bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of Resonance Dechannelling
Authors: Kumakhov M. A. , Wedell R.
Published: July 2007

A theory of non‐linear phenomena leading to the resonance ion dechannelling is developed. These phenomena arise when ions pass from the conditions of axial channelling to those of planar one and also when ions channelled in an axial channel have low energies. In the first case, the wavelength of a particle trajectory in the plane normal to the axial channel becomes comparable with the distances between atomic rows forming an atomic plane. In the second case, the wavelength is comparable with the distance between two neighbouring atoms in the atomic row. In both the cases a resonance arises which is due to the fact, that the ion oscillation period in a channel becomes comparable with the lattice potential period. The general nonlinear equations obtained in a number of cases become the Mathieu equation describing a parametric resonance. The theoretical results are compared with those of computer calculations and with experiment.
Powder Reflection Measurements of Some Divalent Lead Halides
Authors: Eijkelenkamp A. J. H.
Published: July 2007

Powder reflection spectra are recorded of α‐ and β‐PbF2, PbCl2, PbBr2, PbFCl, and PbFBr. The measurements are compared with available absorption and specular reflectance spectra. The characteristic exciton of the lead halides manifests itself as a maximum in the powder reflection spectra owing to a prevailing specular component of reflectance. An additional low‐energy absorption appearing in the powder spectra of PbF2 after a thermal treatment, is ascribed to incorporation of O2− impurity. The low‐energy absorption visible in the powder reflection spectra of PbFCl and PbFBr is attributed to either (hydr.) oxide impurity or nonequimolar composition regarding F− and Cl− or Br−. α‐ and β‐PbF2 are photochemically active like the other lead halides.
Low‐Energy Gap Semiconductor Mobilities in Presence of Spatial Fluctuations of Carrier Density
Authors: Barjon D. , Raymond A. , Robert J. L.
Published: July 2007

It is shown that the ionized impurity mobility is reduced by a factor close to [1 +(Δn/n0)]−l when spatial fluctuations of the carrier density are present in the crystal. This result is valid for degenerated and non degenerated semiconductors. It is also shown, that acoustical or optical scattering modes are not affected by these fluctuations.
Electronic Density of States of Amorphous CuGaS2 and ZnGeAs2
Authors: Braun W. , Cardona M.
Published: July 2007

Measurements of the energy distribution curves of photoemitted electrons are made for amorphous ZnGeAs2, and CuGaS2. The energies of the exciting photons are 21.2 (HeI), 40.8 (HeII), 16.8 (NeI), and 26.9 eV (NeII). The amorphous thin films are prepared by sputtering from polycrystalline material onto aluminium substrates. The photoemission curves of the valence band of ZnGeAs2, show a two peak structure. The upper portions of the valence band (0 to 6 eV) are composed primarily of p‐orbitals of the As and Ge atoms. A doublet structure is observed and attributed to the different strength of the Zn‐As and Ge‐As bonds. The lower portion of the photoemission spectrum is dominated by a strong feature derived from the Zn 3d‐level. The photoemission spectra of CuGaS2, show a strong dependence upon excitation energy which is due to the varying admixture of 3d electrons of the Cu with the p‐valence electrons of Ga and S. The spectra are decomposed into approximate p and d densities of valence states by using experimental atomic cross sections for the corresponding partial excitation probabilities.
An Operator Method for Optical Lineshape Calculations in Non‐Adiabatic Electron‐Phonon Systems
Authors: Sigmund E. , Wagner M.
Published: July 2007

A method has been developed for optical band shape calculations in non‐adiabatic electron‐phonon systems. The basis of this treatment is the calculation of ordered exponential operators by successive partial integrations, which can be summed up in a closed form. The regions of validity are discussed. An application to a doubly degenerate Jahn‐Teller system (E‐e type) is presented. A comparison to experimental results is given.
Susceptibility and Superconductivity of Compounds with A‐15 Structure
Authors: Alekseevskii N. E. , Krasnoperov E. P. , Mydlarz T.
Published: July 2007

The magnetic susceptibilities of the Nb3Al1−xGex, and Nb3Ga compounds are measured in the temperature range 20 to 300 K. It is shown that in the compounds investigated the temperature dependence of the susceptibility is considerably less than in other high‐temper‐ ature superconductors with A‐15 structure. It is difficult to explain the high value of Tc and the small temperature coefficient of the susceptibility by the narrowness of the conduction band. It is assumed that the high coupling constant in these superconductors is determined by the properties of the electron‐phonon interaction arising as a consequence of the nearly one‐dimensional movement of the electrons.
Transport Phenomena and Eigenoscillations of Hot Electrons in Semiconductors with Periodic Structure
Authors: Dykman I. M. , Tomchuk P. M.
Published: July 2007

Semiconductors with periodic impurity distribution are studied in electric fields heating the carriers. Both, elastic and inelastic scattering of carriers by lattice vibrations are con‐sidered. The calculations are carried out in a general form without concrete definition of the energy loss mechanism and without introduction of energy relaxation time. An expression for the additional current due to the sample in homogeneity valid over a wide range of fields is obtained. The sign of this current is opposite to the sign of the current in a homogeneous sample. Its absolute value firstly grows with increasing field and then drops up to zero. It is shown that the semiconductor inhomogeneity noticeably influences the spectrum of the eigenoscillations. The dependence of the frequency of these oscillations ω = ωr + iγ on the field strength and the wave vector is determined. The dependence of ω on the wave vector is resonant in nature. The frequency of the longitudinal waves is found to depend not only on the value, but also on the direction of the wave vector.
Formal Results on the Susceptibility of Dilute Alloys. II. Mean‐Field Approach and Applications
Authors: Bisch P. M. , Gomes A. A.
Published: July 2007

A mean field approach is adopted and a matrix formulation is used to obtain formal results for the susceptibility of a two band system (α, β). This general result includes the intra‐atomic Coulomb repulsions, one‐electron impurity potentials, and the change in the Coulomb interaction and inter‐band exchange at impurity site. The complete evaluation of the partial susceptibilities involves the calculation of inverse matrices, which is difficult to perform in the general case. Expressions for the self‐polarization hyperfine field, the Knight‐shift, and ESR g‐shift are obtained for transition metal like systems (s‐d bands), using previous results obtained in I. It is shown that these quantities may be evaluated in terms of phase‐shift, involving only the density of states and the parameters of the problem.
Biexciton Luminescence with Interserial Transition of the Formed Exciton
Authors: Khadzhi P. I. , Petrashku K. G. , Moskalenko S. A.
Published: July 2007

The process of biexciton radiative recombination is studied theoretically with the interserial transition of the exciton in semiconductors with direct allowed interband transitions. Analytical expressions for the spectral and temperature dependences of the rates of different radiative recombination channels are obtained along with the formation of the exciton in the ground, as well as in the excited states of the discrete or continuous spectrum. Depending on the band structures of the exciton and biexciton the emission band for radiative recombination with the formation of exciton possesses the boundary (threshold) frequency and the long‐wavelength (short‐wavelength) tail. Numerical calculations given for CdS indicate the giant probability of the treated process.
Zero and First Sound in NaF and RbI
Authors: Loidl A. , Jex H. , Daubert J. , Müllner M.
Published: July 2007

Zero and first sound as well as the transition region are investigated theoretically in RbI and NaF. The temperature dependence of the first sound, zero sound, and isothermal elastic constants is calculated on the basis of a breathing shell model for the harmonic lattice dynamics and anharmonic parameters from microscopic Grüneisen constants. The theoretical results are compared with ultrasonic data and inelastic neutron scattering results. Own recent neutron experiments in RbI are concerned with the zero sound region at room temperature while the NaF experiments cover also parts of the transition region between first and zero sound at various temperatures between 295 and 700 K. It is shown that the dispersion of sound is satisfactorily described by use of an averaged thermal phonon lifetime. In NaF for each temperature first sound elastic constants and averaged phonon lifetimes are deduced from an analysis of the experimental data.
Electron Mobility of GaxIn1−xSb at Room Temperature
Authors: Barjon D. , Raymond A. , Joullie A. , Robert J. L.
Published: July 2007

Experimental values are given of electron mobility at 300K in GaxIn1−xSb compounds (0 ≦ × ≦ 0.48). The experimental values of the mobility are compared to those computed in the case of mixed scattering by acoustic, polar optical phonons and ionized impurities. The fluctuations of carrier density which only affect ionized impurity scattering are also taken into account.
Pseudopotential Calculation of the Electronic Valence Charge Density and the Optical Dielectric Constant of NaCl and MgO
Authors: Nagel S. , Maschke K. , Baldereschi A.
Published: July 2007

The pseudopotential method is used to calculate the electronic charge distribution and the optical dielectric constant for NaCl and MgO. The results indicate that a local pseudopotential gives a sufficiently good description of the highly ionic properties of these materials.
Cyclotron Resonance with a Slightly Anomalous Skin Effect
Authors: Kuka G. , Braune W. , Herrmann R.
Published: July 2007

Microwave absorption lines for bismuth and bismuth alloys (with antimony and tin) are investigated on the basis of a conductivity model for Fermi surfaces showing a slight deviation from an elliptic shape. A good fit of cyclotron wave dispersion and agreement of the experimentally determined behaviour of microwave absorption with theory are found.
Transmission and Reflectivity of Highly Excited Semiconductors Due to the Optical Conversion of Excitons into Excitonic Molecules
Authors: Wünsche H.‐J. , Henneberger F.
Published: July 2007

The dielectric function for the optical conversion of excitons into excitonic molecules is derived, using Boltzmann distributions for both the systems. In a second step calculations of the optical constants as well as the corresponding transmission and reflectivity spectra are presented. Appart from some other characteristic features, a region of strong reflection occurs at high exciton densities.
Far‐Infrared Magneto‐Reflection in HgTe
Authors: Suzuki K. , Miura N. , Uchida S. , Tanaka S.
Published: July 2007

The magneto‐reflection spectra in HgTe at 119 μm are investigated over a temperature range from 10 to 77 K in magnetic fields up to 160 kG. Firstly, the magnetic field dependence of the interband Γ8 → Γ8 dielectric function is calculated and its contribution to the intraband Γ8 → Γ8 magneto‐reflection spectra is studied. Secondly, an anisotropy parameter, γ2 → γ2 to be 0.5 ± 0.4 from the measurement of the interband Γ8 → Γ8 magnetoreflection spectra for magnetic field directions parallel to the crystal axes 〈100〉 and 〈111〉 at about 10 K.
Spectrum of Magnetic Excitations of a Ferromagnet with Weakly Bound Antiferromagnetic Interstitial Impurities
Authors: Botvinko M. N. , Ivanov M. A.
Published: July 2007

The spectrum of impurity excitations of a Heisenberg cubic ferromagnet with randomly distributed weakly‐bound antiferromagnetic interstitials is investigated. It is shown that if the impurity concentration c is less than some characteristic concentration c0, the density of impurity states has an exponentially narrow peak at the localized‐level frequency ω0. If, however, c ≫ c0, an impurity band appears between the frequencies ω0 and ω1 = ω0, (1 ‐ c(S1/S)). Impurity excitations in this band are characterized with wave vectork with wave vector k(k ⪅ c1/3a−1), their inverse lifetime being small compared with the band width c(S′/S) ω0. Inelastic scattering of neutrons as well as light absorption are studied in the resonant region near ω0 When c > c0, the spectrum of light absorption due to impurities involves two strongly asymmetrical peaks (not one, as in the case of c < c0). The main peak width (that at ω1)is of order (c/co)2 c(S′/S) ω0 and falls with growth of the impurity concentration.
The electronic structure of trapped hole centres in SnO2
Authors: Zwingel D.
Published: July 2007

In SnO2 single crystals, doped with Al and Ga, the ESR spectra of “trapped holes” are observed after UV excitation. A discussion of the hyperfine and superhyperfine parameters of the spectra shows that, due to the different electron configurations of the impurity ions, the hole is localized at different oxygen anions next to the impurity. Prominent temperature effects of the spectra show that exchange processes between different anion sites have to be taken into account, as well as the coupling to phonon modes of the lattice in order to explain the high‐temperature ESR spectra.
Influence of the crystal field on bent configuration of VK centres in AMF3 compounds
Authors: Rousseau J. J. , Fayet J. C.
Published: July 2007

On calculating the admixture of the 2IIg molecular state in the 2∑u ground state by a C2v crystal field, an attempt is made to determine the angle which characterizes the bent configuration of the VK centres in the fluoperovskite lattice.
Spin‐wave measurements of exchange constant A in Ni‐Fe alloy thin films
Authors: Hennemann O. D. , Siegel E.
Published: July 2007

The ferromagnetic exchange constant is measured using spin‐wave resonance in eight nickel‐iron alloys with a composition range from about 55 to 95% Ni. The exchange constant A varies with composition as A = (2.25 ‐ 1.4CNi)erg/cm. The data are in good agreement with the values published by Wilts and Lai.
Magnetic moment distribution in Cu‐Ni alloys
Authors: Sacchetti F. , De Gasperis P. , Menzwger F.
Published: July 2007

The magnetic moment distribution is measured on two f.c.c. Cu‐Ni alloys with composition of 14 and 45 atyo Cu. The results are compared both with those of other similar materials and with the predictions of the coherent potential approximation (CPA). It is found that the composition dependence of the asphericity agrees with the CPA calculations, as it was found also for Co‐Ni and Fe‐Ni alloys. The spherical form factor is found to be contracted in both the Cu‐Ni and Co‐Ni alloys with respect to pure Ni.
On the change of electronic polarisability of an ion in a crystal
Authors: Chakrabarti S. K. , Sarkar S. K. , Sengupta S.
Published: July 2007

A theory for the change in polarisabilities of ions in crystal environment from their free ion values is presented. The contribution to this change is shown to come from the pure third order perturbation and second order exchange terms in the perturbation energy expression for the system. These contributions are separately estimated. The main contribution is shown to arise from third order perturbation effect. The inadequacies of the earlier treatments by Ruffa [1] and by Ledovskaya [2] are pointed out. Calculations for the change in polarisability due to third order perturbation effects are carried out for cations only. For anions some difficulties in the application of the perturbation theory are discussed. The results are then compared with the empirical values of Tessmann, Kahn, and Shockley [3] and also with those of Ruffa.
Optical spectra of aggregate centres in KCl:Pb crystals
Authors: Mărculescu L.
Published: July 2007

Absorption and emission spectra of the Pb2+ ion‐cation vacancy complexes are measured. The existence of two emission bands due to aggregate centres and their shift to lower energies are explained using a model of Pb2+ luminescent centres in the framework of Johnson and Williams' thermodynamical theory.
Cluster approximation for non‐homogeneous disordered systems
Authors: Zawisza S.
Published: July 2007

A two‐parameter cluster approximation for non‐homogeneous n‐component disordered systems is presented. This allows to consider a wide class of approximations. Generalizations of some known results are derived as special cases of the cluster approximation.
Luminescence of a “Cold” gas of biexcitons in CuCl
Authors: Levy R. , Klingshirn C. , Ostertag E. , Phach Vu Duy , Grun J. B.
Published: July 2007

The emission due to the radiative decay of biexcitons in CuCl is studied at low temperatures. Biexcitons formed of free excitons mainly under LO phonon emission, contribute to the broad emission bands observed. In the case of direct creation of biexcitons by resonant two‐photon absorption, a “cold” biexciton gas is created which is not in thermal equilibrium and gives rise to the narrow emission lines. A Bose‐Einstein condensation of the biexcitons seems to be very unlikely.
Magnetic stability at 0 K of a simple cubic ferromagnetic array of pseudo‐spin S = ½ with anisotropic exchange between nearest neighbours
Authors: Belorizky E. , Casalegno R. , Fries P.
Published: July 2007

The spin wave spectrum is calculated of a pseudo‐spin S = ½ system forming a simple cubic ferromagnetic array, by considering the most general interaction between nearest neighbours allowed by a fourfold order symmetry of the bond ℋ︁ij = J‖SizSjz + J⊥(SixSjz + + Siy,Sjy) for a pair (i, j) oriented along the z‐axis. It is shown that, in zero external magnetic field, a ferromagnetic order is allowed at 0 K only for J‖ and J ⊥ both negative, with the easy direction of magnetization along a 0. [100] axis. The zero point energy of the spin waves and the mean spin deviation in the approximate ferromagnetic ground state are also studied.
Electron spin‐lattice relaxation of Cu2+ ions in planar coordination geometries: Calculation and experimental data of T1 for Cu2+ in Ni(II) bis(diethyl‐dithiocarbamate) crystals
Authors: Aminov L. K. , Kirmse R. , Solovev B. V.
Published: July 2007

A general formula is derived for the direct spin‐lattice relaxation process of paramagnetic ions with S = ½ for arbitrary orientations of an external magnetic field relative to the crystal axes. Using the formula obtained, T1 of Cu2+ ions in square planar coordination geometries is calculated in the point charge approximation. The temperature, concentration, and angular dependence of T1 of Cu2+ ions in Ni(II)bis(diethyl‐dithiocarbamate) single crystals are reported. Including some features of covalency a reasonable agreement between the theoretical and experimental values of T1 is reached.
On the shear modulus of the flux line lattice
Authors: Brandt E. H.
Published: July 2007

The shear modulus c66 of the flux line lattice in type II superconductors can be obtained from a two‐body interaction between the flux lines even a t large inductions B ≈Hc2. The potential is composed of a repulsive and an attractive part and has a range diverging at Hc2. An explicit expression for the Ginsburg‐Landau c66 is given for arbitrary B and x. From Gorkov's microscopic theory c66 is derived for large B and arbitrary temperature. Simple approximate formula are given.
Energy band structure and Ferrni surface of potassium
Authors: Faust A. , Merkel C. , Bross H.
Published: July 2007

Wave functions and energy eigenvalues for potassium are calculated with the MAPW method using a nonlocal screened exchange potential. From the results obtained for several symmetry directions, the Fermi energy, the Fermi surface, its extremal cross‐sections, different masses, and the contribution of conduction electrons to the Knight shift are determined and related to experimental data.
Room temperature optical properties of rare earths
Authors: Krizek J. , Taylor K. N. R. , Krizek H.
Published: July 2007

Method of estimating the free electron contribution to the optical conductivity for the heavy rare earths is presented.
Description of reactions between defects in solids
Authors: Pletnev M. N.
Published: July 2007

A new scheme is suggested for describing the system of reacting particles using the Fock space of distribution functions. This method can be applied to calculate any function of the number of particles in a system, in particular, the moments of any order. Diffusion‐limited reactions and simultaneous generation and annihilation of particles as the result of reaction are considered.
Dynamical properties of crystals of Sr(NO3)2, Ba(NO3)2, and Pb(NO3)2. I. Infrared spectra and structure
Authors: Bon A. M. , Benoit C. , Bernard O.
Published: July 2007

The far infrared reflection spectra of three single crystals of Sr(NO3)2, Ba(NO3)2, and Pb(NO3)2 are measured from 2500 to 10 cm−1 and from 10 to 300 K. The results are discussed on the basis of phonon and polariton theory. A method is found to determine the number of ordinary and extraordinary phonons in the case of general symmetry and a relation is established between the phonon frequencies and the polariton frequencies. These results are applied to discuss the spectra of Sr(NO3)2, Ba(NO3)2, and Pb(NO3)2. Then, among several structures proposed by different authors, the T4 structure is proposed from the results.
Spin—lattice relaxation of several nuclei near the ferroelectric transitions of TGS and TGFBe
Authors: Müller D. , Petersson J.
Published: July 2007

Spin—lattice relaxation time, T1, measurements are reported and interpreted of the nuclei H1, C13, and N15 in TGS and F19 in TGFBe. In particular the behaviour of T1 is investigated near the ferroelectric phase transition temperatures of these crystals. The ferroelectric contribution to the C13 relaxation rate in TGS doped selectively in the CH2 group as well as the ferroelectric contribution to the F19 relaxation rate in TGFBe show an anomalous temperature dependence near Tc which can be described by a logarithmic singularity. The ferroelectric contribution to the spin—lattice relaxation rate of the protons in TGS shows a rather complicated temperature and frequency dependence. T1 of N15 doped TGS is nearly constant in a wide temperature range. The presented experimental results are in accordance with the assumption of an order—disorder model for the motions of the molecular groups which cause ferroelectricity in crystals of the TGS group.
Theory of side‐bands in a cooperative Jahn‐Teller material
Authors: Gehring G. A. , Laugsch J.
Published: July 2007

The theory of optical side‐bands is developed for a crystal with a cooperative Jahn‐Teller phase transition containing rare‐earth ions. A tight binding scheme is used to describe the rare‐earth energy levels over the whole Brillouin zone. The possible two‐ion couplings which can give rise to side‐bands are discussed; these include the diagonal and off‐diagonal contributions of the virtual phonon interaction.
Lattice dynamics of rare‐gas solids by Kihara's potential III. Green functions
Authors: Bohra A. K. , Gupta R. K.
Published: July 2007

The phonon Green functions are presented for solid argon, krypton, and xenon. The Green functions are computed by studying the lattice dynamics of these solids, using the Kihara hard core potential. A set of potential parameters fitted to neutron data is used in the computation. The localized mode due to neon in argon is studied as an example of the use of these Green functions in defect problems.
A study of spin interaction in a manganese‐substituted zinc ferrite by neutron spectroscopy
Authors: Ligenza S.
Published: July 2007

Inelastic neutron scattering is studied on a manganese‐substituted zinc ferrite at temperatures between 5 and 293 K. The obtained neutron spectra are interpreted basing on the 6S5/2‐term splitting of the Fe3+ ion in an octahedral B‐site. The transition energy peaks connected with different magnetic environment of Fe3+ (B) ions are observed. The six‐fold 6S5/2‐term splitting and the excited neutron transition are discussed in terms of a phenomenological spin‐Hamiltonian, assuming combined interaction of a static magnetic field and a static electric field gradient with magnetoelectric moments of the 3d5 electron shell. In addition, a superparamagnetic behaviour is studied by neutron diffraction and Mössbauer effect measurement. Quasielastic coherent scattering of neutrons due to superparamagnetic clusters is observed.
The lattice distortion around interstitials in F.C.C. metals
Authors: Miller K. M. , Heald P. T.
Published: July 2007

The displacement field produced by the body centred interstitial and the 〈100〉 spilt interstitial in aluminium, nickel, and the noble metals is calculated using a discrete lattice model. It is assumed that the host atoms interact amongst themselves through a pairwise potential which extends out to second‐nearest neighbours and the short‐range repulsion between the interstitial and near neighbour host atoms is represented by either a Born‐Mayer or Morse potential. With the exception of aluminium the relaxation volume around the 〈100〉 spilt interstitial is similar to that around the body‐centred configuration and furthermore the differences in the results for the two potentials are quite small. For each metal the relaxation volume around an interstitial is found to be significantly larger than that previously calculated for a vacancy.
Pressure effects on magnetic phase transitions in FeRh and FeRhIr alloys
Authors: Vinokurova L. I. , Vlasov A. V. , Pardavi‐Horváth M.
Published: July 2007

The effect of quasi‐hydrostatic pressures up to 100 kbar is investigated on the phase transitions of Fe1−xRhx alloys by resistivity measurements. The pressure dependence of the first order antiferromagnetic (AF) to ferromagnetic (FM) phase transformation temperature dTk/dp = 4.4 to 5 K/kbar. The pressure dependence of the second order FM to paramagnetic (PM) transition, i.e. that of the Curie‐point is dTC/dp = −0.7 K/kbar. At about 60 kbar the thermal hysteresis diminishes, and above the range of the co‐existence of three phases (AF—FM—PM) the pressure dependence of the AF—PM transition is characterized by dT/dp = 0.6 to 1.0 K/kbar. The AF—PM transition is investigated in an Ir‐substituted alloy (Fe44Rh49.5Ir6.5) also, where dT/dp = 0.6 K/kbar.
Dynamical study of the temperature dependence of grüneisen parameters in alkali metals
Authors: Singh V. P. , Hemkar M. P.
Published: July 2007

Using a non‐central force model the microscopic Grüneisen parameters γq, j and the effective value γG at different temperatures are calculated for the alkali metals in terms of elastic constants and their pressure derivatives. The microscopic Grüneisen parameters γq, j are calculated as a function of wave vector q along the principal symmetry directions [100], [110], [111], [210], [211], and [221] and the values are averaged over the wave vector space in the first Brillouin zone using the modified Houston spherical six term integration method. The results obtained for γG show a fairly good agreement with the experimental observations.
Dynamical properties of crystals of Sr(NO3)2, Ba(NO3)2, and Pb(NO3)2. II. Temperature dependence of the infrared spectra
Authors: Bon A. M. , Benoit C. , Giordano J.
Published: July 2007

Reflective infrared spectra of three single crystals of strontium nitrate, barium nitrate, and lead nitrate are investigated between 10 and 3500 cm−1 at temperatures from 10 to 293 K. The temperature behaviour of the dispersion parameters, frequencies, strengths, and damping obtained from classical dispersion relation is analyzed in order to compare, first the miscellaneous modes of the same crystal and then of different crystals. The case of Sr(NO3)2 which presents an anomaly in the infrared spectra corresponding to a phase transition is studied in more depth.
On the quantum mechanical foundation of classical shell models in lattice dynamics
Authors: Niedermann H. P. , Wagner M.
Published: July 2007

The approach is based on the adiabatic principle and the harmonic approximation. The aim is to find those quantum mechanical approximations which are necessary to derive expressions of the same structural form as the classical shell model equations of the lattice dynamics of ionic crystals. To this purpose the long range interaction of the ionic dipoles is treated in first order perturbation theory, whereas for the short range part of the adiabatic potential a more restricted approximation is used. By this means equations of motions are obtained of the form of the equations of the classical shell models. But in contrast to the phenomenological models by quantum mechanical arguments two different coordinates are introduced for the description of a T1u‐motion of an electronic shell.
Calculation of energy‐dependent photoionization probabilities for valence electrons in diamond and silicon crystals
Authors: Kosakov A. , Unger K. , Leonhardt G.
Published: July 2007

Photoionization probabilities for valence electrons in covalent semiconductor crystals are calculated in order to analyse the connection of the photoelectron spectra with the densities of valence electron states. A model is proposed and tested, which is based on solid state properties (adjusted sp3 tight binding functions and OPW functions are used for the initial and final states, respectively) and which yields purely energy‐dependent photoionization probabilities. Rather good results are obtained for C and Si; discrepancies occurring with Ge are discussed.
Indirect near edge emission in pure AgBr
Authors: Weber J.
Published: July 2007

The emission of pure AgBr single crystals is studied in the vicinity of the indirect band gap from 1.5 to 50 K under various excitation conditions. The spectra are analysed with special respect to properties of the phonons involved in the indirect transitions, the lineshape of the free exciton emission which is found dependent on exciting frequency, and the dependence of some weakly bound exciton lines on temperature and excitation power. Results obtained for a narrow line close to the free exciton emission do not support the proposed model of a recombination of possibly Bose‐Einstein condensed free triplet excitons.
Resonance Raman scattering in a strong electric field
Authors: Bechstedt F. , Enderlein R. , Peuker K.
Published: July 2007

A theory is given for the influence of an external electric field on the allowed one‐phonon resonance Raman scattering. It is shown that the phonon peaks of the scattering spectrum remain unbroadened in the presence of the field. The scattering cross section decreases due to the field induced broadening of the stationary electron energies. The relative field induced changes are remarkably large — of the order of magnitude one — if lifetime broadening effects can be neglected. Results for all types of critical points of the electronic transition are presented. It is shown that the field induced change of the scattering cross section decreases strongly if lifetime broadening effects are important. The results provide the theoretical basis for the application of the electromodulation technique to resonance Raman scattering.
Lattice dynamics of paratellurite
Authors: Durand M. , Ayrault B. , Marqueton Y. , Decamps E. A.
Published: July 2007

With infrared reflectivity and Raman scattering all the predicted modes are observed, except one A1 mode. The analysis of the reflectivity spectra leads to low frequency dielectric constants in excellent agreement with those obtained by other authors. The basis vectors of normal modes are determined at several points of the Brillouin zone and these ones of the Γ point compared with those of the rutile structure (SnO2). The unit cell of this last compound is about half of a paratellurite unit cell.
Electrons in finite one‐dimensional crystals in a uniform electric field
Authors: Yang E.
Published: July 2007

By using the time‐dependent Schrödinger equation, the problem of an electron in a one‐dimensional crystal in the presence of a uniform electric field is treated. It is shown that there are two types of electric‐field‐induced energy levels; one is so called Stark ladders and the other is simply due to different boundary conditions. When “appropriate boundary conditions” are imposed on the wave function it is found that the number of periodic potential cells involved in the calculation plays a vital role in determining the existence of Stark ladders. Also, the dynamics of an electron in a one‐dimensional potential well subject to a uniform electric field is studied.
A‐luminescence of alkali halides activated by monovalent mercury‐like ions
Authors: Trinkleb M. F. , Zolovkina I. S.
Published: July 2007

Polarization is found of the long‐wave A‐luminescence bands in KI:Tl, KI:In, KBr:In crystals at 7 K. It is established that the dependence of the polarization degree on azimuth corresponds to oscillator emission along the crystal axis [111]. These bands are assumed to be due to “trigonal” Jahn‐Teller minima of the activator 3T1u excited states. It is established that the 400 nm band in KBr:T1 crystal represents emission from the trigonal minimum of the metastable 3A1u‐state of the activator. A doublet structure of the short‐wave A‐luminescence bands is found and investigated. The hypothesis is made that the structure is connected with the presence of two types of tetragonal Jahn‐Teller minima of the activator 3T1u excited states.
Green's function treatment of ordered ternary alloy
Authors: Tahir‐Kheli R. A.
Published: July 2007

A Green's function formulation of an ordered ternary alloy is given. The case of a two sub‐lattice system with nearest neighbour pair‐wise, static potentials is studied. A decoupling procedure similar to the molecular field approximation is carried out, and the resultant equations are analysed in detail. Conditions for the ternary order‐disorder transition to be of the second order variety are given, and order parameters are computed for several different potentials and concentrations, for which second order transition occurs. A few remarks concerning possible application of the Green's function method to the study of atomic diffusion and relaxation of spatial order in ternary alloys are also given.
Quantum mechanics of dislocation kinks in covalent semiconductors
Authors: Gosar P.
Published: July 2007

A quantum‐mechanical model of the kink motion in covalent semiconductors is presented. A kink is treated as a quasi‐particle which is strongly coupled to the elastic continuum and can move along the dislocation by tunneling between neighbouring equilibrium sites. Kinks are most likely self‐trapped because of the deformation of the lattice. The calculation of the mobility at high temperatures reveals a thermally activated nature of the kink diffusion process.
Interaction of holes with local modes and one and two optical phonons as revealed in M‐I‐silicon tunneling spectra
Authors: von Baltz R. , Urban B.
Published: July 2007

The self‐energy and the density of states for holes in degenerate p‐type silicon are calculated taking into account the interaction with the local mode of the boron acceptor and one and two optical phonons. The current‐voltage characteristics of a MIS tunnel junction is obtained using the transfer Hamiltonian model. Adjusting parameters in d2IdU2 to experimental data the line width of the optical phonon and the effective two phonon deformation potential is obtained. In contrast to other authors, the calculations demonstrate that even a small but energy independent collision time of the carriers has a minor influence on the structures in d2IdU2. The interaction of holes with the local mode is essentially due to silicon atoms vibrating in ‐the neighbourhood of the boron ion.
A perturbed O molecule ion in SrCl2 single crystals EPB results
Authors: de Siebenthal J. M. , Bill H.
Published: July 2007

A detailed BPR investigation is reported about a perturbed O molecule ion introduced into SrCl2 single crystals. The O is associated with an alkali cation located on a next nearest neighbour lattice site. The local distortions produced by this alkali ion influence the orientation of the O molecule ion.
The “destructive” fermi resonance
Authors: Chisler E. V. , Davydov V. Yu. , Goncharuk I. N. , Ivanova E. A.
Published: July 2007

For the first time the coupling between the fundamental vibration of a crystal and its second‐order vibrational spectrum is observed which causes, due to the interference of the transition probabilities, the decrease of the intensity of second‐order vibrational spectrum and, in certain cases, its almost full cancellation. The “destructive” Fermi resonance is revealed in the Raman and infrared spectra of sodium nitrite. The energy of the resonance coupling and the unperturbed transition probabilities are calculated. The spectroscopic data show, that the electrooptic parameters of the second‐order vibrations and the energy of their interaction with the fundamental vibration are approximately independent of the wave vector within the Brillouin zone.
Semi‐classical and quantum‐mechanical theory of hopping conduction
Authors: Bányai L. , Aldea A.
Published: July 2007

A thorough analysis of the semi‐classical and quantum‐mechanical hopping conduction model is made within the frame of the linear response theory. The approach differs in several respects from the previous ones. A deeper understanding of the conditions under which such a mechanism occurs is achieved from the quantum‐mechanics point of view.
On some possible consequences of exciton‐phonon interaction
Authors: Stojanović S. D. , Šetbajčć J. P. , Škrinjar M. J. , Tošić B. S.
Published: July 2007

An exciton system with strong exciton‐phonon coupling is considered. It is shown that the virtual exchange of phonons leads to an effective exciton‐exciton interaction which cannot cause the creation of biexcitons. However it leads to the formation of exciton drops. The elementary excitations arising in the process of the desintegration of the exciton drops have superfluid properties.
Degenerate hubbard model in a magnetic field. Application to Jahn‐Teller systems
Authors: Khomskii D. I. , Kugel K. I.
Published: July 2007

The Hubbard model in the strong coupling limit is used to study the behaviour of substances containing ions with orbitally degenerate ground state (Jahn‐Teller ions) in a magnetic field. In such substances the orbital occupancy and the magnetic structure appear to be closely connected. It is shown that in these systems a transition of first order may occur between different anti‐ferromagnetic phases as well as jumplike spin‐flip transitions (even for an isotropic exchange interaction SiSj). The field dependence of the magnetic moment may be nonlinear, typical of meta‐magnetic substances. Both a simple model problem and a specific case of pervoskites with Jahn‐Teller ions (such as KCuF3) are studied. The behaviour of quasi one dimensional systems is also considered.
Calculation of the temperature‐dependent DC conductivity of disordered alloys in off‐diagonal CPA
Authors: Kolley E. , Kolley W.
Published: July 2007

In the presence of electron‐phonon interaction the dc conductivity of off‐diagonal random alloys is calculated in a modified CPA including vertex corrections. The numerical conductivity results reflecting realistic features of the model used depend strongly on off‐diagonal randomness and temperature. The combined effect of electron‐phonon scattering and statistical correlations can provide localization of minority‐band states.
Paraelectric resonance of hydrogen bonds in ferroelectries and related phenomena
Authors: Vikhnin V. S. , Gordon A. Ya
Published: July 2007

A theoretical consideration is made of paraelectric resonance (PER) of hydrogen bonds (HB) in KH2PO4‐type ferroelectrics. It is shown that the intensity of PER HB essentially increases in the external hydrostatic pressure field. The broadening mechanisms of the PER lineshape are discussed. The properties of the paraelectric phase with the tunneling states of HB, which occur in the high hydrostatic pressure field are studied. Nonresonance investigation methods of such tunneling paraelectric phase, notably the investigations of heat capacity and dielectric permittivity are considered together with the PER.
Electro‐optic properties of CuCl below the absorption edge
Authors: Mohler E. , Thomas B.
Published: July 2007

The dispersion and temperature dependence of the electro‐optic coefficient r of CuCl‐ are measured for wavelengths between 700 nm and the absorption edge near 400 nm. A method is described to eliminate the influence of residual strain birefringence which otherwise leads to serious errors. The dispersion of the optical dielectric constant and the electro‐optic coefficient is used to determine a schematic spectrum of the imaginary part of the dielectric constant and its changes in an external electric field. A model for the lattice contribution to the electro‐optic effect of CuCl based on the concept of bond polarizability is discussed.
Low‐temperature knight‐shift investigations of yttrium metal
Authors: Karcher R. , Lüders K. , Roth G. , Szücs Z.
Published: July 2007

The Y89 NMR is measured in the temperature range from 8 K to room temperature and in magnetic fields up to 80 kG. The Knight shifts KiSO and Kax are determined analysing the lineshapes. At the temperature T = 13 K. Kiso = (0.327 ± 0.005) % and Kax = (0.026 ± 0.002) %. A linear temperature dependence is found for both the Knight shifts: 1 × 104 % K−1 for KISO and 0.1 × 104 % K1 for Kiso. The results are discussed in connection with other electronic properties. It is possible to get a complete separation of the s‐, d‐, and orbital contributions not only for the Knight‐shift but also for the susceptibility and the relaxation rate.
Recombination luminescence in alkali halides doped with monovalent cations. (VK + e)A‐type emission
Authors: Ikeya M. , Crawford J. H.
Published: July 2007

Two types of emission bands are observed in alkali halides doped with monovalent cation impurities during X‐irradiation; Em‐I which is observed at low temperature where the self‐trapped hole, X is immobile and Em‐II, which is observed above the temperature where X becomes mobile. Em‐I is ascribed to(X + e) recombination perturbed by a monovalent cation. Em‐II is associated with ([X]A + e) recombination in which , is trapped by a monovalent cation.
On the influence of disorder on the electron density of states in a simplified model for a quasi‐onedimensional conductor
Authors: Cubiotti G. , Donato E. , Giuliano E. S. , Ruggeri R.
Published: July 2007

The electron density of states and their localization are investigated on a simple model consisting of single platinum chains surrounded by a halogen lattice whose sites are randomly occupied. The values of the hopping integrals are parametrized and, the localization of the states over certain ranges of energy is studied.
Temperature dependence of the electrical conductivity and hall coefficient in 2H‐MoS2, MoSe2, WSe2, and MoTe2
Authors: El‐Mahalawy S. H. , Evans B. L.
Published: July 2007

The electrical conductivity and Hall coefficient perpendicular to the c‐axis of hexagonal MoS2, MoSe2, and WSe2 are measured over the temperature ranges 120 to 1170 K and 140 to 820 K, respectively. The derived temperature‐dependent carrier densities are described by n ∼ exp (–E/kT) for temperatures less than 800 K indicating (thermal) energy gaps of 0.38 eV in n‐type MoS2, 0.27 eV in n‐type MoSe2, and 0.19 eV in p‐type WSe2. Above room temperature the carrier mobilities are given by μ ∼ T−α, where α ≈ 1.5 in MoS2, MoSe2 and 2.7 in WSe2. The high‐temperature (>800 K) thermal activation energies of conduction correspond to energy gaps of 1.27, 0.95, and 1.33 eV in MoS2, MoSe2, and WSe2, respectively. Similar measurements on MoTe2 crystals grown by bromine vapour transport demonstrate the effects of included bromine on the low‐temperature conduction processes.
Screw‐dislocation‐induced scattering processes and acceptor states in Te
Authors: Farvacque J. L. , Douehan J. C. , von Alpen U. , Gmelin E.
Published: July 2007

Electronic properties of e‐screw dislocations, introduced by shear experiments, are studied by galvanomagnetic and optical absorption measurements. The experiments clearly establish the existence of induced acceptor levels in the band gap. The origin of the dislocation charge its well as the scattering mechanisms induced by the dislocations are discussed with respect to three possible mechanisms: deformation potential, piezoelectric potential and core effect.
Dispersion of bulk phonon polaritons inside reststrahlen bands
Authors: Schuller E. , Falge H. J. , Borstel G.
Published: July 2007

When pure spatial damping is taken into account, i.e. when the wave vector is treated as complex and the frequency as a real quantity, dispersion curves of bulk phonon polaritons exhibit turnabouts caused by limiting momenta for the wave vectors which lead to branches within the frequency range of reststrahlen bands. The evaluation of the dispersion curves from reflection experiments for fixed frequency and swept angle of incidence confirms these features. Depending on the experimental arrangement dispersion curves inside of reststrahlen bands are investigated for α‐quartz. The measurements turn out to be a sensitive aid for the determination of damping parameters.
Linear optical anisotropy investigations of the antiferromagnetic phase transition in MnO
Authors: Treindl A. , Germann K. H.
Published: July 2007

The linear optical anisotropy of MnO single crystals is investigated as a function of the wavelength, the magnetic field, and the temperature near the antiferromagnetic phase transition at TN = 118 K. The linear magnetic birefringence (LMB) and the dichroism (LMD) show pronounced structures between 13000 and 22000 cm+1 which decrease with increasing temperature and vanish above TN. They are connected with the 4T1g‐ and 4T2g‐absorption bands of the Mn2+ ions. In the LMB spectra a temperature‐dependent background is associated with the band gap absorption. Magnetic fields up to 7.5 T have no effect on the LMB and LMD. The temperature dependence of the optical anisotropy shows the behaviour of the long‐range order parameter of the phase transition which is discontinuous at TN. An exchange striction model which includes uniaxial stress and magnetic fields is discussed in the mean field approximation; the results are in quantitative agreement with the experimental data.
Inelastic neutron scattering in SbSi near the ferroelectric phase transformation
Authors: Pierrefeu A. , Steigmeier E. F. , Dorner B.
Published: July 2007

Inelastic neutron studies are performed on SbSI. The (013) Bragg peak intensity is strongly temperature dependent and gives evidence for the first order ferroelectric transition. Experimental reasons (poor statistics, small samples) do not permit conclusions concerning the overdamped and rather broad coupled soft mode‐central peak entity which is observed in light scattering. An underdamped soft mode can be excluded. Acoustic and optical phonon dispersion surfaces are measured around the [0ζ0] direction and polarized in [001], which exhibit deep valleys along [0ζ0]. This correlates well with previous diffuse X‐ray results. The depth of the valleys increases strongly in changing the phase from paraelectric to ferroelectric.
Temperature and hydrostatic pressure measurements of the exciton peaks in BiI3 and SbI3
Authors: Brothers A. D. , Wieliczka D. M.
Published: July 2007

Exciton peak positions are measured as a function of temperature in the range 90 to 270 K and as a function of hydrostatic pressure to 3500 atm in thin film samples of BiI3 and SbI3. Values for (∂E/∂P)T are −8.6 × 10―4 eV/atm and −32 × 10―4 eV/atm for BiI3 and SbI3, respectively. (∂E/×T)P for BiI3 varies from −0.49 × 10―4 eV/K near 100 K to +0.56 × 10―4 eV/K near 200 K, and for SbI3 the value is found to be +7.3 × 10+4 eV/K. Estimates of the electronphonon coefficient, (∂E/∂T)V, are made. Comments are made concerning the nature of the excitons and the band structure in these materials.