bluuebunny/crossref_metadata_2025 huggingface dataset:
Commutator techniques in perturbation theory
Authors: J Killingbeck
Published: July 2002

A simple commutator technique is used to find the first order perturbation correction to the expectation values of certain co-ordinate operators. The method is applied to three calculations involving two-electron ions, and the results are compared with those obtained by variational methods in particular the correlation splitting approach. Some useful hydrogenic integrals are tabulated.
Experimental X-ray mass attenuation coefficients for materials of low atomic number in the energy range 4 to 25 keV
Authors: R H Millar  , J R Greening
Published: July 2002

Experimentally determined mass attenuation coefficients for C2H4, CO2, N2, O2, CF4, Ne, H2S, HCl, Ar, air, Mg, Al, SiO2, (C2H5)3PO4at a series of energies from 4.508 to 25.192 keV are presented in tabular form, together with estimates of total random and systematic uncertainty. Full details of the experimental method are given. This data set enables deduction of a coherent data set for the elements from Z=6 to Z=18 with better accuracy, internal consistency and reliability than any previously published data in this energy and atomic number range.
The L emission spectrum of europium 63
Authors: B D Shrivastava  , R K Jain  , V S Dubey
Published: July 2002

Using a 40 cm bent-crystal spectrograph of the transmission type, a detailed study of the L emission spectrum of europium 63 has been made. Four new diagram lines at 1609.4 XU, 1628.9 XU, 2015.8 XU and 2550.1 XU, corresponding, in that order, to the forbidden transitions L1N1and LiIOII,IIIand the quadrupole transitions LIIMIII( beta17)and LIIIMIII(s) have been observed for the first time. The presence of the quadrupole line LINIV,V( gamma11) and the low frequency satellite L lambda10has been confirmed.
Resonant vibrational excitation of the NO ground state by electron impact in the 0.1-3 eV energy range
Authors: M Tronc  , A Huetz  , M Landau  , F Pichou  , J Reinhardt
Published: July 2002

The cross sections for differential elastic scattering and vibrational excitation of the NO ground state have been studied in an electron impact experiment in the 0.1-3 eV energy range. Strong resonant processes are observed. They are associated with the3Sigma-,1Delta and1Sigma+states of NO-. The vibrational levels of the3Sigma-state can be observed up to about 1.2 eV. Sudden increases are observed in the peak spacings and widths, and also rapid changes of the angular distribution of the ejected electrons, and these can be explained by the formation of the NO-1Delta state ( omegacapproximately=185 meV). In order to explain all the observed structure, the third low-lying1Sigma+state of NO-must be taken into account from 1.120 eV upwards. Interference effects between these three resonant states strongly affect the structure heights from one exit channel to the next.
Radiative transitions induced in heavy-ion collisions
Authors: M Gros  , B Muller  , W Greiner
Published: July 2002

Amplitudes for radiative transitions in fast-rotating molecules and heavy-ion collisions are derived. The effect of the Coriolis force on the electron motion and on the radiative transition amplitude is formulated. The term in the transition amplitude dependent on the angular velocity reflects the influence of the rotation of the states.
Vibrational excitation of polar molecules by electron impact. I. Threshold resonance in HF and HCl
Authors: K Rohr  , F Linder
Published: July 2002

Differential scattering experiments with the crossed-beam method have been performed to study vibrational excitation of HF and HCl by electron impact at very low collision energies. The excitation functions for nu =1 and nu =2 have been measured from threshold to some eV above. In each case, an unusually strong and sharp threshold peak is observed with a broad resonance region following thereafter. The absolute cross sections reach values of the order of 10-15cm2for the threshold peaks in the nu =1 channel. The angular distributions are isotropic in the whole resonance region. The threshold resonances are interpreted as quasi-bound2Sigma+states in the dipole field possibly correlated with H*+X-. The broad resonance region is attributed to a very short-lived resonance state which may be the HX-2Sigma+state correlated with ground-state fragments H+X-.
Vibrational excitation of polar molecules by electron impact. II. Direct and resonant excitation in H2O
Authors: G Seng  , F Linder
Published: July 2002

For pt.I see ibid., vol.9, no.14, p.2521 (1976). Differential scattering experiments with the crossed-beam method have been performed to study vibrational excitation of H2O by electron impact for collision energies from threshold to 10 eV. Differential and integral cross sections for the excitation of the (100, 001) and 010 modes are given in absolute units. Two distinct resonance regions are observed which are superimposed on a background of direct excitation: strong and sharp threshold resonances and a broad resonance region centred around 6-8 eV. The broad enhancement of the cross sections around 6 eV is attributed to a very short-lived2A1state of H2O-. The threshold resonances are interpreted as quasi-bound states in the dipole field of H2O. They are expected to be of general importance in electron-polar-molecule interactions.
The structure factor for liquid metals I. The application of neutron diffraction techniques
Authors: D M North  , J E Enderby  , P A Egelstaff
Published: July 2002

The determination of the structure factor for liquid metals at temperatures up to 1300 °C by means of neutron diffraction techniques is described. Particular attention is given to the corrections which must be applied if reliable structure factors are to be extracted from the measurements, including those for absorption, multiple scattering and inelastic effects. It is concluded that for the heavier elements the neutron method can yield reliable data which are accurate to a few per cent over most of the range of momentum transfer.
Some comments on the theory of space-group representations. III. The completeness of the identification of stars in the representation domain for each space group
Authors: A P Cracknell  , B L Davies
Published: July 2002

For pt.II see ibid., vol.3, p.619, 1970. Published sets of tables of the irreducible representations of the space groups are restricted to k vectors within Omega , the basic domain, whereas for a complete identification of all the irreducible representations it is necessary to consider phi , the representation domain, rather than just Omega . The representation domain, phi , is identified for every space group. A general procedure is described that enables one to identify all those additional special k vectors in ( phi - Omega ) that must be included in order to ensure that one, and only one, k vector from each star is identified.
Theory of the symmetric Wolff model. I. Weak-coupling-long-time-zero-temperature limit
Authors: H C Fogedby
Published: July 2002

The local single-particle and density correlation functions for the symmetric Wolff model are derived analytically. The scattering channels are separated by means of the Stratonovich-Hubbard transformation and the associated one-body problem is solved explicitly in the long-time limit. A stationary phase argument yields expressions for the correlation functions in the weak-coupling limit. The single-particle correlation function acquires a time-dependent phase shift due to the readjustment of the spin-up and spin-down Fermi systems. The spin-up and spin-down Fermi systems relax on a timescale determined by the inverse bandwidth. In the intermediate- and strong-coupling regimes many-body effects intervene and the calculation breaks down.
Heat of transport in solids. II. Harmonic theory for a vacancy in the simple cubic lattice
Authors: M J Gillan
Published: July 2002

For pt.I see ibid., vol.10, p.1641 (1977). A recent theory for the heat of transport in solids is applied to the case of vacancy diffusion in the simple cubic lattice. The harmonic theory of diffusion is used. The theory proceeds via the calculation of the energy moment and current in the limit of large times starting from a specified initial condition. An additional insight into the dynamical processes involved is given by computing the moment and current for general times. Accurate numerical values (within the harmonic approximation) for these quantities and for the model heat of transport Q are obtained. One contribution to Q is the three-dimensional analogue of Schottky's linear chain result. The contribution is an important one. This result contradicts Huntington's argument, according to which the Schottky contribution is insignificant in three dimensions. Huntington's argument (1968) may be fallacious.
Intermediate valence-a view of the theoretical situation
Authors: J H Jefferson  , K W H Stevens
Published: July 2002

The phenomenon of intermediate valence (IV) has recently received a good deal of experimental and theoretical attention, but in spite of all the effort the phenomenon is not well understood, particularly the low temperature observations. The theoretical situation is reviewed, and those areas where the understanding seems to be reasonably satisfactory and also those where it is less satisfactory are delineated. The topic is introduced through a simple description of IV. This is followed by a brief account of some of the experimental results from which it is seen that while there are some phenomena which can be understood in an elementary way, there are others which are quite puzzling. A more detailed study is then made of the type of Hamiltonian which it would seem appropriate to use, and this is followed by an account of the approximations which have been used reasonably successfully (thermodynamic-type treatments and high-temperature thermodynamic perturbation theory). Attention is focused on the problem of the ground- and low-lying energy levels.
Nuclear spin relaxation by translational diffusion in solids. IV. Two-spin systems in FCC crystals and third-nearest-neighbour jumps on a SC lattice
Authors: W A Barton  , C A Sholl
Published: July 2002

For pt.III see ibid., vol.9, p.4315 (1976). The two-spin systems describe the dipole-dipole interactions between stationary host spins and spins migrating amongst either the tetrahedral or the octahedral interstitial sites. The tetrahedral sites in a FCC crystal form a SC lattice and two models, the symmetric and asymmetric jump models, are considered for third-nearest-neighbour jumps on this lattice. Numerical results for the correlation functions relevant for single crystals and polycrystals are presented over the entire temperature range. It is found that the simpler, but unphysical, symmetric jump model is a good approximation to the more complicated asymmetric jump model. Discrepancies between the numerical results of Barton and Sholl and those of Wolf (1975, 1977) are discussed.
A quantum field theoretic formulation of Raman spectral line shapes
Authors: B J McKenzie  , G E Stedman
Published: July 2002

The Raman spectrum of a solid is formulated using quantum field theory. The photons are assumed to couple via localised electronic states, e.g. paramagnetic impurities. The laser is assumed to be of low power and to have a frequency well away from any electronic resonance. It is shown that the phonon-induced shift and width of each of the lines corresponding to an electronic transition, a first-order phonon line, and a vibronic transition is formally the same whether the transition is studied by absorption or by Raman spectroscopy. Width and shift contributions are given to second order in broadening interactions for the second-order phonon Raman spectrum. These results exhibit complications in some attempts to compare broadening parameters in different experiments. Some consequential adjustments to formulations of the spectrum of resonance fluorescence are suggested.
Thermoluminescence and colour centre thermal stability in KCl:Ca and KCl:Sr irradiated at room temperature
Authors: A Rascon  , J L A Rivas
Published: July 2002

The thermoluminescence and the thermal stability of the F and V-centres induced by irradiation in KCl:Ca and KCl:Sr samples have been studied. It has been found that the thermal annealing of both types of colour centres and the thermoluminescence are different aspects of the same basic phenomenon. Namely, as previously proposed, the thermoluminescence is due to the recombination with F-centres of mobile interstitials thermally released from V-centres; at some stage in this process light is emitted. No evidence of thermally stimulated currents has been found. The two types of impurity induce a different pattern of first-order glow peaks. This indicates that each impurity induces several interstitial traps. Their activation energies and pre-exponential factors are given.
A neutron inelastic scattering study of LiNbO3
Authors: M R Chowdhury  , G E Peckham  , D H Saunderson
Published: July 2002

Room temperature measurements of the phonon dispersion relations in ferroelectric lithium niobate have been carried out by inelastic neutron scattering. Most of the measurements were made along the c* direction with two acoustic branches measured along an a* direction. The q to 0 slopes of the acoustic branches are found to be in good agreement with the ultrasonic sound velocity measurements. Results on the phonon frequencies are compared with the optical data. Measurements carried out at elevated temperatures up to 900K failed to detect any softening of the 'ferroelectric mode' (the lowest-frequency optic Gamma1mode), thereby suggesting that the ferroelectric phase transition in lithium niobate is not associated with a soft phonon-mode instability. The possibility that the phase transition is of the order-disorder type, rather than the displacive type, is discussed, and, in this respect, similarities with the isomorphous lithium tantalate are pointed out.
Diffusion thermoelectric power of bismuth in a magnetic field
Authors: O P Hansen  , E Cheruvier  , J -P Michenaud  , J -P Issi
Published: July 2002

The diffusion thermoelectric power of bismuth in a magnetic field is calculated assuming a two-band model with parabolic dispersion for the holes and either parabolic or non-parabolic dispersion for the electrons. The scattering is described by a relaxation time which is assumed to be either dependent on energy as for intravalley acoustic scattering or independent of energy. Below 80K the density of charge carriers is only weakly temperature dependent and the Bethe-Sommerfeld expansion is used to derive expressions for the weak-field thermomagnetic coefficients. Above 80K a re-evaluation of earlier mobility data is performed to take into account the thermal smearing of the Fermi surfaces, and numerical integrations of the transport integrals are used. The results give a good agreement with experimental data, but this agreement is not compatible with the existence of an energy gap in the electron spectrum, such as is found in magneto-reflection experiments.
Dielectric relaxation in methylammonium gallium sulphate dodecahydrate (MGaSD)
Authors: R Jakubas  , H A Kolodziej  , L Sobczyk
Published: July 2002

Values of the complex electric permittivity and relaxation times are measured for MGaSD, and values for the Cole-Cole beta factor, tau 0, and Delta F* calculated and compared with those of MASD and MGaSD. The values of Delta F* are found to be almost the same for the three alums in question.
Phonons in perovskite-type layer structures
Authors: R Geick  , K Strobel
Published: July 2002

The authors present a rigid-ion model calculation for the high-temperature phase of (CH3NH3)2MnCl4(MAMC) with the K2MnF4structure (D4h17). The short-range forces are assumed to be axially symmetric. The 11 disposable parameters of the model are fitted to experimental data, i.e. the frequencies of IR and Raman-active phonons, and to neutron scattering data. The high-temperature phase of MAMC is characterised by a disorder of the orientation of the organic ions. For their model, the authors assume each ion to be located at the symmetrised average position. Further, the organic ions are treated as rigid bodies since the authors are not interested in their internal modes. The results of the above model calculation confirm in MAMC the lattice dynamics are more or less three-dimensional and that the interlayer forces are of the same magnitude as the intralayer forces. The authors conclude that the three-dimensional lattice dynamical behaviour of these layered materials is due to the importance of long-range Coulomb forces in these ionic compounds.
Mossbauer investigation of spin-lattice relaxation of paramagnetic Fe3+ ions in NH4Al(SO4)2.12H2O
Authors: S C Bhargava  , J E Knudsen  , S Morup
Published: July 2002

Mossbauer spectra of Fe3+ in NH4Al(SO4)2.12H2O have been obtained in the temperature range of 79-295K with applied magnetic fields of 5.7-12.3 kG. The spectra exhibit electronic spin relaxation with a relaxation time that depends strongly on the temperature and to some extent on the applied field. The spectra have been successfully fitted using the stochastic relaxation model. The relaxation is explained in terms of two spin-lattice relaxation processes: first-order Raman processes involving an internal vibrational mode of the Fe(H2O)6
3+ complex and a contribution which has been attributed to spin-lattice relaxation processes induced by critical crystal-field fluctuations, associated with the phase transition in the alum.
Impurity induced Raman scattering spectra in zincblende-type crystals: application to mixed indium pnictides
Authors: D N Talwar  , M Vandevyver  , M Zigone
Published: July 2002

A theoretical study of impurity-induced first-order Raman scattering has been reported for zincblende-type crystals using Green function techniques. The perturbation model parameters obtained from the analysis of localised and gap modes have been incorporated to compute the Raman scattering structure for mixed indium pnictides in the low-impurity concentration limit. The calculated results correspond very well with the existing experimental data of Carles et al. (1978).
Thermomagnetic study of the antiferromagnetic solid solutions pMnO-qCoO (p+q=1)
Authors: J Kleinclauss  , R Mainard  , H Fousse  , N Ciret  , D Bour
Published: July 2002

Precise measurements have been made of the specific heats of the antiferromagnetic solid solutions pMnO-qCoO and of the pure oxides MnO and CoO in the temperature range 20 to 700K. From these measurements, the Neel temperatures have been deduced and, through the application of the BPW effective field model, the exchange integrals have been estimated: in spite of the change which occurs in the lattice parameter, these integrals appear to remain effectively constant over the range of compositions. A hybrid method based on a combination of the corresponding states method with the effective phonon spectrum has been found to give consistent results when the lattice contribution is separated for MnO and CoO. There is good agreement between the experimental and theoretical entropy for most of the range of compositions. However, there appears to be some anomalous behaviour for the magnetic energy where there is small substitution of one oxide in the other.
Origin of incommensurate structure in the cooperative Jahn-Teller system K2PbCu(NO2)6
Authors: J D C McConnell  , V Heine
Published: July 2002

The recent and general theory of the origin of incommensurate structures in insulators is applied for the first time to a cooperative Jahn-Teller system. As expected from the general theory, two modes of deformation are found in K2PbCu(NO2)6which, on symmetry grounds, can interact to lower the energy of the incommensurate phase but which cannot interact at the symmetry point in k-space defining the commensurate structure. In this salt the operative pair of modes are the equivalent x2-y2and 3z2-r2types of distortion of the octahedral complex around the copper atom. The role of rotations, displacements and acoustic modes is also considered. On this basis a structure for the incommensurate phase is proposed.
Electronic mobility anisotropy of layered semiconductors: transversal photoconductivity measurements at n-MoSe2
Authors: W Kautek
Published: July 2002

An analysis of the shape of photoconductivity spectral distribution curves of n-MoSe2in the range of the direct and indirect fundamental optical transitions is presented, taking surface and volume recombination of the photogenerated charge carriers into consideration. The c-axis hole mobility, muh,z, is almost as great as that in the layers, muh,xy, at room temperature in accordance with band calculations where the top valence band is three-dimensional, and gives allowance for a low effective hole mass anisotropy. A very large anisotropy of muh,xy/ muh,zof approximately 102-103at 77K can be qualitatively understood in terms of extrinsic hole scattering from ribbons of stacking faults. The surface recombination rate is enhanced by the presence of (1011) faces on the illuminated surface.
Monte Carlo simulations and pair approximations on the phase transition of the restricted orientational lattice model for liquid crystals
Authors: I Ono  , K Ito
Published: July 2002

The nematic-isotropic phase transition on the SC lattice model of uniaxial molecules is studied by Monte Carlo simulations and the pair approximation method. The interaction between nearest-neighbour sites is assumed to be proportional to (1-3 cos2theta )/2 where theta is the angle between the long axes of molecules. The models of restricted orientation with 3,4,6 and 10 directions are systematically investigated. The phase transitions are shown to be of first order. Analytical estimations by a pair approximation are in agreement with Monte Carlo results.
Quasi-elastic light scattering at the order-disorder phase transitions of NH4Cl and NH4Br
Authors: S R Andrews  , R T Harley  , I R Jahn  , W F Sherman
Published: July 2002

The authors report measurements of quasi-elastic light scattering associated with ammonium reorientation at the order-disorder phase transitions in NH4Cl and NH4Br as a function of temperature and hydrostatic pressure. NH4Cl and NH4Br show respectively 'ferro' and 'antiferro' ammonium order at low temperatures and zero pressure. The scattering shows critical divergence in NH4Br where the soft mode is associated with the M points of the Brillouin zone but not in NH4Cl where the soft mode is at the Gamma point. A theory of the quasi-elastic scattering cross section is developed in which the main scattering mechanism is a second-order process involving two ammonium reorientational excitations of equal and opposite wavevectors and the dominant contributions are from M points of the zone. This is an extension of theories that successfully predict the disorder-induced Raman spectra and it gives a good description of the main features of the quasi-elastic scattering.
Spin-lattice relaxation by tunnelling motions of methyl groups in four acetates
Authors: F Koksal  , E Rossler  , H Sillescu
Published: July 2002

The proton spin-lattice relaxation times of KCH3COO, Ba(CH3COO)2, Cd(CH3COO)2and Ca(CH3COO)2have been measured over the temperature range 300-310K at 15, 30 and 52 MHz Larmor frequencies. The relaxation times have been found to be frequency-independent within the limits of the experimental errors and the results exhibit minima at 41, 40, 38 and 52K respectively. The low-temperature results have been explained by tunnelling rotation of the methyl groups and the tunnelling frequencies of the ground and the first excited states of the methyl group rotator levels of the compounds have been determined. Also the energy separations between the ground and the first excited states have been deduced. The experimental results at higher temperatures of T1minima display hindered rotation, and around room temperature nearly free rotation of the methyl groups.
The magnetic phase diagrams of Mn(Br1-xClx)2.4H2O antiferromagnets
Authors: C H Westphal  , C C Becerra
Published: July 2002

The magnetic phase diagrams of Mn(Br1-xClx)2.4H2O obtained from differential susceptibility measurements, with the magnetic field H applied along the easy axis, are presented. It is found that the Neel temperature TNand the spin-flop-paramagnetic transition field at T=0 varies linearly with the concentration x, between the values observed for the unmixed compounds MnCl2.4H2O and MnBr2.4H2O. The authors also found that the region in the H against T plane in which the spin-flop phase exists increases with increasing x. From these results they conclude that the anisotropy varies with the concentration, and that the ratio HA/HEbetween the anisotropic exchange field HAand the isotropic exchange field HEassumes values between 0.25 and 0.73. These are respectively the values for the ratios in Mncl2.4H2O and MnBr2.4H2O.
Proton spin-lattice relaxation in the paramagnetic state of a ferromagnetic Ising-like chain system: (CH3)3NH.CoCl3.2H2O
Authors: H Nishihara  , A M C Tinus  , K Kopinga  , W J M de Jonge
Published: July 2002

The spin dynamics of a quasi-one-dimensional ferromagnetic Ising system (CH3)3NH.CoCl3.2H2O (TN=4.135K) is studied by measuring the proton spin-lattice relaxation rate at 3-300K in externally applied fields of 2.5-33 kOe parallel to the Ising axis. The overall behaviour of the temperature and field dependence including a rate maximum at a temperature well above TNis found to be governed by the q to O and omega to 0 component of the susceptibility.
Zeeman infrared spectra of calcium and strontium fluoride crystals containing cerium and neodymium
Authors: C A Freeth  , G D Jones
Published: July 2002

Infrared and Zeeman infrared measurements of the tetragonal symmetry cerium fluoride ion and neodymium fluoride ion charge compensation sites in calcium and strontium fluoride are reported. The neodymium results are supplemented by optical absorption and fluorescence measurements. Crystal field analyses successfully account for the observed2F72/(4f1) and4I92/,4I112/(4f3) multiplet crystal field splittings and for the measured Zeeman splitting values.
Aluminium overlayers on (110) indium phosphide: microscopic aspects of interface formation
Authors: A McKinley  , G J Hughes  , R H Williams
Published: July 2002

The early adsorption and subsequent film growth of aluminium at room temperature on atomically clean cleaved indium phosphide surfaces has been studied using a range of experimental techniques. These include low-energy electron diffraction, Auger electron spectroscopy, as well as core level and valence band angle-resolved photoelectron spectroscopy with conventional and synchrotron sources. The interface formed is complex. For Al coverages below 0.5 monolayer photoemission from occupied InP surface states is significantly influenced following bonding of Al atoms to the surface In and P sites. For larger coverages film growth is believed to involve cluster formation. At the same time the interface shows considerable disorder and exchange reactions occur which lead to the release of indium from the semiconductor. The pinning of the Fermi level at Al-InP interfaces has been studied and the results obtained considered in the light of recent theories of the imperfect interface. The adhesion of Al to InP surfaces has also been briefly studied.
Defect-induced tunnelling and the conductivity of strongly disordered systems
Authors: D Belitz  , W Gotze
Published: July 2002

For a model describing the motion of a zero-temperature electron gas in an environment causing a fluctuating potential and a fluctuating effective mass, a theory is developed which allows for an approximate calculation of the particle density propagation self-consistently with momentum relaxation and defect-induced tunnelling processes. All correlation functions of the system are expressed in terms of two causal kernels for which non-linear equations are derived and solved. A detailed discussion of the influence of the defect-induced tunnelling on the mobility, in particular on mobility edges, and on the Fermi glass stiffness is given and an anomalous high-frequency tail of the dynamical conductivity is predicted.
The finite temperature properties of the massive Thirring model and the quantum sine-Gordon model
Authors: M Imada  , K Hida  , M Ishikawa
Published: July 2002

The thermodynamics of the massive Thirring model is discussed with the help of the Bethe Ansatz approach in a way different from Fowler and Zotos (1982). In particular a finite Debye cut-off is introduced to investigate effects of the lattice discreteness. The specific heat seems to have a structure as a sum of the Debye-type specific heat and a 'soliton' peak, when the authors take into account the relation of the massive Thirring model to the quantum sine-Gordon model. The finite temperature excitations are calculated. The excitation energy proposed by Fowler and Zotos is shown to have significance which is insensitive to the way of formulation.
The energy-level structure of a Jahn-Teller system strongly coupled to many modes of vibration
Authors: M C M O'Brien
Published: July 2002

A transformation of a Jahn-Teller Hamiltonian with multimode interactions is developed, and used to discuss Raman scattering from an impurity in a crystal. It is shown that the same transformation can account both for the sharp, low-energy features that look like scattering associated with a local mode and the broad band which is typical of impurity-assisted scattering from a spectrum of phonons. The coexistence of these two features is shown to be characteristic of strong Jahn-Teller coupling. Line widths and energies are calculated for the sharp features. The effects of stress and the proper definition and use of Ham factors in the presence of a multimode interaction are discussed.
Mean inner potentials of aluminium and copper
Authors: M A Ochando  , A Sanchez  , C R Serna
Published: July 2002

Reflection high-energy electron diffraction patterns were taken from electropolished (100), (110) and (111) surfaces of aluminium and copper. From the displacements of the spots compared with the Bragg position the mean inner potentials were obtained, being 13.2 V for aluminium and 19.1 V for copper.
Structural relationships between spontaneous strain and acoustic properties in ferroelastics
Authors: W I F David
Published: July 2002

A number of structural relationships between the spontaneous strain and acoustic properties of ferroelastic materials is described. Zero-velocity pure transverse acoustic modes are shown to occur in mutually perpendicular pairs at second-order phase transitions. These soft modes have propagation and displacement directions situated in ferroelastic domain wall planes. The resultant ferroelastic spontaneous strain is shown to result from the appropriate summation of pure shear strains produced by these soft modes and characterised by a two-dimensional acoustic strain tensor which is proportional to the difference between the spontaneous strain tensors of two strain-compatible ferroelastic orientation states.
Photoluminescence of iron-doped KMgF3
Authors: A Poirier  , D Walsh
Published: July 2002

Uniaxial stress applied to KMgF3:Fe samples reveals that the ground state of the ion, which emits photoluminescence peaks in the 14000 cm-1region at low temperature (2K), is a singlet orbital state. X-ray irradiation and heat treatment in O2and H2atmospheres together with high-temperature spectra (30K) of the KMgF3:Fe samples show that the most probable ion is Fe3+, the observed transitions being4T1to6A1.
The incommensurate-commensurate transition in a random-field model
Authors: T Nattermann
Published: July 2002

The effect of random-field-induced domain wall wandering on the nature of the uniaxial incommensurate-commensurate (IC) transition is studied in the framework of a d-dimensional 'random-rod' model. In this model, the random field h(r) couples linearly to the wall elongations. Both short- and long-range field correlations (h(r)h(0)) approximately mod r mod-D+1are considered. It is found that the domain wall density l-1vanishes with the driving parameter delta (e.g. temperature, pressure) as ( delta - deltac)betawith beta =(5-D)/2(D-3) for 3<D<5. For D>d, D has to be replaced by d. If D(d) is linear in d then beta -(dR-d)/2(d-dc-) (*) for dc-<or=d<dR-where dRand dc-denote the upper critical dimensionality for the roughening transition and the lower critical dimensionality for the occurrence of a conventionally ordered phase, respectively-D(dR)=5, D(dc-)=3. For D>or=5 or d>or=dRthe classical result l-1approximately ln-1( delta - deltac)-1applies; for D<or=3 or d<or=dc-there is no IC transition. The results for a corresponding bulk random-field model follow from the 'random-rod' model by means of the Villain relation D(d)=(5+2d)/3 for all d, i.e. from (*) with dR=5, dc-=2. It is argued that the formula (*) applies for a variety of IC transitions with dRand dc-specifying different universality classes.
Resonant broadening of quasi-atomic Auger spectra by elementary excitations
Authors: M Cini  , A D'Andrea
Published: July 2002

The broadening of quasi-atomic lines in the CVV and CC'V Auger spectra of solids is discussed and a new, resonant mechanism is proposed. Explicit expressions are derived that allow this resonant contribution to be computed directly from XPS and Auger spectra. The results are compared with outer contributions arising from lifetime, dispersion and phonon broadening, with special reference to a series of transition metals. When certain conditions are met, as in the case of silver, the resonant broadening makes up a substantial contribution to the experimental linewidths.
Effects of vacuum annealing on the electronic properties of cleaved GaAs
Authors: F Proix  , A Akremi  , Z T Zhong
Published: July 2002

Low-energy electron diffraction, Auger electron spectroscopy and photoemission yield spectroscopy measurements have been performed on GaAs samples cleaved in ultra-high vacuum and subject to in situ isochronous anneals up to 850 degrees C. The properties of the cleaved (110) surface remain unchanged up to about 350 degrees C. Around this temperature, a dissociation occurs inducing a local reconstruction of the surface; at the same time, new states appear which may be attributed to Ga vacancies. Then, around 580 degrees C, the well known dissociation of GaAs sets in with formation of As2, and the Ga-Ga bonds which then form, determine the electronic properties of the sample. Evaporation of arsenic would become dominant around 650 degrees C. Beyond about 780 degrees C, the excess Ga coalesces into Ga-metal droplets and the GaAs substrate undergoes faceting.
Paramagnetic resonance and optical absorption of Co4+in SrTiO3
Authors: K W Blazey  , K A Muller
Published: July 2002

The paramagnetic resonance of low-spin Co4+, 3d5in the oxygen octahedral coordination of SrTiO3at 4.2K is reported. The spectrum is described by a spin-Hamiltonian with S=1/2, mod g mod = mod g/sub ///+2gperpendicular tomod /3=1.7414+or-0.0015, and a large hyperfine interaction constant mod A mod = mod A/sub ///+2Aperpendicular tomod /3=254+or-8*10-4cm-1. Correlated with this centre is an optical absorption band at 2.53 eV. This band is attributed to electron-acceptor charge transfer from the oxygen 2p valence states to the t2g( down arrow ) levels of the Co4+. The EPR and optical results are analysed using theoretical expressions due to Thornley (1968), from which an orbital reduction factor of k=0.64+or-0.04 and an average radial electron distribution, (r-3), reduced by a factor 0.67+or-0.06 with respect to the free ion are derived. These significant reduction factors appear to be partly due to a weak dynamic Jahn-Teller effect. The slight tetragonality of the spectrum is a measure of the coupling, p2/ mue= mod gperpendicular to-g/sub /// mod /3 epsilon =2.14*102, to the intrinsic tetragonal distortion of the crystal. The charge linewidth anisotropy is also accounted for by this coupling to Egrandom strains with a distribution alpha =126 kg cm-2.
Coupled-cluster summation of the particle-particle ladder diagrams for the two-dimensional electron gas
Authors: D L Freeman
Published: July 2002

The correlation energy and antiparallel spin pair correlation function of the two-dimensional electron gas are studied within the particle-particle ladder approximation of the coupled-cluster equations. Unlike the three-dimensional electron gas the correlation energy is shown to be convergent in the particle-particle ladder approximation for the two-dimensional system. The coupled-cluster particle-particle ladder equations are cast into the form of singular integral equations. Using a product integration technique the integral equations are solved numerically. The correlation energy and r=0 value of the antiparallel spin pair correlation function are calculated from the numerical coupled-cluster coefficients for a range of electron densities from rs=0 to 16. The antiparallel spin pair correlation function at r=0 is shown to be positive for all electron densities when a wavefunction accurate to first order in the coupled-cluster particle-particle ladder coefficients is used. The calculated correlation energy is compared with dielectric results of Jonson (1976) and ring results of Freeman (1977, 1978). Good agreement between the dielectric results and the particle-particle ladder results are found for intermediate to low electron densities. The particle-particle ladder coupled-cluster correlation energy is shown to be exact in the high-density limit, although the approach to the limit is not correct owing to the neglect of ring contributions. It is suggested that simultaneous inclusion of particle-particle ladder and ring diagrams will give correlation energies accurate over the entire range of calculated densities.
Dynamical mean-field theory for spin glasses. I. Short-time properties in zero field
Authors: J A Hertz
Published: July 2002

The dynamical properties of a simple model of a spin glass are studied. The model has Gaussian random exchange interactions and Langevin dynamics. Following the ideas of Ma and Rudnick (1978) and of De Dominicis (1978), the spin glass phase is characterised by the presence of a finite limit of the spin autocorrelation function as t to infinity , i.e. by a delta-function part of it Fourier transform C( omega ). With a finite system in mind, this delta function is taken to have a small finite width. Then the fluctuation-dissipation theorem is imposed (before the infinite volume limit), forcing a corresponding low-frequency anomaly on the dynamical susceptibility. Solving self-consistently in mean-field theory for the order parameter, one then finds that the Almeida-Thouless instability of conventional theories is suppressed for all times shorter than the inverse width of the delta function in C( omega ). This theory is constructed for exactly zero external field; problems about the field dependence and possible relevance to experiment are discussed.
Electron spin resonance in spin glasses with Dzyaloshinsky-Moriya anisotropy: a microscopic approach
Authors: P H Beton  , M A Moore
Published: July 2002

Starting from the Hamiltonian for a (metallic) spin glass with Dzyaloshinsky-Moriya anisotropy the authors derive expressions for the static susceptibility and the electron spin resonance (ESR) frequencies. The results agree both with experiment and the results predicted by the 'triad' model, for which a microscopic explanation is suggested. The damping of the ESR frequencies due to the inclusion of a Langevin term in the spin equation of motion intended to model bottlenecked Korringa relaxation is also considered.
Surface acoustic waves in metals
Authors: J Heil  , I Kouroudis  , B Luthi  , P Thalmeier
Published: July 2002

The authors present propagation experiments for surface acoustic waves (SAW) in pure single crystals of aluminium and gallium. Geometric resonances and de Haas-van Alphen oscillations are observed as sound velocity changes. In addition, a typical surface effect, a nonreciprocal effect of the SAW velocity with respect to applied field and propagation direction reversal, is observed. A theory is given which explains this effect quantitatively.
A band model for the understanding of divergences in perturbation theory
Authors: P Erdos  , A Suto
Published: July 2002

A simple theoretical model is constructed on the basis of real many-body systems occurring in solids. One example is an impurity in a host with band structure. The model allows an exact solution to be obtained as well as approximations by perturbational methods such as the Rayleigh-Schrodinger and Brillouin-Wigner techniques. The cause and nature of the divergence of perturbation theory is demonstrated and the utility of partial sums of divergent perturbation series is explained.
Iron spin orientation in a competing anisotropy system: Fe1-xCoxCl2
Authors: B D Howes  , D C Price  , M C K Wiltshire
Published: July 2002

The competing anisotropy system Fe1-xCoxCl2has been studied using57Fe Mossbauer spectroscopy. Spectra have been measured at 4.2K for single crystals across the composition range 0<or=x<1 and for x=0.32, 0.35 and 0.44 as a function of temperature. The authors analyse these spectra to obtain the magnetic hyperfine field and electric quadrupole interaction parameters. Using the rigid-crystal model, they have also calculated these parameters as a function of the direction thetaexof a molecular exchange field, and have used these calculations to deduce the variation of thetaexwith composition x from the Mossbauer data. At 4.2K, the phase boundaries on either side of the mixed phase region are clear. Of these boundaries, one corresponds to the ordering of a three-dimensional (3D) XY system in a random field, the other to that of a 3D Ising system in a random field. The latter boundary is not sharp, suggesting that the 3D Ising system in a random field does not undergo a true phase transition to long-range order. In the phase where the spin components perpendicular to the c axis (Sperpendicular to) alone are ordered, a non-zero value of the spin component parallel to the c axis (S/sub ///) is observed. The authors argue that the coupling mechanism between S/sub /// and Sperpendicular tothat are responsible for the generation of the non-zero local values of S/sub ///, also produce the random fields that broaden the phase transition to the mixed phase.
Multiphonon transitions between adiabatic potential curves
Authors: T Markvart
Published: July 2002

Multiphonon transitions are discussed semiclassically using an extension of the configuration-coordinate diagram to a complex plane of the configuration coordinate. The author considers the tunnelling regime, complementary to the region of applicability of the Landau-Zener formula, where the crossing point of the potential curves is classically inaccessible. An expression is obtained for the vibrational matrix element in terms of a contour integral, which can be evaluated analytically for a wide class of adiabatic potential curves. He obtains a general expression for the multiphonon transition rate at T=0, and this is used to calculate the transition rate between non-intersecting Morse curves.
Luminescence studies of individual dislocations in II-VI (ZnSe) and III-V (InP) semiconductors
Authors: S Myhajlenko  , J L Batstone  , H J Hutchinson  , J W Steeds
Published: July 2002

Results are presented of high-resolution luminescence studies from individual dislocations and related defects in ZnSe and InP performed in a transmission electron microscope. In the case of ZnSe unusual luminescence bands (Y at 2.60 eV and S at 2.52 eV) originally observed in photoluminescence studies are attributed to dislocations. In some instances, complete quenching of the excitonic transitions was observed to correlate with the presence of Y emission from complex dislocation tangles. In the case of individual screw dislocations this quenching of the exciton luminescence was found to be variable; for example reduction of the exciton signal was not always observed. For InP, donor-exciton-related transitions were quenched at individual screw dislocations. Donor-acceptor pair/free-to-bound and deep level (band C) transitions were unaffected. For the case of InP, unlike ZnSe, no dislocation-related luminescence was observed within the system detection limit (0.7-4.0 eV).
A molecular dynamics simulation study of the orientationally disordered phase of sulphur hexafluoride
Authors: M T Dove  , G S Pawley
Published: July 2002

The orientationally disordered phase of sulphur hexafluoride at 150K has been studied here using the molecular dynamics simulation technique, with particular emphasis placed on the origin and nature of the orientational disorder. A model involving the frustration of two competing intermolecular interactions that had been proposed previously has been shown to be a good description of the origin of the disorder through calculations that have given information concerning the correlations between the orientations of neighbouring molecules. It has also been shown that the disorder is dynamic rather than static in nature, and features of the molecular motion that are related to this dynamic disorder have been identified. It is suggested that some of the features of orientationally disordered crystals that have been studied might be observed experimentally in a coherent neutron scattering study, and some preliminary calculations of S(Q) are reported.
Low-frequency relaxation in condensed matter and the evolution of entropy
Authors: A K Rajagopal  , A S Teitler  , K L Ngai
Published: July 2002

A model of low-frequency relaxation in condensed matter is provided. The model involves time-dependent transition rates which are obtained from time-independent transition rates. The time dependence arises because of an environment that provides a time-dependent entropy contribution to the free energy that controls the transitions. The model has two related consequences. Firstly, relaxations at low frequencies conform in general to a fractional exponential decay function, phi = phi 0 exp(-(t/ tau p)1-n), 0<or=n<1, independent of both the nature of primary relaxation species and the type of condensed matter. This fractional exponential decay property of relaxation has been widely verified. The general occurrence of this behaviour allows a confident determination of the key parameter, n, in the fractional exponential decay function, to be made. A second consequence of the model is the concept that the relaxation time tau p observed in a particular experiment is related to a direct relaxation time tau 0 that would occur in a time-independent environment by means of a relation dependent on the same key parameter n determined from the relaxation measurement. The latter relation provides various renormalisation relations appropriate to particular situations.
Reflectivity spectra of Cd1-xMnxTe, Zn1-xMnxS and Zn1-xMnxSe in the 0.7-8.1 eV energy range
Authors: M Zimnal-Starnawska  , M Podgorny  , A Kisiel  , W Giriat  , M Demianiuk  , J Zmija
Published: July 2002

The reflectivity spectra of Cd1-xMnxTe, Zn1-xMnxS and Zn1-xMnxSe in the 0.7-8.1 eV energy range have been investigated and compared with other known results for these mixed crystals. On the basis of the results obtained a model of the electronic structure of these compounds is proposed.
The magnetic transition in the jellium model
Authors: V C Aguilera-Navarro  , M de Llano  , A H Zimerman
Published: July 2002

Two-point Pade approximants are employed to interpolate between the known high- and low-density ground-state-energy expansions for the electron gas problem to predict the paramagnetic-to-ferromagnetic transition point in excellent agreement with recent computer simulations.
The random-field puzzle. I. Solution by equivalent annealing
Authors: M Schwartz
Published: July 2002

Random-field ferromagnetism systems are discussed, using an equivalent annealed system. It is shown how the critical exponents of the random system are related to the exponents of the pure system. The critical exponent eta is expressed in terms of a dimensionality reduction d'=d-2+ eta (d'). The lower critical dimension for the Ising model is two and for the continuous models it is four. Very strong arguments are given for a hyperscaling relation 2- alpha =d'v and for all the exponents to be given by a dimensionality reduction.