bluuebunny/crossref_metadata_2025 huggingface dataset:
On the Influence of Screening on the Ferroelectric Curie Point
Authors: Nattermann Th.
Published: July 2007

A simple model of a uniaxial ferroelectric photoconductor is considered in which the influence of the electronic system is described by an appropriate expression for the dielectric function. On the basis of the Hamiltonian of the soft optical vibration mode the theory of the thermodynamical Green's function is used to derive an equation for the temperature of the ferroelectric phase transition. From this, the shift of the transition temperature and the change of the spontaneous polarization, caused by an excitation of valence band electrons, can be calculated. Under certain conditions the results are shown to be in agreement with those obtained by other authors.
An Elastic‐Modulus Effect Calculation of the Interaction of Point Defects with Extended Dislocations in F.C.C. Metals
Authors: Bapna M. S. , Meshii M.
Published: July 2007

A generalized calculation was carried out on the interaction energy and interaction force of point defects with extended dislocations due to the elastic‐modulus effect. It was shown that the maximum interaction does not occur at the middle of two partials. The results obtained were used to estimate the maximum interaction energy between a vacancy and an extended dislocation and the hardening coefficient, αv, for vacancy‐dislocation interaction. These results were examined in the light of experimental results in gold. Further, it was noted that asymmetry in the sign of the interaction above and below the slip plane will not occur when the interaction between an edge dislocation and a point defect is predominantly of the elastic‐modulus type rather than an elastic‐misfit type.
EPR of the Four‐Copper‐Ion Complex with Spin S = 2 in Triglycine Fluoroberylate Monocrystal
Authors: Stankowski J. , Maćkowiak M.
Published: July 2007

EPR study of Cu2+ in TGFB revealed at high temperatures isolated complexes with square coordination decreasing in number with lowered temperature to yield four‐ion aggregates. In describing the aggregate spectrum, a spin Hamiltonian with effective spin S = 2 is used; its parameters are determined to gz = 2.12, gy = 2.15, gx = 2.08, D = 0.108 cm−1, E = 0.030 cm−1, c = 0.009 cm−1, α = −0.003 cm−1. The hyperfine structure constant of the aggregate is four times smaller than that of the isolated centre. The strong dependence on temperature and high pressure is related with NH3 group rotation in the TGFB crystal.
Résonance électronique de spin et susceptibilité magnétique des composés GdCu et GdAg
Authors: Burzo E. , Urzu I. , Pierre J.
Published: July 2007

Nous avons étudié par R.P.E. les composés GdCu et GdAg. La valeur de l'interaction d'echange Jsf déduite de ces mesures est négative. Elle est comparée à celles déduites de mesures magnétiques et de R.M.N.
Electron Images of Two‐Dimensional Domain Walls
Authors: Hothersall D. C.
Published: July 2007

The out‐of‐focus electron images of the two‐dimensional 180° Bloch‐wall model of Hubert are calculated using a wave optical formulation. The asymmetry of the calculated images is established and qualitative comparison is made with experimental domain wall images from walls in thin sections of Fe and Co. The experimental image of a 1800 wall in a foil of FeSi is compared with theoretical profiles calculated in two ways; the first using a one‐dimensional wall model with a linear rotation of the magnetization, and the second using the two‐dimensional wall model. As a result of these comparisons the electron imaging effects of the two wall models can be tentatively related.
Effect of Slow Vibrational Relaxation on Anomalous Stokes‐Shifted Luminescence and Energy Transfer in Solids
Authors: Dexter D. L.
Published: July 2007

An analysis is presented of the emission spectrum to be expected from model centers in vibrationally excited and electronically excited states. It is shown that for centers which normally exhibit a sizable Stokes' shift there will be, for a short time following excitation, a large component of emission at approximately twice the usual Stokes' shift. This anomalous emission would be detectable by ultra‐fast (pico‐second) excitation and detection, and in selected systems by conventional means. Its measurement would provide an estimate of the vibrational relaxation time of the center. The implications of the relaxation rate on electronic energy transfer between like species are briefly discussed.
A Model for the Phase Diagram of Fe(PdαxPt1−x)3 Showing a Quadruple Point
Authors: Wissel Ch.
Published: July 2007

The pseudo‐binary Cu3Au type ordered alloys Fe(PdxPt1−x)3 are described using a Heisenberg Hamiltonian. The free energy and the sublattice magnetization are calculated in the molecular field approximation at T = 0 and near the transition to the paramagnetic state for all values of x (0 ≦ x ≦ 1). For special values of the parameters full agreement is obtained with the experimental results including the quadruple point. In other cases some other interesting results are obtained.
Electron and Hole Mobilities in Solid and Liquid Benzene C6H6
Authors: Hirth H. , Stöckmann F.
Published: July 2007

The mobility of electrons and holes has been deduced in the temperature range −100 to 60 °C from transit time measurements. The electron mobility in solid benzene can be described by μ ∝ T−2, which may be explained by interaction with acoustic phonons. The hole mobility in solid benzene rises exponentially with increasing temperature. The mobility is controlled by interaction with traps depending on the purity of the samples. Approaching the melting point there is a reduction of the electron and hole mobility. This may be explained by thermally produced lattice defects which show up already some ten degrees below the melting point. In liquid benzene the mobilities are three orders of magnitude smaller rising slightly upon temperature. In the liquid polaron‐like states are formed, where the charge carriers propagate by an intermolecular hopping mechanism.
Hot Electron Kerr Effect in Semiconductors with Non‐Parabolic Dispersion Law of Energy
Authors: Almasov L. A. , Dykman I. M.
Published: July 2007

The phenomena of birefringence and plasma frequency splitting in semiconductors with isotropic non‐parabolic dispersion law of carrier energy in the presence of a dc electric field have been considered on the basis of expressions obtained in a previous work [l]. n‐InSb is taken as an example for the numerical calculations. It is shown that the phase shift due to the birefringence can attain a considerable value. The effect can, in principle, be used for creation of modulators, shutters, and circulators of electromagnetic radiation in the infrared range.
Thermal Conductivity of Ni++ Doped AgCl
Authors: Donecker J.
Published: July 2007

The thermal conductivity of quenched AgCl crystals doped with 190 to 3300 ppm Ni++ has been measured between 2 and 70 °K. The values have been analysed within the framework of the relaxation‐time approximation. It was not possible to describe the temperature dependence of each of the experimental curves or their concentration dependence solely with a Rayleigh‐type phonon scattering mechanism. A perturbation treatment of the scattering by a mass‐defect complex yields deviations from the Rayleigh behaviour at high frequencies. But these theoretical deviations cannot explain the experimental data. It is suggested that resonance scattering of the vacancy is the additional scattering mechanism. The values of the Rayleigh‐scattering parameters of the complex are discussed.
On the Spin Wave and Exciton Density of States in an Antiferrornagnet
Authors: Baryakhtar V. G. , Petrov E. G.
Published: July 2007

The singularities in the spin wave and exciton density of states are considered. It is shown that the strongest singularity in the magnon density of states g(ω) has the form g(ω) ∼ |ω – ωc|−½. This singularity is due to the constant‐energy surface. The strongest singularities in the exciton state density are observed for wave vectors which lie on the straight lines directed from the centre of the first Brillouin zone to the centres of its hexagonal faces.
Study of Colour Centres in BaCl2 and BaBr2
Authors: Houlier B. , Yuste M. , Chapelle J. P. , Taurel L.
Published: July 2007

After additive or electrolytic coloration, BaCl2, and BaBr2, single crystals exhibit F‐centres, whose proper symmetry is C3. The hyperfine spectrum in BaCl2 can be accounted for with the assumption that there exist two types, FI and FII, of F‐centres, and that each hyperfine constant referring to a Ba2+ ion (k) is given by an empirical expression ak = gN A/d, where dk is the distance between the ion k and the F‐centre. The two types of F‐centres have the same g‐tensor with principal values, in the case of BaCl2, gXX = 1.966; gYY = 1.976; gZZ = 1.986. The F‐centres can be optically bleached at room temperature, and then regenerated by heating above 100 °C.
Mössbauer Effect Studies of 57Co and 57Fe in β‐Rhombohedral Boron
Authors: Stanke F. , Parak F.
Published: July 2007

The behavior of 57Fe and 57Co doped β‐rhombohedral boron was investigated by means of Mössbauer spectroscopy. Samples prepared by diffusing 57Fe at temperatures above 1000 °C and 57Co at temperatures of 1500 °C or higher showed a reproducible quadrupole interaction and a strongly anisotropic ƒ‐factor. Using combinations of the different parameters obtained from the spectra it was attempted to determine the position of the impurities. There is some evidence for an incorporation within the 28 atom subgroup. It was shown that the sites occupied by Co and Fe do not have a C5 v point symmetry.
X‐Ray Spectra of Disordered Alloys
Authors: Babanov Yu. A. , Sokolov O. B. , Sorokina M. F.
Published: July 2007

A formula for the X‐ray spectrum intensity is derived taking “cross‐over” transitions into account. The “cross‐over” transition is shown to be only one of possible mechanisms explaining the additional maximum in the X‐ray spectra of disordered binary alloys. The results of the calculation of the intensity by the Green's function method are used for the qualitative interpretation of the Lβ2‐emission bands of Ag and Pd in a set of disordered alloys.
Light Scattering and Infrared Absorption in Cuprous Oxide
Authors: Reydellet J. , Balkanski M. , Trivich D.
Published: July 2007

A great number of bands have been observed by light scattering and infrared absorption in Cu2O crystals corresponding to transitions most of which are normally forbidden in the perfect crystal. An analysis of the symmetry properties of the lattice modes in the presence of defects is given. In particular the role of each symmetry site for the defects is investigated and the observed modes interpreted as being due to actived band modes and localized modes. Double phonon processes are observed as well as the normally allowed optical transtions for the perfect crystal.
Reflectivity and Magnetoreflectivity Measurement on n‐Type PbSe
Authors: Mycielski A. , Aziza A. , Balkanski M. , Moulin M. Y. , Mycielski J.
Published: July 2007

Infrared refectivity and magnetoreflectivity measurement were performed at 30 °K on‐type PbSe samples with electron concentrations rangingfrom 4.5 × 1017 to 4.3 × 1018cm−3. From the best fit of experimental and theoretical curves the electron effective mass and relaxation time were determined as functions of electron concentration. The dependence of the effective mass on electron concentration is in good agreement with Kane's model of the conduction band.
Higher Anharmonic Contributions to Diffuse X‐Ray Scattering
Authors: Reid J. S.
Published: July 2007

An investigation has been made into the importance of higher‐order anharmonic interference terms between the one‐phonon and multiphonon scattering. The second‐order terms are discussed in detail and an exact expression is also given for the general term involving the direct coupling of the one‐phonon and n‐phonon scattering processes. The most interesting second‐order process has been evaluated directly for the alkali halides NaF, NaCl, NaBr, NaI, KCl, and KBr with the scattering vector lying along [100]. It is concluded that the first‐order term is dominant most of the way to the zone boundary at temperatures up to at least 600 °K. This rapid convergence appears to continue for the third and higher‐order diagrams and these are not expected to contribute significantly to experimental intensities. However, near the zone boundary the second‐order term can be a few percent of the total scattering and hence should be considered.
Remarks on Screening of Positive Particles in an Electron Gas
Authors: Stachowiak H.
Published: July 2007

The March‐Murray formalism is used in the first approximation to study two effects which are neglected by the static RPA approximation. These are the exchange Hartree‐Fock correction and the effect of the small mass of the positive particle in the case of a positron and their influence on the screened potential of the positive particle is investigated. The results are of interest for the theory of positron annihilation in metals.
Magnetic and Excitonic Instabilities in a Two‐Band Model
Authors: Teleman E. , Ciobanu G. , Fazakas A. B.
Published: July 2007

A two‐hand model with short range interactions between electrons is studied in order to find magnetic and excitonic instabilities. Using the vertex part equation method, some general conditions for the appearance of instabilities leading to phase transitions are derived. Due to the existence of different coupling constants and the possibility of various band shapes, many cases appear and are discussed.
Debye Theory of Dislocations
Authors: Mihály L. G. , Tichy G. G.
Published: July 2007

Continuum models of dislocation lead to divergencies in the core. Using the Debye model, i. e. a cutoff at the wave vector kD, the divergency, inherent to the continuum approximation, disappears and an explicit expression for the cutoff radius in the coordinate space is obtained. The static Green's function of the elasticity and the stress field of a screw dislocation are calculated and shown that in the classical elasticity theory a singularity appears on the surface along which the dislocation is introduced. This stress however can be neglected because of the atomic structure of the crystal.
The Hindered Rotation in Various Fields of Cubic Symmetry
Authors: Beyeler H. U.
Published: July 2007

The hindered rotation in cubic fields of various shapes and intensities is studied. The “Devonshire Model” is extended to include cubic fields being linear combinations of spherical harmonics of order four and six. In contrast to Devonshire's potential, this two parameter manifold of potentials contains some with the energy minima at the 〈110〉 directions. Agreement is found between the low‐lying rotator levels in a large hindering potential and these of harmonic oscillators localized in the minimum wells of the potential.
Thermomodulation of Optical Parameters in Tellurium
Authors: Herrmann Kabin , Dumröse H.
Published: July 2007

A temprature modulation of the optical absorption, x, and the reflection coefficient, R, in tellurium is made for the first time by a periodicallyi haeating CO2 ‐laser beam. For highlyi doped tellukrium the thermoreflection spectra show a stong temperature and polarisation dependence and are discussed as far as fpossible in terms of xisting energy band models. Thermoabsorption in the optical band‐gap region may be interpreted by indirect transitions and show that the temperature dependence of the band gap is not the dominating process.
Binding Energy and Compressibility of Alkali Metals
Authors: Kachhava C. M.
Published: July 2007

The binding energy and compressibility of alkali metals are calaulated from the Ashcroft's empty core model potential on the basis of core radii obained by the fittings to the phonon data. The importance of overlap exchange repulsion and the correct dielectric screening theory is discussed. On the whole, a very satisfactory agreement between theoretical results and experimental data is obtained from these considerations.
Spectre d'absorption et champ cristallin agissant sur I'ion Nd3+ dans I'oxysulfure de neodyme
Authors: Souillat J. C. , Rossat‐Mignod J. , Linares C.
Published: July 2007

Des spectres d'absorption nous ont permis de préciser la structure électronique de l'ion Nd3+ dans le composé Nd2O2S. Les paramètres de Slater F2 = 326,3 F4 = 47,95, F6 = 5,26 cm−1 et la constante de couplage spin‐orbite ζ = 855,5 cm−1 rendent compte de la structure de l'ion “libre” avec une déviation quadratique moyenne de 67 cm−1. Dans le cadre du couplage intermédiaire les niveaux de champ cristallin issus de 4I9/2, 4F5/2, 4F5/2, 4F7/2 et 4F9/2 nous ont permis de déterminer les paramètres de champ cristallin.
The Raman Spectrum of Amorphous Tellurium
Authors: Brodsky M. H. , Gambino R. J. , Smith J. E. , Yacoby Y.
Published: July 2007

The reduced Raman spectrum of amorphous Te consists of broad maxima at 90 and 157 cm−1. The spectrum reflects the vibrational density of states and is interpretable in terms of disordered chains. The highest vibrational energies of the disordered chains are higher than the corresponding energies of the ordered chains in crystalline Te. The higher vibrational energies in the amorphous form are due to the reduced effectiveness of long range forces which tend to depress vibrational energies in the crystalline form.
Investigation of Hf Interaction of 181Ta Dissolved in the Ni Lattice
Authors: Bożek E. , Hrynkiewicz A. Z. , Goloczewski J. , Goss R. , Styczeń J.
Published: July 2007

The hyperfine field at Ta nuclei diffused into a nickel foil was studied by the PAC method both with and without and external polarizing magnetic field. The value Hhf = (−82.6 ± 1.1) KG is smaller than the reported one of 90 kG, which could be explained by the large field distribution observed. For assumed Lorentzian shape of the field distribution FWHM (δ) = 22%.
Galvanomagnetic Properties of HgTe in High Magnetic Field
Authors: Zakrzewski T. , Dziuba E. Z.
Published: July 2007

The dependence of the Hall coefficient and conductivity of mercury telluride crystals on the magnetic field has been investigated at temperatures 4.2, 77, and 300 K. The results are explained with the assumption that the transport phenomena are caused not only by electrons and holes but also by additional low mobility carriers. These additional low mobility carriers are assigned to an impurity band.
Anomalous Spin‐Lattice Relaxation by Quasi–Spin Waves in K4Fe(CN)6·3H2O
Authors: Kubarev Yu. G. , Moskvich Yu. N. , Zeer E. P.
Published: July 2007

The influence of quasi‐spin waves on the spin‐lattice relaxation time, T1, in ferroelectric K4Fe(CN)6·3H2O crystals is considered. It is shown that the quasi‐spin waves theory explaines the anomalous behaviour of T1 near the ferroelectric transition temperature Tc. The calculated value of the critical shortening of T1 at Tc and the angular dependences of T1 near Curie temperature agree well with experimental data.
On the Bi‐Exciton Formation in Crystals
Authors: Bobrysheva A. I. , Miglei M. F. , Shmiglyuk M. I.
Published: July 2007

The binding of two free excitons into a bi‐exciton due to direct exciton‐exciton interaction is studied. The process of bi‐exciton formation via exciton–phonon interaction is not taken into account. By group theory methods the wave functions of two interacting excitons are constructed for a model applicable to CuCl and Cu2O crystals. Making use of these functions, the Fourier transform of exciton‐exciton interaction energy is calculated. By the Green's function method the dissociation energy of the bi‐exciton is obtained in the adiabatic approximation (the exciton‐exciton interaction energy is much smaller than the ionization energy of the exciton). A numerical estimate and comparison with experimental values is made for CuCl crystals.
Akustische Kernresonanz an Na23 im NaF‐Einkristall
Authors: Grande S. , Shutilov V. A. , Fershtat L. N.
Published: July 2007

Bei Zimmertemperatur wurde die akustische Sättigung der magnetischen Kernresonanz von Na23 mit Hilfe von Impulsverfahren im NaF‐Einkristall beobachtet. Die T2‐Werte sind anisotrop und werden hauptsächlich durch Dipol‐Wechselwirkung bedingt. Die Spin–Gitter‐Relaxationszeit T1 ≈ 5 s wird überwiegend durch Quadrupoleffekte verursacht, zeigt jedoch eine Winkelabhängigkeit. Die Ultraschall‐Sättigung der Übergänge Δm = ±2 erfolgt mit akustischen Längswellen über die Quadrupol‐Spin–Phonon‐Kopplung. Es wird die Frequenz–Amplitudencharakteristik der Sättigung und daraus die Linienbreite der akustischen Sättigung bestimmt. Die Spin–Phonon‐Kopplungskonstante und γ∞ werden errechnet. Aus dem experimentellen Abfall der Sättigung bei Einstrahlung am Rand der Linie wird die Rolle des Dipol‐Reservoirs bei der akustischen Sättigung behandelt.
Optical and Magnetooptical Absorption in Pure HgTe
Authors: Saleh A. S. , Fan H. Y.
Published: July 2007

Infrared absorption in pure, n‐type HgTe has been studied in the wavelength range 3.5 to 35 μm. At ≈ 12 °K, the absorption continues to decrease with decreasing photon energy showing that the energy gap is < 40 meV. The absorption at 300 °K rises with decreasing photon energy after passing a minimum at ≈ 55 meV, due to effect of free carriers. Under an applied magnetic field, up to 30 kG, the absorption at a fixed wavelength shows oscillatory variation with the field strength. These oscillations give a determination of the effective mass and the g‐factor of conduction electrons, and show that the energy gap is close to zero.
Propagation of Surface Spin Waves in the Inhomogeneous Field of Rectangular YIG Samples
Authors: Schilz W.
Published: July 2007

The propagation velocity and the coupling of surface spin waves to the outside rf field in rectangular YIG samples is strongly influenced by the local variation of the internal magnetic field. An experimental study of these effects is made and a clarification of the reflection mechanism of surface modes near the ends of the sample is presented.
Field Dependence of Exciton‐Induced, Contact‐Limited Photocurrents in Molecular Crystals
Authors: Killesreiter H. , Baessler H.
Published: July 2007

The current–field dependence of contact‐limited photocurrents resulting from dissociation of Frenkel excitons at the interface between molecular crystals and a metal or a fatty acid layer is investigated. In the field range 3 × 103 Vcm−1 < F < 105 Vcm−1 both photocurrent and dark current follow a j = j0 (1 + bF) relationship with no indication of a field‐independent limiting current. An explanation is given in terms of the Onsager model for thermally activated diffusional escape of a carrier inside the crystal from the attractive Coulombic potential of its countercharge. At higher fields the Frenkel‐Poole effect becomes operative. It is concluded that exciton dissociation initially produces a thermalized carrier inside the crystal.
The Korringa‐Kohn‐Rostoker and Augmented Plane Wave Method for Non‐Spherically Symmetric Muffin‐Tin Potentials
Authors: John W. , Lehmann G. , Ziesche P.
Published: July 2007

By means of an appropriate description of the scattering states of non‐spherically symmetric potentials a generalized KKR method is derived. As an example a lattice with several atoms per unit cell is considered. Also a generalized APW method is derived.
Zur Ultrarot‐Absorption in Ionenkristallen
Authors: Knohl U.
Published: July 2007

Die ultrarote Absorption und Reflexion von Lithiumfluorid wurden bei 7,5, 70 und 300 °K in einem Schalenmodell mit deformierbaren Schalen [4,5] berechnet, wobei Anharmonizität dritter Ordnung berücksichtigt wurde. Es zeigte sich, daß die anharmonischen Beiträge der Coulomb‐Kräfte von gleicher Bedeutung sind wie diejenigen der Kräfte kurzer Reichweite. Der Einfluß des nicht‐linearen Dipol‐Moments zweiter Ordnung stellte sich als vernachlässigbar heraus bei Frequenzen unterhalb der zweieinhalbfachen Resonanzfrequenz. Die Übereinstimmung zwischen Experiment und Theorie mit einem anpaßbaren Parameter ist sehr gut bei tiefen Temperaturen, wird jedoch schlechter bei höheren Temperaturen wegen der Vernachlässigung biquadratischer Glieder in der Anharmonizität.
Exciton Interaction in CdSe
Authors: Bille J. , Liebing H. , Mengel P.
Published: July 2007

The emission spectrum of CdSe platelets has been observed at 2 K under laser excitation from 0.1 to 10 MW/cm2. By light pulses from a frequency‐doubled Nd:YAG laser (531 nm) hot electrons are created in CdSe, which thermalize into the exciton bands within 10−13 s and decay radiatively within 10−9 s. The spectra, obtained by this excitation, show a considerable increase of the temperature of the crystals. By resonant excitation, i.e. direct excitation of the valence band electrons into the exciton bands by a tunable laser, this temperature increase could be avoided. A Bose condensation of the excitons, as expected by Gross et al., was not observed. Instead, results have been obtained which seem to indicate the formation of exciton molecules and the condensation of the excitons into drops with an electron‐hole pair density of about 2 × 1017 cm−3.
Hartree‐Fock Calculation of Electron States in Small Aggregates of Lithium and Beryllium Atoms
Authors: Stoll H. , Preuss H.
Published: July 2007

SCF‐MO‐LCGO calculations, which have been performed on small periodic aggregates of Li and Be atoms, are reported. It is shown that properties which are characteristic of solids can be found already in comparatively small systems. The variation of these properties with shape and size of the systems is discussed. In addition effects which are attributed to the influence of the surface are treated, e.g. the fluctuation of charge density and the distortion of lattice in the zone near the boundaries.
A Method for Calculating Signals of NMR‐Pulse Experiments in Solids
Authors: Ernst H. , Schmiedel H. , Schneider H.
Published: July 2007

A method for calculating signals of NMR pulse experiments in solids is given which allows to calculate the detailed form of signals between pulses. Applying this method to a multiple pulse cycle given by Mansfield and Waugh‐Huber one is able to discuss the detailed form of the first few echoes. Considering a great number of pulses this method diverges, however.
Spectra of Electronic Excitations in CaF2, SrF2, and BaF2 in the 8 to 150 eV Range
Authors: Frandon J. , Lahaye B. , Pradal F.
Published: July 2007

The characteristic energy losses suffered by a 20 keV electron beam through CaF2, SrF2, and BaF2 films have been measured with a magnetic spectrograph. From the energy loss function — Im ε−1, optical constants are calculated by means of Kramers‐Kronig analysis in the 8 to 40 eV range and are compared with the available reflectance data, especially those obtained with synchrotron radiation. The observed structure is tentatively explained for the three compounds, following a band model based upon symmetry considerations. The origin of the energy losses corresponding to the soft X‐ray range is also briefly discussed.
Optical Phonon Spectrum of Sodium Bromate by Laser Raman Spectroscopy
Authors: d'Andrea A. , Fornari B. , Mattei G. , Pagannone M. , Scrocco M.
Published: July 2007

The optical phonon spectrum of NaBrO3 has been determined by Raman scattering. Comparison with the infrared reflection spectra reported in literature is made. Good agreement is found between the two kinds of measurements. From the polar phonon frequencies a value of 5.63 is calculated for the static dielectric constant, whose experimental value is reported to be 5.7. Dielectric transition strengths and damping constants are also determined from the Raman data.
The Contribution of the Dislocation Forest to the Flow Stress
Authors: Schoeck G. , Frydman R.
Published: July 2007

The formation of junctions between a moving dislocation and the trees of a dislocation forest has been calculated for f.c.c. crystals. In contrast to previous estimates the concept of the orientation‐dependent line energy and a more realistic averaging procedure was used. In a random forest in which the tree length is equal to the tree separation only about 20% of the trees form junctions. The stress to overcome the junctions is higher for screw dislocations than for edges but the exact values depend on the nature of the forest.
A Model Calculation of Optical Absorption in Transition Metals
Authors: Kutty A. P. G. , Manohar C.
Published: July 2007

Using a model Hamiltonian containing a s‐band, d‐band, and s‐d hybridization the optical absorption in the Hartree‐Fock approximation is calculated. It is also shown that RPA corrections to optical absorptions are negligible as long as the s‐band is free‐electron like.
NMR and Susceptibilities of CeAlx Intermetallic Compounds
Authors: Niculescu V. , Pop I. , Rosenberg M.
Published: July 2007

NMR and magnetic susceptibility of CeAlx intermetallic compounds with x = 1, 2, 3, and 4 were investigated. The most important contributions to the magnetic susceptibility are the Curie‐Weiss term, expressing the paramagnetism of the localized f‐electrons and the temperature independent term due to an enhanced Pauli paramagnetism, the enhancement factor increasing exponentially with the cerium concentration. The temperature dependence of the observed strong positive Knight shift is explained by the enhancement of the hyperfine interaction at the Al27 sites, owing to the exchange polarization of the conduction electrons by the localized cerium f‐electrons. The exchange constants Jfs and θp are negative for the four investigated compounds. A positive sign of the sum results which characterizes the oscillating behaviour of the indirect exchange interaction between the rare‐earth ions.
Effect of Trigonal Warping on Landau Levels of Graphite
Authors: Gupta O. P. , Wallace P. R.
Published: July 2007

We have devised a successive approximation scheme for taking account of the trigonal warping (γ3) terms in the calculation of the lower‐lying magnetic levels of graphite at fields ⩾ 20 kG. A two‐component effective Hamiltonian has been introduced for these levels. Starting from analytic expressions in the case γ;3 = 0, straightforward and simple iteration schemes have been devised (i) to take account of the higher levels and (ii) to obtain the γ3 contributions. It is found that the trigonal warping changes the levels only by a few per cent except for the n = 0 levels. The levels n = 0, 1, 2, 3 have been calculated for a range of fields from 20 to 200 kG.
Photoemission Studies of Electronic Colour Centres in Additively Coloured KCl and KBr Crystals
Authors: Kashkai A. D. , Berezin A. A. , Arseneva‐Geil A. N. , Matveev M. S.
Published: July 2007

The spectral distribution of the optical absorption and the photoemission quantum yield and the energy distribution of the photoelectrons are studied in the range of the photon energies of 2.0 up to 5.5 eV on additively coloured KCl and KBr single crystals with a concentration of F‐centres of 1017 up to 1018 cm−3. Several energy levels in the forbidden band, responsible for the photoemission, are found. The positions of these levels in respect to the vacuum level are 2.8, 2.5, and 2.1 eV for KCl and 2.9, 2.3, and 1.7 eV for KBr. These levels are supposed to correspond to the ground states of F‐, M‐, R‐electronic colour centres. The above energy values are compared with the theoretical values of the ionization potential of F‐, M‐, and R‐centres calculated in the quasimolecular model of F‐aggregate colour centres. The calculated first ionization potentials, as well as the experimental energy values, decrease when the electronic colour centre became more complex. In KBr crystals an additional energy level 1.4eV below the vacuum level is found which probably corresponds to the N‐centre.
Mechanische Nachwirkung in Kupfer und Nickel, verursacht durch Hantel‐Zwischengitteratome
Authors: Wanger F. J.
Published: July 2007

Mit Hilfe eines Torsionspendels wurde die Nachwirkung des Elastizitätsmoduls nach plastischer Verformung an Cu und Ni gemessen. Die mechanische Nachwirkung wird durch die beim Verformen erzeugten Hantel‐Zwischengitteratome verursacht, welche im Spannungsfeld einer Versetzung zu einer versetzungsinduzierten Umorientierung (Orientierungsnachwirkung) und Diffusion (Diffusionsnachwirkung) angeregt werden. Aus der Orientierungsnachwirkung ergibt sich die Aktivierungsenergie für die Umorientierung der Hantel‐Zwischengitteratome im Fall von Cu zu 0,67 eV und für Ni zu 0,87 eV. Die Wanderungsenergie der Zwischengitteratome folgt aus der Diffusionsnachwirkung und beträgt für Cu ebenfalls 0,67 eV, für Ni hingegen 0,99 eV. Die Ergebnisse der mechanischen Nachwirkungsmessungen sind in voller Übereinstimmung mit den Resultaten aus inneren Reibungsund magnetischen Nachwirkungsmessungen.
Raman‐and Infrared Reflection Spectra of KBrO3 Single Crystals
Authors: Unger B. , Haussühl S.
Published: July 2007

Detailed Raman‐scattering and polarized infrared reflection measurements are reported on KBrO3 at 300 K and at liquid helium temperature. The angular dispersion of all extraordinary phonons is compared with theory. Also the static dielectric constants are determined from the measured frequencies.
The E(k) Relation for a Two‐Band Scheme of Semiconductors and the Application to the Metal‐Semiconductor Contact
Authors: Flietner H.
Published: July 2007

The E(k) behaviour is calculated for a two‐band scheme as an approximation for the vicinity of the forbidden gap of a semiconductor. An estimate is given for the validity of the effective mass approximation in the forbidden gap. Application is made to the metal‐semiconductor contact and it is shown that this model describes the experiments for semiconductors ranging from covalent to the ionic type.
On the Investigation of Small Dislocation Loops in Cubic Crystals by Diffuse X‐Ray Scattering
Authors: Trinkaus H.
Published: July 2007

Results for analysing point defects in cubic crystals by diffuse X‐ray scattering as given in a previous paper are applied to small dislocation loops. The scattering pattern in the immediate vicinity of Bragg reflections is shown to indicate whether or not the crystal contains predominantly one of several high symmetry configurations. In the presence of dislocation loops with a lower symmetry or of a mixture of different configurations, average fractions of edge‐ and shear‐components can still be determined. The corresponding parameters also give information about the shape of defects in the case of other types of defect clusters.
Spectroscopic Properties of TbAsO4
Authors: Wüchner W. , Bohm W. , Kahle H. G. , Kasten A. , Laugsch J.
Published: July 2007

Absorption spectra of Tb3+ in TbAsO4, indicate that this material undergoes a Jahn‐Teller induced phase transition from tetragonal to a lower, probably orthorhombic, symmetry at TD = (25.5 ± 2.5) K. Zeeman spectra with the magnetic field in the basal plane taken below TD, show a large anisotropy which is caused not only by the g‐factor but also by large internal fields. The g‐tensor of the isolated two lowest singlets, which are separated less than 0.6 cm−1 at 1.74 K, is uniaxial with g110 = 17.0 ± 1.0 in the [110] direction. Measurements down to 0.77 K show a magnetic phase transition at TN, = (1.48 ± 0.04) K to an antiferromagnetically ordered state which increases the splitting of the two lowest lying singlets up to (2.62 ± 0.06) cm−1 at 0.77 K.
Covalency and Deformability of Ag+‐Ions in the Lattice Dynamics of Silver Halides
Authors: Fischer K. , Bilz H. , Haberkorn R. , Weber W.
Published: July 2007

The lattice vibrations of the silver halides AgCl and AgBr have been investigated using a shell model with deformable ions. The deformability of the d‐shell of the Ag+‐ion and its covalent coupling to the neighbours have been considered by introducing two new degrees of freedom in the electronic shell of the Ag+‐ion. They are of quadrupolar (Γ12+) and of rotational (Γ15+) symmetry character. The calculations have been confirmed by comparison with the unpolarized Raman spectra of second order.
Magneto‐Optical Spectra Due to Excitons in Cuprous Halides
Authors: Suga S. , Koda T. , Mitani T.
Published: July 2007

Faraday rotation and magnetic circular dichroism spectra have been investigated in the exciton absorption region of CuCl, CuBr, and their solid solutions. The characteristic temperature dependence of the observed spectra is successfully interpreted in terms of the mixing effect of the Z3 and Z1,2 excitons caused by the magnetic field.
On the Theory of Nonradiative Transitions in Polar Media II. Processes with “Nixing” of Quantum and Classical Degrees of Freedom
Authors: Dogonadze R. R. , Kuznetsov A. M. , Vorotyntsev M. A.
Published: July 2007

The probability of radiationless transition between two multidimensional parabolic terms is calculated for the processes with “mixing” of quantum and classical degrees of freedom. The dependence of the transition probability on the heat of the process is investigated. The transition is shown to be of quasi‐activation less and quasi‐barrier less character at large values (modulo) of the heat of the process.
Mechanische Nachwirkung durch induzierten Snoek‐Effekt
Authors: Hornung W.
Published: July 2007

Für kubisch‐flächenzentrierte Metalle wird die mechanische Nachwirkung aufgrund weitreichender Wechselwirkung zwischen Versetzungen und Zwischengitteratomen in der Hantellage (ZGA) berechnet. In Übereinstimmung mit Experimenten von Wagner ergeben sich zwei Anteile im zeitlichen Verlauf von Moduldefekt und Dämpfung, von denen der eine mit der Umorientierung der ZGA in homogenen Spannungsfeldern, der zweite mit den bei der weitreichenden Diffusion auftretenden Wanderungsschritten verknüpft ist. Dies erklärt das unterschiedliche Verhalten von Kupfer und Nickel in solchen Experimenten.
On the Verwey Transition in Magnetite
Authors: Lorenz B. , Ihle D.
Published: July 2007

The Verwey transition in magnetite is investigated using a band model with interatomic Coulomb energy introduced by Cullen and Callen. This model is treated in the Hartree‐Fock approximation. A tight‐binding calculation of the band structure with Verwey ordering yields a gap in the single‐particle excitation spectrum for M = (U/2|h|) m > 1 (U Coulomb energy, h overlap integral, m ordering parameter). Below the Verwey temperature Tv semiconducting behaviour for M > 1 and a region of gapless Verwey ordering and semimetallic behaviour for 0 < M ⩽ 1 is found.
On the Theory of Parametric Excitation of Magnetostatic Modes in Infinite Ferromagnetic Plates by Parallel Pumping
Authors: Dintner H. , Klupsch Th.
Published: July 2007

Based on the conception of a generalized Polder tensor, the Damon‐Eshbach theory of magnetostatic modes propagating in infinite ferromagnetic plates magnetized parallel to the surfaces is extended including a microwave field parallel to the de field. Restricting to modes obeying the parametric resonance condition, a set of equations determining the mode parameters is derived which exhibit the description of instabilities. The critical microwave field strength hc, for setting‐in the instability with respect to the whole system is calculated for X‐band pump frequencies within the volume mode region using YIG material parameters by introducing a phenomenological damping constant. hc, plotted over the static field strength (“butterfly curve”) appears as a smooth curve because of a favoured excitation of modes situated within a quasi‐continuous part of the spectrum. The possibility of an excitation of surface modes with high pumping efficiencies is suggested.
Spectroscopic Confirmation of a Crystallographic Phase Transition and of Antiferromagnetic Ordering in TbPO4
Authors: Böhm W. , Herb R. , Kahle H. G. , Kasten A. , Laugsch J. , Wüchner W.
Published: July 2007

The optical absorption spectrum of Tb3+ in TbPO4, shows that this substance undergoes a crystallographic phase transition from tetragonal to some lower symmetry at TD = (3.7 ± 0.5) K. This phase transition lifts the symmetry allowed degeneracy of the lowest lying crystal field component. A magnetic field applied parallel to the optical axis cancels the crystallographic distortion. Below TD = (2.15 ± 0.10) K TbPO4, orders antiferromagnetically with the magnetic moments neither parallel nor perpendicular to the optical axis. An induced magnetism is required for this ordering. The spectroscopic measurements indicate that this magnetic transition should be of first order.
Spin Configuration and Intrinsic Coercive Field of Narrow Domain Walls in Co5R‐Compounds
Authors: Hilzinger H. R. , Kronmüller H.
Published: July 2007

In ferromagnetic materials with large anisotropy constants (K1 > 108 erg/cm3) the domain wall width, δB, is of the order of magnitude of a few lattice constants (10 to 30 Å). The spin configuration of such 180°‐walls in Co5R‐compounds is determined within the framework of the Heisenberg model. It is found that the deviations from the micromagnetic solution occur mainly in the centre of the wall. The wall energy was found to depend periodically on the position of the wall centre with respect to the crystal lattice. The oscillations of the wall energy give rise to a coercive field Hc = const (A/D) exp [– π δB/D] (A exchange constant, D wavelength perpendicular to the domain wall). The possibilities to obtain large intrinsic coercive fields are discussed in some detail.
Constant Coupling Approximation for Amorphous Magnets
Authors: Kobe S. , Handrich K.
Published: July 2007

The constant coupling approximation is used to determine the critical temperatures of amorphous ferro‐ and antiferromagnets, for both the Heisenberg and Ising model. Due to the structure fluctuations of the exchange integrals a decrease of the Curie temperature and the Néel temperature, respectively, are found in comparison with the corresponding crystalline case. This result agrees qualitatively with results of both the Green's function and the high‐temperature expansion treatments.
Determination of the Dielectric Tensor of Pyrene (C16H10) by Means of Electron Energy Loss Measurements
Authors: Venghaus H.
Published: July 2007

Electron energy loss spectra from 0 to 40 eV were taken on pyrene single crystals with 60 keV electrons. The energy loss functions Im (−1/εii) (i = x, y, z) were derived from spectra recorded with different orientation of the momentum transferred to the crystal by the electrons scattered inelastically. By means of a Kramers‐Kronig transformation the main axis elements εii of the dielectric tensor were calculated from Im (−1/εii). The results obtained are interpreted in terms of excited molecular states in relation to the results of optical absorption measurements, theoretical work and other electron energy loss experiments.
An EPR Study of a Neutral Manganese Center in NaCl
Authors: Ikeya M.
Published: July 2007

An EPR study of the MnO‐E center below room temperature was made in X‐irradiated and F‐bleached NaCl. In addition to the isotropic spectrum with g = 2.0018 and A = 19.5 × 10−4 cm−1 at room temperature, the spectrum at low temperature shows fine structures with an axial field of D = 160 × 10−4 cm−1 and 〈110〉 and 〈111〉 symmetries, which were analyzed from the shift of (M8 = 1/2 ⇋ M8 = −1/2) transitions. The temperature dependence of the hyperfine coupling constant shows anomalies. These results were discussed based on the assumption that the center is an off‐center.
Fluorine Self‐Diffusion in Pure and Doped CdF2
Authors: Süptitz P. , Brink E. , Becker D.
Published: July 2007

Using F 18 as tracer the fluorine diffusion is measured in single crystals of CdF2, undoped and doped with Na+ or La3+, in the temperature region 270 to 640 °C. The results are discussed with respect to the disorder and the attainable conversion of CdF2 from an insulator to a semiconductor. The fluorine diffusion is large compared with the cation diffusion. Its dependence on doping is characteristic of Frenkel disorder in the anion sublattice, whereby the fluorine vacancies are more mobile than the fluorine interstitials. From the temperature dependence of the diffusion coefficient follows a value of 0.39 eV for the activation enthalpy of motion of fluorine vacancies and 2.1 eV for the formation enthalpy of a Frenkel pair.
The Spectrum of Optical Absorption by Mott Excitons in an Electric Field
Authors: Monozon B. S. , Seysyan R. P. , Shelekhin Yu. L.
Published: July 2007

The wave functions of a Coulomb particle in a weak uniform electric field are obtained by the method of reference problems. Expressions are given for polarized light absorption coefficients for the yellow excitonic series of cuprous oxide. Field dependence of line intensities is established for field induced lines and those allowed in the absence of field. The spectrum was taken of the yellow excitonic series of Cu2O in the electric field at T = 4.2 °K as well as the differential spectrum of this series which was due to the difference in absorption of light polarized in the parallel and perpendicular direction to the electric field. Theory and experiment are in good agreement.
Hole g‐Factor in Sn‐Doped Bismuth
Authors: Giura M. , Marcon R. , Marietti P. , Presutti E.
Published: July 2007

A new method for the measurement of the g‐factor is presented. This gives a more detailed knowledge of the hole dispersion law in Sn‐doped Bi and allows to explain some experimental results obtained by Bate et al. A feature of the method is the possibility of determining whether the Landau level splitting is larger or smaller than the spin splitting.
Directional Dependence of Extraordinary Infrared Oscillator Parameters of Uniaxial Crystals (II)
Authors: Lamprecht G. , Merten L.
Published: July 2007

The classical dispersion equation for the extraordinary IR spectrum of uniaxial crystals is shown to be valid, even if damping is included. The directional dependence of the damping factors γj(ϑ) and their effect on the other oscillator parameters (dispersion frequencies ωj(ϑ), oscillator strengths 4 πσj(ϑ) is investigated in detail. Evidence is given for the validity of a sum rule of damping factors. The theory is applied to MgF2, ZnF2 and α‐quartz.
A Closed Formula for Nuclear Quadrupole Relaxation in Ionic Crystals
Authors: Pietilä A.
Published: July 2007

In the harmonic approximation of lattice dynamics, a closed formula for transition probabilities, which includes all phonon processes appearing in the treatment given by van Kranendonk, has been derived. This has been done by Fourier transforming the field gradient instead of the usual power series expansion with regard to the nuclear displacements. The formulae for the point‐ion and the overlap potential models are presented in detail. Fürther, the expressions are used to demonstrate the validity of the power series method for the proceeding potentials. A shell model is suggested in order to include the dynamical changes of the ionic wave functions in the calculation of the field gradient.
Two‐Phonon Processes in Crystals of D Symmetry
Authors: Bercha D. M. , Puga G. D. , Borets A. N.
Published: July 2007

It is established that the absorption in the intermediate infra‐red region in BiOI, BiOBr, and BiOCl has two‐phonon character. Group‐theoretical investigations of the phonon spectrum and two‐phonon infra‐red absorption and Raman scattering for these crystals are Carried out. The selection rules obtained enables one to ascertain what phonon combinations from one or different branches, in what points of the Brillouin zone of crystals of symmetry DJ are allowed or forbidden.
Optical Properties of CrO Ion in Orthorhombic Sulphates
Authors: Radhakrishna S. , Pande K. P.
Published: July 2007

Vibrational frequencies of CrO ion embedded in orthorhombic sulphates have been determined from optical and infrared absorption studies. In the optical absorption spectra, the electronic levels are found to have a superposed vibration structure, in the regions 2300 to 2750 Á and 3300 to 3850 Á, with alternate strong and weak lines. The set of strong lines is interpreted to be due to the transitions from the ground electronic state to the vibrational levels of the excited state, whereas the set of weaker lines is assigned to the transitions from one of the vibrational levels of the ground electronic state to the vibrational levels of the upper excited electronic state. It is found that the IR forbidden bands become allowed and are observed in these lattices. No splitting is observed in the degenerate modes in IR spectra.