bluuebunny/crossref_metadata_2025 huggingface dataset:
K X-ray emission spectra of Mg and Al
Authors: M O Krause  , J G Ferreira
Published: July 2002

Relative energies and relative intensities of the K alpha satellites and the K beta bands of Mg and Al metals and anodized Al have been measured. Spectra were obtained with a PAX spectrometer which provided high resolution and accurate intensity data (PAX identical to photoelectron spectrometry for the analysis of X-rays). Relative energies agree well with data from the literature. Relative intensities of line groups that correlate with initial KL, KL2and KL3defects are compared with theoretical predictions based on the shake-off theory. Theory underestimates the satellite group intensities by 15% for KL defects and 30-40% for KL2defects. The intensity ratio K beta /K alpha1,2is found to be 1.7% for Mg and 2.4% for Al.
Effective one-electron operators for discrete and ionizing transitions in 'many-body' and 'model one-electron' theories
Authors: H S Taylor  , S W Wang
Published: July 2002

An effective one-electron operator for one-electron transitions in atoms and molecules is derived by using the functional derivative form of many-body theory. The operator is useful in many-body theory itself and in model potential theories of discrete and photoionizing transitions.
Measurement of the lifetimes of ionic excited states using the inelastic electron-photon delayed coincidence technique
Authors: A J Smith  , F H Read  , R E Imhof
Published: July 2002

A modification of the inelastic electron-photon delayed coincidence technique is described which permits the measurement of the lifetimes of ionic excited states. The technique has been applied to measurements in the N2+, CO+and CO2+ions, and the essential results are as follows: N2+B2Sigmau+, v=0, 60.6+or-0.6 ns; CO+B2Sigma+54.3+or-1.8 ns; CO2+A2Piu108.3+or-1.4 ns; CO2+B2Sigmau+117.4+or-2.1 ns. Also presented are two coincidence energy-loss spectra of the continua resulting from the excitation of the A2Piuand B2Sigmau+states respectively of CO2+.
The mobilities of atomic krypton and xenon ions in the2P1/2and2P3/2state in their parent gas
Authors: H Helm
Published: July 2002

The mobilities of atomic ions of krypton and xenon in their parent gas are shown to depend on the spin state of the ion. In both gases the mobility of the ground-state ion (2P3/2) was found to be lower than that of the metastable ion (2P1/2). This difference originates in the different interatomic potentials which arise from the interactions of (2P3/2)+(1S0) and (2P1/2)+(1S0) states respectively. The results may be related to a recent theoretical investigation of a difference in the charge-exchange cross section of both spin states at low ion energies (Sinha and Bardsley 1976). The zero-field mobilities were obtained by extrapolation at 294K in krypton and at 273, 293 and 345K in xenon.
Application of optical techniques in the study of electromagnetic surface waves
Authors: G C Aers  , A D Boardman  , P A Young
Published: July 2002

Theoretical multi-layer techniques, used in the classical optics of thin films are developed for use in the study of surface electromagnetic waves on active dielectric media. Surface plasmon-polaritons are considered, by way of illustration, and new forms of dispersion equation are derived which are expressed in terms of convenient integrals over possible surface density profiles. The solutions of this integral approach are compared with exact multilayer simulations and are shown to be extremely accurate in the retardation regime. Spatial dispersion is included by deriving a novel 4*4 characteristic matrix for which an integral form is also given. The behaviour of certain classes of profiles is discussed in detail and the existence of structure-induced modes is debated.
Band structures and Fermi surfaces for 1T-TaS2, 1T-TaSe2and 1T-VSe2
Authors: A M Woolley  , G Wexler
Published: July 2002

Band structures and Fermi surfaces are presented for three quasi-octahedral layer compounds. The dependence of both properties on the component kzof the wavevector normal to the layers is shown to be important particularly in its effect on the formation of charge density waves. Whereas the nesting of the Fermi surfaces is rather weak in the two-dimensional approximation it becomes very strong if the critical wavevector q is a allowed to have a qzcomponent and equal to one third of the height of the Brillouin zone. Two additional (related) features are discussed namely the three-fold (as opposed to six-fold) symmetry and the small trigonal splitting of the t2gtriplet at the centre of the zone.
Trigonal ammonium fluosilicate studied with nuclear relaxation
Authors: I Svare
Published: July 2002

Proton and 19F relaxation in trigonal (NH4)2SiF6 have been studied down to 30K. Above the phase transition at 38K the activation energies for ionic reorientation are: Ea(NH4
+)=1.11+or-0.03 kcal mole-1 and Ea(SiF6
2-)=15.1+or-0.5 kcal mole-1. Slow cooling gave an ordered phase below 36K with increased Ea(NH4
+) and a change in T1 by a factor 100 at Tc. With fast cooling, the phase change was not observed. Tunnelling reorientations of NH4
+ limit the maximum relaxation rates T1
-1 and T1 rho
-1, and this is used to estimate the tunnel frequency to be about 50 MHz about Tc.
Magneto-optical investigation of the linear chain ferromagnets AFeCl3(A=K,Rb,Cs)
Authors: E Krausz  , S Viney  , P Day
Published: July 2002

The optical absorption and dichroism spectra are reported from 3000 to 300 nm from 1.2 to 300K in parallel and transverse fields up to 5T. The quintet-triplet transitions are assigned in the approximation of an octahedral crystal field and the origin of the broad polarised band at about 540 nm in the Rb and Cs salts is discussed. The temperature dependence of the spin-forbidden transitions enhanced by the exchange-induced electric dipole mechanism is analysed using the classical spin model of Ebara and Tanabe (1974). At the lowest temperatures the observed intensities deviate significantly from the theoretical predictions and do not approach zero. Evidence is presented that this is due to large zero-point fluctuations of the spin system rather than the perturbing influence of the groundstate orbital angular momentum.
The effect of pressure on the optical properties of 2H and 4H SnS2
Authors: M J Powell
Published: July 2002

A study is presented of the optical absorption edge of 2H-SnS2and 4H-SnS2at 77K and 273K and as a function of hydrostatic pressure. The results are interpreted to show that for both polytypes the lowest energy optical transition is an indirect one (Eg) and that the first direct transition (Eo) follows closely to higher energies. The results show that polytypism is significant in determining both the magnitude of the energy gaps and their pressure coefficients. This suggests that layer-layer interactions are very important for the electron states responsible for the band gaps.
Electronic contributions to the elastic constants of tellurium-doped bismuth
Authors: A J M Lichnowski  , G A Saunders
Published: July 2002

Keyes' model for the electronic contribution to the elastic constants has been extended to the rhombohedral group Vb semimetal bismuth, and the magnitude of the effect calculated using parameters taken from the literature. The theory predicts that an increase in the free electron density should decrease C11, C66and C44and increase C12and C14. These predictions have been tested by measuring the elastic constants of single crystals of bismuth heavily donor-doped with tellurium (1019-1020electrons cm-3); the experiments were performed between 4.2K and room temperature by the ultrasonic pulse echo overlap technique. In general, doping of bismuth with tellurium causes the expected decrease in the ultrasonic wave velocities and the elastic constants vary in the manner predicted by the theoretical model. An increase in the free electron concentration causes C11and C66to decrease, C12to increase, while the variation of C33is negligible.
Parametric representation of the equation of state of a Heisenberg system in the critical region
Authors: J E Kirkham
Published: July 2002

A parametric equation of state for the Heisenberg system is given, which has the correct singularities due to Goldstone modes. It is shown to be an exact representation of the epsilon expansion to O( epsilon ) and of the leading order in 1/n expansion, and to be in reasonable agreement with Monte Carlo calculations and experimental data for CrBr3.
An investigation using thermal phonons of the concentration dependence of V3+frequency crossing signals in Al2O3
Authors: L J Challis  , D L Williams
Published: July 2002

Measurements have been made of the concentration dependence of the size of the frequency crossing signals due to V3+ions in Al2O3. These occur in the low-temperature thermal magnetoresistance at 3.1 T for fields along the c axis. The signal height varies approximately as (concentration)0.7and the width as (concentration)0.15and these variations are shown to be consistent with theoretical analysis when account is taken of the hyperfine splitting of the V3+levels. The data confirm that the Delta M=2 transition is much more strongly coupled to the lattice than the Delta M=1 transition and lead to values of (G442+G512)12/=15+or-5 cm-1and (1/2G2+2G152)12/=425+or-210 cm-1.
Spin-transfer due to covalency for the tetrahedral-site Fe3+ions in Fe3O4
Authors: V C Rakhecha  , N S Murthy
Published: July 2002

The authors apply the model due to Hubbard and Marshall (1965) to interpret the experimental tetrahedral (A) site Fe3+form factors in Fe3O4. A large covalency is inferred for the A-site, and it is shown that approximately 28% of the moment density is transferred away to the oxygen ions. The sigma covalency is seen to predominate. A comparison with the Fe3+tetrahedral form factor in a closely related compound yttrium iron garnet (YIG) has been made and the two are seen to agree well with regard to the main conclusions. The A-site is not a centre of inversion and its form factor has an imaginary part also. The role of the latter in the analysis of experimental data is brought out.
One-magnon light scattering in garnets and ferrites with and without compensation temperatures
Authors: A F Khater
Published: July 2002

For pt.I see ibid., vol.11, p.563 (1978). The theory of one-magnon light scattering from a two-sublattice ferrimagnetic system is applied to four ferrimagnetic materials: YIG, GdIG, the Li ferrite Fe(Li0.55Fe1.45)O4and the Li-Cr ferrite Fe(Li0.5Fe0.25Cr1.25)O4. YIG and Li ferrite have no compensation effects, and GdIG and the Li-Cr ferrite do. The results for the different materials are compared with those in scattering for the ferro- and antiferromagnet in T<or=Tc. The compensation ferrimagnets GdIG and Li-Cr exhibit new features in the compensation region absent in both the ferro- and antiferromagnet.
Threshold conduction in inversion layers
Authors: C J Adkins
Published: July 2002

After an introduction to inversion layers and a summary of the theoretical ideas concerning the expected effects of disorder in such systems, the observed behaviour near the threshold of conduction is reviewed. Although the system shows a classic metal-insulator transition, the detailed behaviour is not consistent with an independent-particle mobility edge model. A new model is proposed in which correlation causes the carriers to behave like a viscous liquid near the conduction threshold. The new model provides quantitative explanations of many observed properties. Other experiments are suggested by which it may be further tested.
Theory of the symmetric Wolff model. II. Weak-coupling-long-time-low-temperature limit
Authors: H C Fogedby
Published: July 2002

For pt.I see ibid., vol.10, p.2869 (1977). The local single-particle and density correlation functions for the symmetric Wolff model (1961) are studied in the asymptotic weak-coupling-long-time-low-temperature limit. The scattering channels are separated by means of the Stratonovich-Hubbard transformation and the associated one-body problem is solved explicitly in the low-temperature limit. A steepest-descent argument yields expressions for the correlation functions in the weak-coupling limit. The correlation functions are characterised by a time dependent phase shift which decays on a time scale of the order of the bandwidth. In the intermediate and strong coupling regime many-body effects intervene and the calculation breaks down; in that context comments are made on some recent work by Mattis (1976).
Quantum crossover in the transverse Ising model
Authors: I D Lawrie
Published: July 2002

The critical behaviour of d dimensional Ising models in a transverse magnetic field, Gamma , is investigated using a quantal generalisation of the Landau-Ginzburg-Wilson free-energy functional introduced by Hertz and Young (1976). A variational approximation of the Hartree type allows a detailed analysis: behaviour near the critical line, Gamma = Gammac(T) is governed by the exponents of the d dimensional spherical model (which are mean-field-like for d>or=4). However, the point ( Gamma ,T)=( Gammac(0),0) is multicritical in nature, and characterised by the exponents of the (d+1) dimensional spherical model. Crossover scaling functions, valid near the multicritical point, are found for the longitudinal susceptibility and free energy. They involve the crossover exponent phiT; for 2<d<or=3 this is equal to the (d+1) dimensional correlation length exponent, nud+1=1/(d-1); for 3<d4, no crossover occurs. These results are shown to be consistent, for all d>2, with the finite-size scaling analogy suggested by Suzuki (1976) and others.
The cellular method for non-spherical potentials
Authors: S L Altmann  , W Barton
Published: July 2002

A form of the cellular method is developed in which the non-spherical terms in the potential can be treated exactly. Examples of its use are given for copper metal and graphite crystal. In the latter, eigenvalues change in most cases by 0.2-0.3 Ryd as a result of introduction of the non-spherical potential and it is also found that layer-layer interaction cannot be considered small.
Electronic transport in alkali halides containing F centres
Authors: G T Jenkin  , D W Stacey  , J G Crowder  , J W Hodby
Published: July 2002

The trapping by F-centres of conduction electrons in KBr, KCl, KI, RbCl and RbI is studied via their photoconductivity at low temperature and in high magnetic fields. In zero magnetic field the carrier lifetime before final trapping is found to approach the scattering time at 4K when the density of F centres exceeds 1017cm-3. The trapping process is then important reducing the carrier mobility. In KBr the triplet state trapping cross-section is less than 3% of the singlet state trapping cross-section.
Absorption and luminescence of Cr2+(d4) in II-VI compounds
Authors: M Kaminska  , J M Baranowski  , S M Uba  , J T Vallin
Published: July 2002

Infrared absorption and luminescence measurements of substitutional Cr2+(d4) in the II-VI compounds ZnS, ZnSe, ZnTe and CdTe are reported. The experimental results are interpreted in the framework of crystal field theory with spin-orbit coupling and a substantial Jahn-Teller interaction in the5T2ground state and a much smaller, but significant, Jahn-Teller interaction in the excited5E state. With a semiclassical model and a Monte Carlo computing technique convoluted shapes of absorption and luminescence spectra are obtained. The Jahn-Teller energy for the5T2ground state is found to be approximately 350 cm-1. For the first time a Jahn-Teller energy for the excited5E state is reported, and has a value of approximately 50 cm-1. The inclusion of ligand field effects in the spin-orbit coupling constant was found to be important.
Mean-square relative displacements of nearest-neighbour atoms in Ge
Authors: P Rabe  , G Tolkiehn  , A Werner
Published: July 2002

EXAFS spectra of crystalline and amorphous Ge have been used to determine the mean-square relative displacements sigma 2 of nearest- and next-nearest-neighbour atoms in the temperature range 80-400K. A strong correlation of the nearest-neighbour motion is observed for crystalline Ge, but this correlation decreases in the amorphous material. The results are compared with calculations performed within the Debye approximation. The structural parameters of amorphous Ge are consistent with a continuous random network approach.
Phase diagram and homogeneity gap in microemulsions
Authors: S Ballaro  , F Mallamace  , F Wanderlingh  , D Senatra  , G Giubilaro
Published: July 2002

Microemulsions are investigated for a wide range of concentrations and temperatures. After a direct observation of the samples, light scattering and viscosity measurements are performed. Structural transitions are clearly indicated. In addition, a homogeneity gap is found, in which the sample separates into two phases. Each of these consists of a micro-emulsion but they have different structures.
Structural and electrical properties of layered transition metal selenides VxTi1-xSe2and TaxTi1-xSe2
Authors: F Levy  , Y Froidevaux
Published: July 2002

The structural and electrical properties of single crystals of the systems TaxTi1-xSe2and VxTi1-xSe2have been investigated. Single crystals have been grown by chemical transport reactions over the whole concentration range. Their crystallographic structures can be described on the basis of the 1T polytypic form. The electrical properties of the mixed crystals are interpreted in comparison with the stoichiometric samples. The most interesting effects occur in vanadium-doped TiSe2samples where the temperature of the resistivity anomaly is lowered by additions (0.001<or=x<0.01). For larger x(0.01<or=x<or=0.05) the anomaly disappears and a semiconducting phase is observed down to low temperatures. For increasing x, the temperature dependence of the resistivity has a metallic character.
Impurity scattering of electrons in non-degenerate semiconductors
Authors: H M A El-Ghanem  , B K Ridley
Published: July 2002

A unified approach to the theory of impurity scattering in non-degenerate semiconductors is presented using a simple model for the scattering potential, and partial-wave techniques. The theory describes both charged and neutral impurity scattering and distinguishes between sign of charge. It also allows a novel extension to be made to resonance (or Breit-Wigner) scattering. Third-body screening is taken to provide a bridge between the Brooks-Herring and Conwell-Weisskopf formulae, and its effect on neutral impurity scattering is examined for both Erginsoy and Sclar cross sections. The relationship between scattering and the depth of a bound state is exhibited using quantum-defect wavefunctions. Resonance scattering, if the resonance is sharp, has only a weak effect on the mobility, but a large effect on rH, the ratio of Hall and drift mobilities. The latter reaches the magnitude 3.7 for sharp resonance. The ratio rHis therefore a sensitive detector of resonance scattering.
Lithium ions in aqueous solution
Authors: J R Newsome  , G W Neilson  , J E Enderby
Published: July 2002

The nature of hydration around Li+in two concentrations of LiCl in D2O has been investigated by the methods of neutron diffraction and isotopic substitution. It is found that the hydration number varies with concentration, and that the angle of tilt between the Li-O axis and the plane of the water molecule, though substantial, is less than that expected if a lone pair orbital of the oxygen atom is directed towards the Li+ion.
Neutron diffraction study of liquid mercury using the electron LINAC
Authors: T Matsumoto  , M Senda
Published: July 2002

Liquid mercury was measured at room temperature by time-of-flight neutron diffractometry on the electron LINAC. The presence of a subsidiary shoulder in the structure factor which was not observed in Vineyard's neutron diffraction study (1954) but which has been cited in recent X-ray diffraction studies is confirmed. Furthermore, a molecular dynamics calculation was made to obtain the radial distribution function, and it was compared with the one derived from the neutron diffraction experiment.
Conduction bipolarons in low-temperature crystalline WO3-x
Authors: O F Schirmer  , E Salje
Published: July 2002

Conduction electrons in the low-temperature phase of crystalline WO3-x(x>or approximately=0.0001) are found to be paired diamagnetically, forming bipolarons due to joint lattice distortion. They can be dissociated with light in a broad band of wavelengths peaked at 1.1 eV, as monitored by the appearance of W5+ESR and a corresponding change in DC conductivity. The transport activation energy is 0.18 eV in the paired state and decreases upon dissociation. The bipolaron binding energy is larger than 0.36 eV. Separated electrons recombine in a bimolecular process controlled by diffusion in the conduction band.
The 2 × 1 reconstruction of the Ge(001) surface
Authors: J C Fernandez  , W S Yang  , H D Shih  , F Jona  , D W Jepsen  , P M Marcus
Published: July 2002

Low-energy electron diffraction (LEED) analyses of several structure models for the Ge(001) 2 × 1 surface confirm that the reconstruction involves at least 3 layers, and demonstrate that models with buckled asymmetric dimers are favoured over models with symmetric dimers. Three models with different amounts of buckling are given, which show equivalent moderate agreement with experiment.
The structure around nitrate ions in concentrated aqueous solutions
Authors: G W Neilson  , J E Enderby
Published: July 2002

The first-order-difference method of neutron diffraction has been applied to a 7.8 molal aqueous solution of sodium nitrate in which isotopic substitutions were made on the nitrate ion. The intramolecular distance is found to be 1.23+or-0.02 AA. There are 1.3+or-0.2 water molecules coordinated axially to the nitrate ion with rNDand RNOvalues of 2.05+or-0.02 and 2.65+or-0.10 AA respectively. A model for the longer-range structures is proposed.
Band gaps and asymptotic behaviour of continued fraction coefficients
Authors: P Turchi  , F Ducastelle  , G Treglia
Published: July 2002

Continued fractions are frequently used to approximate densities of states. When the support of these densities is connected (i.e. there is no gap) the coefficients (an, bn2) of the continued fraction converge when n to infinity . In the presence of gaps, they exhibit undamped oscillations. The authors characterise these oscillations completely by studying the spectrum of periodic linear chains. It is found that the coefficients are periodic abelian functions of n (elliptic functions in the case of a single gap), in agreement with recent results by Magnus (1979). The corresponding recurrence laws are derived, and methods for terminating the continued fraction are given. The case of a single gap is treated in detail, and numerous examples and applications are provided.
The chemical potential of radiation
Authors: P Wurfel
Published: July 2002

In a thermodynamic treatment electromagnetic radiation of any kind is described. The difference between thermal and non-thermal radiation is accounted for by introducing the chemical potential of photons. Instead of an effective temperature all kinds of radiation have the real temperature of the emitting material. As a result Planck's law for thermal radiation is extended to radiation of any kind. The concept of the chemical potential of radiation is discussed in detail in conjunction with light-emitting diodes, two-level systems, and lasers. It allows the calculation of absorption coefficients, of emission spectra of luminescent materials, and of radiative recombination lifetimes of electrons and holes in semiconductors. Theoretical emission spectra are compared with experimental data on GaAs light-emitting diodes and excellent agreement is obtained.
Electron diffusion in bond-disordered media
Authors: N I Chekunaev  , Yu A Berlin  , V N Fleurov
Published: July 2002

A theory of electron motion over randomly situated centres is developed. The electron binding energy in the centres is assumed to be constant. By means of the concepts developed in percolation theory all the centres can be separated into two groups. The first group makes a diffusion cluster responsible for the macroscopic transport. The second group of centres, i.e. isolated centres plays the role of the sources owing to their electron exchange with the first type of centre. A modified diffusion equation is derived describing the electron motion in such a system. By means of this equation the electron motion in an external uniform electric field has been analysed.
A mathematical analysis of diffusion in dislocations. III. Diffusion in a dislocation array with diffusion zone overlap
Authors: A D Le Claire  , A Rabinovitch
Published: July 2002

For pt.II see ibid., vol.15, p.3445 (1982). The authors consider diffusion from a surface y=0 into a solid containing a regular array of d dislocations per unit area, all normal to the surface and each represented as a pipe of radius a within which the diffusion coefficient D' is very much greater than that, D, for diffusion in a regular crystal. They calculate the mean concentrations (c(y)) at depths y, as determined in conventional sectioning experiments, for both constant concentration and thin-finite-source conditions at y=0. (c) is evaluated as a function of eta =y/(Dt)12/, alpha =a/(Dt)12/, Delta =D'/D, epsilon2= pi a2d and of epsilon / alpha , the ratio of the mean diffusion length, (Dt)12/, to the effective half-spacing between dislocations, ( pi d)-12/. Full account is taken, through an appropriate Wigner-Seitz-like boundary condition, of the effects of the mutual overlap of the diffusion zones around dislocations. For epsilon / alpha <or approximately=1, values of (c) do not differ by more than a few per cent from those previously calculated by the authors for well spaced non-interacting dislocations; for epsilon / alpha <or approximately=0.4 the two solutions are numerically identical. As epsilon / alpha increases above one the effects of overlap rapidly become increasingly important. They examine the nature of the penetration plots, log(c) versus y2, and the effective diffusion coefficients, Deff, deduced from them. The case epsilon / alpha <or=1 was discussed previously by the authors. The plots show maximum curvature around epsilon / alpha approximately=5 but would be difficult experimentally to distinguish from linear when epsilon / alpha >or approximately=10. For this range Deffis found to be closely represented by the Hart relation, Deff=D(1+ pi a2d( Delta -1)), the condition of validity of which can now be specified more precisely than heretofore.
Nickel and copper on cleaved indium phosphide: structure, metallurgy and electronic properties
Authors: G J Hughes  , A McKinley  , R H Williams
Published: July 2002

Previous studies of Schottky barrier formation between metals and clean cleaved (110) surfaces of n-type InP have shown that chemically reactive metals (Ni, Fe, Al) pin the Fermi level relatively close to the conduction band whereas unreactive metals (Cu, Ag, Au) lead to pinning closer to mid-gap. The authors have selected Ni and Cu, as representative of each group, and studied the detailed interactions between these and the InP surface using a range of experimental techniques. These include LEED, Auger and photoemission using a laboratory source (He I radiation) as well as a synchrotron source. Nickel was found to interact very strongly with the surface resulting in the formation of nickel phosphides at the interface and the metallic indium diffusing out into the Ni contact. Copper interacts much more weakly with the surface, the core level and valence band photoemission spectra contrast markedly to the behaviour observed for Ni. The formation of Schottky barriers has also been probed by monitoring appropriate shifts in the photoemission spectra. The importance of the interactions at the interface on Schottky barrier formation is considered. Interface widths are also discussed.
Field-dependent photocarrier generation in crystalline polydiacetylenes
Authors: U Seiferheld  , B Ries  , H Bassler
Published: July 2002

The electric field dependence of the photocarrier generation rate has been measured for crystalline polydiacetylenes (PTS and DCH) in the range 10<E<105V cm-1. For E<104V cm-1it follows the predictions of the one-dimensional Onsager theory, above 104V cm-1a superlinear increase with no indication of saturation is noted. The high-field behaviour is interpreted in terms of field-assisted ionisation of Frenkel excitation states that are degenerate with conduction band states. Applicability of the 1D Onsager theory to treat geminate pair dissociation is consistent with the recently developed concept of strong electron-phonon interaction leading to short carrier mean free paths. The microscopic anisotropy of the carrier mobility parallel and perpendicular to the polymer chain direction is found to be greater than 104, larger than inferred from macroscopic conduction experiments.
Induced absorption and metastable states in ZnS:Mn and ZnS
Authors: J M Noras
Published: July 2002

The absorption and excitation spectra of a strong photoinduced band found in samples of ZnS:Mn at 6 and 77K are reported. The process involves first the population of a metastable level and subsequently excitation to a higher level, which shows the induced absorption. The centre responsible is not identified.
Effects of chemical environment in the lineshape of silicon L2,3VV Auger spectra of nickel silicides
Authors: U Del Pennino  , P Sassaroli  , S Valeri  , C M Bertoni  , O Bisi  , C Calandra
Published: July 2002

The Si L2,3VV Auger lineshape in nickel silicides of different compositions has been studied both experimentally and theoretically with the purpose of understanding the behaviour of silicon states in compounds where the Si atom has different local chemical environments. The experimental spectra provide evidence of significant modifications in the lineshapes, which are associated with changes in the distribution of the valence electrons. These modifications can be explained by theoretical calculations based on a single-particle description of the bulk electronic structure. Some discrepancies between theory and experiments can be attributed to approximations in the treatment of the final state of the Auger electron and to the neglect of surface effects in the electronic structure calculation.
Further studies of substitutional Cr2+in GaAs: the APR spectrum and its interpretation
Authors: A S Abhvani  , C A Bates  , P Bury  , P J King  , D R Pooler  , V W Rampton  , P C Wiscombe
Published: July 2002

APR experiments on weak n-type samples of chromium-doped GaAs produce a weak set of absorptions consisting of sharp peaks on a broad background when the sample is kept in the dark. A model proposed earlier for substitutional Cr2+ions at Ga sites in which the dynamic Jahn-Teller effect and random strains play a crucial role, has been examined to see whether it can account for the EPR and for this weak APR absorption. Revised electronic and Jahn-Teller parameters are deduced which satisfy the requirements of data from conventional and thermally detected EPR experiments and data deduced from the structure of the zero-phonon line and phonon scattering experiments, while at the same time the model then satisfactorily fits most, but not all, of the APR data. The first-order Ham factor gamma is found to be about 10-4and the implications of this value in understanding the properties of Cr2+are also discussed.
The electronic structure and stability of localised defects in semiconductors. II. Vacancies in silicon, gallium phosphide and zinc selenide
Authors: M J Kirton  , P W Banks  , Lu Da Lian  , M Jaros
Published: July 2002

For pt.I see ibid., vol.17, p.2575 (1984). The authors present the results of their calculations on the ideal and reconstructed vacancy and divacancy in silicon and the ideal and Jahn-Teller distorted cation and anion vacancies in gallium phosphide and zinc selenide. For the silicon vacancy they find that the charge density profile, the defect potential and the Jahn-Teller parameters are all in agreement with the existing results obtained from the self-consistent Green function methods. They also present a consistent description of the silicon dangling bond. An analysis of the localised structure of the divacancy in silicon is given, and they argue that the electron-lattice coupling at the divacancy should be significantly weaker than that at the single vacancy. The authors provide a comprehensive description of the dangling-bond states of vacancies in GaP and ZnSe and a coherent analysis of their results is presented. They predict a strong and systematic variation in the Jahn-Teller coupling constant across the periodic table, and their results are briefly compared with the existing experimental data.
Bethe ansatz and 1/N expansion results for N-fold degenerate magnetic impurity models
Authors: J W Rasul  , A C Hewson
Published: July 2002

The authors calculate the ground state susceptibility chi0, to leading and next-leading order in a 1/n expansion, for the N-fold degenerate Anderson model in the integral valent limit. From this and perturbational results they are able to calculate the Wilson numbers W(N), which relate to chi0and TK, to first order in 1/N.
Nuclear spin stimulated echoes in glasses
Authors: M Engelsberg  , N M Borges
Published: July 2002

The results of measurements of11B and23Na stimulated echo decays in various glasses are reported. In some cases a model of quasi-independently precessing spin-packets perturbed by spectral diffusion appears to be applicable. The data are interpreted in terms of a stochastic model that provides a satisfactory description of the long intrinsic decays prevalent in these glasses.
Mean field theory of Potts spin glass
Authors: E J S Lage  , J M Nunes da Silva
Published: July 2002

A mean field theory of the s-component Potts spin glass model is presented and is shown to reduce to TAP theory for s=2 (Ising). The paramagnetic free energy has instabilities for temperatures which agree with those derived using replica theory. The ordered phases, however, indicate that the transition is first order for s>2, in apparent contradiction with what has been obtained with replica theory.
Angle-resolved photoemission from a copper monolayer on silver (100)
Authors: G C Smith  , C Norris  , C Binns
Published: July 2002

The electronic structure of a Cu (100) monolayer supported on Ag (100) has been investigated using low-energy angle-resolved photoelectron spectroscopy. The measured electron-energy-parallel-momentum (E-k/sub ///) dispersion relation is compared with the published calculation for the isolated monolayer, and normal emission spectra calculated by a one-step photoemission theory based on the time-reversed LEED formalism are presented.
A simple model for excitons bound to axial isoelectronic defects in silicon
Authors: G Davies
Published: July 2002

The axial nature of isoelectronic defects in Si is modelled by a uniaxial stress acting on the crystal's valence band and conduction band extrema. The model provides an approximate description of the energy spacings and relative luminescence intensities of the lowest-energy bound exciton transitions. Experimental data on the effects of uniaxial stresses and magnetic fields can be fitted using three parameters for each, the value of the parameters being predictable to well within a factor of two from the properties of perfect Si.
Interaction of negative ions with vortices in superfluid3He-B
Authors: V P Mineev  , M M Salomaa
Published: July 2002

The interaction potential of negative ions with vortices in3He-B is calculated in the Ginzburg-Landau regime. The authors contrast ion trapping by the two different kinds of possible vortex structures, those with normal and superfluid core. Ion experiments are suggested to provide useful new information on the vortex transition in rotating3He-B.
Transition-metal impurities in III-V compounds
Authors: B Clerjaud
Published: July 2002

A survey of the recent progress on the properties of transition-metal impurities in the most common III-V compounds GaAs, InP and GaP is presented. A summary of experimental techniques available for identifying deep centers and/or locating deep levels in the band gap is given. The author concentrates on 3d isolated impurities, giving the known energy levels of Fe-group substitutional impurities in the three compounds. Complexes involving these impurities are also considered. In addition, 4d, 5d and rare earth impurities are briefly discussed.
An integrable one-dimensional Fermi system with magnetic impurities. II. Low-temperature thermodynamics
Authors: T B Bahder  , E H Rezayi  , J Sak
Published: July 2002

For pt.I see ibid., vol.17, p.893 (1984). Thermodynamic equations are derived, for a model of a one-dimensional metal containing magnetic impurities, using the exact Bethe ansatz solution. Explicit expressions for the magnetisation, susceptibility, and specific heat are given in three low-temperature regions of the H, T plane.
Spectroscopic studies of solids using correlation photoacoustic spectroscopy (CPAS)
Authors: A Mandelis  , J T Dodgson
Published: July 2002

Cross-correlation photoacoustic spectroscopy (CPAS) of condensed matter carries spectroscopic information related to the optical absorption coefficient of solids in both signal magnitude and time delay channels. A one-dimensional theoretical model is presented in which the cross-correlation photoacoustic response to pseudo-random-binary-sequence (PRBS) optical excitation is evaluated for systems of variable optical absorption coefficients. The model is further compared to CPAS data from powders of Ho2O3 in the visible spectral range.
NMR tunnelling effects in solid methyl fluoride
Authors: E C Reynhardt  , J C Pratt  , A Watton
Published: July 2002

Proton and fluoride second moments and relaxation times have been measured as functions of temperature over the temperature range 131>T>7K. The results are dominated by the tunnelling of the CH3groups. The activation energy associated with the splitting between the ground torsional state and the first excited level was found to be 670 J mol-1. An estimate of the temperature dependence of the average tunnelling frequency is obtained from the results. The fluorine spins relax through the tunnelling system of the protons. In the rotating frame the very high tunnelling frequency ( approximately=44 GHz) gives rise to a serious discrepancy between calculated and experimental values since a common spin temperature is not attained.
Anharmonic effects in the thermodynamic properties of solids IV. The heat capacities of NaCl, KCl and KBr between 30 and 500 °C
Authors: A J Leadbetter  , G R Settatree
Published: July 2002

The adiabatic calorimeter described previously has been improved by automation of the adiabatic shield control. The calorimeter has been used to measure the heat capacities of NaCl, KCl and KBr in the temperature range 30 to 500 °C with estimated accuracies of better than ±0·3% forTless, similar 430 °C and ±0·5% at the highest temperatures. The precision of the results, as measured by their average deviation from a smooth curve, is about 0·15%.

Contributions to the heat capacity arising from the thermal creation of vacancies are discussed in relation to the increasing positive curvature of theCp(T) plots above 600-700 °K. Comparison with the results of ionic conductivity experiments shows that in all three cases the contribution from vacancy formation to the apparent excessCpis a very small fraction (similar 1%) of the total, which is primarily a higher-order anharmonic effect.
The electro-reflectance lineshape for a quantum well: the dependence on angle of incidence and temperature
Authors: A P Thorn  , A J Shields  , P C Klipstein  , N Apsley  , T M Kerr
Published: July 2002

The authors have studied the effect of varying both the angle of incidence and the temperature on electro-reflectance spectroscopy (ERS) measurements for a 100AA p-i-n single quantum well structure of GaAs/Ga0.8Al0.2As in which the electric field is applied perpendicularly to the well. The effect of each variation is to change the effective optical path between the semiconductor-air interface and the quantum well. In each case, they observe changes in the lineshape that are consistent with the simple rules reported previously for interpreting ERS measurements on single quantum well structures. They here rederive the rules analytically using the 'etalon' model for the semiconductor structure and demonstrate both experimentally and theoretically the inversion of the ERS lineshape when the optical path changes by a quarter wavelength.
Frequency-domain measurements of reaction kinetics in a LaNi5 hydrogen storage alloy
Authors: E M Gray  , T L Young
Published: July 2002

The traditional method of studying reaction kinetics in metal hydrides, by applying a substantial pressure step and monitoring the amount of gas absorbed versus time, suffers from undesirable temperature changes due to the strongly exothermic/endothermic absorption/desorption reactions. Small-signal measurements in the frequency domain promise considerable practical benefits, including in principle the resolution of rate-limiting and other reaction steps.
Influence of the interaction potential on defect jumps in a Lennard-Jones lattice
Authors: R Vogelsang  , C Hoheisel
Published: July 2002

In a recent series of investigations the dynamic behavior of an argon-like Lennard-Jones (LJ) lattice during defect jumps was studied in detail. The present report is concerned with molecular dynamics results for the jump dynamics, when the LJ potential parameters epsilon and sigma of the interaction of the jumping atom with the host atoms of the lattice are changed. As the effect of the variation of sigma is essentially similar to that of the variation of epsilon , the study was restricted to changes in sigma only. Two main results were found: (i) A slight decrease of sigma alters the jump dynamics appreciably; the jump resembles approximately a 'ballistic flight' and the distinct correlations between jumping atom and nearest neighbours are nearly negligible. (ii) A slight enhancement of sigma leads to slight alterations of the short-time behavior of the jump dynamics; however, correlations in space and time are significantly longer-ranged, as indicated by the distinct correlation functions involving the second-coordination-shell particles.
Image intensifier studies of crystalloluminescence of NaCl
Authors: M A Gibbon  , H Sopp
Published: July 2002

The application of image intensification techniques to the study of the spatial, temporal and spectral characteristics of the light emitted during the crystallisation of sodium chloride is described. The crystallisation is induced by adding concentrated hydrochloric acid to a saturated aqueous solution of sodium chloride. It is found that during an early stage of its growth a homogeneously nucleated crystal emits a burst of some 105 photons (measured in the range 390 to 570 nm). Assuming that this light emission accompanies an amorphous-to-crystalline transition in the growing 'crystal', simple thermodynamic arguments indicate that it occurs when the crystal diameter is of order 10-7 m. Comparisons of the crystalloluminescent spectrum with the photoluminescent spectra of the crystalline and dissolved salt suggest that the light emission occurs from crystalline sodium chloride. Temporal studies revealed some long-lived crystalloluminescent emission (lasting up to several seconds) that are consistent with local high levels of supersaturation. Overall, the results broadly support the conclusions reached in earlier photomultiplier-based work by Garten and Head (1963). Unlike photomultipliers, image intensifier allow the spatial behaviour of crystalloluminescence to be studied and provide a sensitive and accurate way of recording the spectra of these transient and low-level light emissions.
Magnetic field dependence of second harmonic generation, coherence lengths, and second order susceptibilities near the band edge in InSb
Authors: H G Hafele  , R Grisar  , C Irslinger  , H Wachernig  , S D Smith  , R B Dennis  , B S Wherrett
Published: July 2002

Resonant effects in second harmonic generation (shg) in InSb near the energy gap have been studied by radiating with a high power Q-switched CO2laser at 10.6 mu m and tuning the energy gap by applying magnetic fields up to 55 kG in samples of carrier concentrations from 8*1014to 5*1016cm-3at a temperature of 12 K. Absolute, independent measurements of the coherence lengths lc+-were made as a function of magnetic field, using circularly polarized radiation and wedge shaped samples, by a modification of the original 'Maker' experiment. These results, together with measurements of SHG+-and absorption coefficients alpha+-have yielded the magnetic field dependence of the second order nonlinear susceptibilities chi+-at 5.3 mu m. Virtual transition processes have been used to formulate the theory for the resonant behaviour of chi and lcand the effect of blocking by conduction band population is demonstrated experimentally.
Electric field-induced changes in the two-phonon infrared absorption spectrum of diamond
Authors: J F Angress  , A J Maiden
Published: July 2002

Electric field-induced changes in the two-phonon infrared absorption in diamond have been observed in the region 2400 to 2700 cm-1. The induced spectrum is very similar to the second order raman spectrum for a similar experimental configuration. This similarity is attributed to field-induced changes in the selection rules governing two-phonon absorption.
The low temperature thermal expansion of thallous chloride and thallous bromide
Authors: A D Redmond  , B Yates
Published: July 2002

Measurements of the linear coefficients of thermal expansion of TlCl and TlBr are reported between 25 and 260 K. The results are believed to be accurate in an absolute sense to within 2% down to 50K, falling to 6% at 25K. The Gruneisen parameters gamma have been calculated using earlier data on the specific heats and elastic constants, between 260 and 80K. Within this range it is concluded: (i) Any variation of gamma with temperature is within the limits of experimental uncertainty. (ii) Any variation of gamma with ionic mass ratio is within the limits of experimental uncertainty. (iii) The mean room temperature value of gamma for TlCl and TlBr, 2.45+or-0.18, exceeds the corresponding mean value for CsCl, CsBr and CsI, 2.04+or-0.12. Estimates of gamma0based upon the pressure derivatives of the elastic constants at room temperature suggest that for TlBr statement (i) may be extended to T=0, but for TlCl( gamma0- gammainfinity) may exceed unity. Moments of the vibrational frequency spectra of the two crystals have been calculated and varying degrees of consistency with different theoretical models have been observed.
An X ray diffraction investigation of molten copper-germanium alloys by the partial interference function method
Authors: S P Isherwood  , B R Orton
Published: July 2002

Interference functions have been obtained from alloys containing Cu-36 at% Ge, Cu-80 at% Ge and Cu-90 at% Ge at 660, 920 and 933 degrees C, respectively, by x ray diffraction. It has been found that the assumption of concentration independence of the partial interference function between copper and germanium, ICuGe(K), is not valid over wide differences of composition. As the concentration changes ICuGe(K) changes to become similar to the partial interference function of the pure component which has the largest atomic concentration.
Exact solution of the Maier-Saupe model for a nematic liquid crystal on a one-dimensional lattice
Authors: P A Vuillermot  , M V Romerio
Published: July 2002

The planar and spatial lattice versions of the Maier-Saupe model (1959, 1960) for a nematic liquid crystal are exactly solved for a one-dimensional lattice, without periodic boundary conditions. The two-molecule correlation functions are studied, and it is shown that these two models do not exhibit an order-disorder phase transition except at T=0, as is the case for the classical Heisenberg model in one dimension.