bluuebunny/crossref_metadata_2025 huggingface dataset:
The energy distributions of electrons produced in multiphoton ionisation of rare gases
Authors: L -A Lompre  , G Mainfray  , C Manus  , J Kupersztych
Published: July 2002

The energy spectrum of electrons produced in the multiphoton ionisation of rare gases in the 1012-1015W cm-2range displays interesting features. It generally consists of a series of peaks evenly spaced by an amount equal to the photon energy. The number of peaks depends strongly on the laser wavelength. For example, with Xe only one additional peak is observed at short wavelengths, while about ten and even tens of peaks are observed at longer wavelengths such as 1064 nm. These absorption processes can be described in terms of continuum-continuum transitions or, better still, in terms of the absorption of photons by the electron in the field of the ion to which it was originally bound. Furthermore, as soon as the electron-ion pair is formed, the electron acquires a quiver energy Delta in the presence of the EM field. The absorption of additional photons, corresponding to an energy less than Delta , is made energetically impossible. This leads to the suppression of the first peaks of the electron energy distribution. The disappearance of a certain number of peaks depends strongly on the laser wavelength and intensity. The disappearance of nearly 30 peaks has been observed for He at 1064 nm and 1015W cm-2. Finally, additional effects, such as electron angular distributions and space charge effects, can change the relative amplitude of the first electron peaks. These effects must be taken into account before any comparisons can be made between theory and experiment.
Excitation and ionization of atoms and ions in a non-thermal plasma. I. Populations of excited levels
Authors: S Suckewer
Published: July 2002

The author presents formulae for the populations of levels of excited atoms and ions of a plasma in non-thermal equilibrium over a wide range of electron densities. The validity of the method presented is established by comparison of the calculated population of excited levels of hydrogen-like ions with the results of McWhirter and Hearn (1963). For illustration, the population of excited levels of C II ions are calculated. On the basis of this method the partition function for hydrogen-like ions in a non-thermal plasma was obtained.
Low-frequency spectra of L-alanine
Authors: V D Gupta  , M V Krishnan
Published: July 2002

Inelastic incoherent neutron spectra are obtained for L-alanine and the frequency distribution is calculated in the one phonon and cubic approximation. Possible assignments for the peak positions are given and discussed.
Threshold electron impact excitation of the rare gases
Authors: C E Brion  , L A R Olsen
Published: July 2002

Threshold electron impact spectra have been obtained for the rare gases helium, neon, argon, krypton and xenon using a velocity selected electron beam. Zero-energy electrons ejected at excitation thresholds are scavenged by sulphur hexafluoride and the resulting SF-6ions are detected using a masses filter. Optically forbidden transitions are observed to make a large contribution to the total threshold excitation. The shape of the helium scavenging curve above the first ionization potential is discussed with regard to threshold laws for ionization by single electron impact. Excitation of outer s and p electrons in neon, argon, krypton and xenon is observed as well as other structure which is attributed to double electron excitation.
Measured lifetimes of the C3Πustate of N2and the a3Σg+state of H2
Authors: R E Imhof  , F H Read
Published: July 2002

The electron-phonon coincidence technique was applied to the measurement of lifetimes in N2and H2. In this method the inelastically scattered electrons are analysed and used to select the upper state of interest, thus eliminating the systematic errors due to cascading and the spectral overlap of different band systems. The lifetimes of the first three vibrational levels of the C3Piustate of N2were found to be 35.6+or-0.5, 34.9+or-1.8 and 34.5+or-2.3 ns, and those of the first two vibrational levels of the a3Sigmag+state of H2were found to be 11.0+or-0.42 and 10.6+or-0.6 ns.
High resolution L2,3M2,3M2,3 Auger electron spectra of argon
Authors: T Kondow  , T Kawai  , K Kunimori  , T Onishi  , K Tamaru
Published: July 2002

Auger spectra of Ar(L2,3M2,3M2,3) excited by 860 eV electrons were measured by a high resolution and variable angle electron spectrometer. At larger angles, the present spectra agreed with the measurements by Siegbahn et al and Mehlhorn et al, whereas at smaller angles, four extra peaks were discovered. The level width of the L2 was found to be equal to that of the L3 which was 0.10+or-0.02 eV on the basis of the line width measurements of each peak at the highest resolution (40 meV) of the spectrometer. The transition probability of Ar L2,3M2,3M2,3 (1D2) was estimated as (1.5+or-0.3)*1014s-1, which coincided with Rubenstein's calculation.
Scattering mechanisms in low-energy e-H2O collisions
Authors: G Seng  , F Linder
Published: July 2002

Crossed beam experiments have been performed to study elastic scattering and vibrational excitation in e-H2O collisions for the energy range from 1 to 10 eV. The rise of the cross sections at low energies (below 2 eV) is interpreted in terms of a direct scattering mechanism, whereas the broad enhancement of the cross sections around 6 to 9 eV is attributed to H2O-states intersecting the Franck-Condon region in this energy range. Theoretical estimates for the energies of H2O-states are not in agreement with these findings.
Study of plasma-produced H3+ions during the decay period in helium-hydrogen mixtures
Authors: O J Orient
Published: July 2002

The time dependences of the mass-identified H3+ion densities have been measured during the decay period of a plasma produced in helium containing 0.01-0.1% hydrogen. Measurements were carried out at a temperature of 300K and over a pressure range 1.5-100 Torr. The time dependence was found to be exponential during the latter part of the afterglow period. Measurement of the decay time constants yielded the zero-field reduced mobility of mu0=31.6+or-1.0 cm2V-1s-1.
Radiative-lifetime measurements in Si II-Si V
Authors: A E Livingston  , J A Kernahan  , D J G Irwin  , E H Pinnington
Published: July 2002

Radiative lifetimes have been measured for transitions in Si II to Si V in the vacuum ultraviolet region using the beam-foil technique. Lifetime results for Si II-IV transitions resolve existing discrepancies among previous measurements for these ions, and derived oscillator strengths reflect the effects of configuration interaction for several terms in Si III. The oscillator strength for the 1194 AA multiplet in Si II is the first experimental value for this transition that provides consistency with interstellar curves of growth determined recently from observations for the first time, and the first experimental lifetime results for this ion have been obtained.
Application of the model microfield method to Stark profiles of overlapping and isolated neutral lines
Authors: A Brissaud  , C Goldbach  , J Leorat  , A Mazure  , G Nollez
Published: July 2002

Describes a number of analytical and numerical calculations related to the model microfield method applied to Stark broadening (in particular, treatment of strong collisions and angular averages). This method is used for several types of profile: He I overlapping lines, Li I and Mg I resonance lines and Li I and O I non-resonance lines. For each of these, various approximations are discussed and the profiles are compared to those obtained from other theories and to experimental results. In particular, it is shown that the forbidden components cannot be obtained correctly using the usual approximation in which the electronic broadening is calculated independently of the ionic microfield.
Polaritons in LiTaO3
Authors: D Allan  , A P Cracknell
Published: July 2002

A group-theoretical symmetry label assignment is given for phonons in the uniaxial ferroelectric crystal LiTaO3. This is used, with measured phonon frequencies in determining polariton dispersion relations for the crystal, both by direct calculation and by symmetry arguments. In particular, crossing points in the dispersion relations are discussed. The calculated dispersion relations are compared with the available experimental evidence and the agreement is found to be satisfactory, even in the approximation when damping is neglected.
The ground-state energy of a fermion liquid
Authors: G D Buchan  , R C Clark
Published: July 2002

A closed expression for the energy of a fermion system is found using a Jastrow product wavefunction. The energy is expressed in terms of the two-particle and three-particle distribution functions. Using the Kirkwood superposition approximation for the three particle distribution functions and a cluster expansion to derive the fermion two-particle distribution function from that of mass-three bosons the energy of 3He has been evaluated. The results are in reasonable agreement with experiment and with those obtained by the cluster expansion technique. An attempt is made to assess the error introduced by the super-position approximation by making the same approximation for a boson system.
Covalency in cubic lanthanide sesquioxides
Authors: B M Angelov
Published: July 2002

The overlap integrals (2s mod 4f), (2p sigma mod 4f), and (2p pi mod 4f) have been calculated for the entire series of cubic lanthanide sesquioxides (C-Ln2O3) through analytical five-term (2s, 2p: O2-) and four-term (4f:Ln3+) ground-state wavefunctions. The set of distances used for C2and C3ication sites is reported for the first time. It has been found that only pi -type electron delocalisation is feasible within the fragment Ln3+-O2-; the respective overlap integrals vary from 0.12 to 0.03.
Magnetic excitations in diluted ferromagnets
Authors: T Fujiwara
Published: July 2002

The recursion method of Haydock, Heine and Kelly (see ibid., vol.5, p.2845 (1972) and ibid., vol.8, p.259 (1975)) is applied to the study of the excitations in a two-dimensional diluted ferromagnet with an anisotropic exchange interaction. Spectra at various points in the Brillouin zone are presented over the whole range of concentrations of the magnetic ions. The multi-peaked band (near the percolation concentration) and the dangling bond responses are discussed by analysing the wave functions.
Nuclear spin lattice relaxation and activation energies of tunnelling methyl groups
Authors: P A Beckmann  , S Clough
Published: July 2002

The coupling of lattice vibrations with the rotational motion of hindered methyl groups is considered. It is shown that the apparent activation energy for methyl group rotation is determined at relatively high temperatures by the methyl group torsional energy splittings, but at low temperatures it depends on the frequencies of short-wavelength lattice modes which couple relatively strongly with methyl-group rotation. The temperature dependence of the proton spin lattice relaxation time from 6 to 50K is reported for three samples containing methyl groups whose tunnelling frequencies have previously been measured very precisely. They are 4-methyl-2,6-ditertiarybutylphenol (MDBP) and two deuterated derivatives. The activation energy measured at temperatures between 18 and 50K is in good agreement with the splitting between ground and first-excited methyl group torsional states, as computed from the known tunnelling frequencies assuming a three-fold hindering potential. At temperatures below 15K the observed activation energy is only about half this value.
Suppression of nuclear inelastic channels in nuclear resonance and electronic scattering of γ-quanta for different hyperfine transitions in perfect 57Fe single crystals
Authors: U van Burck  , R L Mossbauer  , F Parak  , G V Smirnov  , N A Semioschkina
Published: July 2002

The energy dependence of the intensity of gamma -quanta Laue reflected through nearly perfect 57Fe single crystals was measured for all nuclear hyperfine components in iron. The specific features of the effect of suppression of inelastic channels in the presence of electronic scattering were investigated for the various hyperfine components.
Model nonlocal electrostatics. I
Authors: A A Kornyshev  , A I Rubinshtein  , M A Vorotyntsev
Published: July 2002

An approximate method for calculating the electrostatic potential due to external charges in systems with interfaces, involving components with spatially dispersive permittivities is developed. The method can be applied to systems with an arbitrary type of nonlocal dielectric response of the components and for a broad class of interfaces, separating the 'quasi-uniform' regions. All the solutions are obtained in the form of integrals from functions which are known and functions which can be determined from a Fredholm integral equation of the second kind. For a set of systems, its kernel provides an easy transformation of this equation to a simple differential one, so that the solutions are obtained in an analytical form. As an example, the contact of two media at a plane interface is considered.
Lattice vibrations in a disordered crystal: adamantane in plastic phase
Authors: J C Damien  , J Lefebvre  , M More  , R Foure  , B Hennion  , R Currat
Published: July 2002

Coherent inelastic neutron has been used to measure the high-symmetry dispersion curves in deuterated adamantane (C10D16) at room temperature. The single crystal was grown by slow sublimation. The essential points for a dynamical study of a plastic crystal are briefly summarised and a group-theoretical analysis of the normal modes is given. The existence of librations in adamantane is established. They are found to be severely damped at all wavevectors. A force constant model, allowing for atom-atom interactions between disordered molecules, is then presented in the rigid molecule quasi-harmonic approximation. It accounts quite well for neutron results even for the broad librations and gives interesting information about the interaction mechanism between neighbouring molecules in this orientationally disordered crystal.
Scattering intensity of amorphous silicon in a random stacking model
Authors: Y Akeda  , F Yonezawa
Published: July 2002

The authors extend the formalism of the scattering intensity due to Yonezawa and Birman (1977) which is based on a random stacking model for tetrahedrally bonded amorphous semiconductors. The extension is carried out by relaxing, step by step, several aspects of long-range order inherent in the original random stacking model. By so doing, they can extract microscopic information about the structures which is reflected in macroscopic quantities. By removing some long-range order, they are actually modifying the model so that it better simulates realistic amorphous systems. They apply the modified formalism to the calculation of the diffraction pattern and the radial distribution function for amorphous silicon.
Variational equation of state and entropy of liquid argon
Authors: B N Perry  , M Silbert
Published: July 2002

Two consequences of Gibbs-Bogoliubov variational theory are examined as applied to liquid argon: the choice of a hard-sphere reference system leads to an equation of state if the packing fraction is used as the variational parameter, the non-ideal part of the entropy is only a function of eta . Calculations are performed along the freezing and vapour-pressure lines. Excellent agreement with experimental results is obtained if the Lennard-Jones potential is regarded as an effective pair potential and its parameters are appropriately chosen, and for the hard-sphere reference system the empirical radial distribution function proposed by Verlet and Weis (1972) is used together with the Helmholtz free energy derived from the Carnahan and Starling equation of state (1969).
CPA theory of diluted Heisenberg antiferromagnet with random first- and second-neighbour exchange bonds
Authors: R A Tahir-Kheli  , A R McGurn
Published: July 2002

Extending a recent work (ibid., vol.11, p.497 (1978)) on ferromagnets, a diluted Heisenberg antiferromagnet is studied which possesses both nearest and next-nearest neighbour exchange interactions. As in that work, the coherent potential approximation (CPA) is used and for generality the possibility that exchange bonds between any pair of interacting magnetic atoms may also be random is included. By examining the frequency moment conservation behaviour and the dependence of the Neel temperature on randomness, it is shown that the present CPA is exactly analogous to that given in the previous work for the ferromagnet and, moreover, possesses the same level of accuracy, i.e. to two leading orders in 1/z (where z is of the order of the coordination number). For a variety of cases, numerical results are presented for the coherent exchange integrals, the Neel temperature, the density of states and the magnetic response functions.
Interionic potentials for alkali halides. I. Crystal independent shell parameters and fitted Born-Mayer potentials
Authors: M J Sangster  , R M Atwood  , U Schroder
Published: July 2002

A consistent set of shell parameters for alkali halide crystals of both rocksalt and CsCl structures is reported. The fitting procedure, values of the parameters and their sensitivity are discussed. Short range interionic potentials are determined and are shown to give a good description of both static and dynamic properties of the crystals.
Effects of alloying on the transport properties of amorphous and crystalline Ge films
Authors: P Nath  , V Dutta  , K L Chopra
Published: July 2002

A comparison of the electrical transport properties of amorphous and crystalline Ge films codeposited with Al and Cu is presented. The properties of amorphous Ge-metal films are consistent with the formation of homogeneous alloys and the impurity states introduced on alloying. On the other hand, in crystalline Ge-metal films, formation of alloys and/or heterogeneous Ge-segregated metal/compound phases at high metal concentrations explains the observed electrical properties.
Anisotropic percolation on the Bethe lattice
Authors: L Turban
Published: July 2002

An anisotropic bond percolation problem is solved exactly on the Bethe lattice with z=3 using the 'ghost site' method. The percolation probability P(p) and the mean cluster size S(p) are obtained. The critical components are unaffected by anisotropy except for the expected changes in the one-dimensional limit.
Surface states and photoemission from Mo (001)
Authors: J E Inglesfield
Published: July 2002

With the matching Green function method and a simple surface potential, two sp-d hybridisation-gap surface states were found at Gamma on Mo (001), one very close to the Fermi energy and one at -3.3 eV. Another resonance appeared just under EFon moving away from Gamma . The results are in agreement with directional photoemission experiments. The origin of the Gamma surface states is studied.
ESR on the triplet state of Fz centres in potassium chloride crystals doped with strontium
Authors: H J Paus  , K M Strohm
Published: July 2002

Z2 centres are produced in KCl:Sr by various methods and their optically excited triplet state is investigated by means of ESR measurements. In general the ESR spectra are very complicated and contain a large number of lines. Analysis is only made possible by varying the impurity and F-centre concentrations and by variation of the optical excitation conditions. Five different types of triplet centres are found and interpreted as Z2 centres. Three types have triplet lifetimes of about 20 ms and absorptions in the usual Z2 absorption region >or=1.8 eV. The two others possess triplet lifetimes of 100 ms and 1.6 s, not previously reported. They exhibit additional absorptions at low energies (0.3 cm-1) and none of them show simple axial symmetry. At very low temperatures (1.6K) a remarkable optical electron-spin polarisation can be detected. It is used for the determination of the absolute sign of the axial fine structure parameter of two Z2 centre types.
Specific heat of superconducting and normal Nb20Zr80
Authors: E Gmelin  , K Guckelsberger
Published: July 2002

The specific heat of a quenched Nb20Zr80 crystal has been measured above and below Tc=8.7K. Results and analysis suggest that the material has a well defined transition and is fully superconducting below 8.6K. This is of particular interest in view of the fact that ultrasonic attenuation recently measured by Thomas et al. (1980) on the same sample showed no discontinuity at Tc and decreased only below 3K.
On the sign of alpha in the percolation problem
Authors: M Barma
Published: July 2002

The Harris criterion (1974) is used to argue that the critical exponent alpha is non-positive in the percolation problem if the dimension is greater than one.
Magnetoconcentration and related galvanomagnetic effects in non-intrinsic semiconductors
Authors: S Cristoloveanu  , J H Lee
Published: July 2002

The magnetoconcentration effect, its main subeffects and applications are studied with regard to semiconductor doping for the various parameters: electric and magnetic fields; bulk and surface recombination rates, etc. In comparison with the well known intrinsic case, important consequences are associated with the effect in slightly p-doped semiconductors. It is shown that during the magnetoconcentration effect, doping is responsible for anomalous negative or positive magnetoresistance as well as for the Hall voltage increase, reduction or even inversion. The effect is generally a maximum in p-doped semiconductors; the optimum doping depends on the ratio of the electron and hole mobilities and on the magnetic induction. The experimental data on n- and p-type germanium with resistivity from 10-2to 0.7 Omega are in good agreement with the computed results.
Structural phase transitions in K2CO3(Raman scattering study)
Authors: A Maciel  , J F Ryan  , P J Walker
Published: July 2002

Structural phase transitions in K2CO3have been investigated using light scattering techniques. The Raman spectrum of the high-temperature hexagonal phase consists of a very broad structureless band which suggests that the system is highly anharmonic. Below the hexagonal-monoclinic transition at Tcl=693K the authors observe a soft mode which is assigned to librational motion of the CO3groups. Evidence of a cell doubling transition at Tc2=526K is found in the number of Raman active modes observed at low temperature, although a soft mode is not observed at the transition.
Vibrational fine structure in the optical spectra of NiXF6.6R2O (X=Si, Ti; R=H, D) at low temperatures
Authors: S K Datta  , D De
Published: July 2002

The optical spectra of hexahydrated nickel fluosilicate and fluorotitanate and their deuterated analogues have been studied in the UV-visible and near IR regions at 300 and 80K. The low-temperature spectra reveal a progression of small, nearly equispaced peaks on the higher-energy side of the 'red band' lying around 650 nm. The frequency of the precursor of the progression corresponds to one quantum of an asymmetric normal mode of O-H (or O-D) vibration superposed on the electronic origin of the 3A2g to 3T1g transition and the interval of the progression originates from Ni-O vibration. The values of the asymmetric O-H and O-D vibration frequencies have been estimated and are found to be approximately 100 cm-1 larger in the fluosilicates than in the corresponding fluorotitanates. The results are corroborated by the near IR data.
Computer modelling of simple point defects in rare-earth-doped alkaline-earth fluorides
Authors: R J Kimble Jr  , P J Welcher  , J J Fontanella  , M C Wintersgill  , C G Andeen
Published: July 2002

The results of a package of FORTRAN computer programs for modelling defects in ionic crystals and for fitting experimental data are described. The fundamental concept of the defect simulation is similar to HADES except that the minimisation procedure is different since the package is designed to run on small computers. As an example of the use of this package, the relative stabilities of NN and NNN complexes for various rare earths, Ianthanum and yttrium are considered. First, the data-fitting routine was used to analyse relaxation data for NN and NNN complexes in rare-earth-doped strontium fluoride. The experimental results for strontium fluoride were then used in conjunction with the defect-simulation program to determine potentials for all of the rare earths, yttrium and lanthanum. Those rare-earth potentials were then used in the simulation of calcium fluoride and show that the NNN complex should not be observable except for possibly the smallest rare earths. This implies that the B site of Wright and co-workers or the RII relaxation requires another explanation. Also, the potentials were used in the simulation of barium fluoride, showing that the NN complex should be observable only for the largest rare earths or lanthanum. Next, the enthalpy for NN to NN reorientation via the interstitialcy mechanism was calculated for rare earths in calcium and strontium fluoride. In general, the calculated reorientation enthalpies are larger than the experimental values. However, the variation of the enthalpy with the size of the rare earth is in reasonable agreement with experiment. Finally, the variation of the calculated enthalpy with pressure is found to be in excellent agreement with experiment.
The temperature dependence of inelastic neutron scattering in rotational tunnelling systems. I. Formulation and perturbation theory
Authors: A C Hewson
Published: July 2002

In recent years inelastic neutron scattering experiments have revealed marked variations in the temperature dependence of the rotational tunnelling spectra of molecular groups in various local environments. A new theoretical approach is developed here to explain these results. A microscopic model of a rigid molecular group, whose rotational degree of freedom is coupled to lattice modes, is formulated in terms of standard basis operators for states of the uncoupled system. It is shown that a complete description of the neutron spectrum can be found in terms of the time-dependent correlation functions of these operators. As a preliminary to a fuller treatment of this problem a perturbational approach is developed, which allows the conditions for a treatment-dependent shift to lower tunnelling frequencies, which is commonly observed experimentally, to be thoroughly examined.
Calculation of spin-echo response for interacting pairs of spin-3/2quasi-particles
Authors: M A B Whitaker  , R D Cosby
Published: July 2002

The spin-echo response of interacting pairs of spin-3/2quasi-particles is calculated for 90 degrees - tau - beta90 degreessequences. The calculation is aided by the introduction of spin manifolds of different spin character; each manifold may be considered separately. No truncation of the dipolar interaction is performed; the calculation is in the representation in which the entire Hamiltonian is diagonal. The authors' results may be compared with the numerical calculations of Baianu et al. (1981) and their experimental results for spin-1/2triads. It is found that there is reasonable but not perfect agreement with experiment, and it is suggested that the interacting quasi-particle model may not be absolutely valid for this system.
The measurement of inner potentials for copper, silver and gold
Authors: A Goswami  , N D Lisgarten
Published: July 2002

Measurements of the inner potentials for copper, silver and gold using the Yamaguti method have been made, yielding 13.4, 17.8 and 21.4 volts respectively. These values are compared with previous experimental results and theoretical calculations.
Antiferromagnetism in the Hubbard model in the Roth scheme
Authors: K Eapen
Published: July 2002

The two-sublattice Hubbard model has been solved in the Roth scheme of decoupling. It is found that antiferromagnetic solutions are in present in the strong-correlation limit. The stability of antiferromagnetic solutions is investigated.
A general picture of threshold effects in LEED
Authors: J C Le Bosse  , J Lopez  , C Gaubert  , Y Gauthier  , R Baudoing
Published: July 2002

The authors reinvestigate the interpretation of fine structures associated with beam emergence threshold in LEED. This work is based upon a new expansion of the total amplitude matrix of the crystal in a power series of the off-diagonal part of the bulk amplitude matrix. They clearly show that the pure resonance picture is incorrect, even in the favourable case where the fine structures fall inside a forbidden gap. Their alternative picture is that the dominant mechanism is an interference between the waves directly reflected at the barrier or at the bulk, and the waves associated with processes including on metal-metal reflection at the barrier. This interpretation, which proved to be adequate in their previous study of Al(001) and Ni(001), holds equally well for the fine structure measured by Adnot and Carette (1977) on W(001) in the presence of a band-gap. As a consequence the authors discuss why McRae's method (1967) to extract structural information from a 2D surface band structure cannot be used.
The spin packet lineshape in dilute ruby samples
Authors: R Boscaino  , F M Gelardi
Published: July 2002

The spin packet lineshape of the inhomogeneous resonance lines of dilute ruby samples is experimentally investigated by the saturation transient method. By comparing experimental and theoretical results of the saturation behaviour of these samples both during the transient and the steady state, the authors show that the spin packet lines are well described by a gaussian shape. The relationship between their results and conflicting predictions issuing from different theories is also examined.
Lattice dynamics, lattice instabilities and phase transitions in fluoride perovskites
Authors: N Lehner  , H Rauh  , K Strobel  , R Geick  , G Heger  , J Bouillot  , B Renker  , M Rousseau  , W G Stirling
Published: July 2002

The authors have measured the phonon dispersion curves of the fluoride perovskite materials KMnF3and KZnF3by means of coherent neutron inelastic scattering. The aim of this investigation was to provide the phonon spectrum of KMnF3which undergoes structural phase transitions and which has been studied by various methods in the last decade and, for comparison, of KZnF3in which no phase transitions occur. In addition they have found an anomalous temperature dependence of the R15' and M2modes in KZnF3which are the modes that soften near the phase transitions of KMnF3. This indicates that there is in general a tendency towards an instability against octahedra rotations in all fluoride perovskites. For the analysis of the phonon spectra, they have employed a lattice dynamical shell model under various conditions. In some of the models, they have successfully used interionic potentials which were derived from experimental data in other materials for some of the short-range interactions. Thus it could be shown that these potentials are also valid in the fluoride perovskites with different interionic distances. Furthermore, the experimental data are compared with the results of infrared spectroscopy and Raman studies at low temperatures as a test of the assignment of optically determined phonon frequencies to normal modes which are at the zone centre Gamma at low temperatures and at zone-boundary points in the cubic perovskite structure.
A unified equivalent-circuit approach to the theory of AC and DC hopping conductivity in disordered systems
Authors: S Summerfield  , P N Butcher
Published: July 2002

The Miller-Abrahams equivalent circuit is used to motivate an extension of the pair approximation to the AC hopping conductivity sigma ( omega ) of a random system, which embraces the DC limit sigma (0). The theory is simple to develop and readily yields both analytical and numerical results. The formulae obtained reproduce the pair approximation in the low-density limit and give known exact asymptotic results at high densities and high frequencies. The numerical predictions of the theory are compared with data obtained by direct numerical solution of Kirchhoff's equations for random networks. Excellent agreement is obtained for sigma (0) for energy-independent and energy-dependent hopping in 2D and 3D, and for sigma ( omega ) for energy-independent hopping in 3D.
Elastic behaviour of the chalcopyrite CdGeAs2
Authors: T Hailing  , G A Saunders  , W A Lambson  , R S Feigelson
Published: July 2002

The components (at 290K C11=9.45, C12=5.96, C13=5.97, C33=8.34, C44=4.21, C66=4.08, in units of 1010N m-2) of the elastic stiffness tensor of the chalcopyrite structure crystal CdGeAs2have been obtained between 4.2K and 290K from measurements of ultrasonic wave velocities by the pulse echo overlap technique. These elastic moduli, used to calculate the bulk and Young's moduli, Poisson's ratio and the Debye temperature ( thetaDel=257+or-2K), define the elastic behaviour of an AIIBIVC2Vchalcopyrite. Measurements of the hydrostatic pressure dependence of the ultrasonic wave velocities up to about 108Pa have been used to obtain the pressure derivatives of the elastic moduli at room temperature. Negative values of delta C44/ delta P and delta (1/2(C11-C12))/ delta P show that the long-wavelength transverse q(110), e(110); q(100), e(001) and q(001), e(100) acoustic phonon modes soften under high pressure. Expressions for the acoustic mode Gruneisen parameters in the long-wavelength limit in the anisotropic continuum model have been developed for 4/mmm Laue group tetragonal crystals; the calculated Gruneisen parameters for CdGeAs2establish quantitatively an anomalous decrease of the frequencies of certain shear modes. The high pressure transition at 60 GPa may be associated with weakening of the interatomic binding forces against shear distortion.
Conductivity of a one-dimensional system of interacting fermions in a random potential
Authors: W Apel
Published: July 2002

Impurity scattering is introduced in the Tomonaga model of an interacting one-dimensional Fermi system. The dynamic (frequency-dependent) conductivity is averaged over the random potential resulting from the impurities. To study this function the author exploits connections to two different two-dimensional classical models for which a Kosterlitz-Thouless-like transition is obtained. Thus, the system's conductivity changes from the usual insulating form to the ideal conducting behaviour of free fermions. Whether the system insulates or conducts depends on the sign and the strength of the fermionic interaction in qualitative agreement with the behaviour of the density response function and the pairing susceptibility of the pure system.
A theory of incommensurate phase transitions in anisotropically stressed BaMnF4
Authors: J Lorenc
Published: July 2002

A simplified theory of incommensurate phase transitions in anisotropically stressed BaMnF4is proposed. The equilibrium properties and phase transitions induced by temperature and/or anisotropic stress are analysed in terms of the Landau-Ginzburg theory. A conjecture concerning the phase diagram is presented. The occurrence of critical end points or n=2 component 'renormalised' tricritical points is demonstrated within the renormalisation group approach.
Optical absorption in hydrogenated microcrystalline silicon
Authors: Z Iqbal  , F -A Sarott  , S Veprek
Published: July 2002

Optical absorption measurements between 0.35 and 2.5 eV at 22 degrees C on hydrogenated microcrystalline silicon films prepared in a DC discharge at deposition temperatures (Tdep) between 110 and 450 degrees C, are presented and discussed. Anomalously high optical absorption was observed for films prepared at a floating potential, which is a few orders of magnitude higher than of single-crystal Si for samples prepared at Tdepbetween 260 and 350 degrees C and, in the photon energy range up to 2.0 eV, is also higher than that of amorphous Si. Samples prepared at a floating potential at Tdepor approximately=400 degrees C, and those prepared at a negative substrate bias, show a lower optical absorption, which for the latter films is similar to that of amorphous silicon. Annealing up to 800 degrees C under ultra-high vacuum resulted in a small but reproducible increase in absorption independent of Tdep. Similar annealing experiments followed by absorption measurements in high vacuum indicated no measurable effect of the adsorbed oxygen on the optical absorption. The angular dependence of the forward light scattering from the films was measured at various wavelengths and found to diffuse scattering and absorption in the films. Furthermore, quantitative estimates of the surface roughness of the films from reflection data and scanning electron micrographs were made, and these results are discussed with respect to the enhanced absorption and scattering in the films.
Theory of the intraband magneto-optics of cadmium arsenide
Authors: M Singh  , P R Wallace
Published: July 2002

Using the band-structure model proposed by Bodnar (1977), the authors have calculated the dielectric function of Cd3As2in the presence of a magnetic field. This calculation, which is carried out in full detail only for waves propagating along the direction of the magnetic field, which may, however, be in an arbitrary crystallographic direction, permits the analysis of cyclotron resonance as well as measurements of magnetoreflectivity or absorption. Numerical calculations are presented for propagation along or perpendicular to the c axis.
An interpretation for FEM patterns of a hydrogen-adsorbed tungsten tip and its application to hydrogen adsorption on single-crystal planes
Authors: M Sato
Published: July 2002

An attempt is made to interpret the FEM patterns of the hydrogen-tungsten adsorption system at 300K. The proposed model predicts that the FEM patterns depend on the contours of the adsite-surface distance drawn on a unit stereographic triangle of BCC crystals. The main outlines of the observed FEM patterns are found systematically to be able to be analysed as circles or circular arcs centered at some crystallographic planes on the tip, and they are found to correspond to parts of the predicted contours. An application to the adsorption on single-crystal planes is also discussed.
Ham factors and energy levels in multimode Jahn-Teller systems
Authors: M C M O'Brien
Published: July 2002

A method of calculating the energy levels and Ham factors of a Jahn-Teller system interacting with many modes of vibration is described. The multimode effect in the ground states is dealt with by first doing a transformation to concentrate most of the Jahn-Teller interaction into an effective cluster Hamiltonian, and then allowing for the remainder by introducing one extra phonon mode at a time. Calculations are done using a numerical perturbation theory: energies are calculated by the Lanczos method of diagonalising large sparse matrices, and the small changes in energy resulting from the introduction of small interactions are used to deduce perturbation parameters. This method is applied to E*( Sigmaiepsiloni), with the inclusion of a quadratic Jahn-Teller coupling or warping term. Experimental results on Ni3+:Al2O3are discussed as an example.
Nucleation processes in the charge density wave state of 2H-TaSe2
Authors: P Prelovsek  , T M Rice
Published: July 2002

The authors examine the free energy barrier associated with critical nuclei in the homogeneous nucleation of discommensurations at the commensurate-incommensurate transition in the charge density wave state of 2H-TaSe2. The various terms entering the free energy of critical nuclei are estimated within a Landau theory. The parameters in the Landau theory are deduced, as well as possible, from experiment. The energy barrier that results is reasonable, also somewhat too high to account for the homogeneous nucleation rate observed by electron microscopy. The reduction of the energy barrier due to internal strains associated with the orthorhombic domain structure is estimated.
A new theory of acoustic solitary-wave polaron motion
Authors: E G Wilson
Published: July 2002

The formation and break-up of the acoustic solitary-wave polaron from a free electron and acoustic phonons in one dimension is described. In an electric field the polaron moves as a massive rigid classical particle. Energy dissipation by acoustic phonons is weak, the mobility in the continuum limit is infinite, and the polaron drift velocity is the velocity of sound. The theory is applied to polydiacetylene and explains the experimental ultra-high mobility and saturated drift velocity found in that material by Donovan and Wilson (1981).
'Pseudo-acoustic' magnon dispersion in yttrium iron garnet
Authors: J S Plant
Published: July 2002

Neutron spectroscopy measurements have confirmed an earlier intimation of an 'anomalous' anisotropy in the dispersion of 'acoustic magnon like' excitations in yttrium iron garnet Y3Fe5O12. The present experiments have shown this anisotropy to be large and intricate, with energies at a fixed excitation wavelength 25 Å, varying from 5 to 20 meV. A contention that such anisotropy might arise from strong magnon-phonon coupling effects is not supported by experimental investigation of (i) a spin-reorientation transition in Tb0.3Y2.7Fe5O12or (ii) applying variously directed magnetic fields to YIG itself. The experimental dispersion curves are replicated instead by linear spin-wave theory, with fitted values for all exchange interactions J1ad, J2ad, J3ad, J1aa, Δ1aa, J2aa, J3aa, J1dd, J2dd, J3dd, J4dd, J5ddcloser than a/2∼6.2 Å. In this new model, the experimentally observed 'acoustic magnon' anisotropy is attributed to a complex coexistence of acoustic and low-energy optic magnon branches: ensuing computations suggest that possible 'soft modes' of such 'pseudo-acoustic' branches may relate to previously reported 'cantings' of the ferrimagnetic structure in various substituted garnets. The new model also helps in understanding some reported anomalies in existing magnetisation, specific heat and microwave measurements.
Localisation of states in two-dimensional disordered systems: travelling cluster approximations and scaling theories
Authors: S M Chitanvis  , P L Leath
Published: July 2002

The hierarchy of the travelling cluster approximations (TCA) is used to study the spectrum of two-dimensional disordered electronic systems. Working within the single-band model of a two-dimensional disordered binary alloy on a square lattice with nearest-neighbour hopping, the authors analyse ( mod G00(E0+i eta ) mod2) as eta to 0+within any finite-diameter TCA. They study the behaviour of ( mod G00(E0+i eta ) mod2) as the TCA cluster size goes to infinity for small eta and conclude that for the exact case when all diagrams are summed, there is an absence of extended states in two-dimensional disordered systems, independent of the degree of disorder. In addition they discuss the plausibility that all states can be rigorously shown to be localised by this method. Finally, they present a one-parameter scaling theory for ( mod G00(E0+i eta ) mod2) in analogy with the conductance scaling theory of Abrahams et al. (1979) which predicts pure point localised states in one and two dimensions and a mobility edge in three dimensions.
Anomalous high reflectivity observed in nuclear resonance diffraction in Bragg geometry
Authors: H J Maurus  , U van Burck  , G V Smirnov  , R L Mossbauer
Published: July 2002

Absolute reflectivities were determined for Mossbauer radiation in pure nuclear Bragg reflections of a nearly perfect57FeBO3single crystal. The measured anomalous high reflectivities reached values of 25-35%. The results agree with the dynamical theory for resonance diffraction, where anomalous high reflectivities are explained as a consequence of the suppression effect.
Thermodynamics of the Hubbard chain: temperature extension of ground-state renormalisation for fermions
Authors: C Vanderzande  , A L Stella
Published: July 2002

A recently proposed ground-state renormalisation-group approach to the Hubbard model is generalised to finite temperatures. This new method provides a global and consistent description of the thermodynamics of the model at all temperatures. Application to the Hubbard chain with a half-filled band gives values for various thermodynamic functions which compare remarkably well with existing exact or numerical data, over all temperature ranges. The difficulties posed by Fermi-Dirac statistics in the set-up of the transformation are discussed to some extent and it is shown that these do not prevent one, at least in principle, from extending the approach to higher dimensions.
The ultramicrostructure of commensurate graphite intercalation compounds
Authors: G Timp  , M S Dresselhaus
Published: July 2002

The ultramicrostructure of commensurate graphite intercalation compounds has been investigated using high-resolution transmission electron microscopy. By directly examining the intercalate lattice at atomic resolution, the authors have discovered intercalate domains of nearly macroscopic dimensions (>1000 AA) extending along the graphite basal plane of commensurate superlattice structures. Along the c axis, evidence for long-range staging order is also found (>200 AA). They have found no evidence of stacking faults characteristic of the Daumas-Herold staging domain model for intercalation compounds nor have they found evidence for fractional staging.
Anomalous X-ray scattering factors calculated from experimental absorption spectra
Authors: P Dreier  , P Rabe  , W Malzfeldt  , W Niemann
Published: July 2002

Anomalous X-ray scattering factors at photon energies close to absorption edges are calculated by applying the dispersion relation to absorption spectra. Absolute absorption coefficients are obtained from measured relative spectral distributions normalised to known absolute cross sections. The calculated scattering factors are corrected for relativistic contributions. Results for Ni, Cu, Zn, and Zr close to the K-edges and for Ta, W, Pt and Au near the respective L-edges are presented and compared with published scattering factors.
A local-density functional description of the valence transition in samarium sulphide
Authors: P Strange
Published: July 2002

A self-consistent relativistic band-structure calculation on samarium sulphide using the local-density approximation is discussed. The LMTO method was used and the bands were calculated for several values of the lattice constant. The results of these calculations show that a local-density functional approach can describe the f-d valence transition. The importance of using a fully relativistic treatment is emphasised. In both the high- and low-volume phases SmS shows a band-gap at the Fermi energy. The structural and bonding properties as they change through the transition are discussed in detail. Comparison with experimental measurements of the absorptive part of the dielectric function is made.
Hot-electron magnetophonon spectroscopy on micron- and sub-micron-size n+nn+GaAs structures
Authors: L Eaves  , P S S Guimaraes  , J C Portal
Published: July 2002

Magnetophonon resonance is used to study electronic conduction processes in a series of short n+nn+GaAs sandwich structures with active (n) layer thickness between 0.25 and 9 mu m for E up to 20 kV cm-1and B up to 18 T. The high electric fields induce a new contribution to sigmaxx, quasi-elastic inter-Landau-level scattering. The onset condition for this process can be derived from the properties of the electron eigenfunctions and eigenvalues in crossed E and B fields. A possible relation between this process and the destruction of the non-dissipative current in the quantum Hall effect is discussed. Electric-field-induced damping of the amplitudes of the magnetophonon extrema is reported and discussed in terms of the intracollisional field effect and the theory of Barker.