fully characterised. In this paper, we present a study of MBE grown dilute GaNxAs1−x (x ≈ 0.01) structures grown using two different nitrogen RF-plasma sources. The samples have been characterised using low energy secondary ion mass spectrometry (SIMS), whilst the attributes of the RF-plasma sources have also been fully assessed with this analytical technique. The study shows that low energy SIMS is essential in order to fully characterise the structure and purity of the GaNAs superlattices grown and can also assist in qualification of the RF-plasma source used for growth.

International audience; Silicon wafers have been implanted with hydrogen at high fluence. The kinetic evolution of the buckling structures has been observed in situ by atomic force microscopy during a thermal annealing at 200 degrees C. It is shown that the blistering of the silicon wafers occurs at the first stage of the annealing without any noticeable threshold. The deflection of the blisters continuously increases with time, and some blisters are observed to coalesce. The time evolution of the internal pressure inside the cavities is finally determined from the blister dimensions with the help of an elastic analytical model. (C) 2013 AIP Publishing LLC

In this paper we experimentally investigate the application of selective interdiffusion to p‐type (Zn doped) distributed Bragg reflectors, as employed within a range of vertical cavity devices, as a means of lowering the series resistance. Various rapid thermal anneal temperatures and times are studied, both with and without silica encapsulants. The degree of intermixing, and hence series resistance reduction, is found to be cap dependent and this is verified both by secondary ion mass spectrometry and electrical resistance measurements. Both these techniques suggest that the intermixing, due primarily to Zn enhanced interdiffusion, is increased when no encapsulant is used. In this case a series resistance reduction approaching 50% is achieved within the 14 period GaAs/AlAs Bragg reflector. In the silica encapsulated case Ga vacancies are injected into the structure and these suppress the movement of Zn. The resulting Ga diffusion coefficient in this case is found to decrease by a factor of 3 relative to ...

SiGe/Si multi-quantum wells (MQWs) were grown on ridges formed on terraces between parallel V-grooves along the [110] direction on Si(001) surfaces. The structures were characterized using photoluminescence (PL), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). SEM micrographs indicate that the morphology of the low index side walls of the ridges is significantly better than that of the etched V-grooves. Cross-sectional TEM studies show that SiGe/Si MQW structures are formed at the top of the ridges, which provides possible confinement for quantum wires. Compared with MQWs formed on an unpatterned (001) surface, PL emission corresponding to the no-phonon (NP) and transverse optical (TO) phonon assisted peaks is shifted to higher energy. The position of these peaks appears to be independent of the size of the ridge.

International audience; In this paper an analytical solution for the stress state in a coated stent is presented, with a particular focus on the interface stresses between the coating and stent. As a first step a simplified stent architecture consisting of a hi-layered composite elastic arch is considered. The variations of normal and shear stress at the interface as functions of the boundary conditions at the base of the arch are explored. Depending on applied displacement and rotation, very distinct distributions of stress occur along the interface: dominant shear or dominant normal stress, compressive or tensile normal stress. A bi-layered composite elastic strut is then added to the composite elastic arch in order to create a realistic coated stent geometry. A displacement is applied to the bottom of the strut to simulate stent deployment. The addition of the strut is found to increase the normal stress and decrease the shear stress at all points on the interface. The influence of the various geometrical and material parameters on interface stress is explored using the analytical procedure developed in the paper, providing practical insight for stent-coating design. (C) 2012 Elsevier Ltd. All rights reserved.

In this paper we present results on the spectral response of p-i-n GaAs/InGaP quantum well (QW) material grown on a 10° off GaAs (100) substrate. Room temperature photocurrent spectroscopy shows that although no excitonic features are resolvable at 0 V, clear excitonic transitions are observed on application of an electric field. This effect is accompanied by a blue shift and sharpening of the band edge. Such a phenomenon is consistent with the formation of a narrow, low band gap QW at the InGaP-GaAs interface caused by an indium memory effect during growth.

International audience; Silicon and germanium wafers were implanted with hydrogen at high fluence and then submitted to thermal treatment at low temperature. By means of atomic force microscopy observations and finite elements simulations, the internal pressure under the resulting blisters appearing onto the surface has been determined. It is found that the internal pressure could be strongly overestimated from calculation if the internal compressive stresses in the implanted layers are neglected and/or incorrect boundary conditions are used. (C) 2011 American Institute of Physics. ]

Strain-balanced Si/SiGe multiple quantum wells (MQWs), which are designed to overcome the limitation of the number of wells coming from the strain accumulation, were fabricated, and their optical properties were investigated. X-ray diffraction spectra and cross-sectional transmission-electron-microscope images showed a high-crystalline quality of samples and excellent uniformity of the well width. Well-resolved no-phonon and TO-phonon-assisted transitions from strain-balanced MQWs were observed by low-temperature photoluminescence spectroscopy, and both their temperature and excitation power dependence showed blueshifts due to the delocalization of excitons, the band bending, and/or the band-filling effect.

Atmospheric pressure MOVPE has been used to investigate two GaAs based microcavity resonator devices, the “vertical surface emitting laser (VSEL) and the asymmetric Fabry-Perot modulator”. The building blocks for both devices include (Al)GaAs/AlAs distributed Bragg reflectors (DBRs), GaAs/AlGaAs MQWs and InGaAs/GaAs pseudomorphic QWs. The effect of precursor impurities on device operation is discussed together with the assessment techniques of optical reflection, X-ray diffraction, dissolution profiling and photovoltage spectroscopy. AFPM arrays with 21 isolated devices showed approximately 60% reflection change for 9.5 V bias. 50 μm diameter VSEL devices have been fabricated using DBRs grown with and without intermediate layers. The inclusion of 200 A Al 0.3 Ga 0.7 As at each interface of the DBR dramatically reduces the operating voltage of the laser.

The application of an electric field across a quantum well structure induces a shift in the exciton dominated absorption edge. This effect is exploited in a number of optoelectronic devices including electro-absorption modulators, electron-refraction modulators, and optoelectronic logic devices. We will discuss how such modulators may be designed to operate at low voltages, with large changes in transmission or reflection and at high speeds. The choice of well widths, barrier widths, the use of coupled wells, and the enhancement due to Fabry Perot structures will all be considered. Results will be included for GaAs-GaAlAs, GaInAs-InP, and GaAs on silicon structures. Applications of these devices in optical interconnection of electronic circuits, and optoelectronic logic will be reviewed.

We report the hybrid operation of a computer generated diffractive optical element, or kinoform, and a vertical cavity surface emitting laser, both elements being GaAs based. Using the laser as the optical input source the binary kinoform is shown to produce a focused 4/spl times/4 fan-out spot array, Diffraction efficiencies of approximately 20% and near diffraction limited spots are obtained for single moded VCSELs. The kinoform is also shown to function successfully for larger, multi-moded, VCSELs without significant performance drop.

We show that use of quantum-well absorbers within the Bragg reflectors of GaAs/AlGaAs-based vertical cavity surface emitting lasers results in improved chirp and modulation characteristics. Further enhancement can be obtained using strained InGaAs/GaAs.

We report for the first time broadband DPSK 5.2 GHz wireless over fibre transmission using a normal-incidence reflective InGaAsP/InGaAsP MQW AFPM as both modulator and photodetector for passive picocell applications.

The authors report the electroabsorption response of strained multiquantum well In 0.2 Ga 0.8 As/GaAs structures as a function of increasing barrier thickness. We find that a tradeoff exists between improved structural quality and reduced performance due to the increase in nonabsorbing material. However changing the barrier from 100 to 200 A causes a 25% increase in the maximum absorption change and a significant lowering of the insertion loss

Ridges developing during growth of a Si buffer layer on a Si(001) substrate patterned with V grooves, are utilized as an alternative approach to quantum wire fabrication by gas source molecular beam epitaxy. Secondary electron microscopy indicates the advantages in quality and definition of those microstructures over quantum wells grown in the vertex of V grooves. Photoluminescence results of multiple quantum well (MQW) structures grown on patterned substrates, show the existence of a more spatially confined structure and that the MQWs are almost free of dislocations.

Photoluminescence and cathodoluminescence have been used to investigate the strain in GaAs/AlGaAs multiquantum well structures grown on a thick GaAs buffer layer on a patterned silicon substrate. By growing the epitaxial GaAs only on ‘‘islands’’ of Si the density of microcracks is reduced by nearly two orders of magnitude. Although the biaxial tension is reduced in the vicinity of a microcrack, the strain is almost eliminated at the island edges. Strain is still present in the center of the islands but reduces with decreasing island size offering the possibility of relatively strain and microcrack free, small area optical devices on silicon substrates.

We present results on Resonant Cavity Light Emitting Diodes (RCLEDs) emitting at 650 nm, which have high efficiencies and low voltages. In particular, we report on the angular properties of these devices, and highlight the observation that overall spectral linewidth increases with collection angle. This unusual property of RCLEDs is largely a consequence of employing a microcavity in the design. An additional contributing factor is the relative distribution of gain amongst the cavity modes (i.e. the level of tuning or detuning of the underlying emission, defined with respect to the longitudinal cavity mode). We have used measurement techniques which spectrally resolve angular radiation profiles to determine the (de)tuning directly. Moreover, these profiles demonstrate how the overall spectral linewidth increases with collection angle. To this end, we have developed a semi- empirical method for determining the overall linewidth as a function of emission numerical aperture (NA). A 4 nm detuned device has been investigated and linewidths have been found to increase from 3.1 nm to 13.6 nm over a range of NA approximately equals 0 to NA equals 1, an increase by a factor of around 4. Obviously, a variable linewidth also implies a variable coherence length with NA. Consequently, the coherence length was found to decrease from 30 micrometer to 9 micrometer over the same range. Independent coherence length measurements were carried out by direct interferometric measurements, and confirmed the expected trends.

Conventional LEDs exhibit a broad spectral width but do not suffer with the electrical driving complexity, temperature sensitivity and optical feedback problems found with the use of lasers. Passively filtering the light from a conventional LED can narrow the spectral width but will always reduce the spectral power density of the output light and would require additional packaging and optics. However a new class of structures, namely Resonant Cavity Light Emitting Diodes (RCLEDs), have been developed in the last few years which appear to overcome several of these issues as well as offering better high-speed modulation performance and efficiency. We have been investigating RCLED's in both the GaAs and InP material systems. The quantum well emitters in both of these systems are chosen such that emission may be through the substrate, i.e. strained InGaAs QW (on GaAs operating around 980 nm) and InAsP QW (on InP operating around 1350 nm). The results demonstrate narrower spectral widths, higher peak spectral power densities and lower temperature sensitivity than nonresonant reference devices grown with nominally the same luminescent region. In this report we concentrate mainly on the GaAs structures. (5 pages)

In this paper we will focus on the challenges of fabricating photonic crystals in III-V semiconductors on rib waveguides. While discussing our methods, we keep in mind our target of applying these structures to the control of spontaneous emission as first proposed by Yablonovitch in Phys. Rev. Lett., vol. 58, p. 2059, 1987.

It has been established in previous works that corrosion kinetics in primary water of various zirconium alloys are periodic. Each period is associated with a layer of cracks parallel to the metal-oxide interface. These observations have been made either in autoclave or in pile. This indicates that corrosion processes in autoclave and under irradiation are of similar nature though their absolute kinetics might be different. Taking advantage of this correlation between cracks and corrosion kinetics, the present work aims at identifying the main microstructural parameters controlling cracks appearance in the oxide layer under well-controlled conditions. In order to achieve this, Zircaloy-4 was heat-treated to obtain various metallurgical states (stress-relieved versus recrystallised with different grain sizes) followed by corrosion tests in primary water. The key metallurgical parameters for the various conditions have been analysed (texture, precipitate sizes and grain sizes and distributions) using electron microscopy and synchrotron X-ray diffraction techniques. Corrosion kinetics of the various Zircaloy-4 microstructures are distinct as expected from the literature. Crack morphology in the oxide layer has been analysed and quantified using a dual beam scanning electron microscope/focused ion beam. Crack layers are evident even at small scale of observation. Three dimensional (3D) images of the oxide structure are presented. Cracks observed in this way are typically penny-shaped with a radius of about 100 nm. Near the metal-oxide interface, they are mainly found at the top of metal protrusions in the oxide. The roughness of the metal-oxide interface was measured. It does not exhibit any periodicity. The residual stresses in the oxide layers were measured by high energy (44 keV) synchrotron X-ray diffraction in transmission mode. Large compressive stresses (∼−1 GPa), changing with the metallurgical state and through the oxide scale thickness, were measured. The residual stresses in the oxide layers were measured by high energy (44 keV) synchrotron X-ray diffraction in transmission mode. Large compressive stresses (∼−1 GPa), changing with the metallurgical state and through the oxide scale thickness, were measured. A model of the oxide breaking at the point of transition has been developed. It is based on mechanical considerations and the existence of compressive stress in the oxide layer.

Micro-resonator modulators exploiting the quantum confined Stark shift and incorporating the asymmetric Fabry-Perot havebeen fabricated which exhibit high contrast (15-20 dB), and large reflectivity changes (>50%)atlow voltages (3V-4V). Thispaper will review progress made in the development of these devices and will consider how the performance can be modified, post growth, by use of intermixing techniques to modify the shape of the quantum wells. The asymmetric Fabry Perot structure is also a convenient configuration to investigate the intermixing process itself. Measurement of the lateral extent ofthe intermixing process promoted by vacancies in GaAs/AlGaAs structures obtained using this structure are reported. 1. INTRODUCTION Studies of the effects of electric fields on the optical properties of quantum wells have led to the development of a class ofoptoelectronic devices which relies entirely on the quantum confmement of electrons and holes in the potential wells formedby the heterostructure interfaces between two different semiconductors. Stark effect optical modulators and self electro-opticeffect devices (SEEDs) 2 are two such examples .

The authors demonstrate a voltage-controlled distributed Bragg reflector with a very high off-state reflectivity and low-voltage operation suitable for use within a vertical-cavity surface-emitting laser. The unbiased peak reflectivity approaches 99.8% at 980 nm, which may be modulated through the quantum-confined Stark effect by 1%. The mirror allows measurement of photocurrent to monitor VCSEL output power.

The first observation of electro-absorption in a InGaAsN-GaAs single quantum well p-i-n diode structure is reported. Clear excitonic resonances were observed at room temperature at 1150 nm an a strong Stark shift is obtained for applied fields of up to 2.6 /spl times/ 10/sup 7/ V/m. The field induced changes in refractive index were calculated at 1300 and 1550 nm and it is concluded that low voltage phase modulators could be fabricated using this material.

High efficiencies and low voltages for resonant-cavity light-emitting diodes operating close to 650 nm are reported. Peak external quantum efficiencies of 6% are obtained for a low current injection of ~1 mA, giving an optical power output of 0.1 mW. For higher current injection at 13 mA, 1 mW of power is produced with a slightly lower, but modest, efficiency of 4%. These record high efficiencies mean that high power performance previously achieved in RCLEDs at the inherently more efficient wavelength of 660 nm can now be replicated in 650 nm devices. Forward biased operating voltages are between 1.7 and 1.9 V for a current range of 0 – 60 mA.

The first demonstration of resonant cavity enhancement of optical emission from an InAs-GaAs quantum dot light emitting diode is reported. For emission around 1300 nm efficiency enhancements of greater than 10 for low current injection and greater than 3 for higher current injection for light coupled into a multimode optical fibre are observed.

The post-growth bandgap engineering of a fifty period multiple quantum well is reported. A controlled blue shift of the band edge is achieved through the deposition of a SiO/sub x/ encapsulant followed by rapid thermal annealing giving shifts of up to 16 meV. The expected depth dependence of this process is not observed and thus the blue shifted samples retain clearly resolved room temperature excitonic features and preserve the quantum confined Stark effect.

Zones of total stress tension and steep stress gradient occur in soil structures such as multilayer pavements and earth dams, and direct tension tests and simple bending tests on soils could be used advantageously to investigate soil behaviour under these conditions. The Paper describes the observed behaviour in direct tension and bending of two clays compacted with standard Proctor effort at different moisture contents ranging from below to above optimum. The investigation is in terms of total stress and is concerned with both stress-strain behaviour and strength condition. It is shown that the elastic bending theory is not satisfactory for analysing bending in soils and an alternative differential method of analysis is used which does not require the assumption of any preferred stress- strain laws. The maximum tensile strains at failure are observed to increase with the increase of moulding moisture content irrespective of the type of test or the optimum moisture content. This supports the view that clays should be compacted at high moisture contents when considerable tensile deformation of a soil structure is expected. The values of both the initial tangent modulus and of the secant modulus (at maximum stress) for both clays are markedly greater in tension than in compression, and the relative values are influenced by the moulding moisture content. Thus the use of identical values for tension and compression in analysis of soil structure is not justified. Des zones de contrainte de tension totale et de gradient élevé se produisent dans des structures de sol telles que les multicouches et les barrages en terre. Des essais de tension directe et de simples essais de flexion sur les sols pourraient être utilisés avantageusement afin d'étudier le comportement du sol dans ces conditions. L'étude décrit le comportement observé dans une tension non drainée et une flexion de deux argiles compactées au Proctor standard à diverses teneurs en eau inférieures. et supérieures à l'optimum. L'étude est en termes de contraintes totales et elle est concernée par le comportement de contrainte de tension ainsi que par les conditions de résistance. II est montré que la théorie de la flexion élastique n'est pas satisfaisante pour analyser la flexion dans les sols, et une autre méthode différente d'analyse est utilisée qui n'exige pas l'hypothese d'aucune loi spéciale de contrainte déformation. On observe que les déformations maximum de traction à la rupture augmentent avec l'accroissement de la teneur en eau, indépendamment du genre d'essai ou de la teneur optimum en eau. Cela confirme que les argiles devraient être compactées à des teneurs en eau élevées lorsque l'on s'attend à une déformation de traction considérable du sol. Les valeurs du module initial tangent et du module sécant (à contrainte maximum) pour chacune des argiles sont, d'une façon marquée, plus grandes en tension qu'en compression, et les valeurs relatives sont influencées par la teneur en eau. Ainsi, l'utilisation de valeurs identiques en traction et en compression dans l'analyse de la structure du sol n'est pas justifiée.

Measurements of the intensity dependence of CO 2 laser radiation reflected from a polished Cd 0.23 Hg 0.77 Te etalon at 77 K results in nonlinear features at power levels on the order of 1 mW, in good agreement with a theory which includes bandgap resonant nonlinearities and Auger carrier recombination. A high-reflectivity rear coating enhances these features, giving a characteristic which demonstrates differential optical gain. The effects of heating are discussed in this unusual case in which the electronic and thermal contributions to the intensity dependence have the same sign.

Summary Several theories have been advanced to explain the magnetic properties of rocks. Measurements reported here have been made on specimens from a single lava to investigate the use of the theoretical relationships to determine the amount and nature of the oxides responsible for the NRM, the extent to which the original moment appears to have been disturbed and appropriate models to describe the magnetic particle distribution. Evidence for low temperature oxidation is common enough to call into question whether the theories of TRM are applicable. The oxides in these specimens are changed by heating, so anhysteretic moments were induced in an attempt to compare these with the theoretically expected values and thus to estimate if the natural remanent moment of a specimen is of the size that would be expected for the oxide dispersion.

Specimens of kaolin have been prepared in the laboratory by consolidating from a slurry, both isotropically and anisotropically, to a lightly overconsolidated state within an overconsolidation range of l·0–2·7. Each specimen was then tested in either undrained triaxial compression or extension. This has enabled a study to be made of the influence of preconsolidation pressure and anisotropy on the strength and stress-strain characteristics of lightly overconsolidated kaolin. These results are relevant to the general stress-strain studies being pursued at Cambridge on idealized soils and, to some extent, to the behaviour of soft clay deposits in the field. On a préparé au laboratoire des échantillons de kaolin en les consolidant à partir de boues, de manière isotrope et anisotrope, en un état legtrement surconsolidé ayant un domaine de surconsolidation allant de 1 à 2·7. On a étudié chaque échantillon en compression ou traction triaxiale à teneur en eau constante. Ceci a permis d'étudier l'influence de la pression et de l'anisotropie de surconsolidation sur la résistance et Ies caractéristiques contrainte– déformation de kaolins légèrement surconsolidés. Ces résultats présentent un intérêt pour les études générales de contrainte-déformation de sols idéalisés qui sont en cours à Cambridge, et dans une certaine mesure, pour le comportement de dépaôts d'argiles tendres in situ.

We discuss the origin and characteristics of the very large intensity-dependent changes in the refractive index of the semiconductor cadmium mercury telluride at 10.6 m. With continuous wave (c.w.) radiation and sample temperatures of 77 K nonlinear Fabry-Perot effects can be observed at powers as low as 1 mW. The Auger dominated recombination process and the thermal contributions to the refractive index make observations of optical bistability difficult with c.w. radiation. However, the use of a pulsed CO 2 laser and two-photon absorption of the radiation does lead to the observation of bistability at room temperature. We compare these results with the reported work on InSb.

The achievable electroabsorption modulation in GaAs/(GaAl)As normal-incidence multiple-quantum-well (MQW) diodes with 50-AA and 100-AA well widths is assessed. It is suggested that modulation performance should be characterized by the transmission change (T/sub hi/-T/sub lo/) when the device is used in a simple source-modulator-detector communication link, whereas in more complex applications in which optical signals are merged, the contrast is more appropriate. For either transmission change or contrast there is an optimum number of wells which varies with design voltage, and this is used to calculate achievable modulation in the bias-absorbing mode for 100-AA and 50-AA well widths. The effects of improved smoothness at the interfaces and lower background dopings are investigated. It is found that a transmission change of 40% at 5 V is typical and does not improve significantly with better growth, while the achievable contrast depends strongly on the background doping. The results are compared with those of real modulators. >

A statistical formulation is given explaining the fibrous structure seen in speckle patterns in polychromatic light. It is shown that the appearance of the structure is strongly dependent on the surface roughness of the diffuser. This may be useful as the basis of a determination of surface roughness.

Brief details are given of three methods used to analyse stress in soil beams composed of compacted clay. Two of the methods, direct method and differential method, apply to the calculation of stresses from the applied bending moment and the strains measured by the radiographic technique. In the third method, elastic bending theory method, the stresses and strains are calculated from the bending moment and the observed deflections of the beam using elastic bending theory. A comparison is made of the different methods and it is concluded that the differential and direct methods of stress analysis for bending tests give the same shape of stress-strain curves although not identical curves. Stress-strain curves obtained from the elastic bending theory differ in shape from those obtained by the other two methods. Tensile and compressive stresses obtained by the differential method are lower than those obtained by the other two methods because the differential method allows some plastic redistribution of stress. As failure is approached tensile stresses by direct and elastic bending theory are respectively 1.4 and 1.5 times average tensile stress by the differential method. /TRRL/

A description is given of equipment for the measurement of the Kerr effect in water and other relatively highly conducting media. The equipment uses 2 mu s field pulses up to 8 MV m-1 in amplitude and is sufficiently sensitive to measure the smallest Kerr constants such as that of carbon tetrachloride. Illustrative results are presented for water.

Atmospheric pressure MOVPE has been used to prepare AlGaAs/GaAs MQW pin diodes exhibiting well-resolved room temperature exciton resonances and large reverse field breakdowns. The MQW i regions have been optimised by applying growth conditions which minimised the residual carbon concentration associated with the reagent TMA. Diodes with MQWs of well width 40 to 160 A have been assessed for the quantum confined Stark effect (QCSE) by photocurrent spectra and as absorption modulators. A 75 period 50 A Al 0.3 Ga 0.7 As/60 A GaAs i region gave a transmission change of 20% at 828 nm for 16 V applied bias. Diodes have also characterised for hole and electron ionisation rates when biased as avalanche detectors. An electron-to-hole ionisation rate ratio enhancement of 14 was observed for a 45 period 58 A Al 0.3 Ga 0.7 As/105 A GaAs MQW.

Using synthetic near-ideal graphite obtained by hot-pressing and annealing pyrolytic material, we have prepared graphite nitrates from the first to the fourth sequence, under controlled conditions. For the first sequence compound, present determinations on form I (room temperature) agree with previous publications and confirm the stacking sequence A | A | A | A . Form I sequences 2, 3 and 4 show well defined stacking of the graphite networks, characterized by X-ray methods as follows: sequence network stacking* system unit cell (Å) space group 2 A | AB | BC | CA | A rhombohedral a H = 2⋅46 R 3 ¯ m c H = 33⋅45 3 A | ABA | ACA | A orthorhombic a = 2⋅46 Cmc 2 1 b = 4⋅26 c = 28⋅96 4 A | ABAB | BCBC | CACA | A rhombohedral a H = 2⋅46 R 3 ¯ m c H = 53⋅5 * The vertical lines imply layers of intercalate. In all these compounds, the stacking sequences found can be generated from the stacking in normal graphite, ABAB , if the change AB → A | A on entry of successive intercalate layers involves the movement of a boundary dislocation through the structure. Since it is observed that the end-product (sequence N ) under any given conditions is formed by way of a systematic numerical succession of higher sequence compounds ( N +2, N +1) as intermediates, this fundamental stacking change must reverse so that AB ⇌ A | A . When any of these sequences of graphite nitrate are cooled through a λ transformation around —20°C, increased ordering appears in the low temperature structures (form II). Each intercalate layer now exhibits two-dimensional long range order. In addition, successive intercalated layers within any crystallite are stacked in a non-random way, which in the case of the first sequence compound implies a structure that is periodic in three dimensions. Within any intercalated layer in form I, the molecular packing resembles that in liquids. In a given crystal compound the numerical sequence of such layers is regular, and successive intercalate layers occur after N carbon hexagon planes. In form II, molecular packing in the intercalated layers is crystalline. Some correlation also appears between the occupied sites in different filled layers. Implications of this order-disorder transformation are briefly discussed.