bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical studies of the structure of porous silicon films formed in p-type degenerate and non-degenerate silicon
Authors: C Pickering  , M I J Beale  , D J Robbins  , P J Pearson  , R Greef
Published: July 2002

The detailed structure of porous Si (PS) layers formed in p-type wafers with resistivities 0.01-25 Omega cm has been investigated using reflectance, transmission, ellipsometry and photoluminescence techniques. Marked differences were observed in the optical properties of PS formed in degenerate or non-degenerate Si and these results are correlated with the results of other techniques. The optical techniques together with effective medium modelling have been shown to be useful non-destructive methods for either assessment of PS density or detection of unsuspected phases. The degenerate PS layers consistently showed good retention of the single-crystal characteristics of the starting wafer, only c-Si and voids being detected. For these samples, good agreement was obtained between optical and gravimetric densities. However, the non-degenerate PS had much greater variability, with greater loss of crystallinity and significant incorporation of oxygen, due to partial oxidation having occurred on or immediately after anodisation. Oxide fractions have been determined both optically and gravimetrically, with up to 50% oxide being detected in some samples. Non-degenerate PS samples with high oxygen concentrations appeared to be in the form of a chemical mixture, SiOx, from interpretation of the optical constants. Photoluminescence measurements together with the other techniques indicated a complex mixture of phases in the latter samples-voids, alpha -Si:O (and/or alpha -Si:H), an unknown amorphous phase and silicon oxide. This complex structure probably contributes to the observed instability of thick non-degenerate PS layers when heated in UHV as part of the cleaning procedure prior to epitaxial growth, all degenerate samples being able to withstand heat treatment.
Effect of the variation of the inner potential of the crystal surface in VLEED
Authors: J Lopez  , J C Le Bosse  , J M Baribeau
Published: July 2002

In very low-energy electron diffraction (VLEED), the fine structures appearing below the emergence thresholds in I00(E) are usually used to test surface barrier models. The authors reinvestigate this problem, in particular for the case of a loosely packed surface. Two surface barrier models are tested in the case of a very-low-energy intensity profile (E <or approximately=5 eV) obtained on the W(001) surface for theta =45 degrees along the (10) azimuth: the modified image barrier introduced by Jennings and Read (1974) and a new saturated image barrier. Neither of these models yields a good fit of experimental data as long as one assumes that the barrier joins the bulk inner potential at the top layer edge continuously or with a potential step. Without challenging the validity of the muffin-tin approximation inside the first layer, it can be suspected that the top-layer potential is less attractive than any bulk-layer one. it is found that for both barrier models, shifting the top-layer inner potential upwards leads to a significant improvement of the fit. It is also found that the modified image barrier tends to exhibit a non-physical region of constant potential potential near the top layer and as a consequence the saturated image barrier appears to be more reliable. Finally this work illustrates the fact, that, especially for loosely packed surfaces, the surface barrier concept must not be restricted to the vacuum region but has to be extended inside the crystal region where the bulk potential is modified by the presence of the surface.
Dynamical screening and carrier mobility in GaAs-GaAlAs heterostructures
Authors: X L Lei
Published: July 2002

The ohmic mobility due to acoustic and polar optical phonon scattering in a GaAs-GaAlAs heterojunction is calculated by the use of a formula in which temperature, wave vector and frequency-dependent screening are built in. It is shown that under RPA the enhancement of the polar optical phonon-induced resistivity due to dynamical effects almost compensates the reduction by static screening over the entire temperature region.
Thermal conductivity under pressure and through phase transitions in solid alkali halides. I. Experimental results for KCl, KBr, KI, RbCl, RbBr and RbI
Authors: P Andersson
Published: July 2002

The thermal conductivity, lambda , of polycrystalline KCl, KBr, KI, RbCl, RbBr and RbI has been measured at temperatures (T) from 100-400K at pressures (P) up to about 2.6 GPa, using the transient hot-wire method. Extensive results were obtained for lambda (T,P) of both the low-pressure (NaCl-type) and high-pressure (CsCl-type) phases. There was generally good agreement with previous results for lambda (P) at room temperature, the decrease of lambda through the phase transition and lambda (T) at atmospheric pressure. The heat capacity per unit volume was also measured simultaneously but for technical reasons data could be obtained only near room temperature. Corresponding values of specific heat capacity were in good agreement with previous work over the whole pressure range.
A renormalisation group study of weak disorder in the Hubbard Hamiltonian
Authors: G R Bhat  , V A Singh
Published: July 2002

The authors have undertaken a real-space renormalisation group study of the weakly disordered Hubbard Hamiltonian. They find that the presence of a small correlation leads to an unexpected result: an enhancement of delocalisation which becomes more pronounced as the dimensionality increases.
Exchange interactions between Cr3+ions in magnesium oxide. II. Electron spin resonance spectra
Authors: J C G Carroll  , S M McMurry  , J Corish  , B Henderson
Published: July 2002

For pt.I see ibid., vol.18, p.6397 (1985). Cr3+ions in MgO may cluster in pairs with cation vacancies to form C.MgV"MgCr.Mgneutral dimers, which behave magnetically as exchange-coupled pairs. The authors have identified two different neutral Cr3+dimers in MgO using electron spin resonance (ESR). One is a linear aggregate parallel to (100) directions; the other has a right-angled configuration centred on the vacancy, with each of the impurity-vacancy interactions being in (100) directions. The ESR spectra have been interpreted using exact solutions of the spin Hamiltonian in which the magnetic ions are coupled by two linear terms Js1.s2+De(3(s1.n)(s2.n)-s1Os2). The exchange interactions are both antiferromagnetic with J-values of 0.004+or-0.001 cm-1and 1.5+or-0.05 cm-1for the linear and bent dimers respectively. These values reflect the variation of J with magnetic ion separation in the dimer.
Comments on thermal donor states in silicon and germanium
Authors: C S LaPorta  , J C Kimball  , J T Borenstein  , J W Corbett
Published: July 2002

A simple model that explains the ionisation spectra for thermal donors in silicon and germanium is extended, and it is demonstrated that the model can explain data for both neutral and charged thermal donor states without using additional parameters.
Censored Weibull statistics in the dielectric breakdown of thin oxide films
Authors: S M Rowland  , R M Hill  , L A Dissado
Published: July 2002

Self-healing dielectric breakdown in thin oxide films has been shown to obey Weibull statistics after allowance has been made for both left and right censoring that is intrinsic to the measuring technique. Two overlapping distributions have been found under constant DC voltage stress testing but under ramp stress conditions these were not always observed. Nevertheless it is shown that the two types of test gave consistency in their analyses. Two particular features of the results reported are that the number of potential events, in some cases, was an exponential function of the applied field and that under ramp test conditions the magnitude of the applied potential at the breakdown estimator time appears to be constant and independent of the sample thickness. The implications of these observations are discussed.
Dielectric and optical properties of lead-activated sodium and potassium chloride crystals
Authors: J S Dryden  , G G Harvey
Published: July 2002

As part of a study of the aggregation of impurity ions in alkali halides, measurements of dielectric loss, of optical absorption and of emission spectra have been made on NaCl: Pb2+and KCl: Pb2+crystals. Changes in the optical absorption and the emission spectra with aggregation of the Pb2+-vacancy pairs are more pronounced in NaCl than in KCl. The main conclusions from the NaCl: Pb2+measurements are:

changes in the absorption and emission spectra result from the same aggregation process studied previously in many alkali halide crystals by dielectric measurements;

the emission spectra, but not the absorption, change when the divalent ion-vacancy pairs (dipoles) form the initial aggregates (trimers); later in the aggregation process both absorption and emission change;

the optical measurements show that the aggregation takes place in several definite stages and the results are not inconsistent with the model proposed earlier, viz. the initial aggregates are trimers and these grow into pentamers, and then heptamers etc. by the addition of dipoles, two at a time.

The dielectric absorption measurements give activation energies for the diffusion of Pb2+-vacancy pairs through NaCl and KCl lattices of 0·80 ev and 0·85 ev respectively.
Low-temperature behaviour at the percolation threshold for Ising spins with correlated dilution
Authors: N S Branco  , Raimundo R dos Santos  , S L A de Queiroz
Published: July 2002

The authors study correlated dilution of two-dimensional Ising spins. In this 'bootstrap dilution' model, a (randomly occupied) site only develops a magnetic moment if surrounded by at least m magnetic neighbours. By generalising an ansatz proposed by Coniglio (1981, 1982) for ordinary dilution, they find that for some values of m, temperature is irrelevant (in a renormalisation-group sense) near the percolation fixed point, contrarily to the uncorrelated case. Real-space renormalisation-group calculations illustrate their findings.
The rare earth cobaltates: localised or collective electron behaviour?
Authors: G Thornton  , F C Morrison  , S Partington  , B C Tofield  , D E Williams
Published: July 2002

The electrical conductivity and differential thermal analysis (DTA) characteristics of RECoO3 (RE=La, Nd, Gd, Ho, Y) have been reexamined. In general, the data are quite different from those previously reported. In particular, first-order semiconductor-to-metal transitions of YCoO3, LaCoO3 and HoCoO3 are not observed. Instead, broad, high-order semiconductor-to-metal transitions are observed over the approximate temperature ranges La: 385-570 K; Nd: 400-590 K; Gd: 490-770 K; Ho: 540-795 K; Y: 575-850 K. The form of the transitions is similar to that observed for Ti2O3 where a thermally activated band overlap model has successfully been employed. A similar scheme for the rare earth cobaltates can be constructed in which Co 3d electron character is considered collective. This is contrary to the localised-to-collective phase transition model previously employed. In addition, a first-order semiconductor-to-semiconductor transition of NdCoO3 at 342 K is observed in electrical conductivity data. The tenfold increase in conductivity accompanying this structural transition can also be understood within a collective 3d-electron model.
Analysis of the electron paramagnetic resonance parameters of BaZnF4:Mn
Authors: Zhou Yi-Yang
Published: July 2002

The electron paramagnetic resonance (EPR) parameters of BaZnF4:Mn have been studied theoretically. The substitution of Mn2+ions for the Zn2+ions has been determined by using a method different from that of other workers. The contribution of next-nearest neighbours to the EPR parameters has been discussed and the results obtained are different from those of previous authors. In addition, various mechanisms which contributed to EPR parameters have been considered for comparison.
Direct observation of gap states in a-Si: H through the SiL23soft X-ray emission spectrum
Authors: R S Crisp  , D Haneman  , V Chacornt
Published: July 2002

Soft X-ray L23emission spectra for single crystal and polycrystalline bulk silicon (c-Si) and for amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) films are presented and show a gross change in the general form of the emission band in going from the crystalline to the amorphous forms. The valence band emission for p-a-Si:H differs noticeably from that for i- and n-a-Si:H. Emission is also observed corresponding to filled states lying in the energy gap above the valence band edge; this apparently has its origin in interface states at surface films.
Influence of plastic deformation on the growth of F centres in highly pure KCl crystals
Authors: P C Mehendru
Published: July 2002

The effect of plastic deformation on the growth of F centres in highly pure KCl crystals has been studied from the growth of the thermoluminescence peaks at 135, 190, and 270 degrees C (for 40 degC min-1 heating rate). These are associated with the first and the second stage F centres, and with F centres due to deformation, respectively. The growths can be fitted by the equations of Mitchell, Wiegand and Smoluchowski (1961) and of Frohlich (1961-62) and the values of the appropriate parameters are given and discussed.
Electrical properties of liquid selenium up to 1300 K
Authors: H Gobrecht  , F Mahdjuri  , D Gawlik
Published: July 2002

The thermal emf and the magnetoresistance of liquid selenium were measured with graphite electrodes in a pressure vessel up to 1300K. A sign change of the thermoelectric power was observed at about 990K. Above this temperature selenium is an n-type semiconductor. A relatively high magnetoresistance was measured above 990K. Experiments for measuring the Hall effect with graphite electrodes up to 870K failed. Considering the results of the experiments a model for the conduction mechanism could be suggested.
A selfconsistent theory of localization
Authors: R Abou-Chacra  , D J Thouless  , P W Anderson
Published: July 2002

A new basis has been found for the theory of localization of electrons in disordered systems. The method is based on a selfconsistent solution of the equation for the self energy in second order perturbation theory, whose solution may be purely real almost everywhere (localized states) or complex everywhere (nonlocalized states). The equations used are exact for a Bethe lattice. The selfconsistency condition gives a nonlinear integral equation in two variables for the probability distribution of the real and imaginary parts of the self energy. A simple approximation for the stability limit of localized states gives Anderson's 'upper limit approximation'. Exact solution of the stability problem in a special case gives results very close to Anderson's best estimate.
Damping theory of the magnetophonon structure in the longitudinal Seebeck effect in semiconductors
Authors: J R Barker
Published: July 2002

A resolvent perturbation analysis of the Kubo-Luttinger response formulae is used to predict weak phase shifted minima in the damped magnetophonon structure of the longitudinal thermal EMF in semiconductors.
Reorientation and degrees of binding of X2+-vacancy complexes in KCl
Authors: A Brun  , P Dansas
Published: July 2002

The reorientation of impurity-vacancy complexes in KCl crystals doped with Mg2+, Ni2+, Mn2+, Ca2+, Sr2+, Pb2+and Ba2+was studied using the ITC method. These results are analysed together with those obtained by other authors by ITC, EPR and optical methods on Cd2+, Be2+, Eu2+, Sm2+and Yb2+ions. For the first series of ions and Cd2+, it was concluded that the most favoured vacancy position is the NNN site, and that it increasingly dominates the NN site as one proceeds from Mg2+to Ba2+. The observed reorientation of complexes is mainly governed by the vacancy jump from the NNN to the NN position. In the case of Be2+it was found that the majority of vacancies are in the NN position and the special behaviour of the Eu2+, Sm2+and Yb2+rare earth ions is discussed.
On the narrowing of Mossbauer lines by selective modulation: lifetime distribution of excited nuclei
Authors: P Hannaford
Published: July 2002

Expressions are obtained for the distribution of lifetimes and the mean lifetime of excited Mossbauer nuclei which contribute to gamma -ray emission lines detected by selective modulation. The treatment is restricted to the case of pure natural-broadened line profiles.
The low-temperature thermal expansion and Gruneisen parameters of some tetrahedrally bonded solids
Authors: T F Smith  , G K White
Published: July 2002

The results of thermal expansion measurements using a three-terminal capacitance dilatometer on Ge, GaAs, ZnS, ZnSe and CdTe below 30K and on ZnSe and CdTe between 57 and 90K are reported. Respective values of 0.49, 0.32, -0.14, 0.0 and -0.85 derived for gamma0th, the zero temperature limit of the Gruneisen parameter gamma =3 alpha VBs/Cpare given. These are in good agreement with the values of gamma0elcalculated from the pressure dependence of the elastic constants. gamma (T) as a function of reduced temperature, T/ theta0, where theta0is the Debye temperature determined at the zero temperature limit, is found to have an essentially universal form and passes through a negative minimum at T approximately 0.06 theta0. The implications of this observation and the importance of the degree of ionicity in dictating gamma0are discussed.
The chain method for electron scattering in lattices
Authors: J B Pendry  , P Gard
Published: July 2002

A step by step procedure is suggested for solving electron scattering problems in crystals. First scattering for a single atom is calculated, then for a chain of atoms next for a layer of chains and finally for the entire crystal. The new method has great speed and flexibility. It is applicable to band structure and LEED calculations and is especially useful in the theory of RHEED.
Angular dependence of electron emission from surfaces
Authors: J B Pendtry
Published: July 2002

A close analogy is traced between LEED and emission processes resulting in a generalization of LEED calculations to encompass the emission case. The theory describes all multiple scattering corrections to angular dependence of emission by making use of RFS perturbation theory and is expected to be of use for interpretation of Auger and XPS experiments, especially those using synchrotron radiation.
Proton relaxation by tunnelling in partly deuterated ammonium salts
Authors: I Svare  , D P Tunstall
Published: July 2002

Proton relaxation in partly deuterated (NH4)2SnCl6and NH4ClO4at 4.2K is very much faster than in the non-deuterated salts. The short T1is caused by tunnelling, which for asymmetric NH3D+allows a two-step energy transfer process: Spins to Tunnel states to Lattice. The spin-tunnel transfer is resonant at the level crossings Omega0= Omegatunneland 2 Omega0= Omegatunnel. In symmetric NH4+the Pauli principle requires simultaneous change of spins, tunnel and phonon states, which can only happen through higher-order transitions with small probabilities. A similar relaxation difference is predicted between CH3and CH2D compounds.
Spectral linewidths in acoustoelectric many-wave systems
Authors: W Wonneberger
Published: July 2002

The natural spectral linewidths of individual waves in a stationary (nonlinear) acoustoelectric many-wave system are investigated theoretically within the Fokker-Planck approach to fluctuation phenomena. Using methods of turbulence theory it is found that the wave-wave coupling provides the dominant contribution to the linewidths, thus exceeding the influence of spontaneous emissions and thermal fluctuations. The final result is given in the form Delta omega = alpha /g+ Delta alpha , where the first term is the spectral linewidth of a free Gaussian wave with thermal loss constant alpha and amplification factor g above thermal level. The second term, Delta alpha , is the wave-wave coupling contribution which equals the differential loss per mode at the operating point. Numerical results for the linewidths of modes in the acoustoelectric oscillator operating in several modes, in quasi-stationary fluxes and in stabilized acoustic domains are also given.
Is molten cuprous chloride a molecular liquid?
Authors: J G Powles
Published: July 2002

Recent measurements of the three partial structure factors of molten cuprous chloride by neutron diffraction Page and Mika (1971) have been reinterpreted in terms of a liquid consisting of CuCl molecules with internuclear separation of 2.3 AA and an internal vibration frequency greater than 400 cm-1. The assumption of a plasma-like distribution of itinerant copper ions is shown to be unnecessary. It is suggested that the molecule be searched for by inelastic neutron scattering, by neutron diffraction at both high and low momentum transfer, by spectroscopy and by nuclear magnetic resonance.
Optical pumping cycle of exchange-coupled F+-centre pairs in MgO and CaO
Authors: B Henderson
Published: July 2002

In n-irradiated CaO and MgO crystals, triplet-state defects are produced in which two electrons are trapped in the O2-sites of a linear trivacancy configuration O2--Ca2+-O2-parallel to the (100) crystal direction. The electronic spins are coupled antiferromagnetically to give a ground-state singlet1A1gfor these F+-centre pairs, the first excited level3A2ubeing thermally populated at room temperature. Measurements of the3A2ustate EPR spectrum at 1.4K under optical excitation demonstrates that optical spin alignment memory is induced during the optical pumping cycle.
The Hall conductivity of a disordered alloy in the coherent-potential approximation
Authors: K Niizeki  , K Hoshino
Published: July 2002

The Hall conductivity is formulated within the CPA for the tight-binding model Hamiltonian. Particular care is taken for gauge invariance of the formulation. The present formalism is applicable to the case with off-diagonal disorder in addition to diagonal disorder, and the effect of the off-diagonal disorder on the Hall conductivity is investigated for the model considered by Shiba (1971) and that by Blackman et al. (1971). It is shown that the vertex correction to the Hall conductivity vanishes for these models provided that the lattice has the inversion symmetry. For a numerical application of the present formalism a binary alloy with a simple cubic lattice is investigated.
Polarized Raman scattering in the solid electrolyte RbAg4I5
Authors: D Gallagher  , M V Klein
Published: July 2002

Polarized Raman spectra of oriented single crystals of RbAg4I5 have been measured for all scattering geometries from 448K down to near the second-order phase transition at 208K. Unpolarized spectra were continued down to 136K. The authors have observed both the vibrations of the Ag+ ions relative to their cages of I- ions and the breathing modes of the cages themselves.
The thermal-neutron Milne problem with capture
Authors: M J Lancefield  , P Schofield
Published: July 2002

A variational principle devised for the Milne problem without capture has been extended to include capture. The theory is at present restricted to isotropic scattering. The flux in the energy-dependent problem is expressed in terms of the corresponding one-velocity solution having the same asymptotic spatial dependence and the one-velocity half-space Green function for an isotropic plane source. Then a variational principle for a quantity related to the extrapolation length may be written down and the emergent angular distribution determined by iterating the trial function. Results are presented for some simple scattering models.
Ionization coefficients and sparking voltages in argon and argon-ethane mixtures
Authors: A E D Heylen
Published: July 2002

Townsend primary and secondary ionization coefficients have been experimentally determined in pure argon and in argon mixed with up to 3% of ethane gas for a total pressure in the range 0·5-700 torr, corresponding to 3less, similarE/p0less, similar2000 v cm−1torr−1at 0°C. Ionization enhancement takes place and it is shown analytically that the major contribution to this is due to the Penning process. At an optimum concentration of about 0·66% of ethane in argon the primary coefficient is increased to forty times its value for pure argon. The presence of common impurities increases the coefficient by up to five times, whilst a similar concentration of electron-attaching gases lowers the coefficient by a factor of 10. Because of the quenching nature of ethane it is shown possible to reduce the secondary ionization coefficient of argon by as much as 107times; with the addition of less than 0·03% of ethane it is possible to increase the secondary coefficient at lowE/p0 by a factor of 10 above that for pure argon. At the optimum concentration the sparking voltage is a factor of 2·2 below that for pure argon, whilst the presence of 0·000 03% of ethane still lowers the sparking voltage of pure argon, by a factor of 1·3. All the sparking voltages lie well below that for pure argon.
Determination of temperature in a radio-frequency discharge using a reversal technique
Authors: P D Johnston
Published: July 2002

An absorption technique for determining the temperature in a circularly symmetric discharge has been developed by Elder, Jerrick and Birkeland, and applied by them to a low-temperature discharge using a tungsten ribbon background source. This technique has been extended here to a higher temperature radio-frequency discharge in argon at atmospheric pressure using a condenser discharge background source. The measured off-axis temperature maximum of 9200°K is in agreement with previous measurements. The error introduced by the finite resolution of the spectrograph used is discussed.
An examination of quantitative electron-probe microanalysis accuracy
Authors: W J M Salter
Published: July 2002

Over 900 complete analyses from both metallic and oxide systems have been examined statistically to investigate the reliability of quantitative electron-probe microanalysis. Details of the general experimental procedures adopted and the forms of fluorescence and absorption correction used are given. The majority of the systems examined were multicomponent ones and this necessitated the use of a `matrix method of manual calculation', which is described.
Measurement of the quantum efficiency of noble-gas scintillators for X-rays
Authors: A E Kiss  , N Patla
Published: July 2002

Experimental values of the x-ray quantum efficiency of krypton, argon and neon scintillation counters for chromium, cobalt and copper K radiation are determined by comparison with a proportional counter, and the results analysed in terms of the energy conversion efficiency of the gas atoms and statistical losses in the photomultiplier. The particular geometry and operating conditions described give maximum efficiencies at the Cu Kα wavelength of 14% for krypton, 4·3% for argon and 3·5% for neon, falling with increase of wavelength. Application of a large electric field to the gas results in practical efficiencies approaching 100% due to gas amplification processes.
The determination of some X-ray mass absorption coefficients
Authors: G D Hughes  , J B Woodhouse  , I A Bucklow
Published: July 2002

Experimentally determined mass absorption coefficients for various wavelengths from 0·5 to 10 Å in Al, Ti, Ni, Cu, Zr, Nb, Pd, Ag, Ta and Au are presented in tabular form; an accuracy to about ±1% is claimed. The tables contain both new and revised data which are compared with values published by Heinrich in 1966; there is good agreement between the two sets.
The spectral response of Cu2S-CdS photovoltaic cells
Authors: R J Mytton
Published: July 2002

Measurements have been made of the spectral response of Cu2S-CdS photovoltaic cells, prepared by the chemiplating technique, as a function of the plating solution concentration and the plating time. The results are interpreted qualitatively in terms of simple energy band theory and the resulting model compared with those already proposed.
Simple transient and non-linear analysis of high-field domains in GaAs
Authors: R Harrison
Published: July 2002

An approximate non-linear study of the phenomenological aspects of the Gunn effect in gallium arsenide has been made. We have attempted, with reasonable success, to analyse quantitatively that region of electron dynamics which includes the growing of the electric field domain from some initial electric field perturbation.

From the initial transient analysis we derive two asymptotic forms of field instability for the boundary value problem, and from inspection of the behaviour of the dynamic impedance of the specimen we conclude that there are three conditions for the successful nucleation of a high-field domain. For initial instabilities to commence either the resistivity of the specimen must be less than some critical value or the product of carrier concentration and specimen length (n0l) must be greater than some minimum value. For the domain to be stable in the steady state, the non-linear analysis indicates that the product of the carrier concentration and the square of the domain widthd(n0d2) should be greater than some critical minimum.

Finally, if the non-linear velocity-field characteristic can be approximated by a parabola for fields greater than the threshold field for negative differential resistivity then we can derive a set of approximate analytical results for the rise time, peak field and width of a domain, and also specify the minimum specimen length required for a domain to be fully formed.
The electron drift velocity in dense gases
Authors: V M Atrazhev  , I T Iakubov
Published: July 2002

The conditions are considered when the interaction of electrons with atoms cannot be described as a number of successive independent collisions. This results in an additional density dependence of the drift velocity. The two density effects are taken into account-the interference of electron and the polarizational screening. It allows the authors to discuss the experimental data available and to explain the results in heating electric fields in hydrogen, argon and methane.
Non-linear mechanical response of various metals. III. Swift effect considered in relation to the stress-strain behaviour in simple compression, tension and torsion
Authors: E W Billington
Published: July 2002

For pt.II see ibid., vol.10, no.4, p.533 (1977). The correlation of the stress-strain behaviour in simple compression, tension and torsion is considered for various metals in relation to the Swift (1947) effect, here observed as the permanent lengthening of a tubular specimen along the principal axis about which it is being twisted. The proposed correlation is based on an incremental-type constitutive equation for an isotropic incompressible strain-hardening elastic-plastic solid with large deformation. Formulation of the proposed constitutive equation involves a modified Mises yield condition and the assumption that the total deformation gradient can be decomposed into a matrix product of the local elastic and plastic deformation tensors. Since the Swift effect appears to be absent at high strain rates, the metals of present interest should appear stronger at high strain rates, a conclusion in accord with the observation that the dynamic yield stress is frequently observed to be higher than the static yield stress in all three simple modes of deformation.
The theory of the back-scattering of light by blood
Authors: G Eason  , A R Veitch  , R M Nisbet  , F W Turnbull
Published: July 2002

Motivated by recent experiments on the back-scattering of light by blood the authors have studied three models of the scattering of a cylindrical light beam by a half-space. In each case solution of the photodiffusion equation yields integral expressions for the photon density everywhere in the medium and for the back-scattered flux at a point on the surface. Multipole expansion of these integrals leads to some useful approximations and also provides insight into a 'dipole' approximation used by previous authors.
Non-linear mechanical response of various metals. IV. Swift effect in relation to the stress-strain behaviour and fracture in simple compression and torsion
Authors: E W Billington
Published: July 2002

For pt.III see ibid., vol.10, p.553 (1977). The Swift effect has been considered in relation to the thin-walled tube approximation. It is shown that the thin-walled tube approximation allows the parameters appearing in a particular incremental-type constitutive equation for an isotropic incompressible strain-hardening elastic-plastic solid with large deformation to be evaluated directly from measurements of the Swift effect. These parameters, evaluated directly from experiment, are shown to yield a much improved correlation of the stress-strain measurements made separately in simple compression and simple torsion. The Swift effect has been considered in relation to the fracture of various metals twisted in simple torsion.
Determination of optical constants of real thin films
Authors: J Szczyrbowski
Published: July 2002

A method is presented for determining the optical constants of films with a slightly rough upper surface and somewhat inhomogeneous over their thickness from normal incidence reflection and transmission measurements. It is shown how the film thickness and RMS height of surface irregularities can be calculated on the basis of measured absolute transmittances in the region of interference fringes.
Motion and shape of snowplough sheets in coaxial accelerators
Authors: N F Tsagas  , G L R Mair  , A E Prinn
Published: July 2002

A long coaxial accelerator is filled with helium at initial gas pressure between 0.2 and 4 Torr. When connected to a large capacitor at <or=10 kV a discharge is started at one end; the central electrode has negative polarity. The velocity of the plasma sheet, the snowplough, and its shape have been derived from streak photographs for terminal currents between about 100 and 300 kA. The motion of the sheet has been analysed by balancing the electromagnetic driving force against the inertia of the mass of the gas swept up by a plane sheet taken to be impenetrable to gas atoms. Calculated positions and average sheet velocities are in good agreement with observations at different positions and pressures. Also the shape of the sheet has been derived by allowing for the sheet's curvature in the linear momentum equation while net radial motions causing variations in profile have, at first, been excluded.
The K-shell cross-section for 80 kV electrons in single-crystal graphite and AlN
Authors: C J Rossouw  , M J Whelan
Published: July 2002

The K-shell cross-section sigmak( alpha ) for 80 kV electrons traversing thin single crystals of graphite and aluminium nitride is measured from electron energy-loss spectra generated within an energy-selecting electron microscope. These values, and the variation with collection semi-angle alpha , are compared with values derived from Bethe theory for an isolated atom.
Transient hot-strip method for simultaneously measuring thermal conductivity and thermal diffusivity of solids and fluids
Authors: S E Gustafsson  , E Karawacki  , M N Khan
Published: July 2002

A transient hot-strip method has been developed for use with solids and fluids with low electrical conductivity. The hot strip (thin metal foil) is used both as a constant plane heat source and a sensor of the temperature increase. The accuracy of the method is so good that it might even be used for the measurement of the specific heat especially under difficult experimental conditions when the standard methods cannot be used or would be very inconvenient. This method has been tested in measurements on fused quartz, glycerine and Araldite at room temperature. The experimental conditions that cause deviations from the mathematical solution of the thermal conductivity equation are discussed and estimates for their maximum influence on the measured quantities are given.
An attempt to apply the equation for polymer life to the dielectric strength of polymers
Authors: E Sacher
Published: July 2002

Zhurkov's 'equation for polymer life' (1965) is used to evaluate dipole moments from electrical breakdown data. While that for PET compares with conventional values, that for PE does not. The discrepancy is shown to be due to the electromechanical compression of the PE, which exists in the rubbery phase under the conditions of measurements.
Time-of-flight spectroscopy of a supersonic beam of mercury. Intensities and appearance potentials of Hgxaggregates
Authors: B Cabaud  , A Hoareau  , P Melinon
Published: July 2002

Molecular clusters Hgx(2<or=x<or=25) have been detected in mercury vapour. They were produced in a supersonic nozzle type molecular beam, ionised by electronic impact and analysed in a time-of-flight mass spectrometer. The appearance potentials of Hgx(2<or=x<or=12) have been measured. The ions resulted from direct ionisation of the corresponding neutral molecular aggregates.
Stress intensity factors at V-notched elastic, symmetrically loaded, plates by the method of caustics
Authors: J N Prassianakis  , P S Theocaris
Published: July 2002

The problem of experimental evaluation of the complex stress intensity factors K at the apices of sharp V-notches in homogeneous, isotropic and elastic plates, symmetrically loaded in their planes, is treated by applying the experimental method of reflected caustics. The symmetry in loading and in position of the notch allows the expression of the complex Muskhelishvili stress functions Phi (z) in the close vicinity of the notch as a single power term with exponent of the order of singularity. The method constitutes a generalisation of the method of caustics, which has been up to now applied to problems of cracked plates, where the order of singularity is lambda =0.5, and to problems where lambda1takes values lambda1not=0. While the experimental set-up is simple, the results obtained are accurate because they are derived from the close vicinity of the singularity for Phi (z) at the notch and therefore represent the real state of deformation as close as possible to the singularity. Applications of the method indicate its accuracy.
Filtered electron image contrast in amorphous objects. I. Elastic scattering
Authors: G Zanchi  , J Sevely  , B Jouffrey
Published: July 2002

A general expression of the intensity available in CTEM for each point of the image of amorphous specimens has been calculated from the point of view of the influence of the thickness of the sample, of the value of the objective aperture and of the energy loss window selected for filtered image formation. From this expression, the signal change due to a small mass-thickness variation is deduced and the practical contrast available can be calculated. This is made in the case of elastic scattering. Contrast enhancement possibilities are considered in the case of bright and dark field images of unstained and stained specimens. A contrast enhancement is demonstrated by calculation and also on images in the case of bright field images of thin foils.
Anisotropic X-ray generation in thin and bulk single crystals (of gold)
Authors: B Neumann  , L Reimer
Published: July 2002

Measurements of the anisotropy of characteristic and continuum X-ray emission excited by 20-100 keV electrons are reported for single crystal films and bulk single crystals of gold. A modified theory of the X-ray emission can explain the observed independence of the anisotropy on the X-ray quantum energy, the decrease of anisotropy with increasing film thickness and the increase of anisotropy with increasing electron energy. The anisotropy of X-ray emission of bulk single crystals is of the order of 1%; however, values up to 50% have been observed with thin epitaxially grown gold films. The anisotropy can be neglected, therefore, in the microanalysis of bulk single crystal specimens. A quantitative microanalysis of epitaxial films in single crystal substrates however, has to consider this anisotropy.
Photoinjection into plasma-polymerised organosilicon thin films. I. Surface states
Authors: J Tyczkowski  , M Kryszewski
Published: July 2002

Investigations into photoinjection of carriers from metal (Au, Al) into thin (0.65 mu m) plasma-polymerised organosilicon films have been made. The presence of two photoinjection threshold energies is indicated; the lower threshold is ascribed to holes, the upper one to electrons. The existence of a significant density of surface states at the metal-polymer contact is proved. A chemical nature is attributed to these states. It is shown that thermal treatment of films causes a rapid decrease of the density of these states. Adsorption of oxygen gas at the metal-polymer contact introduces only a small number of additional charged surface states.
Rutherford back-scattering and ellipsometry of selenium implanted InP
Authors: S S Gill  , B J Sealy  , K G Stephens
Published: July 2002

Good correlation has been found between disorder, measured by Rutherford back-scattering and the extinction coefficient obtained from ellipsometry measurements for 200 keV selenium ions implanted up to a dose of 1*1015 cm-2 at room temperature. Both techniques show that InP becomes amorphous for doses greater than 5*1018 Se+ cm-2. The magnitude of the reflectivity, refractive index and extinction coefficient, obtained from ellipsometry, are presented as a function of dose.
X-ray induced changes in the optical properties of SrF2:Co
Authors: J M Orera  , R Alcala  , P J Alonso
Published: July 2002

The optical absorption spectrum of X-irradiated SrF2:Co is reported. It shows two bands at 290 nm and 340 nm. When the irradiation is performed at 70K, a broad band in the 300 nm region is also produced. The 290 nm and 340 nm absorption bands are associated with Co+ions created by X-irradiation, and the broad band is identified as due to VKcentres. The X-ray excited emission and the emission excited by light in X-irradiated samples are reported. In both cases the emission spectrum shows four emission bands which are associated with Co2+electronic transitions. The excitation spectra of all of these bands coincides with the absorption one. A model is proposed to explain the emission properties.
Electron energy distribution in CO and He-CO discharges. II
Authors: Y Kaufman  , Yu M Kagan
Published: July 2002

For pt.I see ibid., vol.14, p.1061 (1981). A method for simultaneously calculating the electron energy distribution function and the vibrational population in CO and He-CO discharges is described. The solution is self-consistent and includes the interdependence between the CO vibrational population and the electron energy distribution function. This mutual influence is accounted for by incorporating the electron-impact excitation of vibrationally excited molecules as well as superelastic collisions. The cross-sections for stepwise excitation are calculated using a model based on experimental data. The electron energy distribution function is given in an analytical form. The calculations are carried out for different He-CO mixtures and for several values of the electron concentration, the gas temperature and the ratio E/N.
Non-linear mechanical response of various metals. VI. Piece-wise continuous yield condition
Authors: E W Billington
Published: July 2002

For pt.V see ibid., vol.12, p.853 (1979). A piece-wise continuous yield criterion is postulated on general grounds which is similar to the classical yield condition of Tresca, but which in contrast comprises twelve segments. However, the proposed yield condition does reduce to a limiting form which is just that of the Tresca yield criterion. The theory is applied to the classical combined-stress experiments of Taylor and Quinney (1931). It is shown that the proposed yield condition, when combined with an appropriate constitutive equation, is in full agreement with the useable experimental data obtainable from these particular combined stress measurements. As a second example, it is applied to the studies of the Swift effect which are primarily concerned with the simple torsion of a thin-walled cylinder. These studies of the Swift effect are shown to be in general accord with the proposed yield function.
The effect of an electric field on contact electrification
Authors: J Lowell
Published: July 2002

An applied electric field has a strong effect on the contact charging of an insulator by a metal. However, the experiments reported show that previous interpretations of the effect of an electric field are probably wrong, and that it is not possible to estimate the depth of penetration of charge from observations of the effect of an electric field on contact electrification.
Transient voltage measurement of primary ionisation in nitrogen
Authors: K R Spriggs  , J Fletcher
Published: July 2002

Electron impact ionisation of nitrogen gas has been studied using an integrated voltage method with the data subjected to a full analysis including the effects of diffusion. Values of the primary ionisation coefficient have been obtained over the swarm energy range 300 nTd<or=E/N<or=2400 Td. Experimental evidence has been obtained that secondary effects contribute greatly to the charge flow at these high values of E/N and that if not recognised may produce false values of alpha /N. The effect of ignoring diffusion at low gas pressures (p approximately=1 Torr) is also considered.
Study of the double layer at an insulator/liquid interface by step voltage transients
Authors: A Saad  , R Tobazeon
Published: July 2002

The electrical behaviour of dielectric liquids is strongly influenced by the double layer at solid/liquid interfaces. When the solid is a metal, the injection of ions depends on the structure of this layer. A new method is proposed which evidences the presence of such a layer at an insulator/liquid interface. A liquid containing a known amount of electrolyte is subjected to a voltage step, the electrodes being covered with polymer films. From the shape of the transient currents, the total charge displaced across the liquid gap is measured, and the corresponding zeta potential calculated. zeta depends mainly on electrolyte dissolved in the liquid for a given insulator/liquid pair and is much higher than the known values at a metal/liquid interface. A simple model considering the diffuse layer as a charged sheet may be applied. However, at voltages high enough, electrohydrodynamic motion of the liquid contributes to the convective transport of charge in a similar way to that already studied in the case of unipolar injection in insulating liquids.
Observation of rectangular cavity modes in Josephson tunnel junctions
Authors: G Jung
Published: July 2002

Observation of rectangular cavity modes corresponding to RF field variations along the length and width of the junction is reported. Voltages associated with these modes are in agreement with the theory of Nerenberg et al. (1976). Rectangular modes are distinguished from the well known one-dimensional modes by their dependence on magnetic field orientation. Rectangular modes were also sporadically observed as zero-field resonances.
Three-body attachment in oxygen and an air-like nitrogen and oxygen mixture
Authors: T Taniguchi  , K Kawamura  , S Sakamoto  , H Tagashira
Published: July 2002

Calculations of the three-body attachment coefficient eta3/N, N being the gas number density, in oxygen and an air-like nitrogen and oxygen mixture, are performed using a Boltzmann equation method. First, an appropriate cross-section Qa3for this attachment process in oxygen is deduced by modifying that of Spence and Schulz (1972) so that the calculated eta3/N agrees well with the experimental values of Grunberg (1969). Then, the deduced Qa3is used for the calculation of eta3/N in an air-like mixture of 79% nitrogen and 21% oxygen. Good agreement is obtained with the eta3/N values measured for dry air by Hessenauer (1967) at low E/N, where E is the electric field. The variation of eta3/N2with N is studied, and it is found that even though the value of Qa3is assumed to be proportional to N, eta3/N2can decrease with increasing N due to the change in the electron energy distribution, caused by the variation of Qa3.