bluuebunny/crossref_metadata_2025 huggingface dataset:
The negative thermal expansion coefficient of GaP crystal at low temperatures
Authors: K Haruna  , H Maeta  , K Ohashi  , T Koike
Published: July 2002

The thermal expansion of the lattice constant of a GaP crystal was measured in the range 4.2-300K using the Bond method. The authors found a negative thermal expansion coefficient for GaP at low temperatures. They also calculated the Gruneisen parameter and this was negative below 50K. These experimental results do not agree with the phenomenological theory in which a positive thermal expansion coefficient for GaP is predicted at low temperatures.
Molecular mechanism determining phase transitions in the 4-methyl-pyridine crystal
Authors: N Le Calve  , B Pasquier  , G Braathen  , L Soulard  , F Filaux
Published: July 2002

Infrared and Raman spectra of the 4-methyl-pyridine crystal, and its deuterated derivative (d7), at various temperatures from 276.8K (melting point) to 5K are presented and vibrational assignments are proposed. Three crystal phases are observed, in the regions 5-100K (III), 100-254K (II) and 254-276.8K (I). Their structures and dynamics are discussed. In phase III, where the site symmetry for the molecule is C1, different potential-shapes are compared and only a nonperiodic function accounts satisfactorily for the five tau CH3and two tau CD3Raman bands. A molecular model is proposed where the methyl groups are correlated along infinite chains in the crystal. In phases II and I, where the molecular site symmetries are C2and C2v, respectively, the methyl group rotation is free. Quantitative models are developed to represent the temperature effect on frequencies or band widths observed below 150 cm-1and molecular mechanisms are proposed for the phase transitions. The ordering-disordering of the methyl groups which characterises the III to or from II transition is monitored by a rotational libration near 61 cm-1at 5K, strongly coupled to the tau CH3mode. The II to or from I transition is driven by a rotational libration near 116 cm-1.
Electroreflectance spectroscopy from quantum well structures in an electric field
Authors: P C Klipstein  , P R Tapster  , N Apsley  , D A Anderson  , M S Skolnick  , T M Kerr  , K Woodbridge
Published: July 2002

Electroreflectance measurements have been performed on GaAs/Ga1-xAlxAs single and multiple quantum well samples for GaAs well widths of 100 AA. The electric field is applied perpendicular to the well using a PIN diode arrangement of a semi-transparent Schottky barrier. Sharp features are observed in spectra measured both at room and at helium temperatures (15K) at energies corresponding to transitions between heavy or light-hole quantum well levels associated with the valence band and electron levels associated with the conduction band. Comparison is made between the energies of optical transitions measured by electroreflectance and by complementary photoluminescence emission and excitation spectroscopy. The energies of optical transitions are observed to decrease with increasing DC electric field at the well and a resonance model has been developed to explain the observed shifts.
The application of pseudopotentials to low-energy electron diffraction I. Calculation of the potential and `inner potential'
Authors: J B Pendry
Published: July 2002

We explain how to set up a potential suitable for low-energy electron diffraction (LEED) calculations. It is found that for off-diagonal matrix elements, the exchange contribution must be calculated from the full Hartree-Fock exchange operator and not from the local Slater approximation. The potential has to be transformed into a pseudopotential to obtain good convergence inkspace. Some numbers, which include the `inner potential', are presented from calculations on nickel, niobium and graphite. Comparison with experiment shows the potentials to be accurate to within ±2 eV.
Far-infrared and Raman investigation of LiNO3and NaNO2in the liquid state
Authors: H Happ  , T Bohm  , A Kasemann  , R Neuerbourg
Published: July 2002

The far-infrared (FIR) reflection spectra of molten LiNO3and NaNO2are measured from 13 to 530 cm-1and from 13 to 350 cm-1respectively. Data from microwave and millimetre-wave measurements between 0.3 and 5 cm-1are added to extend the spectra to zero frequency. The dielectric function epsilon ( omega ) and the optical conductivity sigma are derived from a Kramers-Kronig analysis of the reflection spectra. These functions exhibit a strongly damped oscillatory response. The FIR conductivity can be well described by a generalised oscillator equation containing a sum of two exponentials as the memory function. Results of additional AC measurements up to 10 GHz and the millimetre-wave data reveal the shape of sigma in the transition region from oscillatory to diffusive behaviour. The Raman spectra of the ionic liquids are measured between 0 and 300 cm-1. Scattering of residual lattice modes is observed superimposed on Rayleigh wing scattering. In NaNO2the variation of the spectra of the lattice modes in the solid-to-liquid transition is studied.
Insulation breakdown in a random nonpercolating network of conductors
Authors: B K Chakrabarti  , K K Bardhan  , P Ray
Published: July 2002

For a non percolating network of conductors, the insulation breakdown voltage VBvanishes near cc, the percolation threshold for conductors, as (cc-c)t'. Using a small-cell real-space renormalisation group technique (with scale factor b up to 3) the authors find t' approximately=0.9 for d=2. A Monte Carlo estimate for 20*20 square (bond-diluted) and triangular (site-diluted) lattices gives t'=0.8+or-0.1. The breakdown universality classes for such metal-insulator transition are also discussed.
The Aharonov-Bohm effect in electrostatically defined heterojunction rings
Authors: C J B Ford  , T J Thornton  , R Newbury  , M Pepper  , H Ahmed  , C T Foxon  , J J Harris  , C Roberts
Published: July 2002

Micrometer-sized loops of two-dimensional electron gas have been made on GaAs-AlGaAs heterostructures by electrostatic confinement. A split gate is used to define the loop, allowing the width of the conducting channels to be varied by changing the gate voltage. The magnetoresistance has been measured at low temperatures (T<100 mK) and shows strong Aharonov-Bohm oscillations with amplitudes of up to 7% of the total resistance in the narrowest devices. The oscillations are strong out to B approximately=0.5 T and then die out as B increases to approximately=1 T, with a possible dependence on the channel width. Magnetic depopulation of the ID-sub-bands is also seen.
Transmission electron microscope studies of paraferroelectric phase transitions in BaTiO3and KNbO3
Authors: Chen Jun  , Fan Chan-gao  , Li Qi  , Feng Duan
Published: July 2002

It was found that quasi-periodic microdomains could be produced within a temperature range of several degrees centigrade below the Curie point in KNbO3and BaTiO3. The geometry and the diffraction contrast image characteristics of the microdomains show that they cannot be the normal 180 degrees or 90 degrees microdomains; they are the so-called 'order-modulated' microdomains, which result from the crossover of the 'order-disorder' and the 'displacive' phase transitions in these two materials. A mixed model of both 'order-disorder' and 'displacive' phase transitions was suggested to explain the experimental results obtained with transmission electron microscopy.
Condensed phonon modes and the recently discovered displacive superlattice of nickel arsenide
Authors: R L Withers  , G L Hua  , T R Welberry  , R Vincent
Published: July 2002

Recently a weak, orthorhombic, displacive superlattice was observed in both mineral and synthetic samples of NiAs. The symmetry-adapted displacement eigenvectors and corresponding atomic displacement patterns of the normal modes of the unmodulated sub-cell associated with the observed modulation wavevector (q=1/3 (1120)) are derived. Comparison with an experimental convergent-beam electron diffraction study strongly suggests that the deviation of the modulated structure from the unmodulated parent structure can largely be ascribed to the condensation, or freezing-in, of a q=1/3 (1120), Sigma 6 phonon mode.
The crystal structure of nickel arsenide
Authors: J G Thompson  , A D Rae  , R L Withers  , T R Welberry  , A C Willis
Published: July 2002

The crystal structure has been redetermined using X-ray diffraction data collected from a triply twinned crystal, a=10.8568(8), b=6.2682(5), c=5.0340(7)AA, Cmc21grown using I2as the vapour transport agent. The weak orthorhombic superlattice originally observed by convergent-beam electron diffraction, corresponding to the freezing in of a q=1/3(1120) Sigma6sub-cell phonon mode, is confirmed, and magnitudes of the atomic displacements from their ideal positions refined. Closer inspection of the data revealed the presence of a q=0, Sigma6usymmetry, frozen-in-phonon mode, which depending on its sign when added to the q=1/3(1120) mode, results in two homometric solutions.
The velocity of sound in the liquid In-Te alloys
Authors: Y Tsuchiya
Published: July 2002

The velocity of sound has been measured as a function of composition and temperature in liquid In-Te alloys. The compressibility deduced from the velocity of sound for Te-rich alloys has a maximum when plotted against temperature. This is interpreted as being due to the existence of In2Te3which persists in the liquid and yet which undergoes a type of structural transition or dissociation with temperature.
The positron annihilation anomaly near the superconducting transition temperature in single-phase YBa2Cu3O7-xcompounds
Authors: Zhu Jingsheng  , Song Jianxian  , Wang Jun  , Yang Zhongjin  , Zhang Yuheng  , C W Lung
Published: July 2002

The positron annihilation technique has been used for studying the high-Tc-superconductor YBa2Cu3O7-x. The Doppler broadening lineshape parameter as a function of temperature between 75 and 300 K, and the positron lifetimes of the normal state and of the superconducting state have been measured. A sharp negative anomaly in the S-parameter was observed near the superconducting transition temperature. In addition, it was found that the S-parameter and mean lifetime decrease below the transition temperature Tc. These phenomena vanish for the samples annealed in vacuum at 800 degrees C. It is suggested that high-Tcsuperconductivity in the new oxide is probably accompanied by an adjustment in the positions of the oxygen vacancies and the occurrence of 'valence pairs', i.e. an enhancement of the covalent character of the electrons after the superconducting transition.
Nuclear magnetic relaxation in two ferromagnetic Heusler alloys
Authors: N r Sharpe  , J M Titman  , B G Wood
Published: July 2002

The nuclear magnetic spin-lattice relaxation times of the 63Cu and 55Mn nuclear spins in the ferromagnetic domains of Cu2MnAl and Cu2MnIn are found to be inversely proportional to the absolute temperature, and related to the magnitude of the hyperfine field. In Cu2MnAl ( gamma 2T1T)-1 is 8.4+or-0.2 and 4.7+or-0.1 s Oe2 deg-1 for the 55Mn and 63Cu nuclei respectively, and for Cu2MnIn the same parameters have the values 9.1+or-0.2 and 3.6+or-0.1 s Oe2 deg-1. The transverse relaxation rates and the longitudinal rates associated with the centres of the domain walls are also temperature dependent. The hyperfine fields at the Al and In nuclei are found to be 68.2+or-0.5 and 89.7+or-0.5 kOe respectively.
Optical absorption and photochromism in iron-doped rutile
Authors: W Clark  , P Broadhead
Published: July 2002

Photochromism has been observed in single-crystal iron-doped rutile. Colouring occurs on irradiation with light of photon energy greater than 2.5 eV. The ratio of the activation energies for optical bleaching (2.5 eV) and thermal bleaching (0.5 eV) agrees with approximate theory. The colouring process is interpreted as the optical transfer of an electron from the valence band to an Fe3+ Square Operator centre where Square Operator signifies an iron-adjacent anion vacancy: the majority of the resultant holes are trapped by Fe3+ Square Operator ions on sites without an adjacent vacancy. Bleaching is a combination of optical and thermal release of electrons from Fe2+ Square Operator centres direct and thermal release of electrons from Fe2+ Square Operator centres into the conduction band followed by recombination with the trapped holes, and direct recombination between the Fe2+ Square Operator centres and non- trapped holes.
Mass-velocity correction to the solution of the Schrodinger equation for the Mathieu potential
Authors: M Steslicka  , S G Davidson  , W M Fairbairn
Published: July 2002

Perturbation theory is used to calculate the mass-velocity correction to the energy solution of the Schrodinger equation for the Mathieu potential. It shows that this relativistic effect causes a shrinkage of the energy spectrum.
Two-electron jumps in the potassium Kβ X-ray spectrum
Authors: J Utriainen  , T Aberg
Published: July 2002

It is shown that the low energy structure of the K+K beta1,3line in KF, KCl and KBr is very complex. A possible explanation is given in terms of the simultaneous excitation of an outer shell electron and emission of an X-ray photon.
Excitations from the spin-density wave ground state
Authors: A E K Dowson
Published: July 2002

A derivation is given of the dispersion relation for excitations from the spin-density wave ground state. Working in the random phase approximation, and taking a single tight binding electronic band with a delta function interaction between electrons, the author finds three types of collective mode, of which one is longitudinal and two transverse. Calculation shows that the former has a linear dispersion relation for small wave vectors, and is probably a zero-sound excitation modified by the magnetic ordering. One of the transverse modes is also linear in behaviour, and is identified as a spin wave excitation. The other behaves as omega = omega0+Aq2for small q, where omega0lies in the energy gap; on examining the approach to the ferromagnetic limit, it is demonstrated that omega0approaches zero, and the mode goes over smoothly into a ferromagnetic spin wave.
Selection rules and symmetry for optical absorption spectra in praseodymium:lanthanum ethyl sulphate
Authors: D R Nicol
Published: July 2002

The selection and polarization rules for the optical absorption lines of Pr in LaEtS are discussed and compared with experimental spectra obtained at liquid nitrogen temperatures. It is found that the rules obeyed are those of C3h symmetry rather than those of the D3h symmetry usually assumed in crystal field treatments.
An X-ray diffraction study of liquid mercury between 50C and 150C
Authors: B R Orton  , R L T Street
Published: July 2002

Interference functions from liquid mercury have been obtained at 50, 100 and 150 degrees C, using a specially designed X-ray diffractometer. The important feature of the results is the rapid change of first peak height with temperature. By matching the observed interference functions to those from a hard sphere model it is found that the equivalent packing density of the liquid changes from 0.42 at 50 degrees C to 0.32 at 150 degrees C. From these values of the packing density the compressibility has been calculated at each temperature and all are higher than the published experimental values. It is concluded that there may be an increase in the inter-ionic repulsive force as the temperature increases.
Theoretical determination of the imaginary parts of the atomic scattering factors of Si
Authors: M Calamiotou  , S E Filippakis
Published: July 2002

The imaginary parts of the atomic scattering factors of Si have been calculated for 12 characteristic wavelengths by using both Hartree-Fock and hydrogen-like wave-functions for the bound Si electrons. Since the values resulting from the two calculations do not differ significantly and both agree very well with existing experimental values, the use of the simpler formulae obtained by Wagenfeld using hydrogen-like wavefunctions for bound as well as excited Si electrons is suggested for the calculation of the imaginary parts of the atomic scattering factors without substantial loss of accuracy.
Nonexponential nuclear Zeeman relaxation of tunnelling methyl groups
Authors: M Punkkinen  , S Clough
Published: July 2002

A model is proposed to explain the nonexponential nuclear Zeeman spin-lattice relaxation observed at low temperatures with some samples containing methyl groups. The theory predicts an observable nonexponential relaxation in the region 0.25 omega0< Deltat<6 omega0, where h(cross) Deltatand h(cross) omega0are the tunnelling and Zeeman splittings respectively. Curves are presented which can be used to obtain Deltatfrom the experimentally observed weights and relaxation times of the fast and slowly relaxing components of the magnetization in the case of a very sharp tunnelling-splitting distribution. For Deltat<< omegaoor Deltat>> omegaothe present model agrees with the previous theories in predicting an exponential relaxation.
Neutron crystal-field spectroscopy and susceptibility in ErcY1-cAl2
Authors: H Heer  , A Furrer  , E Walker  , A Treyvaud  , H-G Purwins  , J Kjems
Published: July 2002

Inelastic neutron scattering experiments and susceptibility measurements have been carried out on polycrystalline ErcY1-cAl2. A least-squares fitting procedure has been applied to the neutron data which favours four sets of crystal-field parameters. The results are compared with the measured susceptibility and other bulk magnetic properties. From this it is concluded that the crystal-field parameters x=-0.54 and W=-0.018 meV are the most probable ones.
Temperature and field dependence of the optical absorption edge in amorphous As2S3
Authors: R A Street  , T M Searle  , I G Austin  , R S Sussmann
Published: July 2002

The optical absorption coefficients and the temperature dependence of the edge in amorphous As2S3have been measured at temperatures down to 10 K. The temperature coefficients of the gap at a fixed absorption level ( delta E/ delta T)alpha, can be separated into two components. The first is attributed to electron-phonon interactions and the shift is found to be proportional to the internal energy on a Debye approximation, as predicted from the theory of zero-phonon transitions in colour centres. The second is the temperature broadening of the Urbach tail. The slope of the tail is interpreted in terms of random electric fields arising from disorder and phonons; the former is estimated to be three times larger than the contribution from zero-point motion. The electro-modulation spectrum has been measured in the region of the absorption edge and is discussed in terms of exciton and other models.
Absorption of infrared radiation by ionized donor pairs in GaAs, InP and CdTe
Authors: J Golka
Published: July 2002

A new mechanism of absorption of infrared radiation at low temperatures in n-type GaAs, InP and CdTe is investigated. Photon energies are about two thirds of donor ionization energy. Donor concentrations up to 2*1014cm-3in GaAs, 5*1014cm-3in InP and 2*1015cm-3in CdTe, and compensations up to 0.2 are considered. A model which is based on photon-induced transitions of electrons from 2p sigmau- to 3d sigmag-like states of ionized donor pairs is used. The formula for the absorption coefficient is derived and numerical results are presented. The transitions can be also detected as changes in DC conductivity.
Role of the potential in LEED; application to the Na(110) surface
Authors: P M Echenique
Published: July 2002

A study of the exchange and correlation component of the one-electron LEED potential was carried out. An analysis of low-energy electron diffraction data from the Na(110) face at T=173K and normal incidence was made to test the different model potentials. A good fit to experimental spectra was obtained with a model based upon Hartree-Fock (core) and Hedin and Lundqvist results for the exchange and correlation potential of a uniform electron gas. The inner potential and absorption are those of a uniform electron gas with the same density as Na (rs=4). The surface layer is determined to be neither contracted nor dilated with respect to the bulk (110) planes.
Surface states and the surface potential: the copper (111) face
Authors: S J Gurman
Published: July 2002

Surface-state energies have been calculated for the copper (111) face in the energy region close to the Fermi level, using a parametrised form of surface potential. By fitting the calculated results to photoemission data, the author obtains a surface barrier which has the correct scattering properties for the valence electrons.
The NMR second moment of solid cyclohexane
Authors: F A Rushworth
Published: July 2002

A theoretical value for the intermolecular contribution to the second moment of the NMR absorption lineshape of cyclohexane has been calculated, based on a recent determination of the crystal structure in the lower solid phase. The result agrees very well with the original estimate made by Andrew and Eades (1953) using indirect methods. This agreement supports the use of these indirect methods for the estimation of the intermolecular second moments in other substances whose crystal structures have not been found.
Electron paramagnetic resonance of Mn2+ in CuGaS2
Authors: G L Troeger  , R N Rogers  , H M Kasper
Published: July 2002

The electron paramagnetic resonance of Mn2+ has been observed in three different environments in CuGaS2. Designated MnI, MnII and MnIII, they are interpreted respectively as: (i) Mn2+ ions substitutional at a Ga site: (ii) Mn2+ ions substitutional at a Cu site; and (iii) Mn2+ ions substitutional at a Cu site with a next-nearest-neighbour Cu vacancy.
A study of the errors due to shear and rotatory inertia in the determination of Young's modulus by flexural vibrations
Authors: D Hardie  , R N Parkins
Published: July 2002

Values of Young's modulus have been obtained for commercial 60-40 brass, 0.9% carbon steel, Vitreosil silica, super-purity aluminium and alumina by determination of the resonance frequencies in `free-free' transverse vibration and an attempt made to correct the results for shear and rotatory inertia. However, it was found that an empirical correction depending linearly on the stiffness ratio gives a more dependable result than the various suggested theoretical corrections.
Variable-energy blast waves
Authors: R A Freeman
Published: July 2002

The analysis of Sakurai is extended to include variable-energy flows with particular reference to cylindrical spark channel formation. A new analysis in Lagrange coordinates enables the blast-wave decay to be studied and the number of differential equations to be reduced. A new approximation for finding the velocity of the shock front is presented.
High-speed solid-state thermal switches based on vanadium dioxide
Authors: R G Cope  , A W Penn
Published: July 2002

Using the phase transformation at 68°C found in materials based on vanadium dioxide, devices exhibiting an S-type current-voltage characteristic have been produced. The characteristic has an unstable differential negative-resistance region which leads to fast switching (20 ns). Devices have been made by the fusion of mixtures of vanadium pentoxide with other oxides on to thin wires in a reducing atmosphere, by depositing vanadium dioxide particles between conductors using a dilute varnish suspension, and by reactive sputtering of vanadium in low oxygen pressures to give thin films.

Theoretical expectations based on a thermal explanation are in good agreement with the observed static and dynamic characteristics. A total cycle time of 700 ns has been obtained, most of which is derived from the turn-off time. On/off resistance ratios over 1000 have been observed.
Semiconductor microwave generators
Authors: C Hilsum
Published: July 2002

An account is given of the four types of microwave generator made from semiconductors: the transistor, the varactor, the avalanche diode and the transferred electron diode. The design problems for each device are outlined. A more detailed discussion of the physical principles of avalanche diodes and transferred electron diodes is included. The two types of transferred electron device, the Gunn diode and the LSA diode, are contrasted. Finally, comparisons are made between the performances of all the generators, including power output and noise, and some indication is given of future prospects.
Growth of ionization currents in carbon tetrafluoride and hexafluoroethane
Authors: S E Bozin  , C C Goodyear
Published: July 2002

Values of the ionization coefficient α and attachment coefficient η have been obtained in CF4and C2F6at 20°C using the modified Townsend method. In CF4the reduced fieldE/pwas varied from 40 to 90 V cm−1torr−1and the reduced coefficients α/pand η/pwere found to be independent of pressure from 5 to 20 torr. In C2F6the range ofE/pwas from 70 to 190 V cm−1torr−1and the pressure was varied from 2·5 to 30 torr. Values of α/pand η/pare in good agreement at the lower pressures but at 30 torr it was necessary to assume a small detachment coefficient δ/psimilar, equals2 × 10−3cm−1torr−1, in order to obtain consistent results.
The high-temperature thermal conductivity of sintered uranium dioxide
Authors: J B Ainscough  , M J Wheeler
Published: July 2002

Thermal conductivity values are calculated from 700 thermal diffusivity measurements between 970 and 2020°K on 25 samples of sintered 2·7% enriched UO2·00, 97% of theoretical density. The measurements were made using the modulated electron-beam technique and the thermal conductivities of all the samples, derived by using Moore and Kelley's specific heat values, are represented by k=(0·0227 T+3·38)−1+6·6 × 10−13
T
3 W cm−1 degk−1 (970-2020°K) the 95% confidence limits being ±14½%. This representation of the thermal conductivity is preferred because the values of the constants agree both with theory and the results of other investigators.
Mechanisms of electrical breakdown in mercury vapour
Authors: G D N Overton  , D E Davies
Published: July 2002

Measurements have been made of breakdown potentials, primary ionization coefficients and formative time lags, under uniform field conditions in mercury vapour with a mercury pool as a cathode. Generalized secondary ionization coefficients (ω/α) were calculated as a function ofE/p0 from the breakdown potentials and primary ionization coefficients. This curve is interpreted in the light of calculations made of the relative populations of atoms in the P states per ion pair and the application of Davidson's treatment of the temporal growth of ionization. It is concluded that the dominant secondary processes in mercury vapour for the pressure range 0·5-1·0 torr, when the cathode is a mercury pool, are collision-induced radiation from the destruction of metastables in the gas and positive-ion action at the cathode for values ofE/p0 below 600 V cm−1torr−1. At higher values ofE/p0 the process is one of positive-ion action at the cathode alone, the ion acting by virtue of its potential energy.
The analysis of transients in pulsed LCR circuits with appreciable skin impedance
Authors: R D Medford  , M Pelovitz dagger
Published: July 2002

An analytic procedure is developed for the calculation of current waveforms in pulsed LCR circuits for cases where the skin impedance cannot be neglected.

Design curves are given which enable current waveforms to be determined for circuits of interest in fusion and high magnetic field research. The paper includes worked examples of the use of the curves.
A new synthetic scattering kernel with applications in neutron transport theory
Authors: M M R Williams
Published: July 2002

A new synthetic kernel, based on the secondary model of neutron scattering, is derived. The new model reproduces infinite-medium spectra in 1/v-poisoned media with considerable accuracy and behaves in many ways like a real, chemically bound moderator.

The eigenfunctions of the scattering operator defined by the new kernel are found to be Laguerre polynomials but, unlike the heavy gas model which also has Laguerre polynomial eigenfunctions, the eigenvalues have a limit point behaviour. This fact enables the space eigenvalues of the transport equation to assume a particularly useful form and overcomes the anomalous behaviour observed when the heavy gas model is employed.

The new kernel is more accurate than the heavy gas model, being valid for all values of moderator mass. It is also just as convenient from an analytical point of view, as illustrated by an example based on neutron diffusion in a temperature gradient.
The magnetostatic energy associated with a polycrystalline ferrite
Authors: J E Knowles
Published: July 2002

At the grain boundaries of moderately hard magnetic ferrites there occurs a layer of free pole of r.m.s. surface density ω̄. The magnitude of ω̄ is here estimated as a function of the over-all magnetization, and the magnetostatic energy of a grain in a demagnetized assembly is then found by calculating the energy of a spherical grain carrying free pole ± ω̄0on alternate octants; the μ*effect, the presence of domain walls and the influence of surrounding grains are also considered. The value so found is consistent with that deduced from observation of domain wall spacings in manganese-magnesium ferrite.
Spatial and temporal spectroscopy of a streamer discharge in nitrogen
Authors: P Stritzke  , I Sander  , H Raether
Published: July 2002

The light emitted during the different stages of the spark development in nitrogen has been observed with spectroscopic methods. The first detectable light is emitted with sufficient intensity just after the critical amplification. This type of radiation continues till the start of the heating of the plasma in the predischarge channel. It allows one to determine the electron density and to calculate the current in the predischarge channel. At about 170 ns after the start of the avalanche the dissociation of the molecules and the ionization of the atoms takes place near xcrit in times of a few ns. This causes a rapid increase of the current and the emission of a strong continuum intensity as well as the emission of atom and ion lines. Now the predischarge channel transforms into a plasma channel at a current of about 10 A. Applying plasma diagnostic methods in the spectral range of 3000-8600 AA the temperatures and the electron densities as function of time and radius of the channel can be derived in the plasma channel.
Visual observation of phase domains in dysprosium orthoferrite
Authors: A Maziewski  , R Szymczak
Published: July 2002

The coexistence of the ferrimagnetic and the antiferromagnetic phases has been observed in the DyFeO3 orthoferrite. The observations were made in a polarization microscope by means of the Faraday effect. Several features of the spin reorientation which shed some light on the mechanism of the transition, have been observed.
Three-body attachment in oxygen
Authors: T Taniguchi  , H Tagashira  , I Okada  , Y Sakai
Published: July 2002

The three-body attachment coefficient in oxygen is calculated by a Boltzmann equation method using the electron collision cross section for this process which is proportional to the gas number density. The calculated values of the coefficient agree well with previous experimental values and represent well the gas pressure dependency of this coefficient, suggesting that the present treatment is basically appropriate.
A simple model for cavitation erosion of metals
Authors: J Kalestrup Kristensen  , I Hansson  , K A Morch
Published: July 2002

The effect of cavitation erosion on metals is considered. A model is proposed which offers a relative measure of the cavitation erosion resistance and of the length of the incubation period for different metals. The model predicts that the energy absorbed by the plastic deformation of the material depends upon the hardness of the material, and it finds a qualitative difference between cavitation erosion in different apparatus types. In the case of flow induced cavitation the model finds the cavitation erosion to be proportional to the 5th-7th power of the flow velocity.
Relaxation of an electron beam in a partially ionised plasma
Authors: M Tendler  , A Arora
Published: July 2002

The relaxation of an electron beam, with thermal spread, entering a partially ionised plasma has been studied taking into account all sorts of binary collisions. The kinetic approach has been used and the problem has been solved as a boundary value problem. It has been found that the relaxation of a beam depends on the relative location of the spectrum of excitation levels in comparison with beam electron energy. Moreover, the collisions with excitation energies much less than the beam energy are mainly responsible for the thermalisation of the beam. Numerical calculations have been done for a mercury discharge.
The formation of an aqueous fog in a shock tube
Authors: D L Hastings  , J P Hodgson
Published: July 2002

The extent to which a shock tube could be used for laboratory experiments on the fluid mechanics of two-phase mixtures of droplets in an inert carrier gas was determined. A shock tube high pressure section was charged with moist carrier gas and the flow produced a stationary cloud of water droplets behind the reflected expansion wave. The droplets were formed by homogeneous nucleation and preliminary experiments showed that their diameters were of the same order as the wavelength of light. Subsequently, two-colour light transmission measurements were made and the droplet diameter was determined by using the Mie scattering theory. Attempts to measure the size distribution indicated that the droplets had a narrow range of sizes. Depending on the initial conditions, diameters were in the range 0.3 mu m to 0.9 mu m. The fog persisted for several milliseconds before the arrival of the reflected shock wave from the opposite end of the shock tube. Preliminary observations indicate that the droplets cause considerable dispersion of the shock.
A second-harmonic, ring dye laser for the generation of continuous-wave, single-frequency UV radiation
Authors: C E Wagstaff  , M H Dunn
Published: July 2002

A frequency-doubled, continuous-wave ring laser is described, capable of generating 3 mW of single-frequency UV light, with a tuning range from 292-302 nm using ADA as the doubling crystal. The crystal is Brewster-angled and incorporated into an arm of the cavity compensated for astigmatism and coma. Cavity analysis predicts the stability region, spot sizes and positions of the foci in the dye jet and crystal, and enables mode matching to be optimized. A unidirectional device, used to maintain oscillation of the laser with one direction of circulation is described. The ring configuration enables single mode operation to be maintained with only one, thin, tuning etalon in the laser cavity. Using a servo-control circuit, the laser frequency is locked to an external reference cavity to minimise long-term drift, and to enable the single mode to be scanned continuously through 6 GHz in the UV.
Electron scattering by gas in the scanning electron microscope
Authors: D A Moncrieff  , P R Barker  , V N E Robinson
Published: July 2002

Presents theoretical calculations and experimental observations on electron scattering by gases in the specimen chamber of a scanning electron microscope. The observed scattering is in good agreement with theoretical calculations based on elastic and inelastic electron scattering. For normal surface inspection work, good resolution is retained in back-scattered imaging when gas is deliberately introduced into the specimen chamber. Experimental and calculated beam profiles are discussed for 25 keV electrons in nitrogen gas. Guidelines for choosing operating conditions for higher than normal gas pressures in the specimen chamber are also presented. Micrographs are provided to illustrate the image clarity attainable under high electron scattering conditions.
Growth and electrical properties of deuterated TGS produced by the rotating disc technique
Authors: B J Lillicrap  , J D C Wood  , V M Wood  , N Shaw
Published: July 2002

The growth of fully deuterated D(3) TGS, both alanine-doped and undoped, is described. The rotating disc method of growth was used with modified apparatus to facilitate crystal handling and reduce loss of deuteration in solution. Some characteristic types of cracking occur when highly deuterated material grown on undoped crystal were subjected to extensive electrical measurements. A selection of comparative results is reported, and attention is drawn to the effect of thermal cycling on the doped material.
Charge-exchange mass spectrometry
Authors: J M Tedder  , P H Vidaud
Published: July 2002

Charge-exchange mass spectrometry (CEMS) is a new technique, based on the phenomenon of charge-exchange, for probing molecular structures in the gas phase. A large number of primary positive ions, 40 to 50, of known ionisation potential, (eVi)X, are made to collide in succession with a target gas at low relative kinetic energy. The absolute charge-exchange cross-sections, Q, for the target gas parent ion and its ionic fragments are determined mass-spectromically and plotted versus (eVi)X. Optically forbidden transitions may be observed and the appearance potentials of not just the target molecule parent ion but also its fragmentation products are determined. The technique is explained and discussed and examples of the results obtained are given.
The production, pulsation and damping of small air bubbles in dilute polymer solutions
Authors: W D McComb  , S Ayyash
Published: July 2002

Damping constants and pulsation frequencies of the sound pulses emitted by air bubbles in a liquid were measured using a submerged nozzle (PVC tube 4.7 mm ID) to produce the bubbles. Measurements were first made in water and a light mineral oil to ensure that free-field conditions had been established and that viscous damping effects were in agreement with theoretical predictions. Further measurements were made in dilute solutions of drag-reducing polymers. In Polyox WSR 301 solutions (concentrations up to 6000 wppm), elastic effects completely relaxed the viscous effects due to the macromolecules. This was in accord with the theoretical analysis. However, in the Separan AP273 solutions (concentration up to 2000 wppm) anomalous increases in damping were found and these were attributed to large values of the extensional viscosity. The theoretical analysis indicated an extensional viscosity of about 50 Poise for 1000 wppm solution as compared with a shear viscosity of a few centipoise.
Appearance energies fro tungsten ions field-evaporated from ionic bonding states
Authors: R G Forbes
Published: July 2002

General formulae are derived for the standard field-ion appearance energies associated with desorption from ionic bonding states. Values for tungsten are obtained using the image-potential approximation, and are compared with the measured appearance energy for W3+ions. The measured value is compatible with field evaporation from a primarily ionic bonding state, but a better understanding of correlation-type interactions at charged surfaces is necessary if useful information about initial bonding state is to be derived from appearance-energy measurements alone.
Electron temperature determination in low-density plasmas from the He I 3889 AA and 5016 AA line intensities
Authors: N Brenning
Published: July 2002

The possibility of determining electron temperature by helium spectroscopy in low-density (ne<1020m-3) plasmas is discussed. It is concluded that most lines can only be used at very low densities (ne<2*1016m-3) because the line intensities are highly influenced by secondary processes, such as electron-impact-induced transitions between excited levels or excitations from metastable levels. The density range where measurements are possible can be extended if the influence of these secondary processes on the line intensities can be determined. For most helium I lines this is impossible for lack of atomic data. However, there are two exceptions, the 3889 AA (33P to 23S) and the 5016 AA (31P to 21S) lines. The influence from secondary processes on these lines is calculated, and methods are developed which can be used for measurement of electron temperatures Te<100 eV in plasmas of densities ne<5*1019m-3. The use of the methods is illustrated by an experiment where they have been successfully applied.
On the occurrence of osmosis during water uptake by adhesively bonded titanium joints
Authors: J P Sargent  , K H G Ashbee
Published: July 2002

Measurements are reported of the influence of dissolved inorganic salts upon the swelling behaviour of an adhesive joint. It is found that both the magnitude and rate of swelling associated with water uptake are reduced when epoxy resin anodised titanium/soda-lime glass joints are immersed in saturated solutions of KCl or NaCl. Application of an optical interference method to measure swelling profiles permits determination of the distribution of stress normal to a butt joint. For the case of swelling during immersion in distilled water, the normal stress across a radius varies in magnitude and in sign.
Measurement of particle size and refractive index using crossed-beam laser anemometry
Authors: M S Atakan  , A R Jones
Published: July 2002

The light scattered by spherical particles crossing the test space of a laser fringe anemometer is monitored simultaneously at two angles. From the visibilities of the signals the particle size distribution and one component of the refractive index may be determined. For non-absorbing particles this is the complete refractive index. Measurements were made on a water spray, glass ballotini and titanium and aluminium oxide particles in flames. Spherical metal oxide particles were produced by the combustion of metal powder in an oxygen-rich methane flame. Extension to the measurement of the full complex refractive index is discussed.
Comparison of Maxwellian and bi-Maxwellian expansions with Monte Carlo simulations for anisotropic plasmas
Authors: A R Barakat  , R W Schunk
Published: July 2002

The authors studied the extent to which Maxwellian and bi-Maxwellian series expansions can describe plasma flows characterised by non-Maxwellian velocity distributions. The problem they considered was the steady-state flow of a weakly ionised plasma subjected to homogeneous electric and magnetic fields, and both polarisation and hard-sphere collision models were used. Monte Carlo simulations provided a basis for determining the adequacy of a given series expansion. They found that, in general, the bi-Maxwellian-based 16-moment expansion for the velocity distribution function is better suited to describing anisotropic plasmas than the Maxwellian-based 20-moment expansion. They also found that for large electric fields the distribution function displays toroidal and bean-shaped characteristics, depending on the collision-to-cyclotron frequency ratio.
The effect of amorphous material on the contrast of electron diffraction patterns
Authors: R A A Al-Mausawe  , T F J Quinn
Published: July 2002

Experiments have been performed to study the contrast of transmission electron diffraction patterns. The effect of film thickness, accelerating voltage and amorphous content are discussed. Diffraction patterns, from vacuum-evaporated thin films of aluminium, alpha -iron and silver, on carbon-coated grids were obtained. Experimental measurement of the contrast from all materials showed that: (i) in plural scattering conditions, the contrast diminished with increasing thickness and decreasing voltage; and (ii) the highest contrast is obtained when the thickness equals the mean free path. However, in the region where the mean free path is greater than the film thickness, aluminium and alpha -iron showed that the contrast decreased as the film thickness decreased, whilst silver remained at its maximum contrast. It seems probable that, with very thin films, the lines may be broadened owing to a decreased crystallite size, and furthermore, the increase in the proportion of the amorphous material will produce greater background, which reduces the contrast.
Methods of avoiding edge effects on semiconductor diodes
Authors: P A Tove
Published: July 2002

The importance of diode edge effects in practical semiconductor devices and in test structures for electrophysical investigations, is discussed. Included in the review are power devices, devices utilising avalanche effects, and radiation detectors. The main effects are on reverse characteristics, introducing premature breakdown or excess currents. The discussion starts with experiments clarifying the role of field distribution and surface treatment of the edge, and the application of simple methods of improvement, such as using overlay electrodes or surface protection.
Adhesive contact of sectionally smooth-ended punches on elastic half-spaces: theory and experiment
Authors: D Maugis  , M Barquins
Published: July 2002

Attractive molecular forces give rise to singular tensile stresses and to a discontinuity of displacement at the edge of contacts. These boundary conditions are those of fracture mechanics and correspond to adding a rigid body displacement to the classical elastic solution. The adhesive contact of sectionally smooth-ended punches (sphere or cone) is studied. Stress distribution, shape of the deformed surface, and relation between load and penetration are given either for equilibrium or kinetic conditions. For a flat-ended sphere, the adherence of a flat punch and the JKR theory for sphere are obtained as particular cases; for zero surface energy systems, the solution reduces to that of Ejike. The case of a sphere with a spherical cap of different radius and that of a rounded cone are also examined. Experimental verification has been performed with a glass ball (R=2.19 mm and flats between 100 and 240 mu m radius) on polyurethane. Radii of contact, penetration and kinetics of detachment agree with the theory within 2%.
The application of thin film wavelength-shifting coatings of Perspex to solar energy collection
Authors: M Grande  , G Moss  , S Milwards  , M Saich
Published: July 2002

The theory of solar energy collectors which utilise sheets of fluorescent plastic is described. A method of producing fluorescent plastic by applying a thin film coating to clear plastic is evaluated and found to be superior to conventional doping techniques. Two small scale test collectors were constructed and measurements to assess performance were made. The quantum efficiency of each collector was found to be 15+or-5% in direct sunlight and an energy efficiency estimate of 2.5% was made for the collector/solar cell combination.
Electron drift velocity in hydrogen, nitrogen, oxygen, carbon monoxide, carbon dioxide and air at moderate E/N
Authors: W Roznerski  , K Leja
Published: July 2002

The drift velocity of electrons in hydrogen, nitrogen, oxygen, carbon monoxide, carbon dioxide, and air free from water vapour and carbon dioxide, at ambient temperatures (293-296K) has been measured by the modified Bradbury-Nielsen time-of-flight method over the range of the reduced electric field E/N: 0.05<or=E/N<or=250 Td, 0.1<or=E/N<or=250 Td, 1.0<or=E/N<or=275 Td, 0.1<or=E/N<or=150 Td, 0.6<or=E/N<or=250 Td and 0.05<or=E/N<or=400 Td respectively. The present results are compared with those obtained by other investigators.
The thermal conductivity of Cl2and O2in the dissociation region measured by optical interferometry. I. Experiment
Authors: S Kurtz  , J Mentel
Published: July 2002

The authors have developed a special tube for an arc in corrosive gases suited for 'end-on' interferometric measurements and describe a method to measure index of refraction profiles and their gradients in these tubes using holographic interferometry and differential interferometry. Holography is used to determine the difference of the optical path lengths and their gradients in identical arc tubes with different lengths to eliminate the contribution of the inhomogeneous end spaces.