bluuebunny/crossref_metadata_2025 huggingface dataset:
Cytotoxic T Lymphocyte Epitopes of HIV-1 Nef
Authors: Lucchiari-Hartz Maria , van Endert Peter M. , Lauvau Grégoire , Maier Reinhard , Meyerhans Andreas , Mann Derek , Eichmann Klaus , Niedermann Gabriele
Published: July 2002

Although a pivotal role of proteasomes in the proteolytic generation of epitopes for major histocompatibility complex (MHC) class I presentation is undisputed, their precise function is currently the subject of an active debate: do proteasomes generate many epitopes in definitive form, or do they merely generate the COOH termini, whereas the definitive NH2 termini are cleaved by aminopeptidases? We determined five naturally processed MHC class I ligands derived from HIV-1 Nef. Unexpectedly, the five ligands correspond to only three cytotoxic T lymphocyte (CTL) epitopes, two of which occur in two COOH-terminal length variants. Parallel analyses of proteasomal digests of a Nef fragment encompassing the epitopes revealed that all five ligands are direct products of proteasomes. Moreover, in four of the five ligands, the NH2 termini correspond to major proteasome cleavage sites, and putative NH2-terminally extended precursor fragments were detected for only one of the five ligands. All ligands are transported by the transporter associated with antigen processing (TAP). The combined results from these five ligands provide strong evidence that many definitive MHC class I ligands are precisely cleaved at both ends by proteasomes. Additional evidence supporting this conclusion is discussed, along with contrasting results of others who propose a strong role for NH2-terminal trimming with direct proteasomal epitope generation being a rare event.
On the interpretation of Hund's rules in atomic spectra
Authors: N C Pyper  , I P Grant
Published: July 2002

Term splittings calculated from multiconfiguration self-consistent field models (both relativistic and non-relativistic) have been analysed by several methods. Recent studies by Davidson (1962), Katriel (1972), Colpa (1973) have suggested that the empirical rules of Hund relating to term splittings may be interpreted largely in terms of changes in the total electron-nuclear attraction. It is shown that a more consistent description can be given in terms of the traditional picture of the motion of valence electrons in the field of the nucleus and an essentially frozen electron core.
Analysis of post-collision interactions in Auger processes following near-threshold inner-shell photoionization
Authors: A Niehaus
Published: July 2002

Semiclassical formulae are derived which describe the influence of the slowly receding photoelectron on the Auger decay of an inner-shell ionized atom. The usefulness of the formulae is demonstrated by their successful application to the quantitative analysis of two different 'post-collision interaction' phenomena. It is predicted that under certain conditions the 'post-collision interaction' will lead to observable interference effects which can then be analysed by the derived semiclassical formulae.
Do present 'charge transfer to the continuum' theories correctly describe the production of vi=veelectrons in ion beam-foil collisions?
Authors: W Meckbach  , K C R Chiu  , H H Brongersma  , J W McGowan
Published: July 2002

The authors' study is designed to test whether or not existing 'charge transfer to the continuum' theory adequately explains the production of viapproximately=veelectrons resulting from 0.2-0.5 MeV H+and H2+ion bombardment of thin carbon foils; the work suggests that it does not. The differential energy and angular distributions of the emitted electrons are discussed in terms of the vectorial electron-ion velocity differences (ve-vi). Although the cusp shaped peaks in both angle and energy are observed, (i) the mod ve-vimod-1dependence predicted by all 'charge transfer into continuum' theories is not found; furthermore, (ii) the predicted increase in cusp width when plotted in terms of (ve-vi) and as a function of Eiis not found; finally (iii) it is confirmed that the yield of veapproximately=vielectrons from H+bombardment is not the predicted Ei-5but rather very nearly an Ei-3dependence through the energy range. Furthermore, the cusp widths associated with H+ions are considerably larger than those for equal velocity H2+ions.
Energy transfer in organic systems. XIII. Plastic scintillators
Authors: A Hallam  , J B Birks
Published: July 2002

For pt.XII see ibid., vol.11, no.13, p.2301 (1978). Energy transfer has been investigated in polyvinyltoluene (PVT) scintillators containing pyrene p-terphenyl and naphthalene by measurement of scintillation pulse shapes and yields. It is concluded that there is an initial rapid diffusion of the excitation energy with solute molecules, residual monomer and PVT excimer sites acting as competing traps. At later times energy can transfer from residual monomer and PVT excimer traps to the solute either radiatively or via long-range resonance, and the efficiencies of these processes are governed by the absorption characteristics of the solute.
First ionisation potentials of atoms obtained with local-density schemes
Authors: K Schwarz
Published: July 2002

The first ionisation potentials of the light atoms up to krypton are computed by means of Slater's transition-state method for the following local-density functional models: Slater's exchange potential, the X alpha and X alpha beta methods and the exchange correlation potential due to Hedin and Lundqvist (1972). By taking into account the experimental multiplet structure of the atoms and the corresponding ions, a direct comparison can be made between experiment and theory. For the four potential models, this shows systematic trends, which are explained. In addition, comparison is made with Hartree-Fock results. It is concluded that the best overall agreement is found for the Hedin-Lundqvist potential.
A parameter-free non-adiabatic polarisation potential for low-energy e-H2collisions
Authors: T L Gibson  , M A Morrison
Published: July 2002

The authors have determined in ab initio, approximate, non-adiabatic polarisation potential for the e-H2system. The scattering results using this polarisation potential together with an accurate static model exchange potential are found to be in very good agreement with a variety of recent experimental measurement. In addition, an analytic fit of the polarisation potential is presented.
Predissociation of the D1Πustate of H2near threshold
Authors: F Borondo  , L R Eguiagaray  , A Riera
Published: July 2002

A recent calculation of Komarov and Ostrovsky (1979) seemed to have settled a controversy regarding the different experimental values of the H(2s)/H(2P) sharing ratio in the predissociation of the D1Piustate of H2near threshold. This calculation was based on a correct physical picture of the dissociation process, but the dynamical treatment rests on invalid assumptions. In the present work, a more rigorous quantum mechanical treatment is presented, and a branching ratio of 0.70 is obtained. The authors' results show how dangerous it is to infer the correctness of a calculation solely from the 'goodness' of a single physical quantity.
Oscillatory total cross sections in the (He-Ne)+collision system: I. Optical study of He(31P) exchange excitation (30-2000 eV)
Authors: J P Grouard  , J L Montmagnon
Published: July 2002

The relative emission cross section for He 31P exchange excitation in He+Ne collisions has been measured in the low-energy range (30 eV-2 keV). When the data, including data obtained with a linear polariser, are plotted as a function of inverse impact velocity, a system of at least four different series of Rosenthal-type oscillations is observed. Their origin is discussed within the framework of the diabatic correlation diagram for the (He-Ne)+system.
Resonance and interference phenomena in the photoionisation of a hydrogen atom in a uniform electric field. I. Resonances below and above the potential barrier
Authors: V D Kondratovich  , V N Ostrovsky
Published: July 2002

The photoionisation cross section for a hydrogen atom placed in a uniform electric field is calculated using separation of the variables in parabolic coordinates and the semiclassical approximation with account for the tunnelling and reflection above the top of the potential barrier. The corrections to the semiclassical approximation are established accounting for the peculiarities of the effective potential barrier form. The approximate separation of the centrifugal terms contribution in the phase barrier form. The approximate separation of the centrifugal terms contribution in the phase integrals allows the latter to be expressed via hypergeometric functions. The structure in the energy dependence of the cross section is associated with its complex poles. The motion of the poles under variation of the photon energy is investigated. The equations defining the resonance positions and widths are obtained and analysed for energies below and above the potential barrier. The boundary between these two regions is described. The analytical expressions for the parameters of resonances lying above the barrier are obtained for the first time. In the vicinity of the resonance the cross section can be well parameterised by Fano's formula. The approximate expressions for the profile index are deduced. The analytical results are compared with numerical calculation data. The recent experiments on the photoionisation of rubidium and sodium atoms are discussed.
Production and storage of highly charged ions at room temperature
Authors: D A Church  , K W Jones  , B M Johnson  , M Meron  , I A Sellin
Published: July 2002

Selective production and storage at thermal energies of highly charged ions (e.g. hydrogen-like argon, Ar17+) are imminently feasible. Multi-step photoionisation by wiggler-enhanced synchrotron radiation of thermal ions stored in a Penning trap is discussed. Potential applications of these ions to electron transfer and photoionisation measurements are pointed out.
Global theory of spectral lineshapes in the weak-density limit
Authors: O Vallee  , J Picart  , N Tran Minh
Published: July 2002

Deals with the fully quantum mechanical non-adiabatic theory of the pressure broadening of spectral lines. The authors establish a global spectral lineshape describing both the core and wings, and also the intermediate region by means of a single calculable expression. Originally developed by Szudy and Baylis (1975), their method has the advantage that it still works in the case of mutually overlapping lines. The case of the core of Stark-broadened hydrogen lines is an example for which the adiabatic assumption breaks down.
Theoretical initial l dependence of ion-Rydberg-atom collision cross sections
Authors: R L Becker  , A D MacKellar
Published: July 2002

Classical trajectory Monte Carlo calculations have been performed for ion-atom collisions in the velocity matching region for target Rydberg states of given specific n and l. A classical ensemble derived previously is used, which corresponds to the set of hydrogenic quantum states of fixed n and l. Substantial dependence on the initial l value is found for charge transfer, ionisation, and excitation cross sections. For charge transfer, the distributions over the final n' show peaks in substantial agreement with a recent experiment of Rolfes and MacAdam (1982). Experimental data for the other processes are not yet available.
Isotope shift and hyperfine structure in low-lying levels of molybdenum
Authors: A T Goble  , C W P Palmer
Published: July 2002

Isotope shifts and hyperfine structure in four transitions of Mo have been measured with a pressure-scanning Fabry-Perot interferometer. The isotope shifts have been combined with all other known isotope shifts in transitions connecting the 4d55s7S and5S and 4d45s25D levels with the 4d55p5P and7P levels in a multidimensional King plot. Values of delta (r2) from muonic-atom isotope shifts are used to separate mass and field shifts, giving improved values of delta (r2) for the isotope pairs, A, A'.
Impact Stark broadening of the Lyman lines of hydrogenic ions under dense-plasma conditions: fine-structure effects
Authors: C Stehle
Published: July 2002

Stark profiles of the low Lyman lines of hydrogenic ions are computed in the impact theory for density and temperature conditions of inertial confinement fusion. When the fine-structure effects are neglected, it is possible to test this approach by a comparison with the computer simulation results of Stamm et al (1984). Strong fine-structure effects are then analysed.
On the double ionisation of methane
Authors: U T Raheja  , F A Rajgara  , C Badrinathan  , D Mathur
Published: July 2002

The double ionisation of CH4was measured to be 38.97+or-0.25 eV by means of energy loss spectroscopy of single-electron loss products of 3 keV collisions between CH4+and H2, CH4, Kr and air. The lower limit for the lifetime of CH42+was determined to be 3.3+or-0.17 mu s.
Study of the weak lines in the L emmission spectrum of samarium 62
Authors: B G Gokhale  , S N Shukla
Published: July 2002

The L emission spectrum of samarium has been studied photographically using a 40 cm curved crystal transmission (Cauchois-type) spectrograph. A new quadrupole line at 1626·8 X.U. has been observed for the first time and ascribed to the transition LINIV,V(κ11). The presence of two more lines (LIINVI,VII, LIIINVI,VII) hitherto observed only by Deodhar and Karnatak in 1955 has been confirmed. The measurements of the previously known dipole and quadrupole lines have been compared with those of earlier workers.
Electron detachment from H-ions by electron impact
Authors: B Peart  , D S Walton  , K T Dolder
Published: July 2002

A crossed beam method has been used to measure cross sections for H-+e to H+2e for interaction energies between 12 and 1000 eV. For energies greater than 40 eV there is excellent agreement with measurements of Dance, Harrison and Rundel and when the energy exceeds 200 eV, the measurements confirm calculations by Inokuti and Kim based on Bethe's approximation. No evidence of structure was found in the detachment function.
A perturbation treatment of the unrestricted hartree-fock equations for the ground states of 3- to 10-electron atoms
Authors: R G Wilson
Published: July 2002

The first order unrestricted hartree-fock equations for the ground states of atomic systems with 4,5,...,10 electrons are solved exactly. The solutions are used to evaluate the corresponding second order energies exactly and the third order energies very accurately. A method is described whereby each atom can be treated as a special case of a general scheme. The third order uhf energy of lithium has been recalculated with greater accuracy and comparison is made between the uhf energy values obtained for the ground states of all the second row atoms and those obtained by other methods. The zero-order and first-order terms in the expectation values of the operators l?2and s?2are calculated and tables are given leading to those of 1/r3, 1/r, r, r2and delta(r).
Decisive criterion for determination of spin doublets in X-ray diagram lines
Authors: G B Deodhar  , S Rai
Published: July 2002

A systematic study of spin doublets in X-ray diagram lines is made. A decisive criterion for determination of spin doublets in X-ray diagram lines is found and discussed. On the basis of this criterion the unsettled question regarding some pairs of the categories (c) and (d) as pointed out by Singh et al. (1970) is now finally settled and some new spin doublets are reported for the first time. The nature of variation of Delta lambda with atomic number is discussed.
Band strengths for the CO2+(A2Πuto X2Πg) system produced through photoionization excitation of CO2
Authors: D L Judge  , L C Lee
Published: July 2002

The CO2+ions in the various vibrational levels of the A2Piuelectronic state were selectively produced by vacuum ultraviolet radiation of lambda 703, 709, and 715 AA. The emission band strengths of the CO2+(A2Piuto X2Pig) system were obtained and compared with other previous results established by different excitation methods. Good agreement with the earlier work was found. Using existing Franck-Condon factors, the electronic transition moments for the CO2+(A2Piuto X2Pig) bands are also presented.
Detection of the electric quadrupole transition L2M3(β17) in the L X-ray emission spectrum of lutetium 71
Authors: A N Nigam  , R B Mathur
Published: July 2002

Using (100) reflections in a 40 cm curved mica crystal spectrograph of the transmission (Cauchois) type, the line corresponding to the electric quadrupole transition L2M3( beta17) in the L X-ray emission spectrum of lutetium 71 has been detected at lambda =1486.54 XU. This has been made possible by completely eliminating the unwanted tungsten, nickel and copper spectra.
Statistical mechanics of classical one-dimensional Heisenberg ferromagnets with single-site anisotropy
Authors: K Nakamura  , T Sasada
Published: July 2002

The exact analytical forms of the low-temperature thermodynamic quantities and correlation functions are obtained for classical one-dimensional Heisenberg ferromagnets with single-site anisotropy including both the systems with an easy axis and with an easy plane. The problem has been managed on the basis of the functional integral method. The calculated results have a few leading terms in series-expansion with respect to the reduced temperature, exhibiting the characteristic points borne out in the recent numerical studies. In particular, the longitudinal correlation function and susceptibility of the system with an easy axis show Ising-like behaviours and the transverse counterparts of the system with an easy plane show isotropic XY- or Heisenberg-like behaviours. Physical implications of the calculated results are also discussed.
An infrared study of the shallow acceptor states in GaAs
Authors: R F Kirkman  , R A Stradling  , P J Lin-Chung
Published: July 2002

The shallow acceptor states in gallium arsenide have been studied by far-infrared Fourier transform spectroscopy. The energy level structure observed in photoconductivity is compared with the predictions of recent acceptor state theory and the agreement is found to be generally good. The groundstate ionisation energies are found to be 26.9 meV (carbon), 28.7 meV (magnesium), 30.6 meV (zinc) and 34.8 meV (silicon). The general features of the GaAs spectra are also compared with results previously obtained for Ge by Jones and Fisher (1965) for InSb by Kaplan (1973) and deep manganese acceptors in GaAs by Chapman and Hutchinson (1967). There is strong similarity between GaAs and the other materials. This similarity extends to the Zeeman spectra and is exploited in interpreting the Zeeman components observed in GaAs in terms of theoretical predictions.
Light scattering from BaTiO3: The CLG model
Authors: A M Ozorio de Almeida
Published: July 2002

The model proposed by Comes, Lambert and Guinier (1968) (hereafter called CLG) for all but the lowest phase of BaTiO3 assumes that each Ti atom has more than one possible equilibrium position, but that their configurations in neighbouring cells are such as to form long linear domains. In the limit of infinitely long chains this disorder is static. The harmonic approximation of the movement of the atoms around their disorder configuration leads to the existence of some modes that are identical to the modes of the averaged lattice. These include all the usually Raman active modes in ordered crystals and it is shown that in the present case these same modes account for perfect coherent scattering from all the cells in the crystal. Contrary to the ordered material, however, the CLG crystal can also scatter incoherently from single cells or from whole chains, leading to a diffuse background spectrum. Major features in the spectra of tetragonal BaTiO3 are thus accounted for, but measurements taken just above the ferroelectric transition temperature seem to suggest that in this region the disposition of parallel chains is correlated.
Density of states of substoichiometric TiC1-x
Authors: J Klima
Published: July 2002

The electronic states of substoichiometric TiC1-x, x=0.0, ... , 0.4 were studied by the CPA method and both the partial and total densities of states were calculated. The results revealed that with increasing vacancy concentration x the Fermi level EFmoves to higher energies and the density of states at EF,g(EF), goes up by a factor of four. The partial density of states analysis shows that the increase of g(EF) is solely due to the contribution of d states. The results obtained differ considerably from the predictions of the rigid-band model. The differences should show up in the X-ray spectra as is illustrated by the calculated LIIIemission intensity.
Energy of interaction between two atoms chemisorbed at FCC and BCC transition metal surfaces
Authors: J Lopez  , J C Le Bosse  , J Rousseau-Violet
Published: July 2002

The energy of interaction between two identical atoms chemisorbed on the (100) surface of a BCC or FCC transition metal is numerically investigated using a Green function technique based on the use of the Kalkstein and Soven Green functions for the substrate. These results are compared with previous results (1980) concerning the (100) face of an s band simple cubic crystal and with algebraic predictions. The authors find a good agreement; more precisely the overall qualitative features are the same for the three lattice structures. From this it follows that simple models can be used in chemisorption theory, allowing one thereby to categorise and improve one's understanding of experimental results.
Electron transport in direct-gap III-V ternary alloys
Authors: D Chattopadhyay  , S K Sutradhar  , B R Nag
Published: July 2002

Electron drift, Hall mobility and thermoelectric power in InPxAs1-x, GaxIn1-xAs and InAsxSb1-xhave been calculated by an iterative solution of the Boltzmann equation. A Kane-type band structure corrected for the lattice disorder is assumed. All the relevant scattering mechanisms are included together with band non-parabolicity, wavefunction admixture, degeneracy of the distribution function and screening of the scattering probabilities due to free carriers. Only the ionised impurity concentration (Ni) and the alloy scattering potential ( Delta Ea) have been treated as adjustable parameters to be determined from a fit with the experimental data. Variations of the other different material parameters across the alloy system have been taken from different sources, or reasonable assumptions are made regarding such variations. A large number of experimental results on both mobility and thermoelectric power are considered and good agreement is obtained with Delta Ealarger than that given by the difference in electron affinity between the constituent compounds.
A photoemission study of ultra-thin palladium overlayers on low-index faces of silver
Authors: G C Smith  , C Norris  , C Binns  , H A Padmore
Published: July 2002

The growth mode and electronic structure of palladium adsorbed on the (100) and (111) surfaces of silver at room temperature have been studied using LEED, Auger electron spectroscopy, angle-resolved photoemission and work function measurements. For both surfaces and overlayer grows in a layer-by-layer mode for approximately three monolayers after which it begins to relax to the bulk palladium structure. At submonolayer coverages a p(1*1) lattice gas occurs on the (100) surface whereas flat monolayer platelets are formed immediately on the (111) surface. Electron energy-parallel momentum (E-k/sub ///) dispersion relations were measured for palladium monolayers supported on both surfaces. For the monolayer. The difference in position and width of the resonant bound palladium 4d state on an isolated palladium atom supported on the (100) surface in comparison with an impurity atom in the bulk is consistent with the change in coordination number.
Internal strain in diamond structure elements: a survey of theoretical approaches
Authors: C S G Cousins
Published: July 2002

The various ways in which the internal strain parameter may be extracted from models of the elasticity and lattice dynamics of diamond structure crystals are surveyed. A general potential having six parameters and involving first and second neighbours gives values that agree with observation in silicon and germanium. The four- or five-parameter adiabatic bond charge model indicates a range of values that embraces the observations. Models that are limited to nearest neighbours and those that extend beyond second neighbours appear generally incapable of predicting reasonable values for the internal strain.
Thermal filling effects in capacitance experiments on deep levels in semiconductors
Authors: J M Noras  , H R Szawelska
Published: July 2002

The interpretation of capacitance studies of deep levels in semiconductors is complicated by the presence of a Debye tail of free carriers, thermally excited from the bulk into the depletion region. The concentration of these carriers varies spatially within the depletion region and so produces a variation in the rate constant for capture by deep traps. Unless this effect is taken into account wrong values of concentration of the centre being studied will be obtained. A further consequence is that capacitance transients are in general not exponential functions of time, although in favourable cases an exponential function may be a good approximation. In the general case unreliable time constants may be deduced if capture effects are not eliminated in the reduction of data; this may be difficult to do satisfactorily when measuring initial rates of change of capacitance.
The study of liquid germanium structure
Authors: M Davidovic  , M Stojic  , D Jovic
Published: July 2002

The static structure factor S(q) of liquid germanium is determined at 1123K by neutron diffraction techniques in the range from 0.4 to 8.5 AA-1. The structure factor shows a clear low-lying shoulder on the high side of the scattering angle, near the first peak of S(q). These two peaks indicate a dual structure of the germanium melt. The first one characterises the short-range order and the 'irregular' one is associated with a certain degeneracy of the atomic states of the liquid around the imperfections of the structure, which appear as single vacancies.
Structural phase transitions and superconducting transition temperatures of hexagonal MxWO3compounds
Authors: M Sato  , B H Grier  , H Fujishita  , S Hoshino  , A R Moodenbaugh
Published: July 2002

Neutron scattering experiments have been performed on non-stoichiometric compounds of the hexagonal tungsten bronzes MxWO3(M=Rb and K). A nearly dispersionless phonon branch has been observed in Rb0.33WO3at an energy omegaMapproximately=5.5 meV. This branch reflects the local-mode character of the Rb atoms, which are loosely trapped at their sites within the large channel along the c direction formed by the linkage of WO6octahedra. The x dependence of the energy omegaMof this low-lying phonon (LLP) has been determined by measurements of the peak position in the powder inelastic scattering spectra. Since omegaMdepends only weakly on x, the variation in omegaMappears not to be related to the anomalous x dependences of the superconducting transition temperature Tcobserved previously in RbxWO3and KxWO3. Two kinds of structural phase transitions were observed using powder diffraction measurements. One transition, which takes place above room temperature at Tc1, is due to a distortion of the WO3host cage of MxWO3. It is accompanied by an optical phonon softening at the Gamma point. The second transition is an order-disorder transition of the M atoms. It produces for a certain value of x a long-wavelength incommensurate structure as the low-temperature phase. This is in contrast with the other incommensurate phases such as the CDW states of two-dimensional conductors, where the incommensurate structures generally appear at high temperatures. A local structural excitation model is adopted in the discussion of the superconductivity, and the x dependences of Tcin CsxWo3and RbxWO3are consistently explained in relation to the ordering schemes of the M atoms.
Influence of nondiagonal dynamic susceptibility on the EPR signal of Heisenberg magnets
Authors: H Benner  , M Brodehl  , H Seitz  , J Wiese
Published: July 2002

In EPR with linearly polarised microwaves if just the diagonal contributions chi ++( omega ) and chi --( omega ) of the dynamic susceptibility are considered then a discrepancy occurs between the experimentally observed signal and the predictions of theory. By means of the Mori formalism the authors have calculated the non-diagonal contributions chi +-( omega ) and chi -+( omega ) for insulators with dominating isotropic exchange. For rather small linewidth these terms result in a new kind of dynamic shift of the resonance field, whereas for broad lines they give rise to a drastic anomaly of lineshape. All these effects could be verified experimentally in both one- and three-dimensional spin lattices of axial symmetry in the following ways: the dynamic shift expected for narrow lines could be observed in TMMC; lineshape anomalies were studied in the very broad line of CsNiF3; and in MnF2 the critical increase of linewidth near TN allowed variation between the two extreme cases.
Properties of microcrystalline silicon. IV. Electrical conductivity, electron spin resonance and the effect of gas adsorption
Authors: S Veprek  , Z Iqbal  , R O Kuhne  , P Capezzuto  , F -A Sarott  , J K Gimzewski
Published: July 2002

For pt.III see ibid., vol.16, p.2005 (1983). Measured values of the electrical conductivity, sigma , and electron spin density (g=2.0057) of microcrystalline silicon can be essentially determined by the extent of the contamination of the samples by oxygen unless special precautions are taken as regards the sample preparation and/or handling. For samples deposited at a floating potential, two kinds of oxygen incorporation are identified: irreversible formation of Si-O bonds on the grain boundaries (and on the sample surface) and a reversible absorption which is probably associated with a nondissociative O2delta -(ads) state. The latter results in a decrease of sigmaRTby up to five orders of magnitude, an increase of the activation energy, epsilona, and of the preexponential factor, sigma0, as well as in an increase of the electron spin density. A reversible desorption of oxygen leads to an increase of sigmaRTup to not less than about 10-2Omega-1cm-1and a decrease of the EPR signal below the detection limit of less than 1016cm-3. In order to avoid such effects a negative bias has to be applied to the substrate during deposition. Samples of undoped mu c-Si deposited in this way show neither the incorporation of oxygen into the bulk nor significant changes in the dark conductivity even after long-term exposure to air.
The scattering of phonons by the strain and rotation fields of crystal defects
Authors: R A Brown
Published: July 2002

The scattering of long-wavelength phonons due to the spatial variation of elastic moduli that is produced by the inhomogeneous elastic strain and rotation fields of static crystal defects is calculated in terms of the second- and third-order elastic constants of the undeformed crystal. The present theory is distinguished from those of previous authors by its ability to deal with rotation angles that are not necessarily small, such as those associated with large-angle grain boundaries or macroscopically bent or twisted crystals. For small rotations the classical equations of motion and the second-quantised Hamiltonian obtained by previous authors are rederived and the modifications appropriate to large rotations are discussed. Approximate, but simple, expressions for scattering cross sections, etc., are obtained; these facilitate subsequent calculations of transport coefficients and lead to estimates of lattice thermal resistivities in dislocated crystals that are in good agreement with experiment. Previous theoretical studies of the strain-field scattering problem are reviewed.
Evidence for magnon bound-state excitations in the quantum chain system (C6H11N3)CuCl3
Authors: R Hoogerbeets  , A J van Duyneveldt  , A C Phaff  , C H W Swuste  , W J M de Jonge
Published: July 2002

Ferromagnetic resonance measurements are presented on the quasi-one-dimensional S=1/2ferromagnetic compound (C6H11NH3)CuCl3(CHAC), indicating the existence of magnon bound-state resonances (MBR). In addition to the normal ferromagnetic resonances several extra modes were observed in the temperature region 1.2K<T<4.2K and at various frequencies (18 GHz< nu <75 GHz). It is shown that these extra resonances can be interpreted in terms of Delta m=+or-1 transitions between magnon bound states. The experimental results are compared with three theoretical models: the S=1/2ferromagnetic xxz and xyz models and a semiclassical model which shows the possibility of inducing transitions between envelope soliton states in ferromagnetic resonance experiments. The three models give qualitatively similar results for the low-lying excited states of the magnetic chains in CHAC. The experimental results can be described best by the S=1/2ferromagnetic xyz model.
Semiconductor to semimetal transition in TiS2at 40 kbar
Authors: P C Klipstein  , R H Friend
Published: July 2002

Evidence for p-d band crossing at a pressure of 40 kbar is presented for the layered compound TiS2, based on the variation with pressure of the Hall coefficient, RH, thermoelectric power, S, and resistivity, rho . Below 40 kbar the transport properties may be understood within the framework of an extrinsic semiconductor model with strong electron-phonon scattering. Under pressure the band overlap is estimated to change at the rate of 4.5+or-1.5 meV kbar-1; this implies a band gap of 0.18+or-0.06 eV for TiS2at ambient pressure. Analysis of the quantity -RH/ rho2e as a function of pressure, and solution of the transport equations at 90 kbar are both used to show that in the semimetallic state of TiS2the d-band electron mobility exceeds the p-band hole mobility. In contrast, the hole mobility is the greater at all pressures up to 90 kbar in TiSe2.
Fundamental absorption edges and indirect band gaps in W1-xMoxSe2(O⩽x⩽1)
Authors: K -K Kam  , C -L Chang  , D W Lynch
Published: July 2002

The optical absorption edge (E perpendicular to c) was measured on basal-plane samples of single-crystal W1-xMoxSe2(O<or=x<or=1) at temperatures of 15, 78 and 295K. The data were fitted to several expressions for the shape of the edge, with the best fit being to that expected for indirect allowed transitions. The phonon involved is in an optical branch, and its energy varies smoothly with x. The band-gap energy at any temperature varies smoothly with x, indicating that the band edges are the same for MoSe2, WSe2and the compounds of intermediate compositions.
Density of states of a two-dimensional electron gas in a strong magnetic field
Authors: E Stahl  , D Weiss  , G Weimann  , K von Klitzing  , K Ploog
Published: July 2002

The authors have measured the density of states in the tails of the Landau levels of GaAs-AlxGa1-xAs heterostructures by analysing the variation of the Fermi level position as a function of the magnetic field. The experimental data are best described by a gaussian-like density of states superimposed on a constant background density of states.
Fast hopping of deuterated methyl groups
Authors: D Cavagnat  , S Clough  , F O Zelaya
Published: July 2002

Methyl groups in solids reorientate at high temperature in a manner that is essentially dependent on the temperature of the host lattice but not on its phonon spectrum. The reorientation rate of CH3groups can be easily predicted by evaluating a thermal average over all excited torsional states of the expectation values of the angular momentum, thus making it a calculable function of the hindering potential barrier V and temperature T. The aim of the experiments described was to extend the theory for fully deuterated compounds containing methyl groups; and consequently see how the temperature dependent reorientation rates of CD3could be predicted from measurements of CH3tunnel splittings of the analogous compound at low temperature. Theory-and the subsequent experiments-show how it is not necessary to carry out the full calculation again; instead, it is sufficient to calculate the energy levels of a CH3group in a potential of height 2V and then divide these energies by two. The expectation values of the angular velocities are also reduced by a factor of two. The above can be summarised into what the authors call the 'rule of two', which may be stated by saying that a CD3group hops at half the rate of a CH3group of twice the barrier height at twice the temperature. The result is that the same set of curves of hopping rate ( tau-1) against inverse temperature (T-1) and potential barrier height VHof CH3, can be used for CD3of one halves the ( tau-1) and doubles the (T-1) scales and replaces VHby 2VH. Agreement with this theory is shown to be satisfactory; the data were obtained using the time-of-flight (IN5) and back-scattering (IN10) spectrometers of the Institute Laue-Langevin, Grenoble, for four fully deuterated samples (acetone, nitromethane, iodoethane and toluene). The measurements fall in a region of very fast hopping motion.
Evolution of the granularity of discontinuous metal deposits during covering with a-C (or SiO)
Authors: G Desrousseux  , B Robrieux  , A Carlan  , H Schaffar  , R Faure  , J P Dussaulcy
Published: July 2002

The authors suggest a method which makes it possible to follow continuously, in situ, under UHV, the granulometric change of metallic condensates on carbon films before and during covering with a-C or SiO. For this purpose, they calculate separately the tunnel effect currents which flow between the grains from one interface to the next and directly between the particles. Fixing of the metal distribution does not occur as soon as evaporation of the a-C (or SiO) begins. When this fixing is achieved, a-C (or SiO) fills up the intergranular spaces until the grains are partially or entirely buried. During this burying, the thickness of the a-C layer in the intergranular spaces is compared with the height of the grains. They show that coalescence of the grains precedes the fixing of the deposit geometry. However, this coalescence is less pronounced that that observed between the end of deposit formation and the beginning of the covering.
The LCAO approach to the embedding problem
Authors: G A Baraff  , M Schluter
Published: July 2002

The embedding method invented by Inglesfield (1981) is a method for solving the Schrodinger equation for a small but interesting part of the larger system. It is based on minimising the energy by varying the wavefunction within a closed surface surrounding the region of interest. The correct non-local energy-dependent boundary conditions on this surface then appear as a surface potential term in the functional to be varied. The authors show that in the usual LCAO Green function method there exists a way to accomplish this same embedding using a matrix operator, with structure similar to the boundary condition kernel of Inglesfield's method.
Far-infrared studies at intermediate magnetic fields with the neutral shallow donors in GaAs and InP of transitions not involving the ground state
Authors: C J Armistead  , P C Makado  , S P Najda  , R A Stradling
Published: July 2002

More than thirty sharp lines are observed in the far-infrared photoconductivity from the neutral shallow donors in n-GaAs and InP at wavelengths longer than the longest wavelength that will excite electrons from the ground (1s) state of the donor. Twenty-four of the lines observed can be positively identified as corresponding to transitions given by p to q where p and q are the principal quantum numbers involved and are given by 2, 3 and 4. The n=2 to 2 lines involving the 2s state (2p--2s, 2s-2p+, 2s-2p0) are sufficiently sharp that central-cell structure arising from the chemical shifts of the individual donor species can be observed. The central-cell splitting is found to agree well with that expected from measurements of the splitting on the 1s state. The inter-excited-state transitions are observed under much less restricted conditions of sample compensation, electric field bias, optical excitation and temperature than the broader peaks corresponding to transitions from negatively charged donor-ion (D-) states to the conduction band states.
Quantum well electro-absorption in a strong electric field: a continuum spectrum approach
Authors: Z Ikonic  , V Milanovic  , D Tjapkin
Published: July 2002

Starting from the continuous energy spectrum of a quantum well subjected to a perpendicular electric field, electro-absorption is analysed within the effective-mass approximation. The electric field considered is assumed to be strong enough that the exciton effects may be neglected. The influence of the difference between the well and bulk effective masses on the electron and hole wavefunctions, as well as on absorption, is thoroughly discussed. The absolute values of the well absorption are determined for a couple of examples.
The X-ray K emission spectrum of sodium and of fluorine in some alkali halides
Authors: T Aberg  , G Graeffe  , J Utriainnen  , M Linkoaho
Published: July 2002

The integrated intensities of the K alpha emission satellite groups alpha alpha3alpha4and alpha5alpha6have been measured in secondary excitation for Na+in NaF, NaCl and NaBr and of F-in LiF, NaF and KF. The results are generally in agreement with calculations based on the assumption that the satellites are due to single-electron transitions for KL and KL2states of free ions. However, in LiF and KF the satellite intensities differ from the free ion values; this is attributed to the fact that in fluorides the F-LII,IIIelectrons are valence electrons. The intensity and position of the Na+K beta1line have been determined for the various halides. The K beta1line has been identified as a cross transition between the cation and anion. Its appearance is solely determined by the covalency in the crystal.
Thermodynamical discussion of a linear approximation to the Fermi-Dirac distribution function
Authors: L F Dona dalle Rose
Published: July 2002

A linear approximation to the Fermi-Dirac distribution function is proposed which might be of interest in practical applications. Three different ways of choosing its parameters are suggested. In order to assess the goodness of the different choices, the specific heat curves for a closed parabolic band are calculated within this approximation and compared with the numerically computed, exact specific heat.
Indirect determination of the spin-lattice relaxation time of silicon-29 in trichlorosilane
Authors: A Briguet  , A Erbeia
Published: July 2002

The nuclear Overhauser effect observed on the two lines in the proton spectrum of29Si1HCl3allows the determination of the spin-lattice relaxation time of29Si. The temperature dependence of (T1)29Siis interpreted in term of spin- rotation interaction.
Intensity fluctuations of Brillouin scattered laser light from amplified vibrational waves
Authors: W Wonneberger  , J Lempert  , W Wettling
Published: July 2002

A theory is presented for the intensity fluctuations of Brillouin scattered laser light from stationary intensity-coupled vibrational waves which are amplified far above thermal level by the acoustoelectric effect in piezoelectric semiconductors. The wave coupling modifies the photon counting distribution of Brillouin scattered laser light in a characteristic way giving photon counting distributions different from those of independent thermal waves. The theoretical considerations are supplemented by a numerical analysis of measured photon counting distributions. From Nminteracting waves the subgroup of Nm" waves involved in the scattering is considered. By use of the chi2statistical test combinations of Nmand Nm" are selected which fit the experimental data best. A typical combination is Nm=8, Nm"=5. These numbers are much smaller than expected from the usual conception of Brillouin scattering as well as from nonlinear acoustoelectric wave interaction considerations.
Theory of RHEED
Authors: N Masud  , J B Pendry
Published: July 2002

The so-called 'chain method' of calculating RHEED intensities is investigated. Convergence of the various expansions is tested and the range of feasibility for the full multiple scattering treatment is found to be up to 5 keV incident energy at present. This covers the energy range proposed for determination of atomic coordinates at surfaces, and the sensitivity of RHEED to atomic positions is investigated.
A system for recording temperatures in the vicinity of liquid surfaces
Authors: L A Kuehn  , R S Weaver
Published: July 2002

An apparatus is described for measuring and plotting automatically the temperature distribution in the vicinity of an evaporating liquid surface, with a precision of ±0·01 degc. It consists of a thermistor, which forms one arm of an unbalanced Wheatstone bridge, a mechanism to lower the thermistor vertically through the surface and a coupled voltage divider to monitor its position. The outputs of the voltage divider and the bridge are connected respectively to the X and Y axes of an XY recorder to produce a graph of temperature against thermistor position. Experiments with water evaporating under the atmospheric conditions of 65% relative humidity and 21°C temperature indicate that a temperature depression of 0·83 ± 0·04degc exists at the surface.
Determination of phase transitions through AC conductance
Authors: A Schiraldi  , G Chiodelli
Published: July 2002

A cell design for AC conductance measurements at various temperatures is described. The cell does not require external spring-loaded or screwed supports, permitting the accurate identification of phase transition points irrespective of coincidental large changes in molar volume (e.g. for compounds such as AgI, TlI and organic substances showing phase-mesophase transitions).
X-ray flash techniques
Authors: R Germer
Published: July 2002

The author examines the problems of X-ray flash production and its applications, similar to those of continuous X-radiation. Special problems are the fast production of high X-ray intensities to produce detectable radiation and the high-voltage and high-current discharge circuits required. Anode voltages from some 10 kV to about 1 MV and currents up to 10 kA are normal for laboratory equipment, but 100 MV and 1 MA are possible. The flash time is mostly between 10 ns and 1 mu s. Cineradiography with frame rates up to 106pictures s-1can be achieved.
Vacuum ultraviolet radiance transfer standard based on an argon mini-arc with integral differential pumping unit
Authors: R C Preston  , C Brookes  , F W J Clutterbuck
Published: July 2002

An argon mini-arc is described which has been developed as a high radiance transfer standard in the vacuum ultraviolet between 92 and 200 nm. It incorporates an integral differential pumping unit to interface the high pressure arc to a vacuum spectroradiometric system. It is shown that the radiance is not only extremely stable and reproducible but also calculable in terms of previously determined spectroscopic and plasma properties. The alternative interface method being a magnesium fluoride window has been shown to be severely limited by the formation of colour centres in the window and a gradual accumulation of evaporated material on its surface.
Cryogenic vacuum techniques
Authors: R A Haefer
Published: July 2002

A review on cryopumping is given under the following aspects: molecular flow in non-isothermal vacuum chambers; mechanisms of cryopumping such as condensation, cryosorption at solid gas condensates, cryosorption at porous solids (molecular sieves, activated charcoal), cryotrapping and cryogettering at metal films; calculation of cryopump systems; examples of various types of cryopumps such as liquid-pool cryopumps, continuous-flow cryopumps and refrigerator-cooled cryopumps; various applications.