bluuebunny/crossref_metadata_2025 huggingface dataset:
Charge-density waves in NbSe3 at 145K: crystal structures, X-ray and electron diffraction studies
Authors: J L Hodeau  , M Marezio  , C Roucau  , R Ayroles  , A Meerschaut  , J Rouxel  , P Monceau
Published: July 2002

The crystal structure of NbSe3 has been refined from single crystal X-ray diffraction data. It has a monoclinic symmetry with lattice parameters: a=10.009 AA, b=3.4805 AA, c=15.629 AA, beta =109.47 AA, space group P21/m and six formulae per unit cell. The crystal structure of NbSe3 as determined at 100K shows that the 145K transition in the electrical resistivity is not accompanied by a structural distortion. Except for the decrease due to thermal contraction all interatomic distances are found to be the same. This is compatible with the proposed model of the charge-density-waves formation which has been put forward in order to explain the physical properties of the 145K transition. Electron diffraction pictures taken above and below the transition give direct evidence of the charge-density-waves formation at the transition. At 120K in agreement with Tsutsumi et al. (1977) the pictures contain superstructure spots at the (h,k+or-0.243, l) positions. Contrary to what has been reported by the same authors, the authors have found that at 3K above the transition the electron diffraction pictures do not contain the diffuse scattering streaks corresponding to the planes at k'=k+or-0.243, which would indicate fluctuations of the CDW above the transition.
The temperature dependence of one-magnon light scattering in ferrimagnets. I
Authors: A F Khater
Published: July 2002

A theory is presented of the temperature dependence of one-magnon light scattering in ferrimagnets. The Hamiltonian describing the interaction of light with the magnetic system is taken as linear in the spin operators. Light scattering results are obtained for two-sublattice ferrimagnetic systems with nearest neighbour inter- and intra-sublattice exchange, and are shown to reduce in the appropriate limits to the ferro- and antiferromagnet. Results are also obtained for two-sublattice magnetic systems with extended range exchange in a mixed Heisenberg and Ising representations of order. Formal results are given for one-magnon light scattering from a multi-sublattice magnetic system.
A model calculation of the dielectric function in trigonal Se and Te with local-field corrections included
Authors: F Nizzoli
Published: July 2002

The diagonal as well as the off-diagonal elements of the dielectric matrix epsilonG,G'(q, omega =0) are explicitly evaluated in trigonal Se and Te. A simple tight-binding model Hamiltonian has been used to describe the electron states, while the wavefunctions have been drawn from sp3hybrid orbitals, modified according to the chemical bond. The dielectric matrix has been inverted and the local-field corrections to the dielectric function have been computed. These are found to be very large for q<qfand, at least in the case of selenium, their introduction seems essential to reproduce the experimental dielectric tensor. This conclusion is discussed after the very recent, and disputed, experimental results.
Electrical conductivity of a mixture of conducting and insulating spheres: an application of some percolation concepts
Authors: H Ottavi  , J Clerc  , G Giraud  , J Roussenq  , E Guyon  , C D Mitescu
Published: July 2002

An extensive study of a large, compact, disordered ensemble of plastic spheres of constant diameter, a fraction p of which has been metallically coated is reported. An initial discussion deals with the compactness and average connectivity z of the system. The electrical conductance for completely conducting samples (p=1) has been studied as a function of pressure applied uniaxially to the ends of a cylindrical container, and compared to that of individual contacts in test samples. Although the variation of conductivity with p is consistent with expectations for a site percolation problem, the idea of a simultaneous bond percolation must be introduced to take into account variations in the quality of the different contacts.
Temperature dependence of the mean square relative displacements of nearest-neighbour atoms derived from EXAFS spectra
Authors: W Bohmer  , P Rabe
Published: July 2002

An investigation of the dependence on temperature of the extended X-ray absorption fine structure (EXAFS) above the K-edges of Cu and Co in the metals and above the K-edges of Rb and Sr in the compounds RbCl, SrS, SrF2and SrCl2has been performed. For Cu, Co, RbCl and SrS, the values of the correlated mean square relative displacements of nearest-neighbour atoms derived from EXAFS spectra show a good agreement with those calculated from a Debye model. The experimental data obtained are also compared with available data for mean square displacements of the individual atoms calculated from more-refined lattice vibrational models and with Debye-Waller factors used in X-ray diffraction experiments.
Optically detected magnetic resonance of a self-activated emission at 1.94 eV in gallium-doped zinc sulphide
Authors: J J Davies  , J E Nicholls
Published: July 2002

Optically detected magnetic resonance experiments on gallium-doped zinc sulphide have shown that a band at 1.94 eV is attributable to the radiative recombination of donor electrons with holes on A-centres. The donor are identified as gallium ions substituting at zinc sites to form deep levels which are estimated to lie 0.4 eV below the edge of the conduction band. The depth is consistent with the large gallium hyperfine interaction which indicates that the donor wavefunction is well localised. Donor-acceptor pair recombination also provides an explanation for the previously reported observation of emissive microwave transitions in the Ga2+photostimulated EPR spectrum.
Two effective-medium approaches to linear spin-wave theory for diluted ferromagnets and antiferromagnets
Authors: L M Roth
Published: July 2002

Both single-site (SCP) and multicomponent (MCP) theories are developed to investigate the problem of spin waves in diluted ferro- and antiferromagnets. The SCP theory gives a good description of the effect of a given spin on its neighbours, whereas the MCP theory treats well the effect of neighbour composition on the Ising levels of the given spin. The SCP theory conserves moments of the spectral weight function up to and including the third moment, whereas the MCP theory is only correct up to the second moment. Results for the two approaches are compared with data obtained by computer simulation. For ferromagnetic systems the two theories give rather similar results, but the SCP theory is slightly preferable. For antiferromagnetic systems, however, there are striking differences between the two theories and the MCP theory is clearly superior.
Defect properties of ionic solids. III. The calculation of the point-defect structure of the alkaline-earth oxides and CdO
Authors: W C Mackrodt  , R F Stewart
Published: July 2002

For pt.II see ibid., vol.12, no.3, p.431 (1979). The results of an extensive theoretical study of the point defects in the alkaline-earth oxides and CdO are presented. The study is based on the HADES simulation procedures and uses modified electron-gas potentials described previously. It includes the calculation of perfect lattice properties, point-defect energies and configurations, conduction band edges and thermal band gaps, enthalpies of doping, intrinsic and extrinsic diffusion, oxygen incorporation, the reduction of MgO and CdO and an examination of the OH group and proton migration.
Ion hydration in aqueous CaCl2solutions
Authors: S Cummings  , J E Enderby  , R A Howe
Published: July 2002

The neutron first-order difference method has been applied to a 4.49 molal aqueous solution of CaCl2. Isotopic changes were made in both the Ca2+ion and the Cl-ion so that a complete picture of ionic hydration could be obtained. Both ions coordinate water in a non-dipolar configuration with ion-oxygen distances close to those predicted by gas-phase quantum molecular calculations. The measured hydration numbers for cations and anions (5.5+or-0.3 and 5.8+or-0.3 respectively) imply substantial sharing of water molecules between the ions.
Molten alkali-halide mixtures: a molecular-dynamics study of Li/KCl mixtures
Authors: C Caccamo  , M Dixon
Published: July 2002

Molten Li/KCl mixtures are studied over the full concentration range by molecular-dynamics simulation. The simulations suggest that the lithium ions remain fourfold coordinated by chloride ions throughout the concentration range while the potassium ions are about sixfold coordinated. The diffusion coefficients of both cations remain similar to each other throughout the concentration range. The eutectic composition has been studied at several temperatures. The specific conductivities of the melts are estimated.
The electronic structure of HfN, TaN and UN
Authors: P Weinberger  , C P Mallett  , R Podloucky  , A Neckel
Published: July 2002

Quasi-self consistent relativistic energy band structures have been calculated for HfN, TaN and UN. The chemical bonding in these compounds is discussed in terms of charge transfers and kappa -like charges within the scattering regions for individual one-electron states and in terms of difference charge densities between the final iterated charge densities and initial, superposed atomic charge densities. Difference charge densities are also used to illustrate common features in the electronic structures of the monocarbides and mononitrides of Hf, Ta and U. Finally, in relation to X-ray photoelectron spectra the position of the f bands in the Hf and Ta compounds is discussed in terms of band-structure results and atomic-like relaxation effects.
Enhanced nuclear resonance scattering in dynamical diffraction of gamma rays
Authors: U van Burck  , R L Mossbauer  , G V Smirnov  , H J Maurus
Published: July 2002

The angular and energy dependence of the dynamical diffraction of gamma rays was studied experimentally in the symmetric Bragg case. Mossbauer diffraction spectra were measured with high angular resolution in pure nuclear Bragg reflection of a nearly perfect57FeBO3single crystal. In the spectrum taken at the exact Bragg position, very broad hyperfine resonance peaks were observed. In the spectra just above and below the Bragg angle the reflection peaks were much smaller and were shifted to energy positions above and below resonance respectively. The results were interpreted by means of the dynamical theory and compared with computer calculations. The measurements represent the first direct observation of the enhancement of nuclear resonance scattering in the case of a thick crystal reflection. Furthermore the angular dependence of the resonance scattering by a perfect single crystal has been measured for the first time.
Silver overlayers on (110) indium phosphide: film growth and Schottky barrier formation
Authors: A McKinley  , A W Parke  , R H Williams
Published: July 2002

The early formation and subsequent growth of evaporated silver films on atomically clean cleaved InP (110) surfaces have been probed using low-energy electron diffraction, Auger spectroscopy and angle-resolved photoelectron spectroscopy. Experimental data obtained would suggest that the silver film at room temperature grows initially as islands or rafts with possible nucleating sites at surface steps. Facet formation of the silver film is found to occur where mainly silver (111) facets are obtained. The formation of the Schottky barrier has been probed using photoemission and it is found that in this system the barrier is largely formed for a silver coverage which is equivalent to only about 10-20% of the surface. The abruptness and adhesion at the interface has also been considered. Strong adhesion is obtained for silver deposited at room temperature on the atomically clean cleaved InP (110) surface. This strong adhesion has been attributed to interdiffusion at the interface and for the etched InP surface the oxide layer presents leads to a considerable reduction of the interfacial mixing at room temperature. The mechanisms influencing the Schottky barrier formation have been considered and the defect model possibly involving indium vacancies initiating at step edges is thought the most likely reason for the pinning of the Fermi level at around 0.5 eV below the conduction band edge for silver contacts on the atomically clean cleaved InP (110) surface.
Fluctuations of magnetic moment in spin glass
Authors: P Rusek
Published: July 2002

In the framework of the phenomenological theory the linear response function of spin glass to a magnetic field is determined in the spin wave approximation. The spin wave damping coefficient and the resonance frequencies shift are calculated for a broad range of wavevectors and temperatures. The autocorrelation function of the components of the magnetisation vector that has been calculated on the basis of the response function, in the hydrodynamic limit, is in agreement with the form of the appropriate correlation function calculated in the framework of the hydrodynamic theory. The temperature dependence of the spin glass transport coefficients is determined by comparing both of these functions.
Energy transfer via disturbed host species in a mixed organic crystal system
Authors: O Pekcan
Published: July 2002

Triplet state energy transfer in a pyrene-d10 doped fluorene single crystal was investigated by ESR. The rates of guest triplet absorption signal decays, after cessation of illumination, were determined in two different crystal orientations. The results were interpreted in terms of a particular kinetic model of energy transfer process in the crystal. The depths of the shallow impurity traps due to disturbed host species were found in the temperature range 130-300K. The temperature dependence of the spin-lattice relaxation time was discussed in terms of these shallow host traps. A new mechanism was suggested due to the deep guest and shallow host traps for the relaxation in this mixed crystal system.
Structural phase transitions in ferroelastic RbAlF4. I. DSC, X-ray powder diffraction investigations and neutron powder profile refinement of the structures
Authors: A Bulou  , J Nouet
Published: July 2002

The layer compound RbAlF4is shown to undergo structural phase transitions at 553K and 282K under investigation by DSC and X-ray powder diffraction. The structures of the three phases are determined by neutron powder profile refinement. The high-temperature phase has the ideal TlAlF4structure P4/mmm which corresponds to the tilt system a0a0c0. The intermediate room-temperature phase is confirmed as P4/mbm, the tilt system is a0a0c+. The low-temperature phase is consistent with the space group Pmmn which corresponds to both ap+b0c+(near the transition temperature) and ap+bp+c+(at very low temperature). The 553K phase transition is a non-ferroic phase transition while the 282K transition is ferroelastic. According to Aizu the low-temperature phase is a 4/mmmFmmm species. The temperature behaviour of the structural parameters is discussed, particularly in the framework of ferroelasticity.
Pseudopotential perturbation theory for the surface tension of liquid metals
Authors: M Hasegawa  , M Watabe
Published: July 2002

A theory for the surface tension of liquid metals is developed on the basis of the direct perturbation expansion to second order in the electron-ion pseudopotential. A simple and easily tractable expression for the surface tension is derived from an expression for the Helmholtz free energy which is generalised to a liquid metal with a planar surface. The variational method is used for determining both the electron and ion density profiles at the surface and the bulk radial distribution function is assumed for describing the correlation between ions in the transition zone.
Effects of X-irradiation in europium-doped NaCl
Authors: J O Rubio  , M G Aguilar  , F J Lopez  , M Galan  , J Garcia-Sole
Published: July 2002

The first stage of F- and M-centre colouring has been investigated at room temperature in quenched samples of europium-doped NaCl as a function of the impurity concentration in the range 30-500 PPM. Attention has been paid to determine whether a correlation exists between the change in the concentration of Eu2+-cation vacancy dipoles and the colour centre formation in order to get a deeper insight into the nature of the interstitial halogen traps. The effects of x irradiation are (a) to reduce the number of (I-V) dipoles, (b) to increase the number of trapped electrons, (c) to convert the doubly valent europium ions to the monovalent, as well as to the tripositive state, and (d) to increase the rate of aggregation of dipoles. The number of dipoles destroyed by irradiation was found to be equal to or to exceed the sum of the number of europium ions which had changed their valence state and the number of F centres produced. The results presented in this paper suggest that the (I-V) dipoles act as the dominant traps for the radiation-induced interstitials. It was also ascertained that the M-centre density is proportional to the square of the F-centre density, the proportionality constant being clearly dependent on the inverse of the impurity concentration.
Thermoluminescence of undoped and Mn2+-doped NaF crystals after room-temperature γ-ray irradiation
Authors: S Bhan  , S M D Rao
Published: July 2002

The undoped samples exhibit five glow peaks at 60, 105, 145, 175 and 285 degrees C. The 60 degrees C peak is tentatively attributed to F' centres, the 105, 175 and 285 degrees C peaks to F centres and the 145 degrees C peak to hole centres. Doped samples exhibit six glow peaks at 85, 106, 145, 177, 230 and 320 degrees C. The 106, 145 and 177 degrees C peaks of doped samples are respectively the same as the 105, 145 and 175 degrees C peaks of the undoped samples, and are explained in a similar fashion. The 85 and 230 degrees C peaks are attributed to F centres and the 320 degrees C peak to hole centres. The TL emission spectra of the doped samples consist of two bands at 410-420 and 570 nm at all temperatures, the 410-420 nm emission being due to the intrinsic luminescence perturbed or allowed by the impurities, and the 570 nm emission being due, at least in the case of the first five peaks, to the host-sensitised secondary luminescence with Mn acting as activator. The emission spectrum observed when irradiated doped samples are excited with 342 nm (F band) light also contains bands at 420 and 570 nm.
Superposition model analysis of the short-range ordering for Mn2+in oxide glasses
Authors: J Kliava
Published: July 2002

The superposition model is employed to account for the fine-structure parameters for Mn2+in oxide glasses determined recently by computer simulations of g=2.0 and g=4.3 resonances. Analytic expressions relating distributions of the fine-structure parameters D and E to distributions of ligand coordinates are obtained for sixfold and fourfold coordinations of Mn2+. Quantitative estimates are made of mean distortion parameters and root-mean-square variations of interatomic distances and angles in the environment of Mn2+. It is shown that both the mean distortions from cubic symmetry and the degree of short-range disorder are considerably smaller for the g=2.0 sites than for the g=4.3 sites. A possibility of attributing the g=2.0 and g=4.3 resonances to Mn2+ions substituting respectively for network modifiers and network formers in the oxide glasses is discussed.
Conductance oscillations from Coulomb ordering
Authors: V Dallacasa
Published: July 2002

A possible mechanism for the conductance oscillations observed recently by Pepper (see ibid., vol.12, p.L617, 1979) is proposed on the basis of Coulomb ordering on electrons in a disordered two-dimensional impurity band.
Specific heat and phase diagrams for uniaxially stressed KMnF3
Authors: S Stokka  , F Fossheim
Published: July 2002

The authors have investigated the cubic-to-tetragonal structural phase transition of KMnF3. The phase diagrams as a function of temperature and uniaxial pressure along (100) and (110) have been measured for pressures up to p=0.6 kbar by specific heat measurements. The specific heat curves show Landau temperature dependence but the Landau theory does not explain the observed phase diagrams. The variation of the hysteresis of the phase transition temperature and the Landau parameters as a function of p along (100) indicates the existence of two consecutive tricritical points joined by a second-order line. For p along (110) the measurements suggest that the phase transition is second order above a tricritical Lifshitz point at p=0.45 kbar.
One-electron states in materials with structural disorder
Authors: J P Hernandez
Published: July 2002

A theoretical study of the one-electron states in materials composed of a single atomic species but structurally disordered is developed. The disorder is characterised probabilistically through the joint probability density of finding a given configuration; the 'uniform' case is used in calculations. The influence of system dimensionality, atomic scattering strength and density are examined as they affect the density of electron states per unit volume and localisation as a function of energy. The calculations are variational and designed to study trends. Band tails are obtained numerically for the case of an electron-atom interaction which is fully rotationally symmetric, short ranged and repulsive. Furthermore, the calculated results are interpreted as implying that all one-dimensional states are localised, in two dimensions the implication is of localised and delocalised states while in three dimensions localised and delocalised states are deduced but it is also suggested than an intermediate 'percolative' regime may exist. The energies separating the various localisation regimes are estimated and vary continuously with coupling constant.
Optical properties of copper-related centres in InP
Authors: M S Skolnick  , P J Dean  , A D Pitt  , Ch Uihlein  , H Krath  , B Deveaud  , E J Foulkes
Published: July 2002

A new Cu-related photoluminescence band in InP is reported. It consists of a sharp zero-phonon line at 1.2889 eV with sharp LO and 'gap mode' local phonon replicas to lower energy superimposed on a broad vibronic background. Zeeman studies show that the recombination process arises from an exciton bound to a neutral isoelectronic centre. Electron and hole g-values of 1.27 and 2.12 respectively are deduced, with both electron and hole having spin1/2. The very strong diamagnetic shift of the spectrum is found to be equal to that of a shallow donor electron in InP. This demonstrates clearly that the hole is tightly bound in the short-range potential of the centre with binding energy EBh=127.5 meV and that the electron is then weakly bound in the Coulomb field of the hole (EBe=7.3 meV), so that the overall behaviour is of an isoelectronic donor. The thermal activation energy of the vibronic sideband of the luminescence is found to be 135+or-10 meV, in good agreement with the spectroscopic value of the hole binding energy.
Thermoreflectance study of polydiacetylene-bis (toluene sulphonate) single crystal (PDA-TS)
Authors: S Stizza  , I Davoli  , V Prantera  , A Bianconi  , J Glinski  , J Kalinowski
Published: July 2002

For the first time studies have been made of the polarised thermoreflectance of an oriented PDA-TS single crystal. The spectra in the photon energy range 1.9-2.5 eV are taken at room and liquid nitrogen temperature. From the different sensitivities to temperature the structures at 2.00 eV and 2.44 eV are assigned to excitonic and interband transitions, respectively. The binding energies of the excitons at room temperature (0.44 eV) and those corresponding to the low-temperature split band gap have been estimated to be nearly identical: 0.43 eV and 0.44 eV. The main thermoreflectance structures are assigned to a Frenkel exciton interacting with phonons.
A quantum field theoretic approach to the calculation of reduction factors in Jahn-Teller systems. II. E7otimes;( Σ(α1+ ε)) in octahedral symmetry
Authors: S H Payne  , G E Stedman
Published: July 2002

For pt.I see ibid., vol.16, no.14, p.2679 (1983). The formalism of the preceding paper is applied to doublet levels in octahedral symmetry in interaction with many lattice modes of all frequencies through linear, nonlinear and anharmonic couplings. It is shown explicitly that when non-linear and harmonic couplings are present, the interaction with symmetric modes (which have been ignored in the Jahn-Teller problem up till now) will contribute to reduction factors. The authors give the constraints upon the topology of any field theoretic diagram for it to contribute to a given reduction factor. These permit the generalisation of some results to all orders of perturbation. Explicit formulae are given for each reduction factor and for all temperatures and all fundamental interactions, up to fourth order and in some cases sixth order of perturbation. For example, at low temperatures the temperature-dependent part of a reduction factor varies at T2in second order, while at fourth order it will vary as T4, T5-T6or T9depending on whether linear, non-linear or both of these mixed with anharmonic effects are responsible. The conditions under which the sum rule q=1/2(1+p) breaks down are discussed in detail. For example, it is valid for all linear couplings at all temperatures if only one phonon frequency is chosen (Einstein lattice), while in a realistic (multifrequency) lattice contributions from nonlinear and anharmonic coupling violating the sum rule appear at sixth order.
The conductance of a disordered wire
Authors: R Johnston  , H Kunz
Published: July 2002

From an analysis of the scattering theory for a model of a wire with a disordered section, the asymptotic properties of the electrical conductance are derived as the length of the disordered section tends to infinity. It is proved that the conductance decreases exponentially with the length of the disordered section. It is shown that the exponent is two times the smallest Ljapunov exponent associated with a product of random matrices derived from the stationary Schrodinger equation for the system. The distribution of the conductance is asymptotically log-normal. These results are derived from similar properties obtained for the elements of the transmission matrix. The results for the transmission matrix hold equally for the disordered harmonic strip and bar.
Examination of the statistics of dielectric breakdown
Authors: R M Hill  , L A Dissado
Published: July 2002

The use of a compound Weibull distribution in both time and electric field for the description of dielectric breakdown data is examined. It is shown that it is insufficient to measure breakdown experimentally using only a single variable such as a constant electric field, since the evaluation of the minimum number of parameters required to characterise the simplest system requires four independent pieces of information. Applicable statistical relationships are derived and applied to a wide range of experimental data. It is shown that, in general, the information that has been published on dielectric breakdown is limited and does not allow full assessment of the breakdown mechanism. Such information as is available is discussed and some broad conclusions about breakdown mechanisms are proposed.
Computer simulation of interacting donor electrons in the Anderson-localised regime of semiconductors
Authors: T Takemori  , H Kamimura
Published: July 2002

A computer simulation on a cluster model of n-type doped semiconductors is performed at the impurity concentration corresponding to 1.7*1018cm-3of Si:P, with the aim of testing the general physical picture and theoretical treatment in the 'transfer-diagonal' formulation. A simple Gaussian function is used as the basis of one-electron states, in order to calculate and take into account exactly all the matrix elements including those of the electron-electron interaction. The energy levels and wavefunctions of the ground and excited states of the clusters consisting of six randomly distributed donors are obtained. It is found that there are a large number of states which are occupied by single electrons and that the spins of these singly occupied states interact either by ferromagnetic or antiferromagnetic exchange interactions. The numerical results on specific heat and spin susceptibility averaged over 76 clusters reproduce fairly well the characteristic features observed in experiments on Si:P. The microscopic mechanism of these remarkable features is clarified by examining the many-electron wavefunctions obtained and the treatment in the 'transfer-diagonal' formulation is confirmed.
Energy fluctuations of an isotropic ferromagnetic chain in an applied field
Authors: V Tognetti  , A Rettori  , M G Pini  , J M Loveluck  , U Balucani  , E Balcar
Published: July 2002

The energy fluctuations of a classical Heisenberg ferromagnetic chain are investigated both using a nonperturbative Green function approach and by computer simulation techniques. Particular attention is focused on nonhydrodynamic cases, which can safely be described in terms of undamped boson propagators. As previously found for the magnetisation fluctuations, the interacting spin wave theory can be used to describe the competition between the thermal disorder and the ordering effect of the magnetic field through a thermally induced coupling between magnetisation and energy fluctuations. The spectra of the energy fluctuations are calculated and compared with the corresponding simulation data for different values of fields and wavevectors. Furthermore, thermodynamic quantities such as static energy correlations and specific heat are evaluated and found to be in very good agreement with new transfer matrix calculations. The limits of validity of the theory for very low fields are also analysed.
The intra-collisional field effect in semiconductors. II. Numerical results
Authors: S J Till
Published: July 2002

For pt.I see ibid., vol.15, p.5411 (1982). Numerical calculations appropriate to intra-valley scattering in silicon using the Airy function formalism of Herbert and Till validate the predictions about field suppression of Airy-Airy scattering made by these authors, but to discuss band-tail scattering it is essential to consider scattering from electron wavepackets. These calculations with exact Airy functions indicate that the field-induced band-tail scattering rates closely compensate the field-suppressed Airy-Airy scattering with the result that the total scattering rate is independent of electric field at high kinetic energy. The formulation has been extended to consider a simple inter-valley scattering model and the scattering rates are found to have a similar field dependence to those for intra-valley scattering. The effective mass in the satellite valley scales the scattering rates in the way predicted by the theory and the principal effect of the energy difference of the valley minima is to set a field-dependent threshold for the onset of inter-valley scattering. The parameters have been chosen so that the calculations are relevant to the Gamma to L transition in GaAs.
A theory of the properties of anisotropic magnets
Authors: S M Rezende  , D W Hone
Published: July 2002

Single-ion anisotropy interactions in magnets are notoriously difficult to treat beyond the single-spin-wave approximation, because of the intrinsic important on-site spin correlations. Here the authors propose a Green function decoupling scheme specifically tailored to retain self-consistently the proper static on-site spin correlation functions. The approach is applicable to spins of arbitrary magnitude; the anisotropy contributions do vanish identically for S=1/2, as they should. The magnon interactions arising from exchange terms are treated within the random-phase approximation, and the use of the Dyson-Maleev boson representation limits the accuracy of the results at exceedingly high temperatures. However, the authors find excellent agreement, in some cases greatly improved relative to earlier approximations, with experiments on the prototype uniaxial antiferromagnet FeF2for the temperature dependence of the magnon energies, sublattice magnetisation, parallel susceptibility and spin-flop field up to T approximately=60K approximately=0.76 TN.
A controversial problem: modified Ising model in a random field
Authors: J Villain  , B Semeria  , F Lancon  , L Billard
Published: July 2002

The effect of quenched impurities on a system having a commensurate-incommensurate (CI) phase transition is treated by real-space rescaling. A simplified form of the free energy is treated explicitly. New terms generated by the rescaling procedure do not modify the results. In D=3 dimensions, the Bragg peak shift (misfit) varies linearly with the chemical potential near the CI transition. The CI transition is killed for D<or=Dc=2. At the critical dimension, the misfit decreases exponentially for large negative chemical potentials. The model treated here may be considered a modified Ising model in a random field. The modification is a strong anisotropy of the interfaces separating the up and down domains.
Observations on 'An exact solution of diffusion via the vacancy mechanism'
Authors: L Gunther  , B Gralla
Published: July 2002

For original paper see ibid., vol.16, p.1863 (1983). The authors point out that in the time regime of diffusion via the vacancy mechanism which is probed by the Mossbauer effect, the exact solution of L. Gunther and B.G. Gralla simplifies considerably, producing lorentzian spectra. Their solution thus corroborates the so-called 'encounter theory' approximation of M. Eisenstadt and A.G. Redfield (1963).
Carrier concentration under high magnetic fields
Authors: S Cristoloveanu  , A Chovet  , S Lakeou
Published: July 2002

Carrier concentration under crossed electric and magnetic fields is analysed for the first time in the case where the magnitudes of both fields can be large. Carrier distribution, surface concentration and average concentration are studied for intrinsic semiconductors; similar or contrasting behaviour of the effect with increasing magnetic or electric fields are underlined. Influences on the Hall voltage and magnetoresistance measurements are then considered; it is shown that, quite generally, the influence of a (high) magnetic field on the transverse diffusion currents cancels the usual magnetoresistance effect. It follows that the apparent conductivity is only related to the mean carrier concentration. Simple analytical expressions are derived that allow the authors to propose simplified methods for the determination of bulk and surface recombination parameters, and to obtain the optimum parameters (field intensities, sample thickness...) for sensor applications. Experimental data for Ge and InSb with fields in the ranges 0-12 T and 0-104V m-1agree well with theoretical results.
Lattice dynamics and structural phase transitions in perovskite-type layer compounds. II. The analysis of the temperature-dependent Raman spectra of (CH3NH3)2ClCl4
Authors: R Mokhlisse  , M Couzi  , P L Loyzance
Published: July 2002

For pt.I see ibid., vol.16, p.1353 (1983). The low-frequency (0-500 cm-1) Raman spectra of (CH3NH3)2CdCl4single crystal and of its deuterated homologues (CD3NH3)2CdCl4and (CH3ND3)2CdCl4have been recorded at various temperatures covering the domains of existence of the disordered orthorhombic room-temperature (ORT) and tetragonal low-temperature (TLT) phases. An assignment of the observed Raman-active lattice vibrations is proposed; it is based upon the observed polarisation selections, the comparison of the spectra of the various isotopic derivatives and of the spectra previously analysed for the monoclinic low-temperature phase. It is concluded that in the ORT and TLT phase disorder involves not only the orientation of the methyl ammonium groups but also the CdCl6octahedra sublattice. The peculiar selection rules observed for a low-lying overdamped mode, assigned to oscillations of the rigid CdCl6octahedra, are interpreted using the hypothesis of existence of short-range correlations. A new qualitative approach for the transition mechanism in this system is tentatively given.
Localisation with phase correlations and the effect of periodic cycles
Authors: C J Lambert
Published: July 2002

This paper examines anomalies that arise in the transport properties of a disordered solid. The anomalies are associated with the period-r>or=2 marginally stable cycles of a keyphase recurrence relation. Their presence is signalled by divergences in standard nondegenerate perturbation theory. A 'degenerate' perturbation theory is developed within which anomalies can be computed by expanding about the cycles of interest. The low-order anomalies at energies corresponding to r=2 and 3 are evaluated and a possible extension to a system with an incommensurate potential is discussed.
Magnetoelastic coupling at Fe2+ions in antiferromagnetic CoCl2
Authors: D C Price  , B D Howes  , M C K Wiltshire
Published: July 2002

The57Fe Mossbauer spectrum of a single crystal of (Fe0.002Co0.998)Cl2at 4.2K has been recorded and analysed in terms of the effective-field approximation to the hyperfine interactions. At this temperature, CoCl2(space group R3m) is antiferromagnetically ordered, with moments aligned in the a-b plane of the hexagonal unit cell. It was found that the Mossbauer spectrum, which indicated both magnetic dipole and electric quadrupole splitting, could not be adequately explained using a static crystal-field model (a 'rigid-crystal' model) for the wavefunctions of the Fe2+ion in a site of 3m (D3d) symmetry. In order to obtain a satisfactory interpretation of the data it was necessary to allow the orbit-lattice interaction to generate symmetry-breaking strains at the Fe2+site as a result of the transverse exchange interaction. The electric field gradient asymmetry parameter eta proved to be a sensitive indicator of the presence of such strains. The theory of calculating these strains is outlined. The magnitudes of the strains required to satisfy the Mossbauer data are consistent with the calculations, although estimates of the values of the orbit-lattice coupling parameters are very approximate. These results are analogous to the magnetoelastic effects produced in the uniaxial antiferromagnet FeCl2by a transverse applied field, and they indicate the possible observability of magnetoelastic effects in susceptibility and Mossbauer spectra of non-magnetic crystals of appropriate symmetry.
Determination of electron effective mass, bandgap energy and excited impurity states by photoluminescence experiments
Authors: H Venghaus
Published: July 2002

The paper reviews refined photoluminescence experiments. The first part deals with magnetoluminescence yielding the conduction band electron effective mass via free-to-bound transitions from different Landau levels or alternatively from the Zeeman splitting of the shallow donor 2p state. The latter method relies essentially on selective luminescence excitation at excited donor bound exciton states. The concept of deliberate choice of excitation energy is the focal point of the second part, where excitation spectroscopy on donor-acceptor pair transitions is outlined in detail as a powerful technique to derive excited impurity states and identify impurities in moderately doped semiconductors. Selective pair luminescence as a method complementary to excitation spectroscopy is described briefly and finally it is shown how the bandgap energy can be determined with high accuracy from excitation spectra.
An infrared and neutron scattering analysis of the precipitation of oxygen in dislocation-free silicon
Authors: F M Livingston  , S Messoloras  , R C Newman  , B C Pike  , R J Stewart  , M J Binns  , W P Brown  , J G Wilkes
Published: July 2002

The precipitation of oxygen in silicon has been studied in the temperature range 650 to 1050 degrees C using the techniques of chemical etching, IR absorption applied to the 9 mu m band (4.2K), and small-angle neutron scattering (SANS). The IR data and etch pit counts, relating to the number densities of precipitate particles, have been fitted to Ham's theoretical model for the diffusion limited growth of randomly distributed particles. Full allowance is made for the increase in size of the precipitates with increasing time and good agreement is found over the complete period starting from zero time. Values of the diffusion coefficient of oxygen so determined are in excellent agreement with other data obtained by different methods, including SANS measurements at 750 degrees C described in the present work. The combined data give D=0.11 exp(-2.51 eV/kT) cm2s-1. The SANS data also indicate that the precipitates are not spherical in shape after a short initial period, and they show that the initial concentration of particles nucleated decreases with annealing time. The solid solubility cs(T) is determined at each temperature and compared with previous measurements. The authors conclude that the best combination of results leads to cs(T)=2.6*1022exp(-1.4 eV/kT) down to 850 degrees C. At lower temperatures csdecreases more slowly with decreasing temperature, indicating a possible change in the structure or the form of the precipitated SiO2phase.
ODMR study of the recombination processes in GaP:O
Authors: Y Dawei  , B C Cavenett
Published: July 2002

The local symmetry and transient properties of the ODMR signals related to the oxygen centre in GaP have been studied in detail. The associated impurity centre was observed to have an orthorhombic or lower symmetry and one order of magnitude difference of lifetimes between the 0.841 eV emission and the ODMR signals of this emission were found. Possible recombination processes involving the Jahn-Teller effect are discussed.
Calculation of the mean inner potential
Authors: A Sanchez  , M A Ochando
Published: July 2002

The mean inner potential (MIP) has been calculated for a number of elements and compounds by using the atomic electrostatic potential of Sanchez del Rio (1977). Four models have been assumed for the solid and comparisons with experimental measurements are made. The results have also been compared with the computations of the MIP made by using electronic density data from Moruzzi et al. (1978).
Frequency moments of the dielectric function for an inhomogeneous electron gas
Authors: M Taut
Published: July 2002

A unified treatment of the arbitrary frequency moments Iqq(n)of the longitudinal dielectric matrix epsilonqq'( omega ) and of the moments Fqq(n)of epsilonqq'-1( omega ) is given for the inhomogeneous electron gas. The Iqq(n)are in the RPA, whereas the Fqq(n)are exact in the non-relativistic limit excluding phonons. Furthermore, the author derives formulae allowing the Fqq(n)to be calculated from Iqq(n)and vice versa for a given approximation, which obeys the Kramers-Kronig relations. Using inequalities between the moments the author derives an inequality relating the static dielectric function of a non-conductor and the electron density at the nuclear sites.
Fermi surface measurements of YB4
Authors: T Tanaka  , Y Ishizawa
Published: July 2002

The first measurements of the Fermi surface properties of YB4that involve the use of the de Haas-van Alphen effect are presented. Many frequency branches have been observed in the frequency range from 2*102to 14.5*102T. Cyclotron masses as small as 0.1 to 0.4 m0have been measured for some branches. It is proposed that YB4is a semimetal with an electron (hole) concentration of about 0.03 per unit cell.
X-ray attenuation of silicon in the energy range 25-50 keV
Authors: J F Mika  , L J Martin  , Z Barnea
Published: July 2002

Precise measurements of the X-ray attenuation coefficient of crystalline silicon have been made in the energy range 25 to 50 keV. The results are compared with theoretical predictions and earlier measurements. A systematic discrepancy between theory and experiment is observed. The most likely cause of the disagreement is thought to be the uncertainty in the estimation of the thermal diffuse scattering cross section.
Determination of the structure of the orthorhombic commensurate superlattice state of 2H-TaSe2by convergent-beam electron diffraction
Authors: D M Bird  , S McKernan  , J W Steeds
Published: July 2002

Based on the analysis of higher-order Laue zone diffraction by Bird (1985), the structure of the commensurate superlattice state of 2H-TaSe2is determined by convergent-beam electron diffraction. It is shown that the most general displacements allowed by the orthorhombic space group can be described by a set of displacement waves. Only those waves with wavevector1/3a* and Sigma1symmetry have significant amplitudes. The final displacement pattern shows that each layer is strongly orthorhombic, in contrast with some recent Landau theory predictions.
Hubbard chain with reflecting ends
Authors: H Schulz
Published: July 2002

The one-dimensional Hubbard model bounded by infinitely high potential walls is exactly diagonalised by Bethe-Yang ansatz techniques. The 'surface' energy is studied for the half-filled band and in the Fermi gas limit. The analysis starts with a solution of a more general model in which the particles also interact with their mirror-image sites. In the linear band limit the states of this system become the Bethe eigenstates of the Kondo model at zero coupling.
Coupling between the Zeeman and tunnel energies in pentamethylbenzene: measurements of the proton spin-lattice relaxation rate T1-1
Authors: B Gabrys  , Lieven Van Gerven
Published: July 2002

The proton spin-lattice relaxation rate T1-1of powdered pentamethylbenzene has been measured in the temperature range 4-80K, at two spectrometer frequencies, 40 and 60 MHz. The measurements are analysed using a simplified coupled relaxation theory of methyl groups. Coupling between the Zeeman and tunnel energies, indicated by the appearance of a tunnelling frequency nutwas used to interpret the data, two distinct correlation times for methyl group reorientation being found necessary.
Magnetic structure and magnetic properties of the spinel solid solutions ZnCr2xAl2-2xS4(0.85⩽x⩽1). I. Neutron diffraction study
Authors: M Hamedoun  , A Wiedenmann  , J L Dormann  , M Nogues  , J Rossat-Mignod
Published: July 2002

The spinel solid solutions ZnCr2xAl2-2xS4have been studied by means of neutron diffraction in the concentration range 0.85<or=x<or=1. For the pure compound (x=1), several powder samples with different heat treatments and a single crystal were studied. At TN(x=1)=15.5, TN(x=0.9)=14.5 and TN(x=0.85)=14K a helical structure with k1=(0, 0, 0.79) and mk1perpendicular to (001) is built up in a second-order phase transition ( beta =0.3). At T0=12K, a first-order transition takes place and the helical structure begins to transform into two collinear structures corresponding to k2=(1/2,1/2, 0), mk2//(110) and k3=(1,1/2, 0); mk3//(001). At low temperature, the three phases coexist in different percentages depending on the number of sulphur vacancies. This mixed phase results from a competition between the ferromagnetic first- and third-nearest-neighbour couplings (j1approximately=2K and J3approximately=1K, respectively) and the antiferromagnetic second-nearest-neighbour interaction (J2approximately=-1K).
Rotating transverse nuclear helimagnetism in CaF2. II. Theoretical approximations
Authors: M Goldman  , J F Jacquinot  , C Urbina
Published: July 2002

For pt.I see ibid., vol.19, p.2275 (1986). The authors describe two approximate methods for the study of rotating transverse helimagnetism in high field in CaF2: the random phase approximation (RPA) and a physical model based on the fact that the Weiss field at any nuclear site is produced by distant spins. Both methods predict the helical structure to be stable in zero effective field, and the physical model predicts the occurrence of a mixed longitudinal and transverse phase in non-zero effective field. The rate of thermal mixing upon irradiation of the rare 43Ca spins with the 19F spins is analysed through RPA, and the shape of the NMR line of the ordered 19F spins is analysed through the physical model.