bluuebunny/crossref_metadata_2025_split huggingface dataset:
Evidence for zero‐field fluctuations in Cr3+ near the phase transition in NH4Al(SO4)2 12 H20
Authors: Owens F. J. 
Published: July 2007

AbstractThe effect of the phase transition of NH4Al(SO4)2 12 H2O on the EPR of the Cr3+ ion is studied. The axial zero‐field tensor shows a strong linear temperature dependence with a change in slope occurring at −160°C which correlates to the onset of a marked broadening of the high and low field resonances as well as with an unusual increase in the intensity of the center resonance. The effects are attributed to fluctuations in the local symmetry of the H2O molecules about the Cr3+ ion. The strong temperature dependence of D is argued to be due to an unusually large coupling of the paramagnetic ion to local vibrations of the complex.
A molecular model for the emission process of FA(II) ‐centres in alkali halides
Authors: Kung A. Y. S. , Vail J. M. 
Published: July 2007

AbstractEmission by FA(II)‐centres in alkali halides is investigated by treating the centre as a molecule immersed in a dielectric medium. The molecule consists of a single electron in the field of two unit positive charges and a central anion of effective charge (‐Ze). Its ground and first excited states are estimated variationally as a function of interionic separation and of Z. Lattice relaxation is treated in the rigid point‐ion model with Coulomb and nearest‐neighbour Born‐Mayer interaction. Only displacements of the saddle‐point ion and its two nearest neighbours are considered. These displacements determine Z, which gives a single‐parameter characterization of the defect. Three new FA(II)‐CentreS are postulated, namely in NaF:Li, RbF:Li, and RbF:Na, and their emission energies are estimated from a semi‐empirical scheme to be 0.915, 0.513, and 0.591 eV, respectively.
The influence of dipolar energy in nuclear double‐resonance
Authors: Whitaker M. A. U. 
Published: July 2007

AbstractNuclear double‐resonance is discussed in terms of cross‐relaxation in the rotating frame between Zeeman and secular dipolar reservoirs for each species. Temperatures are assumed for each reservoir, and the treatment limited to the case where interactions between reservoirs are less effective than those inside reservoirs. A set of four first‐order linear simultaneous differential equations is obtained for the development in time of the properties of the system.
A theoretical study of the absorption of photoelectrons created by X‐rays in semi‐infinite targets
Authors: Lantto V. 
Published: July 2007

AbstractWith the use of the diffusion model developed by Bethe et al. under conditions where the distribution of electron velocities is almost random, the absorption of photoelectrons, due to energy loss and scattering, is discussed quantitatively. An exponentially decreasing isotropic source is assumed for the photoelectrons created by X‐rays in a semi‐infinite target. Different boundary conditions are used for the total density of electrons on the plane surface. As in a work of Meister for monoenergetic β‐rays, the analysis is also carried out for a model where a straight motion of electrons changes into diffusion by a direct transition after a fixed value of the path length. Several expressions are introduced to describe the energy distribution and the total yield of the photoelectrons emerging from the target. These expressions are applied to the determination of the energy distributions and total yields of the photoelectrons for some practical cases.
Helicon damping in semiconductors in the quantum limit
Authors: Aronzoh B. A. , Metlikhov E. Z. 
Published: July 2007

AbstractResults are given of theoretical and detailed experimental studies of alternating magnetic field regions with weak and strong helicon damping in semiconductors in the quantum limit for InSb at 1.7 ≦ T ≦ 78 K. The positions of strong and weak helicon damping region boundaries are investigated as functions of the temperature and dimension of the sample and also wave frequency and power. The agreement of the theory with experimental data is satisfactory.
Extra‐atomic relaxation and X‐ray spectra of narrow‐band metals. II. Results
Authors: Grebennikov V. I. , Babanov Yu. A. , Sokolov O. B. 
Published: July 2007

AbstractThe results of a numerical analysis of the effect of the hole potential on the form of the X‐ray spectra of narrow band metals are given. The dependence of the spectra on the magnitude of conduction band filling as well as on the magnitude of intratomic interaction between conduction band electrons and the hole is investigated. It is shown that when the band is filled, the emission and absorption spectra vary from a form of the spectra, which occurs if the hole potential is engaged adiabatically, to a form where the interaction with the hole is not taken into account. In the first case the spectra reflect the local impurity density, and in the second one the density of states of the unperturbed matrix. If the hole potential is strong enough, then the formation of a local level is possible. The most interesting consequence of this is the presence of two lines in core XPS. The systematic change is recorded of the intensities of these lines as the band is filled. The results of the investigation are drawn into a qualitative interpretation of the width of emission bands and of the shift of the inner lines of d‐metals.
The influence of plastic deformation of the flux‐line lattice on flux transport in hard superconductors
Authors: Schmucker R. 
Published: July 2007

AbstractIn hard type‐II superconductors flux lines may overcome pinning obstacles by plastical flow of the flux‐line lattice around the pinning centres. The conditions for this process to occur are investigated. The results for the volume pinning force obey a scaling law and show a specific dependence on the anisotropy of the pinning obstacles and on the Ginsburg‐Landau parameter.
Frequency‐dependent exchange correction to the dielectric function of the electron gas (II)
Authors: Brosens F. , Devreese J. T. , Lemmens L. F. 
Published: July 2007

AbstractThe influence of the exchange interaction on the dielectric response of the homogeneous electron gas is investigated. Starting from an expression for the dielectric function obtained earlier, the function G(q, ω) that describes the effect of exchange, is analysed. It is shown that G(q,ω) has a logarithmic singularity if ω = h (qkF ± q2/2), which makes the full frequency dependence of this function very significant. The dielectric function with exchange differs from the Hartree approximation by a very peaked frequency dependent structure. It even has a pole of first order. Furthermore, due to exchange, the plasmon branch is substantially changed and does not penetrate in the pair creation region. Finally, the exchange interaction induces a zero in the dynamic structure factor, where in the Hartree approximation only a discontinuous derivative with respect to frequency occurs.
Optical investigation of the electrical anisotropy of (SN)x single crystals
Authors: Geserich H. P. , Möller W. , Scheiber G. , Pintschovius L. 
Published: July 2007

AbstractPolarized reflectance spectra of (SN)x single crystals are investigated in the range from 0.15 to 4 eV. For light polarized parallel to the polymeric chains the Drude behaviour found previously is confirmed. The optical properties perpendicular to the chains, however, are explained by a modified Maxwell‐Garnett model. This model takes into account the fibrous structure of (SN)x crystals and allows to separate the intrinsic and extrinsic transverse transport properties.
Lattice dynamics of lithium. A theoretical microscopic calculation
Authors: Weilacher K. H. , Roth‐Seefrid H. , Bross H. 
Published: July 2007

AbstractThe microscopic theory of lattice dynamics is employed to calculate the phonon dispersion curves of lithium. Instead of pseudo wavefunctions, Bloch wavefunctions are used that are solutions of the Schrödinger equation with a realistic crystal potential. The requirement of translation invariance leads up to a sum rule for the density‐density correlation function (q + Q, q + Q') with q=0. For arbitrary vectors q any approximation could be avoided for by means of interpolating the density of the conduction electrons. The resulting phonon spectrum is in good agreement with experiment.
Theory of allowed resonance Raman scattering of first and second order in a magnetic field
Authors: Enderlein R. , Bechstedt F. 
Published: July 2007

AbstractThe effect of a magnetic field on first‐ and second‐order resonance Raman scattering from nonpolar optical phonons is investigated theoretically. Both the Landau and spin quantizations are considered within a simple spheric and parabolic two‐band model. Magnetic field effects on the dispersion of the scattering amplitude can be observed which are similar to those of the dielectric susceptibility in magneto‐optics. New effects appear which have no counterpart in magnetooptics if the ratio of phonon frequency ω0 and cyclotron frequency ωc is an integer. For ωc = ωo e.g., in first‐order scattering all Landau resonances of the scattering amplitude, except one, compensate each other. The magnetic field dependence of the scattering amplitude is similar to its dependence on the incident frequency, so that resonance Raman scattering can be investigated experimentally by changing the magnetic field strength instead of the laser frequency. The two processes of second‐order Raman scattering show essential different dispersions in the presence of a magnetic field. The magnetic field induced circular anisotropy of the scattering cross section is considered and the polarization of the scattering light is investigated.
Determination of the velocity of mobile dislocations by nuclear spin relaxation measurements
Authors: Hackelöer H. J. , Selbach H. , Kanert O. , Sleeswyk A. W. , Hut G. 
Published: July 2007

AbstractWolf's general theory of rotating‐frame nuclear spin‐lattice relaxation is applied to the effect caused by dislocations moving at an average velocity v. Equations are presented for the dependence of the NMR relaxation rate R on the mean waiting time τ of a dislocation before an obstacle, valid in both the weak‐and the strong‐collision regions. In the latter the Slichter‐Ailion‐Rowland‐Fradin theory applies. Nuclear spin relaxation tests on 23NaCl and 23NaF single crystals deformed at constant strain rates ϵ confirm the theory. The experimental data for the predicted maximum in R (reached only for NaCl at ϵm = 20 s―1) yield directly τm = 2.8 × 10―5 s, the mobile fraction of dislocations Qm/Qt = 0.26. The value of ϵm, together with the mean distance between obstacles of ≈ 2 × 10―4 cm, derived from the slope of the curve in the strong‐collision region, leads to vm = 7.1 cm s―1, corresponding to a mobile dislocation density ϱm = 1.4 × 108 cm−2.
Thermal vibration amplitudes of TlCl between 296 and 534 K
Authors: Haav A. A. , Peljo E. , Suortti P. 
Published: July 2007

AbstractThe thermal motion parameters, B, of the Tl+ and Cl― ions of TICI are determined by X‐ray diffraction from powder samples between 296 and 534 K. The B‐values increase linearly with temperature from BTI = 3.39 Å2 and BCI = 2.09 Å2 at 296 K to BTI = 5.68 Å2 and BCI = 3.38 Å at 483 K. This corresponds to a constant Debye temperature of ΘM = 95 K. The results suggest strong second‐neighbour interactions and a mutual cancelation of the anharmonic effects.
Dynamical properties of no impurity molecules in alkali halides. I. Theory
Authors: Zavt G. S. 
Published: July 2007

AbstractThe effect of an impurity molecule on the lattice vibrations is studied. The translational and librational motion of the molecule and lattice vibrations are considered from a unified point of view. Green's functions describing all types of motions as well as their mixing are calculated in the harmonic approximation. Optical response functions (far infrared absorption coefficient and Raman scattering tensor) are found. It is shown that, unlike the case of point impurities, the spectral functions can display interference features, such as asymmetric peaks or antiresonances.
Vibrations of Isotopic Impurities in Crystals Built up by Complex Ions
Authors: Belousov M. V. , Pogarev D. E. , Shultin A. A. 
Published: July 2007

AbstractThe IR reflection and Raman spectra of NaNO3, Ba(NO3)2, KNO3, and NH4Cl crystals containing isotopic impurities of 15N and 18O are studied. The experimental data on local vibration modes are compared with those calculated by the method of Green's functions, the density of vibration states being determined from two‐phonon spectra, as well as those following from the model of additive refraction (AR). The latter is found to yield satisfying results provided the isotopic shift exceeds the vibrational bandwidth. A nomograph is provided by means of which the deviation of frequency and intensity of a local mode calculated in AR approximation can be estimated. The absence of isotopic structure in vibration spectra of some crystals studied by other authors is also explained.
The Optical Properties of MnAs and MnAs0.85P0.15
Authors: Bärner K. , Braunstein R. , Chock E. 
Published: July 2007

AbstractThe reflectivity spectra of MnAs (hexagonal NiAs structure) and MnAs0.85P0.15 (orthorhombic lattice) are measured in the energy range 0.3 to 14 eV at room temperature and a Kramers‐Kronig analysis is performed. It is found that below 1 eV plasma effects due to free d‐electrons dominate. At energies from 3 to 8 eV plasma resonances due to bound d‐electrons are observed. In the intermediate region, the imaginary part of the dielectric constant ϵ2 reveals structure due to transitions between d‐states and from the valence band into d‐ and sp states. Analysis of the combined density‐of‐states function ϵ2E2 yields 3.0 eV for the energy gap between bonding and antibonding sp states for both compounds. Also, from interpretation of ϵ2 together with ϵ2E2 the energy separation between the Fermi level EF and the bottom of the minority spin e‐band and the distance of the valence band edge from EF are obtained.
Spin‐Dependent Luminescence of Donor‐Acceptor Pairs in Semiconductors. I. General Theory
Authors: Thuselt F. 
Published: July 2007

AbstractBy use of orientation parameters of electrons (spin 1/2) and holes (angular momentum 3/2) expressions are given for the angular dependence of both the intensity and polarization of the luminescence of donor‐acceptor pairs in semiconductors with diamond and zincblende structure. The luminescence light may be polarized either linear or circular depending on the observation direction. Its degree of polarization depends on internal or external fields as well as on the eventual polarization of the excitation light.
Electrical Conductivity in the Hubbard Model Including Electron‐Phonon Interaction
Authors: Ihle D. 
Published: July 2007

AbstractUsing the Hubbard model including weak electron‐phonon coupling the electrical conductivity σ is calculated for U ≫ t (U intra‐atomic Coulomb energy, —t nearest‐neighbour hopping integral) and low frequencies. Especially the dc conductivity is investigated for the half‐filled band and T ≫ TD (TD Debye temperature). It is compared with the dc conductivity in the phononless Hubbard model calculated in the approximation lim (σ/t2). The inclusion of phonons results in t → 0 a finite transport relaxation time the temperature dependence of which leads to a shift of the conductivity maximum temperature to a lower value, the sharpening of the maximum, and to the decrease of the conductivity for high temperatures according to T−2.
Nuclear Acoustic Resonance Absorption and Dispersion in Vanadium via Coupling to the Magnetic Dipole Moment
Authors: Müller V. , Schanz G. , Fischer E. , Unterhorst E. J. 
Published: July 2007

AbstractThe first observation of both NAR absorption and NAR dispersion in a single crystal vanadium due to the coupling of longitudinal acoustic waves to the magnetic dipole moment of the V51 nuclei is reported. The experiment is performed in the temperature range of 5 to 294 K. The observed angular dependence and NAR line shapes are in good agreement with the formulas derived in this paper.
Exciton Dissociation in the Electric Field of the Charged Defect
Authors: Petelenz P. , Petelenz B. 
Published: July 2007

AbstractDissociation of Frenkel excitons in the neighbourhood of a charged defect in anthracene crystals is investigated in relation to the recent experimental results of Fort and Coret. The trimer model is adopted, and the charge‐transfer integral relevant to the exciton decay is calculated in a simple semiempirical scheme. The dissociation probability is found to be enhanced due to the interaction with a cationic defect. An alternative interpretation of the experimental results in terms of the mixing of the exciton and charge‐transfer states is proposed. A possible role of both the mechanisms is discussed.
Spin‐Dependent Luminescence of Donor–Acceptor Pairs in Semiconductors. II. Application to Inner Strains
Authors: Thuselt F. , Unger K. 
Published: July 2007

AbstractThe influence of inner strains in semiconductor crystals caused i) by impurity misfit and ii) by bending of epitaxial layers on the polarization properties of the emitted pair radiation is theoretically studied with the aid of the general theory developed recently. It is shown that anisotropic pair distributions in the crystal can be detected in this manner. The predictions of the theory for bended epitaxial layers fits well the observations of Vekua et al.
Energy Bands in Cubic Ice. Ab Initio Calculation Using the Method of Linear Combination of Molecular Orbitals
Authors: Resca L. , Resta R. 
Published: July 2007

AbstractAn LCMO calculation is made of the energy bands of cubic ice with a suitable model structure. With respect to a previous tight‐binding calculation important terms are included consistently with an ab initio derivation and the matrix elements are obtained at any k vector in the Brillouin zone. The problem of the excitonic levels is discussed. The influences of crystal structure features on the electronic states are evaluated. A comparison with very recent experimental VUV, XPS, UPS data is performed and a better understanding of the electronic properties of ice is achieved.
Scattering of DC Electrons by a Spherical Macroscopic Potential Including Screening
Authors: Gerlach E. , Rautenberg M. 
Published: July 2007

AbstractUsing a transport theory based on the equivalence of energy loss and Joule's heat the scattering of electrons by a spherical macroscopic potential is studied which serves as a simple model for radiation damage, inhomogeneities, or precipitations. It is shown that screening may strongly effect the resistivity if the potentials are spatially slowly varying. Further the model calculations predict the possibility of geometrical resonances for a degenerate carrier distribution.
Optical Side Bands in a Cooperative Jahn‐Teller Material: TmAsO4
Authors: Laugsch J. , Gehring G. A. 
Published: July 2007

AbstractOptical side bands are observed in the spectrum of TmAsO4. Measurements of their polarisation and intensity are made as a function of magnetic field. These results are interpreted using a tight binding theory developed earlier. It is found that the most intense side band is caused by off‐diagonal exchange (Γ3 − Γ1 and Γ4 − Γ1) between the ground doublet and the excited singlet arising from virtual phonon exchange. The less intense side bands are caused by a large number of exchange interactions and the analysis is less clear.
Absorption Spectra of S2 Molecules Embedded in van der Waals Solids
Authors: Arai T. , Martin T. P. 
Published: July 2007

AbstractThe \documentclass{article}\pagestyle{empty}\begin{document}$ {}^3\sum\nolimits_{\rm u}^ - {} \leftarrow {}^3\sum\nolimits_{\rm g}^ - {} $\end{document} transitions are investigated of sulphur dimers, matrix‐isolated in Ar and Kr. Each vibrational subband of the electronic absorption appears to consist of several superimposed narrow bands. The separation between the narrow bands is determined to be 60 cm−1 in Ar and 50 cm−1 in Kr. The complex structure is interpreted to be caused by phonons of the matrix materials. The potential parameters of sulphur dimers in Ar and Kr are also calculated.
Ideal Thermal Resistivity of Lithium, Potassium, and Rubidium
Authors: Gupta O. P. , Kharoo H. L. , Hemkar M. P. 
Published: July 2007

AbstractThe temperature variations of the thermal resistivity of lithium, potassium, and rubidium is calculated within the free‐electron approximation using the anisotropic continuum dispersive model proposed by Sharma and Joshi for the lattice dynamics of metals. The theoretical results are compared with available X‐ray measurements. Except for lithium, there is reasonable good agreement between theory and experimental data.
Piezoelectric Coupled Soft Mode in CsH2AsO4 and CsD2AsO4 Studied by Brillouin Scattering
Authors: Azoulay J. , Gerlich D. , Wiener‐Avnear E. , Pelah I. 
Published: July 2007

AbstractThe Brillouin scattering from shorted and free single crystals of CsH2AsO4 and CsD2AsO4 is studied as a function of temperature. Utilizing the (z + x)[y, (z + x)] (z − x) scattering geometry, the softening of the elastic constant C is observed on approaching the Curie temperature from above. The results, which are also compared with similar data on KH2PO4 and KD2PO4, indicate a strong piezoelectric coupling between the acoustic shear mode and the soft optical ferroelectric mode. The temperature of the Brillouin shifted frequencies for all the KDP isomorphous crystals can be computer fitted with a Curie‐Weiss law for the soft mode. The difference between the extrapolated temperature of the shorted and free crystals are also explained on the basis of the thermodynamic relations of the coupled modes and are thus similar to those obtained for the clamped and free dielectric responses.
Plasma Wave Damping in a Layered Electron Gas with Random Inhomogeneities
Authors: Abramovich B. S. , Ignatov A. A. 
Published: July 2007

AbstractThe propagation of plasma excitations in a layered electron gas with random inhomogeneities of the conductivity of the separate layers is investigated. An expression is obtained for the effective dielectric permittivity of the given system at weak fluctuations of the equilibrium carrier concentration. The mechanism is studied of plasma wave damping associated with the energy transformation of the mean field into chaotic self‐oscillations of the system. Decrements of the mean field damping both in the centre and on the boundary of the Brillouin zone are calculated. A strong dependence of the damping character on the degree of fluctuation correlation between different layers is found.
Host and Impurity Excitations in Antiferromagnetic and Paramagnetic FeCO3 in the Far Infrared
Authors: Eremenko V. V. , Maslennikov A. I. , Naumenko V. M. 
Published: July 2007

AbstractThe host and impurity excitations in FeCO3 containing 3% Mn2+ are studied in the range from 12.5 to 500 cm−1. The host excitations are observed both in the antiferromagnetic and the paramagnetic temperature range and are well described for a wide temperature range with a single ion Hamiltonian, including the interactions between all sublevels coming from the ground term 5D of the Fe2+ ion: the temperature dependent trigonal field, the spin–orbit interaction, and the Ising molecular field. For the first time in an Ising antiferromagnet a pair mode (host excitation plus impurity excitation) characterized by the absence of magnon‐magnon interaction is observed. The g‐factors of the observed lines are measured. A sharp drop of the g‐factors near TN cannot be explained by an existent theory of the g‐factor renormalization.