bluuebunny/crossref_metadata_2025_split huggingface dataset:
Phonon Dispersion in the Layered Compound PbI2
Authors: Dorner B. , Ghosh R. E. , Harbeke G. 
Published: July 2007

AbstractPhonon dispersion curves up to a frequency of 2.5 THz have been measured by inelastic neutron scattering in a sample of Pbl2 with D structure for the three principal symmetry directions. No appreciable two‐dimensional character has been detected in the dispersion of the acoustic modes, and the elastic constants derived from the measurements show only slight anisotropy. Comparisons are made of the present optic mode determination with previous measurements.
Double γ‐Acoustic Resonance in Crystals
Authors: Gabbielyan R. G. , Kocharyan L. A. , Mkrtchyan A. R. , Tumanyan S. S. 
Published: July 2007

AbstractA theoretical analysis is made of nuclear transition probability from the ground state to the sublevels of the first excited state under the influence of γ‐quanta and external acoustic excitation. When the electric field possesses an axial symmetry, Heff = 0 and the ultrasonic oscillation frequency Ω satisfies the resonance condition between the nuclear sublevels, therearetwo possible cases of transition if the energy of the monochromatic γ‐quanta satisfies the resonance. The two cases are analyzed depending on the ultrasonic oscillation power and the angle between γ‐bunch and the direction of ultrasonic excitation. It is shown that for A0/λ≪ 1 and ψ = π/2 the 1 → 2 → 3 transition is more plausible. When Ao/λ ≧ 1, θ = 0, φ = 0, the 1 → 3 transition is more probable.
Crystal–Impurity Energy Transport by Molecular Excitons in the Wave‐Like Limit I. Capture Mechanism
Authors: Markvart T. 
Published: July 2007

AbstractThe treatment, by conventional diffusion methods, of exciton transport from the host crystal to impurities is proved to be valid only in the limit of strong exciton–phonon interaction. I n the present work a possible solution to the problem of crystal–impurity exciton transport is presented in the opposite limit. The assumption of rapid exciton transport within the crystal is used to explain the capture mechanism. Other assumptions are used to bring the final result to a form similar to Forster's for the electronic energy transfer between two molecules.
Far‐Infrared Interband Magnetoabsorption and Band Structure of Hg1−xCdxTe Alloys
Authors: Kim R. S. , Narita S. 
Published: July 2007

AbstractMagnetoabsorption experiments on n‐type Hg1−xCdxTe alloy crystals (x = 0, 0.07, 0.126, 0.148) have been performed in the Faraday geometry in the spectral region 4 to 40 meV at 4.2 K and with magnetic fields up to 50 kOe. In the analysis of the spectra we have used the formulae collectively including the exact interactions among the T8,T6, and T7, bands. Our experimental results and the computed energies indicate that „the modified Luttinger parameters”︁ of the T8 bands are common to samples with various x‐values (x < 0.158).
To the Theory of the ESR Lineshape of Triplet Excitons in Molecular Crystals: Influence of the Quasi‐Incoherent Exciton Motion and Comparison with Experimental Data
Authors: Reineker P. 
Published: July 2007

AbstractIn a recent paper, on the basis of the Haken‐Strobl model for the coupled coherent and incoherent motion of excitons in molecular crystals, an equation of motion for correlation functions has been derived, describing the influence of the quasi‐incoherent motion of triplet excitons on the ESR lineshape. From this equation analytic expressions for the positions and widths of the ESR lines have been derived by second order perturbation theory. In this paper it is shown that these expressions are in excellent agreement with results obtained from exact computer calculations. The dependence of the positions and the widths of the ESR lines on the orientation of the magnetic high field in the crystal and their extreme values are discussed. From the comparison with experimental data of Haarer and Wolf for the line positions averaged values for the fine structure parameters in anthracene are derived as D = 765 G and E = −98.8 G. An effective hopping rate of T1 Y 4.4 × 1010 s−1 has been estimated from the line widths. Finally it is argued that the influence of librations on the ESR linewidth may be as important as that of the motion of the excitons between inequivalent molecules.
Magneto‐Optical Measurements on Two‐Dimensional Magnetic Model Systems
Authors: Arend H. , Von Känel H. , Wachter P. 
Published: July 2007

AbstractThe optical absorption of the two‐dimensional antiferromagnets (CH3NH3)2MnCl4 and (C2H5NH3)MnCl4 has been measured between 0.5 and 5.4 eV in the temperature range 4.2 to 300 K. An absorption maximum at 4.96 eV which only appears at low temperatures is proposed to originate from an inter‐3d transition between antiferromagnetically aligned Mn ions. The temperature variation of the absorption power of this maximum follows the spin correlation function for the two‐dimensional Heisenberg antiferrormagnet. Furthermore the energy of one of the intra‐3d transitions is determined as a function of temperature. However, a separation of the spectral shift of the absorption line into a Zeeman‐ and a crystal field component is not unambiguous.
The Electron and Magnon Spectra of Ferrimagnetic Semiconductors
Authors: Kozintseva M. B. , Zilbervarg V. E. 
Published: July 2007

AbstractA calculation of the magnon spectrum is carried out for a degenerate ferrimagnetic semiconductor. The spin‐polarized conduction electrons indirectly effect the exchange between the local magnetic moments. As a result, in the case of antiferromagnetic direct exchange, the acoustic branch becomes harder with increase in the carrier concentration, whereas the optical branch becomes softer, if the signs of the s‐d exchange integrals are the same in both sublattices. With a certain concentration reached, the initial ferromagnetic ordering becomes absolutely unstable with respect to the long‐wave optical magnons. Also a study is made of the electron spectrum at the Néel point. In spite of the absence of the long‐range order, the short‐range order in the paramagnetic proximity still remains. Therefore, the conduction electrons move in such a manner that their spin follows the direction of the local magnetic moment slowly changing in space. This enables the electron to obtain a considerable gain in energy. It follows that a gigantic conduction band edge shift should also occur above the Néel point.
Dislocation Symmetries in the Integral Theory
Authors: Kirchner H. O. K. 
Published: July 2007

AbstractThe symmetry of the displacement field is investigated for the case that a dislocation runs normal to an axis of twofold symmetry. The three components of the displacement field are found to be even and odd functions of the three components of the Burgers vector. Calculations are done in the “integral theory”. The forms of the basic matrices Q, B, and S for the symmetry considered are found as well.
Role of the Jahn‐Teller Effect in the Excited States of KBr: Tl‐Type Crystals
Authors: Giorgianni U. , Mondio G. , Saitta G. , Vermiglio G. 
Published: July 2007

AbstractParticular attention is paid to the role of the magnetic and thermal perturbations which allow for proper assignment of the B band and quantitative evaluation of degenerate energy peak separations. On the basis of experimental magneto‐optical rotation results an analysis of the effects of the electron‐lattice coupling upon the electronic impurity states is carried out by determining a more comprehensive expression for the excited states of Tl+ in KBr.
Vibrational Spectra, Hydrogen Bonding, and Ferroelectricity in the PbHPO4 Family
Authors: Blinc R. , Arend H. , Kandušer A. 
Published: July 2007

AbstractThe infrared and Raman spectra of polycrystalline PbHPO4, PbHAsO4, CaHPO4, and BaHPO4 were measured at room temperature in the range between 4000 and 30 cm−1, and the temperature dependence of the dielectric constant of these materials was studied. The OHO hydrogen bonds in PbHPO4 and PbHAsO4 are found to be very similar to those in KH2PO4 and KH2AsO4 and the XO4 groups (X = P, As) are – in contrast to CaHPO4 and BaHPO4 – as well close to being tetrahedral. The symmetry of the soft mode in PbHPO4 is deduced and it is found that the ferroelectric transition could be of second order as far as group theory is concerned. A microscopic model of the ferroelectric orderdisorder phase transition based on the interaction between the chains of OHO bonds linking the PO4 groups is proposed.
Fermi Surface Anisotropy and Properties of Tungsten Crystals
Authors: Dekhtyar I. Ya. , Silantev V. I. , Sakharova S. G. , Fedchenko R. G. , Patoka V. I. , Kolesnik V. N. 
Published: July 2007

AbstractA study is made of the anisotropy of the properties of tungsten single crystals for equivalent crystallographic orientations relative to the growth axis. An essential difference in the interplanar spacings for samples with equivalent faces perpendicular and parallel to the growth axis was found. For the compared equivalent planes differences in the positron annihilation parameters were observed which point to an anisotropy of the Fermi surface caused by carbon atoms with an anisotropic electron configuration. The sublimation energy for equivalent faces is also shown to be anisotropic.
Non‐Parabolicity of the Highest Valence Band of Bi2Te3 from Shubnikov‐de Haas Effect
Authors: Köhler H. 
Published: July 2007

AbstractThe valence band mass parameters are evaluated from the Shubnikov‐de Haas effect in stationary fields up to B = 10T magnetic induction. The hole concentrations of the samples ranged from 5 × 1017 to 1.1 × 1019 cm−3. The ellipsoidal six‐valley model is confirmed, but in contradiction to earlier reported results the cyclotron masses depend on energy, though the shape of the Fermi surfaces does not vary with increasing oarrier concentration. The extrapolated band edge cyclotron mass for B parallel to the trigonal axis is 0.08%m0 ± 5%, for the density‐of‐states mass of a single valley at the valence band top 0.106%m0 ± 10% is obtained, which corresponds to the integral density‐of‐states mass at the band edge of 0.35%m0 ± 10%. The tilt angle ν = 31.5° ± 10% of the ellipsoidal main axes relative to the crystal axes is higher than published earlier.
Phonon Frequencies of InP Derived from Two‐Phonon Absorption
Authors: Ulrici B. , Jahne E. 
Published: July 2007

AbstractInfrared absorption measurements have been performed on InP at various temperatures from 20 to 300 K in the spectral range from 370 to 700 cm−1 in which the absorption is due to two‐phonon absorption processes. The ir absorption peaks as well as the second‐order Raman scattering data by Alfrey and Borcherds are analyzed in terms of critical point phonon frequencies. The consistency of the derived phonon frequencies is checked from different points of view, above all by using the temperature dependence of the peaks. Especially the LA frequencies turn out to be larger than those of earlier assignments indicating a smaller frequency gap between the acoustic and optical phonons than that calculated by Talwar and Agrawal. Recent neutron scattering data support the assignment given here.
Size and Shape Effects on the Broadening of the Plasma Resonance Absorption in Metals
Authors: Ruppin R. , Yatom H. 
Published: July 2007

AbstractSize and shape effects on the broadening of the plasma resonance absorption of light in metallic samples are investigated. The relation between three different quantum mechanical methods which have been used in the past to calculate this broadening, is clarified. The general theory is applied to various specific geometries, and some physical implications are discussed.
Symmetry Properties of the Planar Antiferrornagnet K2NiF4
Authors: Deonarine S. , Joshua S. J. 
Published: July 2007

AbstractUsing well established group theoretical methods a symmetry analysis is made of magaons and paramagnetic spin waves in antiferromagnetic K2NiF4. Excitons and single ion transitions are also considered and Selection rules for first‐order transitions are derived.
SpinLattice Coefficients of Eu2+ in SrF2
Authors: Kuriata J. , Guskos N. , Rewaj T. , Szumowski J. , Falkowski K. 
Published: July 2007

AbstractEPR spectra of Eu2+ in SrF2 under uniaxial stress are observed. The full set of second‐and fourth‐order spin–lattice coefficients is determined. Line shifts of 15 different hyperfine transitions under uniaxial stress are measured. Five coefficients are evaluated by solving a set of 15 equations, some of them are compared with results of other authors. The differences are explained as being due to the introduction of fourth‐order terms in the spin‐Hamiltonian. The results are found to support Stedman and Newman's model. The correlation with the temperature dependence of b is discussed.
The Correlation between Optical Absorption Spectra, Carrier Mobility, and Phase Transition Temperature in Some Ferroelectrics
Authors: Bursian E. V. , Girshberg Ya. G. , Ruzhnikov A. V. 
Published: July 2007

AbstractA relation between absorption band frequencies connected with free carrier absorption the IR‐region, mobility values, and phase transition temperatures in titanates, tantalates, and niobates is discovered. The existence of a single mechanism determining both phase transition and carrier kinetics in the conduction band is supposed.
Generalised Viscosity and Dislocation Damping Constant in Diatomic Crystalline Solids
Authors: Martin J. W. , Paetsch R. 
Published: July 2007

AbstractUsing Brailsford's model for the anharmonic dislocation drag, generalized viscosities and the damping constant have been calculated for a diatomic solid, represented by a modified Debye‐Einstein type phonon spectrum. The generalized viscosity and its temperature dependence have been examined in detail. It is shown that the effect of phonon dispersion is to increase the region in which the thermoelastic effect makes an appreciable contribution to the viscosity. Consequently, the dislocation drag constant for the Debye‐Einstein model is found to be enhanced by a factor \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{1}{2}q_D l_A $\end{document} compared with the Debye model (qD is the Debye wave number, lA the mean free path of the acoustic phonons). It is also shown that in general the damping constant is very sensitive to temperature, shape of phonon spectrum and phonon lifetimes and it is difficult to predict its temperature dependence. The Debye‐Einstein model gives B α Tm, where m < 1 for intermediate temperatures and tends to one for high temperatures.
On the Existence of Spin Waves in a Model Transition Metal with Finite Anderson Mixing Potential
Authors: Zieliński J. 
Published: July 2007

AbstractInterband s–d exchange interaction is added to the transition metal model Hamiltonian involving the Anderson mixing term. Spin waves are shown to exist even when the Anderson mixing potential remains finite. The value of the exchange coupling constant which makes this possible depends on the mutual localization of s‐and d‐like bands and on the shape of the density of states function of the s‐like band.
Intraband Conductivity and Thermopower of Semiconductors with Slowly Varying Gaussian Random Field
Authors: Zhumatii P. G. 
Published: July 2007

AbstractIntraband conductivity and thermopower of a unipolar semiconductor with a slowly varying Gaussian random field are computed. The calculations are made on the basis of the Kubo formula and a semiclassical theory of the particle motion in a random field. Explicit expressions are obtained for the do conductivity, thermopower and mobility of the charge carriers under various conditions related to different physical situations. Comparison is made with the available experimental data.
Neutron Spectroscopy of Naphthalene Crystal Internal Phonon Modes
Authors: Bokhenkov E. L. , Natkaniec I. , Sheka E. F. 
Published: July 2007

AbstractSpectra of the incoherent inelastic neutron scattering from naphthalene crystals in the region of internal phonon modes (170 to 1300 cm−1) are studied at the temperatures 296, 80, and 4.7 K. The spectra obtained are quantitatively processed by means of a direct spectral procedure. The ambiguity present before in the frequency spectrum of the internal phonon modes of the crystal in the region from 170 to 1300 cm−1 is eliminated basing on these experimental results together with results on Kaman‐Mandelstam scattering spectra and infrared absorption. The au mode frequencies inactive in optical spectra are determined. The possibility is indicated to use the displacement amplitude of the free molekule hydrogen atoms in order to describe the investigated phonon modes in the crystal.
The Valence Band of Os Studied by Electron Spectroscopy
Authors: Nilsson R. , Berndtsson A. , Mårtensson N. , Nyholm R. , Hedman J. 
Published: July 2007

AbstractThe valence band of osmium is investigated using an HP5950A ESCA spectrometer. The electron spectrum is compared with density of states functions from the literature, one obtained from a linear muffin tin orbital calculation for osmium and one from an APW calculation for rhenium.
Surface Plasmon‐Polaritons in a Spatially Dispersive Inhomogeneous Medium
Authors: Boardman A. D. , Paranjape B. V. , Nakamura Y. O. 
Published: July 2007

AbstractThe dispersion of surface plasmon‐polaritons and their associated fluctuation densities and electric fields are calculated for a spatially dispersive semi‐infinite electron plasma, with, parameters appropriate to the metallic state. The model is hydrodynamic and the electron density is assumed to be uniform and equal to the bulk value, except for a small region near the surface. In this surface neighbourhood the electron density is assumed to fall in a step‐function manner as in previously reported successful models of surface profile properties. The momentum range covers that which is experimentally available and includes both the retardation and electrostatic regimes. It is emphasised that the inclusion of spatial dispersion, by the introduction of a pressure gradient, is an essential feature. This is because the distance over which the surface density changes is such that the transit time of the carriers is the same order of magnitude as the inverse plasma frequency. Resonances are possible, therefore, which are entirely absent in pure retardation models which ignore spatial dispersion. Fluctuation densities, electric fields, and polarisation plots are presented. The previously reported assertion that the charge distributions can be rigorously classified into multipoles is investigated numerically.
Transformation of Incoherent Phonon Flux into a Coherent Acoustical Signal as the Result of the Nonlinear Amplification of Acoustical Phonons in Semiconductors
Authors: Gantsevich S. V. , Gurevich V. L. , Kagan V. D. , Katilius R. 
Published: July 2007

AbstractA theory of nonlinear interaction of sound waves in a noise beam is developed. The case is considered where the intensity of the beam grows along a semiconducting crystal in a do electric field. It is demonstrated that if the nonlinear contribution to the amplification constant is negative the transformation of a noise beam into a coherent sound signal with fixed amplitude is possible during a single transit through the crystal. This occurs if the linear amplification constant has a sufficiently sharp maximum while the nonlinear attenuation constant is a smooth function of the wave vector.
The Formation Energy of Vacancies in Aluminium and Magnesium
Authors: Tzanetakis P. , Hillairet J. , Revel G. 
Published: July 2007

AbstractThe formation energy of vacancies is measured under conditions of very low vacancy concentrations for ultrahigh purity aluminium, after quenching down to 4 K. A value of (0.69 ± 0.03) eV is found. Similar experiments made using pure magnesium yield a formation energy of (0.79 ± 0.03) eV. It is shown that in magnesium vacancies are mobile well below room temperature.
The g‐Factor of the Holes in the Highest Valence Band of Bi2Te3
Authors: Köhler H. 
Published: July 2007

AbstractThe Shubnikov‐de Haas effect is investigated on p‐type Bi2Te3 in the concentration range from 5 × 1017 to 1.1 × 1019 cm−3, in magnetic fields up to 10 T flux density, between 1.6 and 4.2 K. From spin‐splitting of the magnetoresistance maxima the product M = gmc/(2m0) is found to be strongly anisotropic in contrast to the results on other semiconductors. Hence the g‐factors may be determined, by inserting the known cyclotron masses mc, yielding a considerable energy dependence and anisotropy of the second rank tensor components of g. From values for |M| ≷ 1 a positive sign for the g‐factor (g > 0) is deduced. Assuming ϑo = 31.5° ± 10% (ϑ0 tilt angle of ellipsoidal principal axes of the Fermi surfaces relative to the crystal axes) to fit also to the corresponding g‐tensor ellipsoids, the principal values g′1 (parallel to the binary axis) and g′2 (in the mirror plane, tilted by ϑ0 relative to the bisectrix axis) are of about one order of magnitude larger than g′3 ≈ 2 (within the regarded mirror plane, tilted by ϑ0 relative to the trigonal axis). The results correspond to the data of the lowest conduction band of Bi2Te3 in every respect, and are discussed qualitatively in a six‐band model.
Displacement of Impurities through Interaction with Radiation‐Induced Interstitials in Alkali Halides
Authors: Matsunami N. , Yokoyama Y. , Itoh N. 
Published: July 2007

AbstractOptical absorption and channeling studies of irradiated Pb++‐doped KCl are made. It is found by channeling experiment that Pb++ is displaced nearly to the face center by irradiation at room temperature. Optical absorption studies indicate that the position of the peak of the V‐type centers produced in Pb++‐doped KCl is different from the V centers produced in pure KCl and the A band for Pb++ becomes broad by irradiation. It is concluded that the interstitials are associated with the impurities and cause the displacement of the latter. A possible model for the complex of the interstitials and the impurity is suggested.
Non‐Thermal Stimulation of Three‐Level Impurities by Phonons
Authors: Michel H. , Wagner M. 
Published: July 2007

AbstractA phonon system is considered, which is coupled to three‐level‐impurities (e.g. tunneling levels of rotator molecules in alkali halides). The kinetic equations for both the phonons (Peierls‐Boltzmann equation) and the occupations of the impurity levels are established. They are solved in a self‐consistent manner for the stationary case. Computer calculations yield the non‐thermal (i.e. finite flux) heat conductivity as well as the scattering and absorption of monochromatic phonons. Finite flux heat conductivity measurements are proposed and the expected deviations in the heat conductivity curves are drawn.
Theory of Electron–Hole Plasma in CdS
Authors: Zimmekmakn R. , Rösler M. 
Published: July 2007

AbstractA finite temperature theory of the electron‐hole plasma in highly excited direct semiconductors is presented using Green's functions. The self‐energy is calculated within the single‐plasmon pole approximation. Results are given for CdS with respect to modified energy bands, free energy, and emission intensity. Special attention is paid to band anisotropy and the participation of two split valence bands.
Adsorption‐Induced Changes of Electron Levels
Authors: Brodskii A. M. , Urbakh M. I. 
Published: July 2007

AbstractThe electron spectrum of a crystal with an adsorbed centre is considered using the mathematical techniques of the quantum mechanical theory of scattering. A special pole approximation is developed which is substantiated by specific features of the problem, Formulas are derived for the adsorption‐caused changes of thermodynamical characteristics and compared with the experimental data.