bluuebunny/crossref_metadata_2025 huggingface dataset:
A lattice dynamical shell model for crystals with molecules having fixed multipole moments
Authors: G S Pawley  , J W Leech
Published: July 2002

The shell model for molecular crystals proposed and tested by Luty and Pawley (1975) has been extended, and is now appropriate for molecules which have a dipole (or higher) moment in the free state. The equations are given whereby the extra interactions between the fixed molecular moments and the induced moments can be added to the dynamical matrix. It is hoped that the equations are in a form clear enough for easy programming. The necessary extension of the Ewald transformation from that given by Born and Huang (1954) to higher orders is given.
Magnetic circular dichroism of M- and R-centres in KCl. III. Calculation of the magneto-optic parameters of the R-centre in the H3-molecule model
Authors: J Margerie  , F Martin-Brunetiere
Published: July 2002

For pt.II see ibid., vol.6, no.17, p.2745, 1973. Using one of Silsbee's models of the R-centre (1965) (an H3-molecule in the equilateral triangle configuration), the authors have calculated the spin-orbit coupling constants and the g-factors for the ground and excited levels of the R-centre in KCl, as well as the oscillator strengths of the optical absorption bands of this centre. The agreement with experiment is poor. A possible improvement of the model is suggested.
Small objects in superfluid 3He
Authors: D Rainer  , M Vuorio
Published: July 2002

Distortions in the superfluid order parameter around a small object in 3He are calculated, together with the supercurrents and the angular momentum induced by it in the liquid. The forces acting on the impurity due to the liquid texture structure are also considered.
Optical absorption of the d4ion Fe4+in pleochroic amethyst quartz
Authors: R T Cox
Published: July 2002

Optical absorption spectra have been taken at 300K for samples of amethyst quartz that show anomalous pleochroism instead of dichroism. The transition moment of the 945 nm band is parallel to a lattice C2axis, called the 'optical C2axis'. The strong visible band which produces the purple colour can be decomposed into three overlapping components B0, B1, B2at 527, 538.5 and 599.5 nm respectively. B0is polarised parallel to the optical C2axis whereas B1, B2are polarised perpendicular to this axis at theta =+53 degrees , -57 degrees to C3respectively. The absorption coefficient in the visible was proportional to the concentration of a recently discovered C2symmetry S=2 centre through a series of annealing and re-irradiation experiments. This work establishes that the S=2 centre produces the optical absorption and that it is an Fe4+ion on a silicon site, verifying Lehmann and Moore's model (1966) of the colour centre.
Atomic dipole theory of photoemission from molecules adsorbed on surfaces
Authors: N J Shevchik
Published: July 2002

A theory for describing the angular dependence of the photoemission intensity emitted from small molecules adsorbed on single crystal surfaces is presented, in which the influence of the atomic potentials upon the excited electronic states is taken into account. It is assumed that the photoemission process is described by a coherent sum of spherical waves emanating from each of the atomic sites contributing to the initial state. The dependence of the photoemission intensity upon the direction of the electric field can be described in terms of a dipole vector associated with the symmetry of every atomic orbital in the initial state. The theory differs from those based upon a plane wave final state in that (i) the photoionisation process is governed by atomic-like dipole selection rules, and (ii) the phase of the outgoing wave from atom is perturbed by the atomic potential. Examples are discussed that illustrate how the details of the wavefunctions and structure of the molecules can be determined from the angle-resolved photoemission intensity.
Changes in diatomic bonding energy due to van der Waals interactions with other atoms
Authors: D D Richardson
Published: July 2002

The author considers the effects of the presence of other atoms on the bonding between two atoms are studies in detail the changes in binding energy of a Van der Waals diatomic molecule in the presence of a hexagonally packed layer of atoms. For the diatom aligned parallel to the surface it is found that as the diatom is moved out from the surface (for fixed diatomic separation) the bond strength is at first enhanced due to two-atom interactions and then weakened as three-body forces dominate. This result would not be possible from a model treating the surface as a flat continuum and demonstrates the importance of structure in interactions near a surface. For the diatom aligned normal to the surface the bond strength is always enhanced. The importance of treating the many-body problem to all orders is noted. Computer calculations are made for rare gas atoms near a layer of the same type and estimates of the magnitudes of the changes in binding energy for various orientations of the molecule are given.
Transverse diffusion currents as an important source of error in magnetoresistance measurements
Authors: S Cristoloveanu  , A Chovet
Published: July 2002

The consequences of the magnetoconcentration effect on the longitudinal current in semiconductor slabs are investigated over a wide range of magnetic field B. It is shown that the current component resulting from the action of the Lorentz force on the transverse diffusion currents is comparable to the conduction component in Ge and especially InSb, even for moderate values of B. The influence of the transverse diffusion remains remarkably important even in the weakest electric fields. Experiments performed at room temperature show good agreement with the developed theory.
Healing and relaxation in flows of helium II. III. Pure superflow
Authors: R N Hills  , P H Roberts
Published: July 2002

For pt.II see J. Non-Equilib. Therm. (1978). The healing phenomenon is studied for steady superfluid flows in stagnant normal fluid, that is, persistent currents. These include rectilinear flow down uniform pipes, the thinning of films by superfluid motion and rectilinear vortex structure.
Empirical relations and the X-ray absorption edge-shifts
Authors: M N Ghatikar  , B D Padalia
Published: July 2002

All the existing qualitative rules and the empirical relations in the light of the available data on the X-ray absorption edge-shifts have been tested. Although the qualitative rules are still valid, the empirical relations are found to be inadequate in interpreting the data on edge-shifts. New empirical relations based on the concept of the effective ionic charge are proposed and tested. The success of these new relations in the case of each of the 3d10, 4p6, 5s2, 6p6, 4d10and 4f14isoelectronic series of compounds is established. The present results suggest that the edge-shifts are mainly governed by ionic charge and these depend linearly on the product of effective ionic charge and the electronegativity of the anion.
Spin state of the Co3+ion in RCoO3compounds with the GdFeO3structure
Authors: I G Main  , J F Marshall  , G A Robins  , C E Johnson  , G Demazeau
Published: July 2002

X-ray-excited Co 2p photoemission lines from YCoO3, HoCoO3, GdCoO3and Sr2(SbCo)O6are broadened asymmetrically by multiplet splitting. The spectrum from the high-spin compound Sr2(SbCo)O6is independent of temperature. In the other three cases the line broadening shows a marked increase with increasing temperature. This effect is attributed to a decrease, caused by thermal expansion, in the value of the octahedral ligand-field splitting parameter 10Dq, which lies not far above the value at which crossover to the high-spin state occurs. Magnetic susceptibility data for YCoO3are shown to be consistent with this interpretation. There is no evidence in favour of an ordering transition, or of electron transfer between high-spin and low-spin Co3+ions.
The colouring problem on a randomly occupied lattice: Existence of a critical concentration for long-range order
Authors: M Devoret
Published: July 2002

The problem of completely frustrated spin systems with occupancy disorder-an example of which is the orientational problem in solid hydrogen-can be approached at T=0 through the simpler problem of colouring a randomly occupied lattice. Within the framework of this colour model, it can be proved that there exists a critical concentration of filled sites below which long-range order disappears. A numerical estimate of this critical colouring concentration in the triangular lattice in pcc=0.72+or-0.03. This model termed 'percoloration' is applicable to the prediction of the existence of a critical concentration for long-range order in a random array of quadrupoles such as solid hydrogen.
Birefringence studies of the phase transition in RbH2PO4
Authors: F Troussaut  , M Vallade
Published: July 2002

The authors have measured the temperature and field dependence of the birefringence Delta n6of RbH2PO4in the vicinity of its ferroelectric phase transition. Their results are consistently fitted by assuming that this birefringence is proportional to the order parameter and that its variations are given by the Landau theory where the coefficient of the quantic term in the free-energy expansion is allowed to vary linearly with the temperature. The transition is found to be second order but close to a tricritical one.
Occupancy correlation function for classical particle hopping: small concentration limit in cubic Bravais lattices
Authors: R A Tahir-Kheli  , R J Elliott
Published: July 2002

Coupled equations for the time- and space-dependent occupancy correlation function of selfavoiding classical particles hopping in periodic lattices have recently been derived by Tahir-Kheli, Elliott and Kaski (1982). These equations are exact in the limit of low particle concentration. Here the authors present solutions for the correlation function in cubic Bravais lattices. Their results are applicable to a variety of physical systems, including the case of hydrogen diffusion in metal hydrides such as TiHxand PdHx.
The dynamics of molecular reorientation in the disordered phase of bicyclo(2.2.2)octane
Authors: A J Leadbetter  , J Piper  , R M Richardson  , P G Wrighton
Published: July 2002

X-ray powder diffraction measurements have been made on both ordered and disordered phases of bicyclo-octane. It is concluded that the structure of the ordered phase is hexagonal, in agreement with earlier work. The high-temperature disordered phase is known to be face-centred cubic. Incoherent neutron quasi-elastic scattering measurements have been made throughout the disordered phase. These have been successfully analysed to show that the dominant motions are uniaxial rotations about the unique molecular axes (adequately described as C6 rotations) plus C4 rotations by which the unique molecular axes can reorient along the eight different diagonals (111) of the cubic cell. The correlation times are described by: tau 6
-1/s-1=(1.1+or-0.3)*1012 exp((-5.3-or+0.5)kJ mol-1/RT) tau 4
-1/s-1=(3.7+or-1.5)*1011 exp((7.6-or+1.5)kJ mol-1/RT).
Incoherent scattering function in simple classical liquids
Authors: G Wahnstrom  , L Sjogren
Published: July 2002

Presents a microscopic theory to calculate the incoherent scattering functions Ss(qw) in simple classical liquids. The halfwidth at half maximum omega12/(q), of this function is calculated for liquid argon and rubidium, and the results are compared with corresponding MD results. The observed oscillatory behaviour in Delta (q)= omega12/(q)/Dq2, with D with diffusion constant, is explained in terms of the physical processes in the medium surrounding the self-particle.
Electronic structure of disordered alloys I. Self-consistent cluster CPA incorporating off-diagonal disorder and short-range order
Authors: V Kumar  , A Mookerjee  , V K Srivastava
Published: July 2002

The authors have developed a self-consistent coherent potential approximation generalised to take the effect of clusters into account. Off-diagonal disorder and short-range order are taken into account. A graphical method married to the recursion technique enables them to work on realistic three-dimensional lattices. Calculations are shown for a binary alloy on a diamond lattice.
Calculations of phenomenological coefficients for matter transport by the matrix method of random walk theory
Authors: Y Okamura  , A R Allnatt
Published: July 2002

Linear response formulae for the phenomenological coefficients LAA, LABand LBBfor matter transport by single vacancies in a very dilute isotropic alloy of solute B in solvent A are studied by the matrix method of random walk theory familiar in the calculation of impurity correlation factors. A symmetry classification of vacancy jumps based on symmetry with respect to the diffusion axis is introduced and applied to the linear response formulae in order to reduce them to compact forms. For typical models these forms contain a small number of functions Kijwhich are linear combinations of vacancy migration Green functions for paths between sites of symmetry types i and j that do not pass through the impurity. The Kijare calculated by inversion of a certain matrix of jump probabilities. Detailed calculations are made for models with first- and second-nearest-neighbour vacancy-impurity interactions for BCC, FCC and diamond lattices. The calculations of LABand LAAare seen to be straightforward extensions of those already familiar in the calculation of LBBand the impurity correlation factor. This suggests that extensions to other crystal structures and migration mechanisms can be made.
An even simpler method for calculating X-ray absorption parameters
Authors: S J Gurman
Published: July 2002

A simple scheme is described whereby X-ray absorption spectra, in clouding the very strong, rapidly varying structure near an absorption edge, may be calculated using only a short program and small amounts of computer time. Comparison with experiment shows good agreement right across the periodic table for K and L edges, with rather poorer agreement for M contributions. The method is of use in situations where a knowledge of the fine detail of the absorption spectrum is not needed, but where an accurate reproduction of the absorption over a wide energy range, and of the major structure near the edge, is. It is also of use in calculating matrix elements for use in EXAFS calculations.
The decay of infrared luminescence in II-VI compound semiconductors doped by 3d transition elements
Authors: R Renz  , H -J Schulz
Published: July 2002

The decay processes of infrared emission after laser excitation have been studied in a sampling technique for several crystalline II-VI materials: ZnS and/or CdS doped by V, Cr, Co, Ni or Cu. From the exponential decay curves with time constants 0.5<or= tau <or=50 mu s recorded at low temperatures, transition probabilities have been derived and compared with calculated electric-dipole matrix elements. Two different interactions are considered by which the parity-forbidden d-d transitions of 3d impurity ions become allowed: in a covalent model, the contribution of the wavefunctions of neighbouring ions is treated by LCAO-MO methods. In a configuration mixing model, lattice vibrations induce a mixing of 4p states of the transition-metal ion with the 3d levels. Configuration mixing appears to be the prevailing mechanism for the internal transitions considered here. The temperature dependence of the lifetime from 4 to 300K is also studied and can be interpreted by means of a configuration-coordinate model.
Ferroelastic phase transition in BiVO4: V. Temperature dependence of Bi3+displacement and spontaneous strains
Authors: W I F David  , I G Wood
Published: July 2002

For pt.IV see ibid., vol.16, p.5119 (1983). Measurements of the Bi3+displacement and the primary and secondary spontaneous strains in the ferroelastic material BiVO4have been made as a function of temperature by single-crystal X-ray diffraction. The results are discussed using both macroscopic and microscopic mean-field theories of the ferroelastic phase transition. Measurements of the temperature dependence of the optical absorption edge have also been made, which show that the band-gap in BiVO4varies markedly with temperature. A method for the analysis of structural distortions in ferroelastics based on bond-length/bond-strength relationships is described. A discussion of the structural mechanism of the phase transition in BiVO4is presented, indicating that the lone-electron pair on the Bi3+ion plays a prominent role.
The solubility and diffusivity of In in CdTe
Authors: E Watson  , D Shaw
Published: July 2002

The solubility and diffusivity of In in CdTe has been investigated in the temperature range 200-850 degrees C, under Cd-rich and Te-rich conditions, using a radiotracer method. The diffusivity is independent of In concentration and below 400 degrees C is also largely independent of temperature. Above 400 degrees C the solubility and diffusivity depend on both temperature and non-stoichiometry. At Te saturation D(In)=6.48*10-4exp(-1.15 eV/kT) cm2s-1and at Cd saturation D(In)=117 exp(-2.21 eV/kT) cm2s-1. The results are discussed in terms of the defect chemistry of CdTe.
Evidence for 3dnto 3dn-14s transitions in magnetite and in lithium and magnesium ferrites
Authors: Xu-Xin Zhang  , J Schoenes  , W Reim  , P Wachter
Published: July 2002

Optical reflectivity and magneto-optical polar Kerr effect measurements in the photon energy ranges from 0.006 to 12 eV and 0.5 to 5 eV, respectively, are reported for magnetite and for Li and Mg ferrites. The diagonal and off-diagonal elements of the dielectric tensor are derived. From a comparison of the experimental results with the cation distribution formulae of the materials the excitations occurring below 2 eV can be assigned to transitions from Fe2+in octahedral sites and the excitation at about 3.9 eV to Fe3+in tetrahedral sites. Energy level schemes are deduced and are supported by a model describing the shape and the sign of the various magneto-optical transitions. In this model 3dnto 3dn-14s orbital promotion processes for the three kinds of iron ions are considered. Above 5 eV oxygen 2 p band to iron 4 s band charge-transfer transitions are shown to be responsible for the very strong optical absorption.
Molecular dynamics and structures of amine boranes of the type R3N.BH3. II. NMR investigation of H3N.BH3
Authors: E C Reynhardt  , C F Hoon
Published: July 2002

For pt.I see ibid., vol.16, p.6129 (1983). Proton second moment, T1H.T1 rhoHand T111Bmeasurements over the temperature range 330K>T>46K are reported. At low temperatures tunnelling of the protons of the NH3group influences the second moment and T1 rhoHresults seriously. An analysis of the results has yielded a tunnelling frequency ft=1.4 MHz at 85K. In the low-temperature orthorhombic phase (T<225K) threefold classical reorientations of the NH3and BH3groups have been identified. The activation energy associated with the BH3reorientation is 6.0+or-0.1 kcal mol-1while that of the NH3group is 2.3+or-0.1 kcal mol-1. In the tetragonal phase (T>225K) the BH3groups and possibly the NH3groups rotate freely. The activation energy associated with the BH3motion is 1.4+or-0.1 kcal mol-1. The high degree of plasticity of the crystals in this phase is attributed to the increased disorder. Strong evidence for piezoelectricity has been found in the tetragonal phase. The orientation of the B-N bonds is the same in both phases and, except for a very slow motion that starts at about 300K, no evidence has been found for a reorientation of the entire H3N.BH3unit. The temperature dependence of the unit-cell dimensions is discussed in terms of the reorientational rates of BH3and NH3groups.
Dynamical mean-field theory for spin glasses. II. Approach to equilibrium in a small field
Authors: J A Hertz
Published: July 2002

For pt.I see ibid., vol.16, no.7, p.1219 (1983). The response of a simple infinite-range model of a spin glass to the imposition of a small external field is studied. The field is turned on at t=0, and the evolution of the spin correlations is followed as processes involving flips of successively larger and larger clusters of spins occur, until equilibrium is achieved at t= infinity . The evolution from one state to the other proceeds through a quasi-continuum of intermediate metastable states labelled by the fraction y of spins that flip simultaneously on that time scale. The time scale for such processes grows very long as the size N of the system increases (possibly like exp((Ny)12/Tg/T)), so in the infinite system, only an infinitesimal fraction of this evolution can be observed. The generalised equation of state describing the evolution of correlations has the form derived by Sompolinsky, but the physical interpretation is different. In particular, the author does not postulate any violation of the fluctuation-dissipation theorem in equilibrium; the terms in the equations which correspond to those which violate it in his theory have the form they do because a non-equilibrium situation is being described. The physical interpretation of the variable y also allows an additional relation between initial- and final-state correlation functions which leads to the Parisi equation for his order parameter q(x), with x=1-y.
The electronic structure and the solid state properties of the one-dimensional tetraza porphin nickel (II ) system. A self-consistent-field Hartree-Fock crystal-orbital approach based on a semiempirical Hamiltonian
Authors: M C Bohm
Published: July 2002

The electronic structure and the solid state properties of tetraza porphin nickel (II), 1, have been studied by means of self-consistent field (SCF) Hartree-Fock (HF) crystal-orbital (CO) calculations that are derived in the framework of a semiempirical INDO Hamiltonian. The iterative SCF procedure is based on an algorithm where Hartree damping of the bond-order matrices and a direct steepest-descent formalism have been combined in order to avoid SCF difficulties in the CO method as a result of the closely spaced occupied and virtual Fermi seas. The diatomic intercell energies are mainly determined by electrostatic contributions. The NiNi coupling is highly repulsive, large stabilising two-centre pairs are found for the interaction between the 3d centre of the reference cell and the heteroatoms (N) in the neighbouring fragments. A large number of ligand, pi , lone-pair and sigma bands are predicted to be on top of dispersion curves with substantial Ni3d character. Oxidation processes in 1 are ligand-centred leading to organic pi radical cations. The INDO CO results for 1 are compared with experimental data derived for nickel phthalocyanine, NiPc, and halogenated NiPc derivatives.
Doping and gap states in amorphous silicon
Authors: J Robertson
Published: July 2002

The author shows that the decline in doping efficiency in a-Si:H with increased doping level arises because the dopants act as negative U centres and pin Efduring deposition. He also shows that only deep states from dangling bonds and dopants respond to coordination changes so unlike in chalcogenides, Efis pinned in a finite density of shallow tail states. Defect bonding can now be expressed in a single scheme valid for all amorphous semiconductors.
Theory of the bound state between phonons and a CEF excitation in CeAl2
Authors: P Thalmeier
Published: July 2002

The coupling of lattice vibrations to the crystalline electric field (CEF) states of 4f electrons plays an important role in rare-earth compounds. It is shown that under favourable conditions this magnetoelastic interaction can lead to a 'bound state' between CEF excitations and lattice vibrations. The existence of this bound state in the intermetallic Laves phase CeAl2explains the results of inelastic neutron scattering investigations which have not been understood before. In this compound, a Gamma7- Gamma8CEF excitation and low-lying vibrational modes of the Ce sublattice are involved in bound-state formation. The properties of this new type of excitation and its consequences for thermodynamic quantities are investigated. In addition the phonon softening at low temperatures found in CeAl2is discussed.
Raman scattering and infrared absorption spectra of Co2+ions in CsMgBr3and CsCdBr3
Authors: C W Tomblin  , G D Jones  , R W G Syme
Published: July 2002

Polarised Raman scattering spectra of the disordered crystals CsMg1-xCoxBr3and CsCd1-xCoxBr3at 15K are reported for the concentration range 0<or=x<or=0.4. The Co2+ion electronic spectra show features that can be assigned to single Co2+ions and Co2+ion pairs. Crystal-field analyses of the single-ion spectra are compared with previous studies of Co2+ions in CsMgCl3and the crystal-field parameters are discussed in terms of a superposition model. The Co2+pair spectrum for CsMg1-xCoxBr3is analysed using a two-parameter exchange Hamiltonian. The lattice phonon spectra for CsMg1-xCoxBr3exhibit normal one-mode behaviour for the concentration range studied, but for CsCd1-xCoxBr3some two-mode behaviour is evident. Additional phonon spectra results are reported for CsCdCl3, RbMgBr3, RbMgCl3and CsMnCl3.
Electron-phonon coupling in intermediate valence
Authors: D G Cantrell  , K W H Stevens
Published: July 2002

It is well established that the phonon spectra of intermediate-valence materials are anomalous. This paper discusses the properties of SmB6, by investigating the energy changes which occur when a static strain, of the same symmetry as an X-ray phonon, is imposed. It is shown that energy changes proportional to the square of the displacement can arise from changes in the energies of localised (4f) states and from interplay of the changes in the conduction band states and the transfer processes between localised and conduction states. These effects are peculiar to intermediate valency.
Structural transitions and ionic conduction in Na2UBr6
Authors: A W Hewat  , J C Taylor  , M Gaune-Escard  , J P Bros  , W Szczepaniak  , A Bogacz
Published: July 2002

The behaviour of the new material Na2UBr6has been studied as a function of temperature by differential thermal analysis, electrical conductivity and neutron diffraction. The large increases in electrical conductivity at 652 and 663K accompany structural phase transitions, with the space group changing from P3m1 at room temperature to Pmnn at 652K and finally to Im3m at 663K. As the material is heated, libration of the UBr4octahedra becomes important, allowing Na+ions to hop between equivalent holes along the z axis in the orthorhombic phase. At higher temperature, the UBr6octahedra begin hindered rotation, forming a simple body-centred cubic lattice with the Na+ions moving between all available holes. The libration of octahedra, dynamically opening up channels for cation migration is an especially interesting feature which allows this material to conduct at a lower temperature than would otherwise be possible.
Determination of the W(001) and W(110) surface potential barriers
Authors: J -M Baribeau  , J Lopez  , J -C Le Bosse
Published: July 2002

Very low-energy electron diffraction fine structure profiles measured on W(001) and W(110) are presented. These data are analysed using a new surface potential barrier and by taking into account a possible variation of the inner potential inside the atomic top layer. From a thorough comparison of several measured and calculated spectra it is found that a good overall fit of fine structure data can be obtained on both surfaces using a unique one-dimensional barrier model. Optimal values of the model parameters (the top layer inner potential Vsand the centre of mass of the screening charge z0) are however quite different on the two surfaces. On W(001), Vsis nearly 4 eV less attractive than the bulk inner potential. On W(110), this value is only about 0.4 eV. Also, z0is about 0.5 AA farther from the centre of the topmost layer on W(001) than on W(110). These results are explained by the difference in the atomic density on these two tungsten surfaces. This work shows that, especially for loosely packed surfaces, possible variation of the ion-core potentials near the surface has not to be overlooked in low-energy scattering on surfaces.
Approximate kinetic energy functionals for atoms in extended systems
Authors: M L Plumer  , M J Stott
Published: July 2002

The applicability of approximate kinetic energy functionals of the electron density n(r) for use in problems involving atoms added to extended host systems is investigated. As well as reexamining the von Weizsacker gradient expansion correction to the Thomas-Fermi kinetic energy, a new gradient correction is proposed which is based on electron gas results and utilises the dominant wave-vector suggested by Langreth and Mehl (1983). These approximate kinetic energy functions are first tested in fully variational calculations on free atoms (Z=1 to 50) with results for ground state energies, (r2) and n(0) compared with corresponding Hartree-Fock values. The models are then used to calculate the embedding energies Delta E of atoms in jellium at density n and the resulting energy curves Delta E(n) for H through Si, Cl and Ar are compared with the curves from corresponding Kohn-Sham-type calculations. Energy curves predicted by these models for Ni, Zr and Pd are also presented. A principal conclusion of this work is that although the gradient corrections do not appear to form universal kinetic energy functionals, models of this type contain features which should prove useful for the study of atoms in inhomogeneous host systems.
Interface in Potts ferromagnet: phase diagram and critical exponents
Authors: L R da Silva  , U M S Costa  , C Tsallis
Published: July 2002

Within a real-space renormalisation group framework, the authors treat the q-state Potts ferromagnet in a simple cubic lattice consisting of two different semi-infinite bulks (respectively characterised by the coupling constants J1and J2) separated by a (1,0,0) interface (characterised by Js). The use of a rather sophisticated two-terminal cluster allows a quite reliable discussion of the phase diagram and its universality classes. Four physically different phases are obtained, namely the paramagnetic and the double-bulk, single-bulk and surface ferromagnetic ones. The critical surface contains a multicritical line (associated with J1not=J2) which in turn contains a special point associated with J1=J2.
Theory of inelastic light scattering off acoustic modes in superlattices
Authors: M Babiker  , D R Tilley  , E L Albuquerque
Published: July 2002

A theory of inelastic normal-incidence backscattering of light off the acoustic modes of a two-component superlattice is considered. The appropriate Green function, derived in an earlier paper (see ibid., vol.18, p.1269, 1985) is used here to derive the scattering cross-section for two cases. In the first, the absorption of light by the superlattice is assumed to be negligibly small. With such a simplification it is possible, as a useful guide, to evaluate the intensities of the light scattering peaks and to explore their variations with the superlattice acoustic mismatch parameter. In the second case the opacity of the medium to light is taken into account. The spectrum predicted by the theory is presented for a GaAs-Ga1-xAlxAs superlattice with x approximately=0.4 and equal superlattice layer thicknesses, and exhibits a number of features which are pointed out and discussed.
The strong-coupling theory of the superconducting critical temperature
Authors: Weng Zheng-yu  , Wu Hang-sheng
Published: July 2002

A Tcformula is derived analytically from the Eliashberg equation by an iterative method. The Karakozov et al. Tcformula is only a weak-coupling limit of it. The values of Tccalculated from this formula are in agreement with the experimental values for both crystalline and amorphous superconducting materials. Certain reasonable approximations are made to render the Tcformula simplified and several useful formulae for Tccontaining only a few parameters are obtained, one of which is just the Rowell empirical Tcformula. The Tcformulae obtained in the present paper are compared further with some empirical ones proposed by previous authors and some comments are given.
Mössbauer spectroscopy of protohaem and mesohaem cytochrome c peroxidases and their fluorides
Authors: G Lang  , T Asakura  , T Yonetani
Published: July 2002

The Mössbauer spectra of cytochrome c peroxidase (CCP) containing protohaem and mesohaem have been observed in a variety of physical conditions. At very low temperature intricate paramagnetic spectra are observed, while at 195°K magnetic effects are almost completely suppressed and two quadrupole pairs appear. All spectra indicate the presence of both high- and low-spin components in agreement with susceptibility results. For the proto and meso forms in 0·2M pH 7 buffer the high-spin fractions at 4·2 °K are 0·25 and 0·4 respectively, while corresponding values at 195°K are 0·63 and 0·67. In both cases the low-spin component is in rough agreement with knowngvalues. The high-spin components are also consistent with electron spin resonance results, and the field dependence of the Mössbauer spectra indicate that the axial term in the spin Hamiltonian is 2D= 30 ± 6 cm-1. The high-spin fraction increases with increasing buffer strength, and is freezing rate dependent at very low or zero buffer strength. Fluoride complexes of both enzymes also show low-temperature paramagnetic Mössbauer spectra. These appear to be entirely high spin with 2D= 14·6±0·6 cm-1and a quadrupole interaction ½QVzz= 0·85 mm s-1. The zero-field spectra at low temperature are diffuse and consistent with the assumption of a large neighbour-nucleus interaction. At 195°K paramagnetic effects remain large and spectra are diffuse both with and without applied fields.
The thermal expansion of palladium-silver alloys at low temperatures
Authors: A C Bailey  , N Waterhouse  , B Yates
Published: July 2002

Interferometric measurements of the thermal expansion of palladium, silver and five alloys of these metals are reported in the temperature range 30 to 270 °K. The experimental uncertainties rise from approximately 1% at 270 °K to 15% at 30 °K. The variation of thermal expansion with concentration is non-linear and the form of the variation is not substantially affected by temperature in the range covered. A degree of correlation has been found with related thermodynamic properties of the alloys, and the need has been established for a rigorous theory of the thermal expansion of binary alloys on an atomic scale.
Thermoluminescence of pure and impurity-doped KBr and NaCl crystals
Authors: P C Mehendru  , S Radhakrishna
Published: July 2002

Two thermoluminescence peaks are observed at 90 and 150 °C in highly pure KBr crystals (background divalent cation impurity not greater than 1 p.p.m.). From the growth of F centres with the time of x-irradiation these two peaks have been interpreted in terms of the two types of F centres postulated previously. The presence of Cd impurity in KBr suppresses the formation of both the types of F centres while the presence of Zn impurity enhances the first-stage F centres but suppresses the second-stage F centres. Optical absorption studies of the KBr crystals doped with Cd and Zn are correlated with the thermoluminescence data. Plastic deformation of highly pure KBr crystals considerably increases the area under both the thermoluminescence peaks. No additional glow peak is observed either in plastically deformed or Cd- and Zn-doped KBr crystals.

Thermoluminesence experiments performed with highly pure NaCl crystals (background divalent cation impurity not greater than 1 p.p.m.) also show two peaks at 170 and 210 °C. However, in analar purity NaCl crystals (background divalent cation impurity about 20 p.p.m.) or in NaCl crystals containing Ca and Cd, an additional thermoluminescence peak at 80 °C is also observed.

The activation energies of both the types of F centres in KBr and NaCl and of the impurity centres in NaCl are evaluated from the thermoluminescence results using the `initial rise method'.
Symmetry properties of the localized potentials in the electron-phonon interaction
Authors: J Ginter  , J Mycielski
Published: July 2002

The electron-phonon interaction in a metal or semiconductor was expressed in terms of the matrix elements of the 'localized potentials', the localized potential being equal to the displacement of the jth nucleus of the basis multiplied by some vector field Wj. The symmetry properties of the Wjfields and some sum rules for these fields were investigated. The results were applied, in particular, to the long range multipole interactions in semiconductors; it was shown that the Wjfield decreases-for increasing distance from the displaced nucleus-no faster than this distance to the power of minus four (which corresponds to the octupole interaction). The symmetry properties of the matrix elements of the Wjfields were studied in both the Bloch and Wannier representations.
Magnetic properties of assemblies of single domain elliptical platelets
Authors: R Heuberger  , I Joffe
Published: July 2002

Magnetization and remanence loops for a uniform spatial distribution of noninteracting single domain thin discs of elliptical shape are calculated. It is found that the relative remanence of such a system has magnitude 0.638. The temperature dependence of hcand hRis investigated and found to be approximately linear over a temperature range depending upon the particle volumes and anisotropy. The values of hcand hRare found to be 0.562 and 0.648 at T=0K, respectively. The results are compared with those obtained for a uniform distribution of uniaxial particles.
Ultraviolet reflectivity of NH4I in the NaCl and CsCl structures
Authors: G Stuhmer  , E Rieflin
Published: July 2002

Reflectivity spectra of NH4I are measured in the NaCl as well as in the CsCl structure between 4.5 and 10 eV. A comparison shows a shift of the first peak towards lower energy, an enhancement of the spin- orbit doublet separation, and a region of largely decreasing reflectivity going from the NaCl to the CsCl structure. These effects are discussed by means of band structure calculations on CsI in both structures.
The average nuclear effective charge in tetrahedral semiconductors
Authors: L Garbato  , P Manca  , G Mula
Published: July 2002

An average nuclear effective charge, deduced from atomic experimental data, is introduced to describe the electronic properties of the tetrahedrally coordinated semiconductors. Ionization potentials, energy gaps, dielectric constants, cohesive energies and heats of formation can then be calculated, for all the AIIIBV and AIIBIVC2
V compounds, with an overall accuracy of the order of 7%.
A nuclear resonance and infrared investigation of the motional states of methyl groups in solids
Authors: P S Allen  , A J Snell
Published: July 2002

It is demonstrated unambiguously that a single methyl group can be responsible for two well resolved minima in the temperature dependence of its proton spin-lattice relaxation time. The analysis of seven infrared absorption bands verifies that these minima are due to (i) hindered threefold reorientation of the methyl group and (ii) transitions of the methyl group between its ground and first excited torsional state. The investigations were made on the isolated methyl group in p-tert-butyltoluene, which was found to have a threefold potential barrier hindering its reorientation of amplitude (4.13+or-0.07)kJ mole-1.
The structure of cubic ZrO2:YO1.5solid solutions by neutron scattering
Authors: D Steele  , B E F Fender
Published: July 2002

The structure of solid solutions of ZrO2containing 18%, 22% and 26% YO1.5as well as 26% YbO1.5have been examined by Bragg neutron diffraction and the diffuse scattering of long wavelength (7 AA) neutrons. The evidence points to a relaxation (0.36 AA) of the six nearest-neighbour oxygen ions surrounding an anion vacancy and there are probably similar but smaller relaxations of the four neighbouring cations which include two Y3+(or Yb3+) ions to maintain local charge neutrality. At the highest trivalent ion concentration studied there are signs of preferential association of oxygen vacancies and at the lowest vacancy concentration the presence of some tetragonal phase is indicated, possibly as an intergrowth.
Ultrasonic attenuation in GaAs from 2K to 300K
Authors: R I Cottam  , G A Saunders
Published: July 2002

Ultrasonic attenuation measurements have been made in GaAs (n-type carrier density 1.3*1016cm-3) between 2K and room temperature at 40 MHz 80 MHz 160 MHz and 320 MHz. The ultrasonic attenuation is dominated by damping due to phonon-phonon interactions and is interpreted in terms of the Woodruff-Ehrenreich theory (1961). In GaAs the attenuation frequency exponent approaches 2 at the highest temperatures and falls towards 1 in the region omega tau =1 (about 30K) these results confirm the predictions of the Woodruff-Ehrenreich theory. Details are given of the Gruneisen parameter average which is operative in this model for the phonon damping.
On the anomalous paramagnetism of Co(II) perchlorate hexahydrate at low temperatures
Authors: B K Chaudhuri
Published: July 2002

Like the hexahydrated perchlorates of Fe2+and Ni2+, the isomorphous Co2+salt is observed to show thermally-reversible anomalous magnetic properties at Tc=(166+or-1)K. Mossbauer studies of this salt diluted with Fe2+ions, however, show an anomaly in the quadrupole splitting ( Delta EQ) in the range from 140K to 160K. This difference in the temperature of the anomaly is probably due to dilution. Again similar to the Fe2+salt, the authors find in Co(ClO4)26H2O a change in sign from positive to negative and an increase in magnitude of about 50% of the axial field parameter Delta below the transition temperature. As in the other isomorphous perchlorates of this group, it is observed for Co(ClO4)6H2O that K/sub ///>Kperpendicular toabove Tcand Kperpendicular to<K/sub /// below Tc. The experimental data in the region of transition can be satisfactorily fitted with theory only by reducing the values of Delta , keeping the other parameters more or less constant. This peculiar phase change in Co2+perchlorate is also associated with the appearance of anisotropy in the ab plane accompanied by a change of asymmetric field from trigonal (above Tc) to tetragonal (below Tc) symmetry.
Relativistic corrections to the LEED intensity profiles for W(001)
Authors: P J Jennings  , M N Read
Published: July 2002

Relativistic corrections to the muffin-tin potential are shown to produce a significant improvement in the agreement between the measured and calculated LEED intensity profiles for normal incidence and low primary beam energies on W(001).
Experimental method dependent Zeeman relaxation of nontunnelling methyl groups
Authors: M Punkkinen  , L P Ingman  , D G Taylor  , P S Allen
Published: July 2002

The nonexponentiality in the Zeeman spin-lattice relaxation of nontunnelling methyl groups is shown to be closely related to the fact that the relaxation depends on the experimental method. Experimental results on CF3COOAg demonstrate that the half-amplitude times by the saturation- tau -90 degrees method can be, provided the saturating 90 degrees pulses are properly spaced, twice as long as those by the 180 degrees - tau -90 degrees method for the magnetic field perpendicular to the fluorine plane above the temperature of the T1minimum. The exponentiality of the relaxation below the T1minimum temperature, independent of the crystal orientation, implies that the intergroup induced transitions are able to equalize the populations of the degenerate Eaand Eblevels in a time much shorter than any of the time constants describing the Zeeman relaxation. The value 25 kJ mol-1is obtained for the activation energy of the CF3group reorientations. Experiments on polycrystalline acetone show a similar but somewhat smaller dependence on the experimental method.
The imaging of amorphous specimens in a tilted-beam electron microscope
Authors: S C McFarlane
Published: July 2002

The transfer function of an electron microscope used in the tilted-beam interference mode is investigated. It is shown that tilting in the presence of spherical aberration introduces terms into the real part of the transfer function having the forms associated in the axial mode with overfocusing and axial astigmatism. The effect of the phase part of the transfer function can be seen as a combination of shift and blurring of the image. Attempts to improve the transfer function by use of the microscope stigmator can at best restore it to its axial (untilted) form. The implications for the tilted-beam 'lattice imaging' of amorphous semiconductors are discussed. The results explain recent experimental data on noncrystalline materials.
Transmission of negative ions through the liquid-vapour surface in neon
Authors: I Bruschi  , G Mazzi  , M Santini  , G Torzo
Published: July 2002

Experiments on the transmission of negative ions from liquid neon to the vapour have been performed at various temperatures and at various liquid levels. The results show that the barrier is due to the dielectric discontinuity. The smoluchowski equation gives a good description of the results, provided that for the electric field e, the values of the field el which exists at the liquid side of the interface are used.
The partial densities of p and d states in α-MoTe2by photoemission using a synchrotron source
Authors: I T McGovern  , R H Williams
Published: July 2002

Photoelectron spectra have been recorded for vacuum-cleaved alpha -MoTe2crystals for a range of excitation energies. The different spectral dependences of the photoionization cross sections for p and d states allow the form of the partial p- and d-state densities to be determined.
The measurement of the magnetic torque acting on a rotating sample using an air turbine
Authors: A Stephenson
Published: July 2002

In the method described, the torque is proportional to the rotational hysteresis loss per revolution and is measured by the change in rotation frequency of the rotor of an air turbine which contains the sample. The method has been used to determine the peak in the rotational hysteresis curves of two basalt samples and is also sensitive enough to measure sandstones. The minimum measurable torque exerted on a sample up to 12 g in mass is about 10-7Nm but the system is capable of further improvement in sensitivity if required.
'Spot welding' of thin copper sheets
Authors: D F Dance  , C Duncan
Published: July 2002

A technique is described for joining together thin sheets of copper using equipment which is readily available in many workshops and laboratories.
Modified double-jet electrolytic thinning technique for transmission electron microscopy
Authors: A Mastenbroek  , G Hamburg
Published: July 2002

A simplified double-jet technique has been developed for preparing transparent specimens for transmission electron microscopy. This technique makes it possible to limit the production of radioactive waste. In addition it is easy to apply and reliable to produce specimens of high quality.
A new method for the measurement of the thermoelectric power of sintered semiconductors
Authors: A Crucq  , L Degols
Published: July 2002

A method of measurement of the Seebeck effect, where a thermocouple is used at the same time as a heating source and as a temperature measuring device of the hot point, is described. Small volume realization and the possibility of irradiation with low energy X-rays (or any low penetration radiation) are the main advantages of this method.
Preparation of nonconducting samples for the scanning electron microscope
Authors: S Chatterji  , N Moore  , J W Jeffery
Published: July 2002

Nonconducting samples for examination in the scanning electron microscope require a coating of conducting material to remove charge from the surface. Normally a layer of gold-palladium, nominally 500 AA thick, is evaporated on to the sample. Experiments using carbon as an alternative coating material showed that an approximately 800 AA thick carbon film can be used instead of metal film The results indicate that the carbon film of this thickness is nearly transparent to the secondary electrons used by scanning electron microscopes for image formation and as a consequence the substrate surface becomes visible.