bluuebunny/crossref_metadata_2025 huggingface dataset:
Collective excitations in the Van Vleck enhanced nuclear antiferromagnet HoVO4. I. Effective nuclear Hamiltonian
Authors: B Bleaney  , G J Bowden  , R G Clark
Published: July 2002

The problem of interacting Van Vleck ions at low temperatures is examined in some detail using both simple intuitive arguments and a high-order perturbation approach. Particular attention is paid to the Van Vleck enhanced nuclear antiferromagnet HoVO4. As a simple example, the problem of a neighbouring pair of Ho3+ions in host YVO4is solved exactly. It is shown that in addition to the expected dipole-dipole interactions between enhanced nuclear moments, there are subtle effects, due to the presence of dipole field-induced moments on the 4f shells, which strengthen the interaction between the two sites. Following a derivation of the many-body Hamiltonian for HoVO4, it is shown that the Hamiltonian can be expressed either in the form of a continued equation or as the sum of 'effective' two-body interactions, between enhanced nuclear moments, which take into account the role played by all the dipole field-induced moments in the lattice. Finally it is demonstrated that the mean field energy eigenvalues and eigenfunctions used by Bleaney (1980), in his discussion of the nuclear ordered state, can be used in conjunction with Green function methods of Haley and Erdos (1972) to compute the excitation energies epsilon (q) versus q in the nuclear ordered phase of HoVO4.
Magnetisation density distribution in Mn1/4TaS2: observation of conduction electron spin polarisation
Authors: S S P Parkin  , E A Marseglia  , P J Brown
Published: July 2002

The authors have determined the magnetisation density distribution in the ferromagnetic layered compound Mn1/4TaS2, using the classical polarised neutron technique. These studies show that although the magnetisation is principally localised on the manganese sites, there is a significant spin polarisation of the conduction electrons, extending throughout the unit cell. These measurements thus provide direct evidence of an important interaction between the Ta conduction electrons and the localised moments on the Mn ions, confirming previous speculations based on transport and optical data. Moreover such an interaction suggests an obvious mechanism, the RKKY interaction, for the exchange interaction between the Mn moments. The moment on the Mn sites is depressed, as a result of this interaction, by about 15% compared with that expected for a Mn2+d5ion. The Mn ions are surrounded by an antiferromagnetically polarised cloud of electrons and the maximum spin density within the TaS2layer is about 3% of the maximum spin density on the Mn site. There is no evidence from these experiments that the S atoms play a role in the magnetic Mn-Mn exchange interactions.
Observation of the KL2to L2V X-ray satellite multiplet structure in metallic Mg
Authors: O Keski-Rahkonen  , E Mikkola
Published: July 2002

The K betaVII,VIIIX-ray satellite band spectrum is measured in metallic Mg with high resolution and interpreted in terms of KL2to L2V transitions from the initial multiplets of the KL2configuration.
Low-temperature behaviour of the random-field Ising model
Authors: A J Bray
Published: July 2002

The random-field Ising model, with a normally distributed random field, is considered for systems with large, but finite, coordination number z. The conventional loop expansion is shown to fail below a temperature T* approximately=Tc/z, where Tcis the (ferromagnetic) transition temperature of the pure system. Resummation of the most divergent terms in the expansion yields a specific heat anomaly in the temperature region T<or approximately=T*. It is conjectured that the temperature T* has dynamical significance.
Spin-phonon Hamiltonians for 4fnions in cubic sites
Authors: S C Chen  , D J Newman
Published: July 2002

Tables are given of the isomorphisms relating real and fictitious operators for the ground (J) states of all 4fnions in cubic symmetry. These tables are then used as a basis for the construction of superposition model expressions for spin-phonon Hamiltonians. Some specific examples are given relating this general approach to expressions for spin-Hamiltonian parameters that have been obtained by previous workers.
Valence states and magnetic properties of LaNi1-xMnxO3 (for 0⩽x⩽0.2 and x=0.5)
Authors: N Y Vasanthacharya  , P Ganguly  , J B Goodenough  , C N R Rao
Published: July 2002

Single-phase LaNi1-xMnxO3 samples in the compositional range 0<or=x<or=0.2 were prepared by the decomposition of precursor hydroxide solid solutions, and La2NiMnO6 was prepared from a mixture of carbonates. All the samples contained a disordered array of nickel and manganese ions. X-ray absorption and XPS measurements indicate the presence of Mn(III) and Ni(III) valence states at room temperature for x=0.1, 0.2 and 0.5. Magnetic susceptibility data suggest Mn4+ solute ions and Stoner-enhanced Pauli paramagnetism of the metallic solvent for x=0.01 with a smooth transition to Mn3+ solute ions and a spontaneously magnetised solvent conduction band at x=0.05. Below 200K, the x=0.05 sample forms superparamagnetic clusters, and below 40K there is evidence for an antiferromagnet spin-density wave. Comparisons with LaCo0.95Mn0.05O3 and La0.98Sr0.02CoO3 confirm that the long-range magnetic coupling occurs via solvent electrons in a narrow conduction band. The conductivity changes from that of a narrow-band metal for x<0.01 to that more characteristic of diffusive motion for x>0.05, but any motional enthalpy appears to remain small ( Delta Hm approximately=0). The x=0.1 sample exhibits ferrimagnetic spin glass behaviour below 40K, and the ferromagnetic interactions increase with manganese concentration. The oxide with x=0.50 is ferromagnetic with a well defined Curie temperature.
High-frequency conductivity of a polar semiconductor with anisotropic conduction valleys
Authors: S Goettig
Published: July 2002

The systematic study of the high-frequency wavevector-dependent conductivity in the presence of static scatterers is presented for the multicomponent anisotropic plasma, coupled with optical phonons in the polar way. The kinetic equation is solved in the self-consistent-field approximation, expanding the one-particle density operator in powers of the screened static imperfection field and radiation electromagnetic field. The conductivity is determined by the current quadratic in the imperfection field and linear in radiation field, averaged over the statistical distribution of imperfections. The exact expression for the complex conductivity tensor, which properly accounts for collective effects, is obtained for the quantum plasma. In the zero-wavevector and isotropic limits with the neglect of coupling with phonons or making some assumptions about the impurity statistics, it agrees with the results of Ron and Tzoar (1963), or of McCumber (1966).
The influence of exciton-phonon coupling on resonant exciton transport in aromatic mixed crystals at very low temperatures
Authors: R Brown  , P Pee  , F Dupuy  , P Kottis
Published: July 2002

A numerical study of the master equation for triplet exciton donor-acceptor transfer at low temperatures in naphthalene is used to show that the microscopic resonant transfer rates must vary very rapidly with temperature. A model of exciton-phonon coupling using the true electronic eigenstate basis is given to explain this result. It is found that the predominant scattering process involves two optical phonons. It is also pointed out that residual long-range couplings due to spin-orbit triplet-singlet mixing are needed to understand the high yield of acceptor emission at low donor concentration. Arguments are given in favour of the existence of an Anderson type transition between localised and extended states, in the experimental system naphthalene-h8in naphthalene-d8.
Pd M4,5N4,5N4,5Auger spectra of Ag80Pd20and Cu95Pd5: the influence of the conduction band densities of states
Authors: M Davies  , P Weightman
Published: July 2002

The spectral profiles of Pd M4,5N4,5N4,5transitions of Ag80Pd20and Cu95Pd5are calculated using the Cini-Sawatzky model for a filled band system the multiplet structure obtained from atomic structure calculations and single electron densities of states obtained from the results of SCF-KKR-CPA calculations. Reasonable agreement with experiment is obtained for Ag80Pd20but not for Cu95Pd5. Agreement with experiment is improved if the calculated intensities of states at the bottom of the band, associated with bonding states, is arbitrarily reduced and the M5N4,5N4,5:M4N4,5N4,5intensity ratio is modified to take account of the influence of M4M5N4,5Coster-Kronig transitions.
Motion on inequivalent lattice sites: NMR theory and application to LaF3
Authors: G A Jaroszkiewicz  , J H Strange
Published: July 2002

A general theoretical treatment is developed for NMR spin-lattice (T1 and T1 rho ) and spin-spin (T2) times for relaxation due to modulation for dipolar interactions by diffusion of atoms on inequivalent lattice sites. This extends previous calculations of T2 to the other relaxation parameters and gives rate equations for a single nuclear species moving over several inequivalent lattice sites and interacting with other (static) nuclear species. The temperature dependences of the relaxation times are shown to have specified characteristics, such as minima in T2 and multiple minima in T1 rho . The formalism is then used to analyse data obtained for LaF3 to evaluate details of fluorine-ion motion. Its applicability is successfully demonstrated.
A molecular dynamics study of the glass transition in binary mixtures of soft spheres
Authors: B Bernu  , Y Hiwatari  , J P Hansen
Published: July 2002

Molecular dynamics simulations of supercooled and glassy states of binary 'soft sphere' mixtures are reported. The transition to the amorphous solid phase appears to be smooth and takes place at a roughly concentration-independent value of an effective coupling parameter derived from a simple 'one-fluid' model.
The KβX-ray satellite spectra in sodium, magnesium and aluminium: the KAhhypersatellite spectrum in aluminium
Authors: O Keski-Rahkonen  , E Mikkola  , K Reinikainen  , M Lehkonen
Published: July 2002

The K betaIII,IV, and K betaVII,VIIIX-ray satellite groups have been measured with high resolution in Na, Mg and Al metals as well as in Na2O. The spectra interpreted in terms of KLnto LnV(n=1, 2) transitions, are analysed in detail using experimental initial multiplet energies from the KLnto Ln+1X-ray transitions. The final-state energies are taken from optical spectra presuming fully screened Lncore holes. The results show this assumption to be justified. The K betaVII,VIIIband is here reported for the first time for Na, and also in Mg and Al it has a peak not found earlier. In metals these spectra allow new testing possibilities for the final-state rule. In Na2O the K betaIII,IVband is a clear duplet of K beta0cross transitions but the K betaVII,VIIIband spreads out to a diffuse hump due to interaction between the sodium L2holes and the localised oxygen valence orbitals. The K2Lnto KLn+1(n=0, 1)hypersatellite spectra overlapping with the K betaVII,VIIIbands in aluminium were separated from the latter and the energies of the multiplet transitions were determined and found to agree with theoretical energies calculated using the multiconfiguration Dirac-Fock method.
The structure of a highly concentrated aqueous solution of lithium chloride
Authors: A P Copestake  , G W Neilson  , J E Enderby
Published: July 2002

The structure of a concentrated aqueous solution of LiCl has been investigated by neutron diffraction. The methods of first- and second-order differences were used. It is concluded that structural features characteristic of both the molten salt and the dilute solution coexist in concentrated solution and that each Li+ion is surrounded, on average, by three water molecules and three chloride ions.
Nucleation, pinning and hysteresis effects at the incommensurate-commensurate transition
Authors: T Nattermann
Published: July 2002

Thermal hysteresis effects due to nucleation and pinning of two- and three-dimensional incommensurate structures are considered at the IC transition and inside the incommensurate phase. A possible disorder-induced domain wall interaction is taken into account. Various equilibrium and non-equilibrium effects due to randomly frozen-in impurities are critically summarised. The results allow the interpretation of unusual hysteresis effects of the dielectric constant observed recently in improper ferroelectrics. It is argued that nucleation is the decisive mechanism for the change of the domain wall density in incommensurate systems.
Neutron scattering structural investigations of the solutions of alkali metals in molten salts. I. Small-angle scattering in the vicinity of the liquid-liquid critical point
Authors: J F Jal  , J Dupuy  , P Chieux
Published: July 2002

With the help of density measurements obtained from neutron transmission data an attempt is made to describe the authors' previous small-angle scattering critical fluctuations in K-KBr, in terms of an improved order parameter, i.e. the volumic fraction. Critical scattering data on the system K-KF are presented and analysed in this framework. A qualitative estimate of the excess-volume behaviour of these solutions is then given.
Perturbation model for the thermal-donor energy spectrum in silicon
Authors: J T Borenstein  , J W Corbett  , M Herder  , S N Sahu  , L C Snyder
Published: July 2002

The general features of the repulsive potential introduced by oxygen atoms successively agglomerating around 450 degrees C thermal donors is determined by fitting the ladders of energy levels for both charge states observed by infrared spectroscopy and accounting for electron-screening effects in the neutral charge state. Recent evidence suggesting that the thermal-donor ground-state wavefunction is constructed from a pair of (100) valleys has been used to determine qualitative features of the perturbing potential. Directional agglomeration is seen to reproduce the experimental energy shifts, a result in accord with the view that thermal donors are nucleation sites for the subsequent formation of coesite.
On the excited and metastable states of Cr-related double centres in GaAs
Authors: K Kikoin  , P Lyuk  , L Pervova  , R Vanem
Published: July 2002

The energy levels of Cr-related trigonal centre in GaAs are studied both experimentally and theoretically. New lines and absorption bands in the energy interval of 0.9<h nu <1.4 eV are discovered in the p-type GaAs:Cr by the method of light-induced modulation of absorption. These lines are ascribed to the intra-centre transitions with charge transfer at a (Cr-X) trigonal defect, (Cr-X+) to (Cr0X0), where (Cr0X0) are metastable vibronic states with a large relaxation time which is found to be 10-1s at room temperature. The microscopic theory of such a double defect is developed with the assumption that X is the arsenic vacancy and the strong lattice relaxation is originated from the pseudo-Jahn-Teller effect due to the tunnelling of an electron between CrGa- and VAs-related deep levels of the double-well effective potential.
Inelastic neutron scattering study of the rotational potential of NH4
+ in (NH4)2PdCl4 and (NH4)2PtCl4
Authors: M Prager  , G Badurek
Published: July 2002

A consistent description of the rotational excitations in (NH4)2PdCl4 and (NH4)2PtCl4 is given in the model of a one-dimensional rotation in a nearly fourfold rotational potential, which is supported by the crystal structure. In such a potential, large tunnel splittings are combined with high-lying librations and moderate activation energies. The estimated model potentials are V( phi )=70 cos 4 phi +0.0004 cos 2 phi (meV) for (NH4)2PdCl4 and V( phi )=66 cos 4 phi +0.0008 cos 2 phi (meV) for (NH4)2PtCl4.
The Jahn-Teller effect of octahedrally co-ordinated 3d4ions
Authors: J R Fletcher  , K W H Stevens
Published: July 2002

The chromous ion in magnesium oxide is studied and an interpretation is given of the acoustic paramagnetic resonance results of Marshall and Rampton. Because of the very strong orbit-lattice interaction it is important to take full account of the Jahn-Teller effect. The cluster approximation justified previously is used, and then are added the spin-orbit coupling, anharmonic forces, external magnetic field and random lattice strains. The resultant secular equations are usually solved numerically in terms of parameters to describe the strains and anharmonic couplings. It is postulated that the observed line shapes reflect random strain distributions and that peaks in the lines are correlated with turning points in the strain-resonance field curves. Parameters are chosen to fit the observations for a definite field orientation and it is shown that a good interpretation can then be given for the results for a variety of orientations. The energy-level diagram of the chromous ion is shown to be quite complicated, and it is pointed out that many more transitions than have yet been observed can be expected.
Diffuse scattering in nickel arsenide, NiAs
Authors: G L Hua  , T R Welberry  , R L Withers
Published: July 2002

The characteristic diffuse scattering distribution that is observed on electron diffraction photographs of nickel arsenide, NiAs, has been interpreted in terms of a force-constant model of the structure. Thermal diffuse scattering intensities for the system have been obtained, both by direct calculation from a force-constant model and also with the use of Monte Carlo simulation in combination with optical transform techniques. The diffuse scattering can largely be explained in terms of simple nearest-neighbour central-force constants between Ni-Ni (in the c direction and in the basal ab plane), Ni-As and As-As, and does not appear to be closely associated with the formation of the recently reported orthorhombic displacive superlattice, which has been attributed to the condensation of a q=1/3(11-20) Sigma6phonon. From the derived force model it is shown that the main driving force required to obtain the superlattice must be a force tending to make the Ni atoms form zigzag chains along c (i.e. alternative Ni atoms are displaced in opposite directions in the basal plane). Additional, albeit small, second-nearest-neighbour forces must also be invoked for the Sigma6phonon to condense at1/3(11-20) rather than at1/2(11-20), which would correspond to the MnP superlattice distortion.
The resistivity and thermopower of liquid mercury and its alloys
Authors: R Evans
Published: July 2002

Model potentials for zinc, indium, thallium, lead, cadmium and mercury are constructed using an adaptation of Shaw's energy-dependent formalism which allows the effect of high d-core states to be included. In the case of mercury this leads to a striking change in the shape of the screened form factor. Numerical calculations of the thermopower in liquid zinc, indium, thallium and mercury are in better agreement with experiment than previous calculations which did not include the energy-dependent term. Calculations of the resistivity of alloys of mercury with indium, thallium, zinc and lead are presented. These show a sharp decrease of the resistivity on alloying which is in reasonable agreement with the experimental results
The small-angle scattering of electrons in thin films of evaporated carbon
Authors: R E Burge  , D L Misell  , J W Smart
Published: July 2002

A study has been made of small-angle (1*10-4-8*10-3rad) electron scattering in thin films of evaporated carbon for a series of incident electron energies (30-80 keV). The angular distribution of the electrons scattered in such films has been interpreted in terms of plural scattering of the incident electrons within the specimen film. The plural scattering calculations were based on both the free atom theory and the Bohm-Pines plasma oscillation theory. The two sets of theoretical calculations both agree with experiment to within a deviation of 20% for angles of scattering in the range 5*10-4-5*10-3rad and for films thinner than about 1000AA.
Ion core scattering and low energy electron diffraction. I
Authors: J B Pendry
Published: July 2002

The construction of accurate ion core scattering factors is discussed, and a Hartree-Fock treatment of exchange found to be important. A formalism is constructed for making accurate, fast, nonperturbative calculations of LEED spectra within the framework of the ion core scattering model.
Measurements of acoustoelectric gain and lattice attenuation in n-GaAs
Authors: Y Shapira  , B Fisher  , B Pratt  , A Many
Published: July 2002

Measurements of the non-ohmic behaviour associated with acoustoelectric amplification were employed to derive the frequency dependence of the small signal electronic gain parameter alpha0and the lattice attenuation alphalin n-GaAs. Samples of different carrier concentrations at different temperatures were used to vary the frequency of the amplified flux. The use of a single frequency model in the derivation of alpha0and alpha1is justified in the text.
The excitation spectrum of a model rare-earth alloy
Authors: R C Jones
Published: July 2002

A ferromagnetic lattice of localized electronic spins coupled by a Heisenberg exchange interaction is considered. The electronic spins interact via a hyperfine interaction with the nuclear spins, and a model is chosen in which only a fraction c of sites is occupied by nuclear spins. The coupled mode spectrum is calculated for c<<1 and is shown to be identical with the results yielded by molecular field theory. An analysis of the terms in the self-energy of the electronic spin propagator shows that because the hyperfine coupling is very much less than the exchange coupling, these molecular field theory results must hold for arbitrary c.
Electronic states in a finite linear crystal in an electric field
Authors: M Saitoh
Published: July 2002

Detailed numerical analysis is given to a tight-binding model of a finite linear chain in an electric field. Eigenenergies and wavefunctions are computed for both periodic and aperiodic boundary conditions, and the criteria for the existence of Stark ladders are critically examined.
Symmetrization of low-energy electron diffraction patterns
Authors: J Rundgren  , A Salwen
Published: July 2002

Using the layer-by-layer approach to low-energy electron diffraction (LEED) the authors explain how to symmetrize the beam scattering matrix for layers. The coincidence structure formed by the layers may be dense or open and may possess any of the 17 two-dimensional space group symmetries. The beams of the electron field become diffracted by the layers in a manner determined by von Laue's diffraction conditions. These conditions turn out to arrange the beams in mutually independent bundles. After establishing the transformation properties of bundles of beams under the point group part of the two-dimensional space group the authors derive a scheme for finding symmetrized combinations of interacting beams. The formalism is applied to the layer-KKR method for LEED calculation. In particular the symmetrization of the beam patterns created by three overlayer structures on surfaces of face-centred cubic crystals are described: the primitive 2*2 structure and the ( square root 2* square root 2)45 degrees structure on the (100) surface together with the ( square root 3* square root 3)30 degrees structure on the (111) surface.
A renormalization group theory for percolation problems
Authors: A P Young  , R B Stinchcombe
Published: July 2002

A renormalization group approach to two-dimensional bond percolation problems is developed using a scaling transformation in real space. A cluster calculation for the correlation-length exponent nu gives 1.28 for the triangular lattice and 1.31 for the square lattice in good agreement with recent series results of nu =1.34+or-0.02.
Surface states of the (110) surface of GaAs
Authors: C Calandra  , G Santoro
Published: July 2002

Results are presented of a new calculation of the surface states at the (110) surface of GaAs. By using Koster-Slater parameters fitted to bulk optical and photoemission data, the surface band structure has been calculated with a LCAO method. The main conclusion is the existence of two surface bands, one inside the band gap at about 1 eV from the top of the valence band and the other a few tenths of an eV below it. The results show good agreement with the available experimental data.
Thermoelectric power of the superionic conductors Ag4MI5(M=K and Rb)
Authors: K Shahi  , S Chandra
Published: July 2002

The thermoelectric power ( theta ) of Ag4KI5and Ag4RbI5is measured between room temperature and few degrees below their melting points using silver electrodes. The behaviour of theta with temperature can be expressed by the equations - theta =0.088(103/T)+0.310(mVK-1), (40-240 degrees C) and - theta =0.086(103/T)+0.338(mVK-1), (40-225 degrees C) respectively, for the two solids. A theoretical analysis of the data gives 0.088 and 0.086 eV for the heats of transport of Ag+ions which are very close to the respective activation energies, in accordance with the prediction of the free-ion theory of Rice and Roth (1972) as well as that of the earlier theory on the heats of transport.
Calculation of the electro-optic effect for finite crystals
Authors: A Rabinovitch
Published: July 2002

The optical absorption coefficient is calculated for a crystal in a finite-range electric field. The origin of the 'red shift' in the presence of the field is explained by the decrease of the band gap. The importance of the potential difference across the sample is discussed. Deviations are obtained from the hitherto existing formulae for the high-field thin-crystal region and a possible experiment is proposed.
A Green function technique for calculating total energy distributions of field-emitted electrons
Authors: A Modinos
Published: July 2002

A formalism is developed which effectively reduces the problem of evaluating the escape probability due to field-assisted tunnelling, of an electron with a given energy and a given wavevector parallel to the surface, to that of evaluating the reflection matrix for an electron with the same energy and wavevector incident on the metal surface from outside. The possibility then arises of using in field-emission calculations some of the approximate methods developed in relation to the theory of low-energy electron diffraction.
Cr2+ excitation levels in ZnSe and ZnS
Authors: G Grebe  , G Roussos  , H -J Schulz
Published: July 2002

The excitation spectrum of the IR emission 5A1(5E) to 5B2(5T2) of substitutional Cr2+ in ZnSe displays, as in the case of ZnS, band-band transitions, photoionization of activator centres, and a structured region of inner transitions towards higher levels of the Cr2+ ions. In the crystal-field approximation, a fitting of the eigenvalues of the Tanabe-Sugano matrices to the excitation bands supplies the crystal-field parameter Dq and the Racah parameters B and C. This fitting and the fact that both for ZnSe and for ZnS, one specific excitation band depends on pre-treatment, suggest a slightly modified interpretation of the ZnS excitation spectra.
Boundary conditions for the liquid-vapour interface of helium II
Authors: H Wiechert
Published: July 2002

Following the principles of the thermodynamics of irreversible processes of discontinuous system a theory of boundary conditions for the liquid-vapour interface of helium II is developed. Neglecting the influences of the coefficients of viscosity in the liquid and the gas, the surface tension of the liquid and surface currents, boundary conditions are inferred from the conservation laws of mass, momentum and energy including all non-linear terms. It is shown that two further boundary conditions for the net mass and energy transfer rates through the free surface of helium II can be obtained from the entropy balance. In the case of non-equilibrium processes occurring at the surface, net flows of mass and energy through the phase boundary result.
Computed peak shapes of mass spectra
Authors: B S Prahallada Rao
Published: July 2002

A method described in earlier papers has been used to determine the ion density distribution in the images formed by radial electrostatic and homogeneous magnetic field analysers and the peak shapes resulting from a scan of such a beam across a slit. The effects of varying some of the field parameters on the density distribution and peak shape are described.
Electrical properties of Hg3Te3-In2Te3alloys
Authors: D A Wright  , S L Dahake
Published: July 2002

Further studies have been made of single-crystal specimens of alloys of Hg3Te3with In2Te3. Samples were annealed in Hg until extrinsic n-type properties were obtained between 77 and 250°K, and until the electron mobility was almost independent of temperature in that range. From 0-15% In2Te3the electron mobility at 100°K was between 5000 and 20 000 cm2v−1s−1. However, from 37·5 to 75% it was less than 500 cm2v−1s−1. Evidence was obtained at several compositions that in this state the scattering parameterswas near zero, and the density of states effective mass was calculated on this assumption from the Hall and Seebeck coefficients. For the 0-15% alloysme/m0was 0·03 at a carrier density of 3 × 1018electrons/cm3; for 37·5-75% alloysme/m0was 0·06 at 1017electrons/cm3.
Moiré patterns formed by inclined gratings
Authors: P S Theocaris
Published: July 2002

Moiré patterns formed by two inclined gratings of identical frequency are characterized by a spatial location which depends on the pitch of the gratings and their dihedral angle. A theory on the formation of moiré fringes is developed, based on the geometric obstruction of light and on the assumption that the viewing direction is perpendicular to one grating. Two extreme cases are studied where the lines of the gratings are either parallel or normal to the dihedral corner.
Relaxation between the hyperfine levels of iodine atoms in the2P1/2state
Authors: W Thieme  , E Fill
Published: July 2002

The relaxation time between the hyperfine levels of iodine atoms in the upper state of the iodine laser transition was measured. For conditions typical of an iodine laser amplifier a time constant of 30 ns is obtained. The time constant does not depend on the argon buffer gas pressure.
Ultrasonic studies of the elastic coefficients of vitreous selenium about Tg
Authors: S Etienne  , G Guenin  , J Perez
Published: July 2002

The authors measured by ultrasound the relative variation of the elastic coefficients (shear modulus G, Young's modulus E, bulk modulus B, Poisson's ratio nu ) of vitreous selenium, either as a function of temperature in the glass transition range, or as a function of time, the material being in different states. During aging at room temperature after quenching from a temperature higher than Tgthe moduli E, G and B increase, nu decreases and the packing of Se atoms in the structure increases, causing the process to slow down. When a sample, aged at room temperature, is brought to a temperature of the order of Tg, the inverse effect is observed. The drastic variation of the elastic coefficients observed about Tgduring a temperature increase (or decrease) is related with a structure relaxation.
An analysis of solid particle erosion mechanisms
Authors: D R Andrews
Published: July 2002

The erosion of a surface by a stream of solid particles is examined by considering the arrival times of the particles on the surface. The treatment first proposes a new concept, an 'impact zone', and then considers the arrival of the particles on one zone as a queuing process. The rate of erosion of ductile metals is predicted using simple arguments from probability theory. A comparison of the mechanisms of erosion has been made using this analysis as a basis for models. Results indicate that not all of the mechanisms are capable of predicting the incubation time effects which have been reported previously. Also discussed is the importance of the duration of the test and the concentration of eroding particles, or flux, striking the target. A recommendation is made for a new standard to be used when quoting flux levels in future publications.
Polarisation reversal in trissarcosine calcium chloride
Authors: S Fjuimoto  , N Yasuda  , K Takagi
Published: July 2002

The temperature T, electric field E and pressure p dependence of the switching parameters of trissarcosine calcium chloride (TSCC) such as the switching time tsand the maximum switching current imaxare measured. The mobility mu of the domain wall motion is expressed by the relation mu varies as exp(-W/kT), where W is the activation energy for the domain wall motion. The value of W is 0.05 eV at p=0 kbar. The effect of pressure on the switching parameters is dominated by the pressure dependence of W and alpha , and delta W/ delta p is 0.03 eV kbar-1and delta x/ delta p is 0.14 kV cm-1kbar-1at -156 degrees C, 0.18 kV cm-1kbar-1at -161 degrees C and 0.22 kV cm-1kbar-1at -166 degrees C. The symmetry m of the switching current transient is almost constant ( approximately=0.5) at fields below 2.5 kV cm-1and increases approximately with E according to the relation m varies as E0.3at fields above 2.5 kV cm-1. Such a switching current transient is explained by the phenomenological theory of the polarisation reversal.
Gas desorption from electrodes and electrical breakdown in vacuum
Authors: V A Nevrovskiy  , V I Rakhovskiy  , V G Zhurbenko  , V A Zernaev
Published: July 2002

Reports results of an experimental investigation of the influence of gas desorption from electrodes on electrical breakdown in high vacuum (P=10-9 Torr). The gas composition was studied by a quadrupole mass-filter. Thermo-desorption of hydrogen from the anode was the dominant effect. A close correlation between gas evolution from the anode and changes in the slopes of Fowler-Nordheim plots for prebreakdown currents was established. Pressure bursts due to gas released from the anode at breakdown were observed.
Analysis of transient voltage measurements from electron swarm experiments including ionisation and attachment
Authors: H A Blevin  , K J Nygaard  , K R Spriggs
Published: July 2002

A comprehensive theoretical analysis of the form of measurements obtained in the electron swarm experimental method due to Grunberg is presented. The analysis covers the general case of such experiments in which electron diffusion, electron collisional ionisation and electron attachment are present. The approach is based upon the scalar form of the Maxwell-Boltzmann continuity equation; the concept of energy supplied from a uniform drift field; and, the fact that typical experimental measurement information may be resolved into distinct electron and ion components. A generalised voltage ratio expression is derived in a form which is directly applicable to the characteristics of integrated voltage transient measurements. A special case of the analysis is shown to be consistent with the existing experimentally tested, simplified theory. The generality of the analysis provides the opportunity of designing new experimental situations in which drift velocities, diffusion coefficients, ionisation coefficients and attachment coefficients may be analysed from the same set of measurement data.
Excitation mechanism of the Cd(II) 5s22D5/2state in the positive column Ne-Cd+laser discharge: an interpretation of the difference of 441.6 nm laser output powers in Ne-Cd and He-Cd discharges
Authors: K Hane  , T Goto  , S Hattori
Published: July 2002

It is shown that the stepwise excitation process from the Cd(II) ground state is the dominant one for the upper state of the 441.6 nm laser line in the positive column Ne-Cd+laser discharge. The much smaller laser output power in Ne-Cd than that in He-Cd cannot be explained by the nonexistence of the Penning excitation process for the laser upper state in Ne-Cd. It is explained by the smaller stepwise excitation rate for the laser upper state in Ne-Cd than that in He-Cd and the influence of both the laser lower state density and the loss of the cavity.
Thermally stimulated discharge currents in polyvinyl butyral films
Authors: J L Gil-Zambrano  , C Juhasz
Published: July 2002

The persistent electrical polarisation in polyvinyl butyral was investigated by measuring the thermally stimulated discharge (TSD) current at different polarising fields, temperatures and heating rates. A peak was observed at about 331K which has the characteristics of a dipolar relaxation process. Thermal sampling technique showed that the relaxation is distributed. Differential thermal analysis gave a second-order transition at about 334K. Because of the good correlation between both thermal techniques it is concluded that the TSD peak is associated with the glass transition of the polymer, and therefore it involves the motion of large parts (sequences of 20-50 carbon atoms) of the polymer chains.
A Monte Carlo analysis of an electron swarm in a nonuniform field: the cathode region of a glow discharge in helium
Authors: J P Boeuf  , E Marode
Published: July 2002

A Monte Carlo method for studying the steady-state behaviour of charged species under the influence of a non-uniform electric field is described. This method, using a null-collision technique based on determination of the increase in kinetic energy between two collisions, avoids calculation of the time variations of the phase space coordinates of the charged species. The electron behaviour in the cathode region of a glow discharge has been analysed using this method; spatial variations of the energy and angular electron distribution functions, and of the macroscopic electron parameters are discussed.
Density perturbations induced by a discharge in a laser cavity
Authors: R J A Tough  , D V Willetts
Published: July 2002

A linearised hydrodynamic analysis of the density perturbations induced in a laser cavity by short, sustained and exponentially decaying discharges is described. The results obtained are compared with, and found to differ from, those of Longaker and Litvak (1969). The authors results are discussed, special emphasis being placed on those features which are characteristic of the essentially two-dimensional radial propagation of sound waves in the cavity and illustrative numerical results are presented.
Depolarisation thermocurrents in oil-in-water emulsions at subzero temperatures
Authors: P Pissis  , L Apekis  , C Christodoulides  , G Boudouris
Published: July 2002

Stable paraffin oil-in-water (OW) emulsions with Tween 20 (polyoxyethylene sorbitan monolaurate) as emulsifier have been prepared over a wide range of concentration. The dielectric behaviour of the emulsions has been studied by the depolarisation thermocurrent (DTC) method in the temperature range 85-250K, to obtain information about the state of water in the emulsion. Four peaks, numbered I-IV, have been observed at temperatures of about 120, 135, 210 and 235K respectively. The DTC peaks I and II have been attributed to the reorientation of free and bound or hydration H2O molecules in the emulsion respectively. Evidence has been obtained that DTC peaks III and IV are due to Tween 20.
Comparison of models of CO2laser impedance fluctuations
Authors: A L S Smith  , S Moffatt
Published: July 2002

There is a large opto-galvanic effect (OGE) in CO2-N2-He laser mixtures and this is exploited in laser frequency and power stabilisation systems. Two substantially different theories have been advanced to explain the effect. The two models are compared and it is concluded that the multi-step ionisation model is not adequate to describe the OGE in CO2lasers, but the temperature perturbation or discharge cooling model describes the phenomenon with considerable precision.
Importance of attachment cross-sections of F-formation for the effective ionisation coefficients in SF6
Authors: M Hayashi  , T Nimura
Published: July 2002

The importance of the attachment cross-section for F-formation for the effective ionisation coefficients in SF6is discussed. The values of the dissociative attachment cross-section qafor F-formation for electron energies from 2 to 15 eV are very small compared with the values of qafor SF6-formation at low electron energies. However, the effect of the values of qafor F-formation on the effective ionization coefficients is very great. Calculations were carried out by a two-term Boltzmann equation analysis. It is pointed out that the measured grand total electron-SF6cross-sections were inconsistent with the sum of elastic and inelastic cross-sections used by many workers.
A statistical analysis of the frictional forces generated between monofilaments during intermittent sliding
Authors: B J Briscoe  , A Winkler
Published: July 2002

The friction between poly(ethylene terephthalate) monofilaments (about 20 mu m diameter), known by the acronym PET, with a range of titanium contents has been measured. In addition, some experiments were carried out with PET fibres sliding over glass and human hair. The configuration used produces a nominal point contact and involves a taut fibre against which a second fibre is brought to form a cantilever. Movement of the first fibre invariably generates discontinuous or stick-slip motion in the cantilevered fibre. The equipment developed for friction measurements was capable of recovering and storing data from a large number of stick-slip events. Much of the static frictional force data could be fitted (at the 95% confidence limit of the chi2goodness-of-fit test) to the gamma, Weibull or log-normal distributions. The exact form of the distribution depended on the conditions of the experiment and, additionally in the case of human hair, on the direction of sliding. Some of the data which could not be modelled by these distributions were found to be associated with time-dependent effects. This non-stationarity, which arises from a low-level wear process, increased with the imposed sliding velocity and the total load at the contact point. By the use of simple regression models of frictional force against time, it was possible to 'correct' some of the non-stationary data so that they became (nominally) stationary. Distributions fitted to these modified data showed good continuity in both the functionality and the numerical values of the distribution parameters with the distributions fitted to the stationary data. The study also revealed the occurrence of a type of microslip prior to gross slip. This microslip manifests itself as a continuous velocity of the contact point of the fibre. Microslip was found to occur under nearly all circumstances and is dependent on a large number of factors. There was a close relationship between the extent of microslip and the degree of non-stationarity.
Electrical properties at very low frequencies of alpha PVDF subjected to thermal cycling
Authors: M -C Felix-Vandorpe  , M Maitrot  , R Ongaro
Published: July 2002

Very low frequency dielectric spectra (103-10-4Hz) show space charge build-up in alpha PVDF under the influence of a low field as the temperature is raised to 80 degrees C or above. Preheating changes very little the medium frequency alternating conductivity when gold electrodes are used, whereas with aluminium electrodes the conductivity increases substantially as excess fluorine collects under the electrodes. Experimental results are interpreted through a two-carrier (H+and F-) conduction model based upon three assumptions: (i) an increase in H+ion concentration during the first heating: (ii) partial deep trapping of H+ions at high temperature; (iii) field-induced extraction of F-ns from the bulk leading to H+enhancement of pre-exponential mobility factor. Orders of magnitude are proposed for H+and F-ion activation energies.
The use of liquid metals for contact-charging experiments
Authors: A M El-Kazzaz  , A C Rose-Innes
Published: July 2002

The contact charging of insulators is often investigated by contacting the insulators with metals. However, the use of solid metal contactors introduces difficulties due to the hardness of the metal, the uncertain area of contact, and charge accumulation which occurs when contacts are repeated. These difficulties can be avoided by the use of liquid metals to contact the insulator. This paper reports an investigation of the use of liquid metals as contactors in contact-charging experiments. Liquid gallium may be more suitable for some measurements than mercury because, as the Appendix shows, the work function of mercury changes with time but that of gallium does not.
Failure modes of hybrid composites consisting of carbon fibre bundles dispersed in a glass fibre epoxy resin matrix
Authors: M J Pitkethly  , M G Bader
Published: July 2002

Tests have been conducted on single carbon fibres, bundles of 1000 carbon fibres impregnated with epoxy resin and on a glass/carbon hybrid composite containing similar bundles interspersed in glass/epoxy resin. The influence of the bundle dispersion on the strength of the carbon bundles is discussed and suggestions for the optimum dispersion are made. The behaviour with respect to the weakest-link hypothesis is also discussed and an explanation is proposed for the observed deviation from the theory for the bundles.
Conventional magnetic shielding
Authors: T J Sumner  , J M Pendlebury  , K F Smith
Published: July 2002

A general review of conventional magnetic shielding is presented. Several possible definitions of the term 'shielding factor' are given, taking into account the ferromagnetic nature of shielding materials. An alternative treatment of single-layer axial shielding for a cylindrical shield is presented and compared to a previous approximation. A new algorithm for calculating multi-layer axial shielding factors is derived and shown to be an improvement on existing methods when applied to some assemblies, including one reported for the first time.
Dielectric behaviour of mineral-oil-based magnetic fluids-the cluster model
Authors: P Kopcansky  , J Cernak  , P Macko  , D Spisak  , K Marton
Published: July 2002

A value of about 20% relative change of the dielectric constant of mineral-oil-based ferrofluids was observed in an external magnetic field. The cluster model, based on a long-chain model, was proposed for the explanation of experimental data.
Electron attachment in water vapour and ammonia
Authors: J E Parr  , J L Moruzzi
Published: July 2002

Electron attachment has been studied in water vapour and ammonia over the rangeE/N=0-91 Td. It has been shown that attachment proceeds via the dissociative attachment process forE/Ngreater than 40 Td in water vapour and 30 Td in ammonia. With the use of published values of electron drift velocities for these gases the attachment rate coefficient has been calculated. No evidence for any low-energy three-body attachment processes was observed.
Finite ion cyclotron radius effects upon the ion probe current
Authors: G Peter  , M Sato
Published: July 2002

Resonant effects upon the ion current to electrical probes in a magnetic field are presented. When the ion cyclotron radius becomes equal to an even fraction of the distance between the double probes, or between the probe and the wall, a sudden decrease or increase in the ion current collection by the probes takes place as a result of resonant plasma depletion or supply between the interacting parts (ion cyclotron radius resonance).
The effect of surface layers on the spontaneous polarization of triglycine sulphate
Authors: A Mansingh  , S S Bawa
Published: July 2002

The spontaneous polarization of the triglycine sulphate in the form of compressed powders with varying particle size has been measured from 0 to 55 degrees C, and it is very much lower than that expected for a polycrystalline sample. This, along with the evidence that spontaneous polarization decreases with the decreasing particle size, indicates that the surface layers may have lower values of spontaneous polarization than bulk material.
The interpretation of the stress intensity term in the 'equation for polymer life'
Authors: E Sacher
Published: July 2002

The stress intensity term is interpreted to be the volume of activation for the submicroscopic crack formation which initiates fracture.