bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of the electron phase relaxation rate in In0.53Ga0.47As: a possible diagnostic test of compositional disorder
Authors: D J Newson  , M Pepper  , H Y Hall  , J H Marsh
Published: July 2002

The authors present low-field magnetoresistance measurements made at low temperatures (<or=4.2K) on n-type samples of the semiconductor alloy In0.53Ga0.47As doped in excess of the metal-insulator transition. The samples were strongly degenerate at the temperatures of the measurements. The low-field magnetoresistance is found to be governed by quantum interference (weak-localisation) effects. The authors find that good fits to the data are obtained with a phase relaxation time as the sole variable. This time is much shorter than the calculated magnetic or spin-orbit scattering times, supporting their assertion that these are only minor perturbations.
Impurities effects on the U6+luminescence in LiF
Authors: A Lupei  , V Lupei  , I Ursu
Published: July 2002

A complex investigation including various methods (luminescence, absorption, laser excited luminescence, EPR) has been performed to elucidate the nature of U6+luminescent centres in LiF. By varying the preparation conditions, especially co-doping with cations or OH-, several centres have been isolated. EPR data on U5+in gamma -irradiated crystals sustain uranate structures for all of them. Models for three luminescent U6+centres are proposed. It is concluded that in the authors' pure crystals there is a basic structure, the completely charge compensated (UO5F)5-complex (emission at 5185 AA and 5278 AA). Impurities or defects can associate to it, producing other centres. Cationic impurities promote also a different type of uranate complex, (UO6)6-.
An NMR study of internal motions and phase transitions in NiTiF6.6H2O
Authors: Hsueh Mei Cheung  , R L Lichti
Published: July 2002

The NMR linewidths of hydrogen and fluorine in the complex ionic salt NiTiF6.6H2O have been examined from 77 to 400K. Separate abrupt changes in the1H width (122-126K) and the19F width (131-134K) confirm the existence of two transitions in the known structural transformation region, and an anomaly near 205K suggests a possible additional transition. The overall temperature dependence is discussed in terms of reorientations of the octahedral complex ions.
Dielectric measurements and domain structure in LiKSO4at low temperatures
Authors: R Cach  , P E Tomaszewski  , J Bornarel
Published: July 2002

Dielectric measurements in the temperature range 33-300K with simultaneous optical observations on LiKSO4slabs perpendicular to the c hexagonal axis have been performed. A few dielectric anomalies and domain textures are obtained according to the different thermal treatments. A specific thermal treatment has been found allowing reproducible results. Previous contradictory results in the literature can therefore be explained.
Correlation theory applied to the spin dynamics of the dimer compound Cs3Cr2Br9. Self-consistent lineshapes of the singlet-triplet excitations
Authors: B Leuenberger  , H U Gudel
Published: July 2002

The effects of spin fluctuations on the magnetic excitations in the slightly under-critical ground-state system Cs3Cr2Br9, which were recently studied by neutron scattering, are investigated by using correlation theory to extend the RPA by one order. The theory renormalises the single-triplet separation from the RPA value of 1.03 meV at 1.6K to the more reasonable value of 0.58 meV. The temperature dependence of the excitations is calculated self-consistently and compared with experiment. From the calculated density of states the lineshapes of the magnetic excitations are calculated and compared with the experimental lineshapes obtained from neutron scattering. The agreement is very good.
Collinear and non-collinear magnetic structures in anisotropic crystals with non-primitive Bravais lattices: the cubic case
Authors: P Moachesi
Published: July 2002

The classical anisotropic exchange Hamiltonian is used for the analysis of the low-temperature magnetic structures of polyatomic crystals. The magnetic order is described by the wave-vector k of the Brillouin zone associated with the Bravais lattice in direct space. For cubic crystals it is shown that anisotropic terms of tetragonal symmetry in the exchange may determine the direction of the spins with respect to k and to the crystal axes. It follows that the experimental observation of longitudinal or transverse polarisation indicates that the spin-spin interaction is not of Heisenberg type. It is also shown that anisotropy is the basic mechanism for the formation of multiaxial non-helicoloidal spin configurations. The author has found two kinds of such spin arrangements, depending on the symmetry of k and on the configuration of the unit cell. Calculations of the exchange energy in the approximation of the next-nearest-neighbor interaction for crystals of cubic symmetry Oh with four atoms per unit cell are presented. A detailed discussion of multiaxial configurations of the triangular and multiple-k-type is given. Their occurrence in MnPt3 and in uranium monopnictides is accounted for in the framework of the present model.
Solution of the generalised random energy model
Authors: B Derrida  , E Gardner
Published: July 2002

The generalised random energy model (GREM) is a spin-glass model which can be solved exactly. One can impose arbitrary pair correlations between the energies of configurations. For several examples (the Sherrington-Kirkpatrick model, the p spin-glass model, the Potts glass, spin-glass models on finite-dimensional lattices) the authors calculate the pair correlation between energies and solve the corresponding GREM. In all cases, the free energy of the GREM corresponding to a spin-glass model on a given lattice, has a simple expression in terms of the specific heat of the pure ferromagnetic model on the same lattice. Lastly they compare the correlations between three energy levels in the GREM and in spin-glass models.
Ordering kinetics and scattering function of neutrons in a layered antiferromagnet Rb2Co0.7Mg0.3F4
Authors: H Ikeda
Published: July 2002

The author presents a neutron scattering result on a time evolution of scattering function in a layered dilute antiferromagnet Rb2Co0.7Mg0.3F4. Time dependence of the interplanar correlation function after quenching from a disordered high-temperature state was directly observed. The decrease of the width of the interplanar correlation function with time obeys a logarithmic law, which is consistent with a previous result. It was also found that the scattering structure function reveals a scaling property within a measured time interval. An anomalous lineshape of the observed scattering function is well described by the generalised interacting kink model of Kawasaki and Nagai.
Correlation theory of ferromagnetism in a narrow energy band
Authors: M J Morris
Published: July 2002

A unified theory of ferromagnetism is developed for a system containing a single electronic energy band with delta-function interactions between electrons. Using a Green function technique an approximate solution of the correlation problem is obtained, the decoupling scheme being uniquely determined from the random phase approximation. Spin waves are thus taken into account. Conditions are derived for the existence of saturated solutions to the self-consistency equations, and these appear to be satisfied in nickel. The mechanism for the existence of ferromagnetism in nickel is then simply explained on the basis of the theory. An analytic expression for the low-temperature magnetization of a saturated ferromagnet is calculated.
Magnons in antiferromagnetic NiO, MnO and α-MnS
Authors: J A Reissland  , N A Begum
Published: July 2002

Calculations are presented for the energies of magnons in NiO, MnO and α-MnS. The density of magnon states of these crystals is found to have two peaks labelled here as E1and E2. The dispersion relations exhibit the symmetry predicted by Cracknell and Joshua. An anisotropic contribution arising from magnetic dipole interactions has been included and compared with a uniaxial anisotropy.
Tricritical behaviour in lamellar phases
Authors: D Sornette
Published: July 2002

The author presents a theoretical study of the stability, unbinding and phase separation of lyotropic smectic liquid crystals. The complex interplay between the bare interactions, namely the van der Waals, hydration or electrostatic forces, and membrane fluctuations is analysed and discussed within a linear functional renormalisation group (FRG). Tricritical behaviour is found, dependent on the two parameters epsilon =(kT-W)/kT, where W is a Hamaker constant, and lambda , which describes the resistance to membrane overlap. For lambda <0 (strong exclusion forces), the unbinding transition first predicted by Lipowsky and Leibler (1986) with a mean spacing z diverging as z approximately (- epsilon )-1is recovered. For lambda >0 (soft membranes), a line of first-order transitions between a finite z system and a phase of isolated membranes (z= infinity ) is found. The existence of particles unable to enter the smectic phase induces an osmotic pressure which leads to the existence of two coexisting lamellar phases, with largely different spacings z1and z2>>z1.
The spin-1 Ising model with a random crystal field: the mean-field solution
Authors: A Benyoussef  , T Biaz  , M Saber  , M Touzani
Published: July 2002

A spin-1 Ising model with a random crystal field Deltaihas been studied within the mean-field approximation. For the particular two-valued distribution P( Deltai)=p delta ( Deltai- Delta )+(1-p) delta ( Deltai), three types of diagrams have been found in the ( Delta ,T) plane, the type depending on the concentration p.
The dynamic behavior of a crystal during defect jumps. III. Distinct correlations during vacancy jumps from molecular dynamics calculations
Authors: R Vogelsang  , C Hoheisel
Published: July 2002

For pt.II see Z. Naturf., vol.a41, p.1051 (1986). The authors have studied the distinct correlation functions of the particles involved in defect jumps in a Lennard-Jones lattice. The Bennett method was used for their molecular dynamics simulations. These distinct correlation functions provide a detail picture of the momentum transfer during the defect jumps.
Spin waves in the oblique antiferromagnetic phase of K2CoxFe1-xF4: a mixed magnetic system with competing spin anisotropies
Authors: S A Higgins  , W A H M Vlak  , M Hagen  , R A Cowley  , A F M Arts  , H W de Wijn
Published: July 2002

The spin waves in the d=2 mixed antiferromagnet K2CoxFe1-xF4have been investigated using inelastic neutron scattering for a sample with a concentration x=0.27. K2CoxFe1-xF4is a system with competing orthogonal spin anisotropies and for temperatures below TL=27+or-2 K the x=0.27 sample is in the oblique antiferromagnetic (OAF) phase. Emphasis has been placed on the low-temperature spin wave dispersion relation which is revealed to have at least two low-frequency branches, and on the effect of temperature on the lower of these branches at small reduced wave-vector as the temperature approaches TLfrom below.
Quasi-crystals-random structures or twins?
Authors: J Wolny  , L Pytlik  , B Lebech
Published: July 2002

This paper discusses different ways of tiling a plane without defects by using two decorating elements. Four classes of structure with different tiling rules and different relative concentrations of the two elements are compared: Penrose tiling, microcrystalline twins, a random structure and a precipitated structure. The four structures have similar diffraction patterns exhibiting tenfold symmetry, but they differ in many essential details. The similarities and differences are discussed with special emphasis on the possibility of distinguishing between the four types of structure.
Paracrystals representing the physical state of matter
Authors: A M Hindeleh  , R Hosemann
Published: July 2002

It has now been established that paracrystals are the 'building blocks' of many materials including polymers, biological materials, colloids, molten metals and alloys, and catalysts. The concept of paracrystallinity and the main equations which are needed for the practical application of the theory to X-ray, neutron and electron diffraction are presented in a simplified style which can be easily grasped by scientists from all disciplines. Examples of the application and usefulness of the theory which support the fact that paracrystals are the building blocks of condensed matter are presented. X-ray diffraction, neutron diffraction, electron diffraction, scanning electron microscopy and transmission electron microscopy are among the techniques which have been used for the assessment of the paracrystalline nature of the materials given in the examples.
An embedded-molecular-cluster method for calculating the electronic structure of point defects in non-metallic crystals. I. General theory
Authors: L N Kantorovich
Published: July 2002

A new approach, called the embedded-molecular-cluster method, is proposed to calculate the electronic structure of point defects in a non-metallic crystal in the framework of the molecular cluster model. The method is applicable to defects with any perturbation radius of the crystal and is based on the concept of the latter as the sum of weakly overlapping structural elements, (atoms, ions, bonds, molecules, etc.). It accounts for Coulomb and exchange interaction between the cluster and the rest of the crystal, as well as (self-consistently) for polarisation of the latter. The present work contains a calculation of the full energy of a crystal with a point defect and arbitrary types of structural elements.
Anomalous transmission of Mossbauer radiation in pure nuclear Laue diffraction. III. Anisotropic scattering
Authors: G V Smirnov  , U van Burck  , R L Mossbauer
Published: July 2002

For pt.II, see ibid., vol.19, p.2567 (1986). The angular dependence of the anomalous transmission of polarised Mossbauer radiation through a perfect single-crystal platelet of57FeBO3was studied. Rocking curves of the Laue reflected and of the Laue transmitted beams were simultaneously recorded for the pure nuclear reflection (001). Striking asymmetries were observed between the transmitted and the reflected intensities for the two polarisation components. The asymmetries were caused by the strong anisotropy of the nuclear resonance scattering. The asymmetry between the total intensities (transmitted plus reflected beam) for the two polarisation components was related to a strongly asymmetric division of the incident radiation into the two wavefields built up inside the crystal. The asymmetry between the intensities of the transmitted and reflected beams revealed a strongly asymmetric composition of the wavefield experiencing anomalous low resonance absorption. The observed anomalous transmission of such a strongly asymmetric wavefield uniquely demonstrated the origin of the suppression of the incoherent reaction channels, i.e. the cancellation of the total amplitude for the individual nuclear excitation (the Kagan-Afanasev rule) rather than the cancellation of the total amplitude of the radiation field at the nuclear position.
Anomalous diffusion by tunnelling on a square lattice: trapping and fusion of triplet excitons in mixed crystals of naphthalene
Authors: R Brown  , J -L Garitey  , F Dupuy  , P Pee
Published: July 2002

This paper presents numerical simulations of dispersive hopping transport by tunnelling between random centres on a square lattice. The results are compared with two approximations, the continuous-time random walk and anomalous diffusion by hopping on percolation clusters. The continuous-time random walk is qualitatively and semi-quantitatively adequate for high concentrations, short times and weak disorder, but diverges from the simulations at long times. Hopping on percolation clusters does not adequately describe the results. Diffusion is in general anomalous, r2(t) infinity tx, 0<x<1, where the upper and lower bounds correspond respectively to weak and strong disorder. The simulations show the dependence of the apparent luminescence decay of a disordered material on the source of excitation. The stretched exponential decay of donor phosphorescence and the algebraic decay of delayed fluorescence of isotopically mixed crystals of naphthalene at 1.6 K are studied as a practical illustration.
Dynamics of the rutile structure. I. Space group representations and the normal mode analysis
Authors: R S Katiyar
Published: July 2002

Multiplier group approach has been used to analyse the normal vibrations of the rutile structure. The irreducible representations at various critical points in the Brillouin zone of the crystal are listed. The basis vectors for the stud of the lattice vibrations have been derived using the projection operator technique and are tabulated. The general form of the atomic displacements at the critical points has been discussed.
Vibronic properties of the F+absorption band in calcium oxide
Authors: C Escribe  , A E Hughes
Published: July 2002

The shape and first four moments of the F+absorption band in CaO have been studied as a function of temperature between 4.2 K and 300 K. Analysis of these results shows that the properties of the band can be accounted for very well in terms of Jahn-Teller coupling of the excited2T1ustate of the F+centre to both Egand T2gvibrational modes, with weaker coupling to A1gmodes. The shape of the band agrees remarkably well with the calculations of O'Brien (1969). Values for the Huang-Rhys factors of S=3.3 for the Egand T2gmodes and SA=1.6 for the A1gmodes are derived from a model where all modes have the same effective frequency of h(cross) omega =264 cm-1. An attempt to allow different frequencies for the various modes is only partially successful, due to the lack of detailed information on the frequency spectra of the coupled modes. Nevertheless such an analysis does suggest that the vibrational sidebands observed at frequencies of about 205 cm-1and 302 cm-1may be associated with A1gand (Eg, T2g) modes respectively. Measurements of the intensity, position and halfwidth of the zero phonon line as a function of temperature cannot be interpreted adequately in terms of conventional theories, unless the frequency of the interacting modes is made much smaller than the value used for the broad band.
Nuclear quadrupole resonance Zeeman spectra of chlorine in 3,4-dichloronitrobenzene. (Crystal structure)
Authors: D Sasikala  , C R K Murty
Published: July 2002

Detailed Zeeman studies of nuclear quadrupole resonance of the 35Cl nucleus in a single crystal of 3,4-dichloronitrobenzene have been made. Two resonance frequencies corresponding to the two positions were observed. The crystal class was identified to be tetragonal. From the Zeeman analysis the field gradient directions, minimum number of molecules per unit cell, asymmetry parameter and hence C-Cl bond characters were determined. The equations for molecular planes were also derived using least squares analysis.
The scattering of light by liquid helium close to the λ line
Authors: C J Palin  , W F Vinen  , J M Vaughan
Published: July 2002

Observations are reported of the total intensity of light scattered by liquid 4He along isobars close to the lambda line. As expected, a peak in the intensity has been observed, and the shape of this peak is compared with that of the peak known to exist in the isothermal compressibility.
Resistivity of liquid Mg-Sn
Authors: G Paasch  , A Schwerdtner  , P Trepte
Published: July 2002

The Faber-Ziman theory for the resistivity of liquid alloys is applied to Mg-Sn, for which the resistivity isotherms exhibit a well marked peak at the concentration of the compound Mg2Sn which exists in the solid phase. Concentration dependent distorted partial interference functions are used, which are capable of matching the X ray diffraction measurements in terms of one adjustable parameter. The calculated resistivity shows a peak which is almost as sharp as that observed.
ESR hyperfine structure of free radical solutions
Authors: B N Misra  , S K Gupta
Published: July 2002

The mechanism of spin exchange in liquids has been studied by observing the hyperfine structure of two free radicals (1-1 diphenyl-2-picryl hydrazyl and picryl-N-amino carbazyl) in solutions of pyridine and benzyl chloride.
Lattice dynamics of rubidium halides
Authors: H H Lal  , M P Verma
Published: July 2002

Lattice dynamics of rubidium halides have been studied on the basis of a theoretical model recently developed by Verma and Singh (1969) by including the effect of three body forces into the framework of the shell model. The results of the calculation are found to agree fairly well with the available experimental data on dispersion relations, second order Raman spectra and specific heats.
The elastic constants of GaAs from 2 K to 320 K
Authors: R I Cottam  , G A Saunders
Published: July 2002

The adiabatic elastic constants of GaAs (n type carrier density 1.3*1016cm-3) have been obtained between 2 K and 320 K from measurements of ultrasonic wave velocities by the pulse superposition technique. One aim has been to compare the results with the somewhat scattered data obtained at room temperature by previous workers: particular attention has been paid to estimation of the overall uncertainty in the elastic constants. At 298 K, C11=1.1841+or-0.0037, C12=0.537+or-0.016 and C44=0.5912+or-0.0018 in units of 1012dynes cm-2. The Debye temperature (346.8 K) computed from the data extrapolated to 0 K is in excellent agreement with that (346.7 K) derived from low temperature specific heat measurements. The temperature dependence of the elastic constants have been fitted by an anharmonic oscillator model. The force constants and effective charge have been calculated on the basis of a rigid-ion model. The Gruneisen parameter, particle displacement and energy-flux vectors and cross-sections of the Young's modulus and wave velocity surfaces have been compiled.
Thin ferromagnetic film: magnetization and spin waves
Authors: J C Levy  , J L Motchane  , E Gallais
Published: July 2002

A Green-function decoupling method for spin 1/2, similar to that used by Tyablikov and Bogoliubov for infinite samples, is developed in the thin-film case. To describe the 'surface condition' the exchange integrals are allowed to vary near the surface, and in the Heisenberg hamiltonian a new term, the surface anisotropy coupling ( Sigma f,gIf,gSz
fSz
g) is included. The spin-wave spectrum and the localized magnetization are studied over a wide range of temperature (from 0K to T(s)
c, the sample Curie temperature). The pinning effect and the appearance of surface spin waves are connected to the 'surface condition' by means of a complex phase shift characteristic of the waves. The non-uniformity of the magnetization distribution at finite temperature is demonstrated to occur and to depend on the surface condition at 0K.
Electroabsorption in disordered solids: selenium
Authors: G G Roberts  , B S Keating  , A V Shelley
Published: July 2002

Electroabsorption data are presented for a number of amorphous chalcogenide semiconductors. Particular emphasis is placed on those obtained for amorphous selenium for which evidence is presented which suggests that the electroabsorption is associated with a quadratic Stark effect of localized excitations in the Se8 rings. For this element the electroabsorption curves reveal subtle differences in the properties of evaporated films prepared under different conditions. The possible application of the electroabsorption technique to determine whether or not the boundary between extended and localized states in amorphous materials is sharply defined, is also discussed.
Three-spin-state generalized Ising model
Authors: S Alexander  , G Yuval
Published: July 2002

Second-rank tensor order parameters appear for liquid crystals and crystallographic transitions. The simplest model with a disordered high-temperature state has interactions invariant under cubic spin transformations. It is shown that for spin 1 this leads to the 3-state Potts model. It is suggested that this model is therefore relevant to the understanding of phase transitions with such an order parameter. A formalism for series expansions and a new proof of the Kramers-Wannier symmetry on the square lattice are presented. Pade approximants for the series expansions including a procedure explicitly utilizing the symmetry are discussed. The negative interaction 'orthogonal' model analogous to the Ising antiferromagnet is also discussed and shown to have a highly degenerate ground state. On the square lattice this is the degeneracy of Lieb's ice. Relationships to ferroelectric models and the three-colour problem are also discussed.
Evidence for thermal quenching of methyl-group tunnelling rotation
Authors: S Clough  , J W Hennel
Published: July 2002

The proton spin-lattice relaxation time of 4-methyl-2,6-ditertiarybutylphenol is measured over the temperature range 12-50K where the relaxation is almost entirely due to rotation of the 4-methyl group. The object is to search for a feature predicted to occur when 2 pi nuttau approximately , where nutis the frequency of coherent tunnelling rotation and tau is the time constant for randomly occurring rotations. A feature of the expected kind is found near 33K leading to a value at this temperature of nut=1.6+or-0.3 GHz. This is much less than the value of 9.4+or-0.1 GHz found by different methods in the range 4-9K. This result is in good quantitative agreement with the theoretical expectation that with increasing temperature, admixture of excited methyl-group torsional oscillator states leads to a reduction of nutthrough a process of thermal averaging.
An investigation of the recombination radiation from the electron-hole condensate in intrinsic silicon
Authors: M A Vouk  , E C Lightowlers
Published: July 2002

Recombination radiation from free excitons and electron-hole droplets in high-purity silicon has been investigated in the temperature range 4.9 to 14.1K using 50 keV electron excitation. The spectra have been analysed to yield the temperature dependence of the density of electron-hole pairs, the sum of the electron and hole Fermi energies, the condensation energy and the chemical potential, and values have been obtained for the ground-state energy of the electron-hole condensate and the sum of the exchange and correlation energies. This is the first experimental determination of the temperature dependence of the electron-hole droplet parameters. The extrapolated zero-temperature values are in reasonable agreement with other experimental determinations and theoretical calculations.
Calculation of some variational bound results for lattice thermal conductivity of Ge
Authors: G P Srivastava
Published: July 2002

The author used a displaced Planck's distribution function as a variational one-parameter trial function to calculate some complementary bound results for the lattice thermal conductivity of Ge. An improved lower bound due to Benin (1970) is shown to be higher than the lowest bound (due to Ziman 1960) by 1.4% at 300K to 3% at 900K. Also, an upper bound due to Jensen, Smith and Wilkins (1969) is shown to be lower than its value in 'zeroth approximation' by 2.5% at 300K to 3.5% at 900K.
Phase transition measurement of pyrene and phenanthrene based on a rotary polarization method
Authors: A Matsui  , T Tomotika  , K Tomioka
Published: July 2002

The phase transition temperature in pyrene and phenanthrene crystals has been measured by a polarization modulation method, which enables one to detect the variation in the refractive index in the transparent region of a crystal. The phase transition occurs at 120K in pyrene and between 210 and 260K in phenanthrene.
Correlation effects in small metal clusters
Authors: M Torrini  , E Zanazzi
Published: July 2002

A model Hamiltonian, in which correlation may be treated exactly, is used to discuss the stability of neutral clusters composed of three and four monovalent atoms. Electron repulsion is shown to have a relevant effect on the relative stabilities of four-atom aggregates of different geometries. In particular, it is found that the more stable structure of a neutral four-atom cluster depends upon the ratio between the repulsion energy of a pair of electrons sharing the same atomic orbital and the hopping integral between nearest neighbours. A transition point in the zero-temperature ground state occurs for one of the four-atom structures at a given value of this ratio.
ESR of an S=2 centre in amethyst quartz and its possible identification as the d4ion Fe4+
Authors: R T Cox
Published: July 2002

A paramagnetic centre having spin S=2 and C2symmetry has been detected in natural amethyst crystals. The only ESR transition observed was the Delta M=+or-4 transition between the lowest two energy levels M=+or-2. Line position measurements at 35 GHz and 16 GHz gave the g-tensor principal value for the C2direction gzz=1.9876+or-0.0006 and the zero-field splitting of +or-2, Delta2=10.166+or-0.005 GHz (at 77K). The constants D and E of the second-order zero-field splitting operator DSz2+E(Sx2-Sy2) could not be determined; however, -D must be greater than 80 GHz and mod 3E/D mod <0.3. The paramagnetic centre could be an Fe4+ion on the tetrahedral silicon site of the quartz lattice.
A study of conduction-band L-states in GaSb through observations of the influence of uniaxial stress on its infrared free-carrier absorption
Authors: A K Walton  , S F Metcalfe
Published: July 2002

Measurements have been made of the optical absorption in n-type GaSb doped approximately 2.5*1017cm-3as a function of high (111) uniaxial stress. Specimens were held at temperatures of 18.5K and 79K and results obtained over the wavelength range 7-22 mu m for radiation polarized parallel or perpendicular to stress. Free-carrier absorption was found to dominate in the wavelength range investigated and although high stress caused large changes in its magnitude, the wavelength dependence lambdapwas independent of stress and polarization with p=2.7+or-0.2 at 18.5K and 3.0+or-0.2 at 79K. Important features of the observed absorption behaviour at 18.5K were a threshold stress below which there was no change, opposite changes for the two polarizations above the threshold, and saturation of the variation at high stress for radiation polarized perpendicular to stress.
Nuclear spin-lattice relaxation of tunnelling molecular groups
Authors: S Clough
Published: July 2002

A theory for nuclear spin-lattice relaxation of molecular groups undergoing tunnelling rotation in solids is described. It is based on the theory of Haupt (1971) but employs a spin Hamiltonian formalism which more clearly displays the relationship to the classical theory of Bloembergen, Purcell and Pound (1948) as further developed by Look and Lowe. The latter approach enables all dipole-dipole interactions to be taken into account. Relaxation does not follow a simple exponential behaviour since the nuclear magnetisation is dynamically coupled to other variables connected with the angular momentum of the rotating groups. The rate constants which describe relaxation appear as sums of easily evaluated traces of nuclear spin operators multiplied by spectral density functions which depend both on rotational tunnelling frequencies and on correlation times for incoherent rotational motion. The classical theory results from equating the tunnelling frequencies to zero.
Thin permalloy films: optimization of properties for computer memories
Authors: R M Pickard  , J A Turner  , J K Birtwistle  , G R Hoffman
Published: July 2002

The vacuum deposition of Ni-Fe elements on glass for use in a 6000 bit, 100 ns cycle time memory is described. Measurement of the magnetic properties of some 20 000 elements has enabled reliable correlation to be made with film composition and deposition conditions.

Memory testing of elements made with calculated optimum dimensions show that films 100 nm thick have wide operating limits because of high coercivity and freedom from cross-tie walls.

Tapering the ends of the elements increased the domain nucleation coercivity above the wall coercivity. Although producing films with rectangular hysteresis loops, tapering did not increase the creep disturbance threshold.

Films thicker than 180 nm show anomalous domain configurations in hard direction fields together with a steep rise in wall coercivity and increased dispersion.
Water movement in porous materials III. Evaporation of water from soil
Authors: D A Rose
Published: July 2002

The equation of vertical flow is adapted to describe the evaporation of water from a semi-infinite porous material during the second stage of drying when water loss is limited by the distribution of water in the material. Flow measurements on long columns of aggregates of six soil materials confirm the theory, and enable the transport parameters controlling flow to be found over the range of volumetric water content θ (cm3 water/cm3) from aggregate saturation θC to dryness (i.e. from near pF 2 to near pF 7).

Evaporation from porous materials initially at a uniform water content θC produces profiles of θ invariant with zt
−1/2 (where z is the depth in cm and t the time in seconds) which differ for each material. As θ decreases from θC diffusivities (D cm2 s−1) calculated from these profiles fall to a minimum, then rise to a subsidiary maximum close to dryness, before falling again as θ approaches zero. Such behaviour accords with prediction. Hydraulic conductivities (k cm s−1) fall continuously as θ decreases from θC to zero.
The calculation from mass spectrometric measurements of the abundances of oxygen isotopes in carbon dioxide
Authors: A H Turnbull
Published: July 2002

When the isotopic abundances of the oxygen in carbon dioxide are to be determined from the mass spectrum of the latter, it seems desirable at first sight to base calculations on the three largest peaks in the spectrum. In certain cases, however, this procedure produces inaccurate answers. Taking a range of carbon dioxides whose oxygen isotopic abundances vary widely enough to cover all realistic cases, examination of the contributions of the oxygen isotopes to the six CO2
+ peaks in the mass spectrum indicates how to choose three peaks on which to base abundance calculations which will avoid these inaccuracies. `Best expressions' for oxygen isotopic abundances have been tabulated against CO2
+ peak patterns.
The form effect in prolate spheroids of magnetically saturated polycrystalline materials: I
Authors: R R Birss  , S R Adamson
Published: July 2002

The deformation that accompanies a change in the magnetization of a ferromagnetic specimen is partly shape-dependent (the form effect) and partly shape-independent (the magnetostriction). This paper investigates the form effect in prolate spheroids on the basis of the theory that assumes that a non-uniform strain is set up by prescribed surface stresses. An explanation is given of how the strain due to the form effect may be calculated. Numerical results are presented in the companion paper by Birss and Hegarty (II).
The temperature dependence of sonoluminescence from water
Authors: K J Taylor  , P D Jarman
Published: July 2002

The intensity of sonoluminescence from circulating, air-saturated water was found to be a maximum at about 13°C; it decreased steadily below this temperature and at 1°C was about one eighth of the maximum value. This temperature dependence is similar to that found for cavitation erosion, and may possibly be due to the onset of ice nucleation, as suggested by Hickling. The conditions necessary to obtain reproducible results are described.
Further characteristics of a molecular beam maser oscillator operated with two cavities in series
Authors: A L S Smith  , D C Lainé
Published: July 2002

The characteristics of an ammonia maser operated with a molecular beam which passes through two resonator cavities in succession are discussed. Experimental observations on the oscillation amplitude in the second cavity as a function of tuning of the cavities, separator voltage and beam flux are given in detail and related to previous work; some discrepancies are noted.
Electrical conduction in liquid air and liquid nitrogen
Authors: S W Byatt  , P E Secker
Published: July 2002

The electrical conduction of liquid air and liquid nitrogen has been measured in very simple diode cells. The current is found to require several minutes to increase to an equilibrium value which is then directly proportional to the electrode surface area. Selection of different electrode materials does not affect the current magnitude, and current-induced liquid motion appears to play only a minor role in the conduction process. A tentative model is proposed in which the charge carriers are assumed to be negative oxygen ions.
Light-scattering photometer calibration
Authors: B R Jennings  , H Plummer
Published: July 2002

A Sofica commercial light-scattering photometer has been critically calibrated using three procedures:

Comparing the angular scattered intensity with the turbidity of a solution of suitable Rayleigh scatterers.

Comparing the experimental and theoretical scattered intensities of a solution of suitably chosen Mie scatterers.

Comparing the scattering from benzene with data in the literature.

Two silica sols, Ludox HS and Syton 2X, when used in method (i), gave identical calibration constants. These values agree with those obtained using method (ii) if polystyrene latex particles, which act as the Mie scatterers, are carefully chosen for their size, polydispersity and truly spherical shape. The procedure for this method appears to be entirely novel. Method (iii) showed that then2refraction correction which is often used seems to be in error, at least for the cylindrical cells and optical arrangement employed in this photometer. A correction innmore nearly fits the data. This was confirmed by measuring the molecular weight of a narrow fraction of polystyrene (Mw= 59 000) in three solvents of different refractive index.
Early stages of light interaction with solids at extreme intensities
Authors: B Steverding  , F P Gibson
Published: July 2002

Electron and radiation transport of energy is treated by self-similar methods in aluminium targets exposed to extreme radiation intensities. Detailed numerical solution of this phenomenon are given including the effects of incomplete ionization on the conduction processes and the internal energy.
A study of a gas scintillation multiwire proportional counter
Authors: S S Al-Dargazelli  , T R Ariyaratne  , J M Breare  , B C Nandi
Published: July 2002

A gas scintillation proportional chamber has been built to investigate its behaviour as a high-intensity low-energy X-ray detector using a gas mixture of argon and nitrogen. An X-ray source (55Fe) and an alpha -particle source (241Am) have been used to test the chamber. The scintillation pulse height has been studied for several anode voltages and for various concentrations of nitrogen. The maximum scintillation light output has been obtained for a nitrogen concentration of 2%. The energy resolution of 32.5% has been obtained using 5.9 keV X-rays for a gas multiplication factor of about 400. Very little change in the characteristics of the chamber have been observed up to rates of about 106 per second.
The spatial and temporal development of the early phases of homogeneous discharges in N2
Authors: O R Ahrens
Published: July 2002

The spatiotemporal development of electron densities within homogeneous overvolted discharges in N2is investigated by optical means during the early build-up phases. The use of photomultiplier techniques rather than image devices enables a quantitative comparison between experiment and one-dimensional model calculations. The influences of space-charge fields and photosuccessors on discharge development are investigated. Considerable electron densities are also detected in front of the primary avalanche even during the early, field-distortion-free stages of the development.
Wave-front distortions due to combined photolysis and gas-dynamic effects in a light amplifier operated with C3F7I as the active medium
Authors: K J Witte
Published: July 2002

The smooth variation of the refractive index occurring in the central part of an amplifier operated with C3F7I as the active medium is theoretically and experimentally investigated. It is shown that the perturbation of the refractive index arises from spatially inhomogeneous pumping and can be explained as a combined photolysis and gasdynamic effect. The wave-front distortion of a resulting laser beam is calculated. Its influence on the focusing properties of the beam is also discussed.
Determination of the dynamic flow characteristics of an electro-hydraulic servo-valve using a laser velocimeter
Authors: W Weston  , M J Lalor
Published: July 2002

The dynamic flow characteristics of an electro-hydraulic servo-valve have been determined experimentally using a laser velocimeter. The volume flow rates were predicted from centre-line velocity measurements using a finite difference digital computer analysis of the Navier-Stokes equation adapted for incompressible flow along a tube. The analysis was first verified at low pulsatile system excitation frequencies by making velocity measurements at radial positions across the flow tube and comparing the profile thus generated with that from the computer analysis. The results show that the attenuation in flow amplitude with increasing frequency could not be accounted for by inertia effects alone and could be explained by changes in the contraction coefficient of the valve spool orifice. An expression is given for the change in flow amplitude as a function of the valve non-dimensional inertial loading.
The determination of basic quantities during glow-to-arc transition in a positive point-to-plane discharge
Authors: F Bastien  , E Marode
Published: July 2002

The discharge is operated in oxygen with a 1 cm gap between a positive point and a plane at a pressure near 300 Torr. The discharge has been found to have a behaviour similar to that in atmospheric air. By introducing traces of hydrogen it has been possible to study the Halphaand Hbetalines broadening which is mainly connected to the Stark effect. However, due to the presence of a macroscopic field the usual theory does not apply, i.e. it is shown that the Stark profile is related to the electron current density j rather than to the electron density Ne. The current density j is derived and from total current i a characteristic radius r0of the filamentary discharge is obtained. A value of the macroscopic field Feis then determined from a numerical simulation of the discharge, leading in turn to the electron density. All these measurements are time resolved and are given as a function of time during the glow-to-arc transition. Typical values are j approximately 3*103A cm-2, r0approximately 20 mu m, Neapproximately 1015cm-3. Some indications concerning the primary streamer, the nature of the discharge plasma and the glow to arc transition are derived from these measurements.
The flow characterisation of a high-gradient magnetic separator
Authors: W A Bullough
Published: July 2002

A review is given of what is required from flow-pressure-drop tests on a multi-fibre high-field-gradient magnetic filter mesh and suggests methods of how the desired results may be obtained. The salient features of the problem are high-lighted so as to aid the production of meaningful data and its presentation to the filter designer in as general and useful a form as possible. Techniques and instrumentation suitable for experimentation in this type of flow problem are outlined and the advantages of air as a test medium are explained. The conditions for successful model testing and the relating of model results to full size filters are discussed (with special attention to the problem of the flexure of low-solidity filters) as are the design of a suitable test rig and the sizing of associated equipment.
Energy dependence of excitation in sputtering
Authors: R S Bhattacharya  , D Hasselkamp  , K -H Schartner
Published: July 2002

Bombardment of Si by 50-500 keV Ar+ions results in the emission of optical lines in the spectral range of the visible and vacuum ultraviolet. The authors present results of the measurements on the energy dependence of various lines emitted by sputtered atoms and ions. The results indicate that the photons are emitted by those atoms which are sputtered in a few collisions from the near-surface region.
The layergram iterative method of reconstructing three-dimensional refractive index distributions
Authors: K A R Kinloch  , S M Fraser
Published: July 2002

The reconstruction of three-dimensional refractive index distributions from interferometric fringe data is required for interpreting interferograms. The layergram iterative method of reconstruction represents a new application of computer analysis techniques which is shown to have good convergence and is economical in computer store requirements.
Non-linear mechanical response of various metals. V. Yield condition for isotropic hardening
Authors: E W Billington
Published: July 2002

For pt.IV see ibid., vol.11, p.459 (1978). The parameters appearing in a particular incremental type constitutive equation for an isotropic incompressible strain hardening elastic-plastic solid with large deformation have been computed directly from measurements of the Swift effect by way of the thin-walled tube approximation. It is shown that these parameters, determined for thin-walled tubes of aluminium, copper in the fully annealed state, and copper and iron both in the as-received state, can be used to evaluate the particular form of the associated yield function for a wide range of the available plastic deformation. The yield function, evaluated in this way, is shown for an initial range of response to be a particular form of modified von Mises yield function. This particular form of modified von Mises yield function is used to compare the predictions at initial yield in simple compression and simple torsion with the predictions obtained from the Swift effect.
The multi-atmospheric pressure operation of a gain-switched iodine laser
Authors: H J Baker  , T A King  , W G McNaught
Published: July 2002

The operation of a 1.315 mu m atomic iodine laser in the gain-switched mode at multi-atmosphere buffer gas pressures is described. Characterisation of the first gain-switched pulse in terms of pulse delay, energy and peak power has been carried out in the high-pressure regime, and factors influencing the output beam directional stability and optical quality have been investigated. Shock waves generated at the tube walls and photolysis-induced density gradients progressively destroy the optical homogeneity of the gain medium during pumping, but gain-switched, single-transverse mode, operation is possible up to 7 atm maximum pressure. Optical inhomogeneity caused by gain saturation is not detected.
Method for determining the spatial distribution of emitting species
Authors: J Godlewski  , J Glinski  , J Kalinowski
Published: July 2002

A method for determining the spatial distribution of radiation-emitting species has been developed based on absorption properties of the substance comprising the emission object and/or finite lifetime of the particles emitted. The method employs a mathematical analysis containing a solution of an integral equation by the method of statistical regularisation (STREG). The method has been applied to the experimental results reported for photon-emitting singlet excitons in anthracene single crystals.