bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of mass defect disorder on resonance Raman scattering
Authors: Kleinert P. , Bechstedt F.
Published: July 2007

A theory is developed for the influence of mass defect disorder on the one‐phonon Raman scattering. By using a Green's function formalism the scattering cross‐section is calculated in the case of mass defect disorder which influences the phonon subsystem only. An explicit expression for the scattering spectrum is given for monatomic and diatomic materials. The impurities substituted in the host crystal cause the occurrence of shifted frequencies in the scattering spectrum and the modification of the intrinsic Raman lines. The dependence of the scattering spectrum on the concentration and the mass of the impurities as well as the frequency of the incident light is studied.
On the existence of a spin‐glass state in an ising alloy model with mixed nearest‐neighbour exchange interactions
Authors: Medvedev M. V. , Rumyantsev E. L.
Published: July 2007

The possibility is investigated of a spin‐glass state as the ground state of a binary Ising magnet AB which is characterized (in the nearest neighbour approximation) by the competitive exchange interactions: IAA > 0; IAB > 0; IBB > 0. It is shown that due to increase of the antiferromagnetic component B concentration c in an intermediate concentration region either a spin‐glass state occurs or the first‐order phase transition from a random ferromagnetic state into a random antiferromagnetic one occurs depending upon exchange parameter ratios and crystal lattice type.
On the decay of the luminescence emission of Pb2+ centres in alkali halide crystals
Authors: Mǎrculescu L. , Vlahovici N. , Velula T.
Published: July 2007

Some experimental results concerning the decay times of the luminescence emission of Pb2+ centres in KCl and NaCl crystals are reported and discussed in order to clarify the origin of the two emissions excitable in the A absorption band. The experimental features support the luminescent Pb2+ centre model implying a resonant energy transfer.
Delayed fluorescence excitation spectra in doped anthracene crystals
Authors: Graber N. , Zschokke‐Gränacher I.
Published: July 2007

The triplet excitation spectrum of the delayed fluorescence is measured in anthracene crystals doped with tetracene. It is shown that below 150 K the quantum yield ratio of delyed host and guest fluorescence depends on the wavelength of the exciting light within the triplet absorption. This effect is attributed to direct excitation of traps below 150 K. Although these traps are not connected with the teracene guest their influence on the triplet excitation spectrum of the anthracene host can be observed only in doped anthracene where the delayed host fluorescence intensity is sufficiently quenched by the guest.
EPR of lithium in mechanically affected silicon
Authors: Höhne M.
Published: July 2007

An intense narrow EPR spectrum is observed at 20 and 1.5 K in lithium doped silicon, which has been mechanically affected. The spectrum exhibits an angular dependent, only partly resolved structure. The spectra at the different temperatures can be explained by assuming lithium donors in rhombic strain fields of intermediate strengths. A Dyson effect yields the line shape at 1.5 K.
Isotropic ferromagnets with biquadratic interactions
Authors: Chaddha G.
Published: July 2007

A spin‐one Heisenberg ferromagnet with biquadratic exchange is investigated, using Green's function technique in the Callen decoupling approximation. Assuming that the transition from the dipolar or quadrupolar state to the disordered phase is of second order, the transition temperature is found to decrease with the biquadratic exchange parameter α for 0 ≦ α ≦ 1 and increase for α > 1. The results for 0 ≦ α ≦ 1 are found to be significantly different from the earlier results in RPA and in Anderson‐Callen decoupling approximation.
Channel model in isotropic exchange theory (III)
Authors: Eremin M. V. , Rakitin Yu. V.
Published: July 2007

The exchange channel model developed earlier is applied to interpret the data on exchange interactions in ion pairs containing ions in excited states. The model is shown to give a satisfactory description of the available experimental data on doping pairs in perovskite‐type crystals provided the resonance excitation transfer effects are taken into account. The relationships based on the exchange channel model are used to calculate the superexchange contribution to the spectroscopic splitting parameter, 10Dq.
Nonrandom strain in „crushed”︁ silicon. EPR of thermally excited lithium donors
Authors: Höhne M.
Published: July 2007

Si:Li single crystals with „crushed”︁ surface layers are investigated by EPR. Measurements at 1.5 K show that these layers, which contain in the outer part also the dangling bond centre with g = 2.0055, nearly preserve their crystalline order, but exhibit strains preferentially perpendicular to the surface. At 20 K thermal population of higher strain‐split levels feigns equal probability of all strain directions. A dynamical averaging process is discussed.
The OCN− vibrational spectrum and the phase transitions in KCNxKCl1−x single crystals
Authors: Castro J. C. , de Souza M.
Published: July 2007

The OCN− bending mode and Fermi resonances are studied in pure KCN and in mixed KCNxKCl1−x crystals. The two phase transitions in these salts are followed as a function of the Cl− concentration. By the application of uniaxial stress in the mixed crystals almost single domain regions are formed that allow to verify that the CN− and OCN− alignment in the orthorhombic phase is along the (110) direction. These single domain regions keep their alignment without applied stress by temperatures below the cubic orthorhombic phase transition. No change in dichroism is found in cooling the crystal to very low temperatures.
Pressure dependence of the electrical conductivity in 2H‐MoS2 and 2H‐WSe2
Authors: El‐Mahalawy S. H. , Evans B. L.
Published: July 2007

The electrical conductivity of synthetic crystals of n‐type MoS2 and p‐type WSe2 over the range 300 to 870 K are measured at pressures up to 30 kbar. The derived values for the pressure‐dependent activation energies are assessed in terms of existing energy band models.
Two‐photon resonant Raman scattering via biexcitons
Authors: Phach Vu Duy , Bivas A. , Hönerlage B. , Grun J. B.
Published: July 2007

Two‐photon resonant Raman scattering processes are studied in different scattering configurations in CuCl. Observation of Raman lines from the front face of the crystal (backward scattering) yields new information on the energy of the transverse T5 exciton and on the biexciton damping. In transmission (forward scattering), two new emission lines can be identified as due to the Raman effect. They show a strong angular dependence because of the polariton nature of the particles involved. Their characteristic features contradict an interpretation in terms of Bose‐Einstein condensed biexcitons.
Investigation of the simultaneous absorption of three CO2‐laser photons by tellurium
Authors: Pöhler M. , Staupendahl G.
Published: July 2007

The selection rule of the simultaneous absorption of three CO2‐laser photons (ThPA) by tellurium is experimentally determined by a two‐beam method. These experiments yield a new proof for the existence of a strong ThPA in Te. At Δ = 10.8 μm the ThPA absorption rates C⊥ ≈︁ 5 × × 102 cm3/W3 s and C∥ ≈︁ 8 cm3/W3 s are estimated. For the first time a three‐photon spectroscopy of the absorption band of Te (E ⊥ c) is reported. The absorption edge shows a distinct exciton peak. Finally, measurements of the intensity‐dependent transmission (E ⊥ c) of Te under various conditions demonstrate the considerable ThPA reduction at wavelengths ⪆ 11 μm.
Force‐correlation function method applied to the electrical resistivity of many‐band metals
Authors: Christoph V. , Schiller W.
Published: July 2007

The method of calculating the electrical resistivity by force–force correlation functions is generalized to many‐band metals. For two‐band metals a formula is derived from which the electrical resistivity R can be calculated by means of a simple perturbation treatment. For the phonon assisted s‐d scattering the temperature dependence of R is calculated for the case that the d‐electrons do not remain in thermal equilibrium. The two‐current formula for the electrical resistivity of ferromagnetic metals and alloys is derived.
Electrical conductivity in the strong‐coupling non‐half‐filled band Hubbard model
Authors: Ihle D.
Published: July 2007

The chemical potential μ and the electrical dc conductivity σ in the one‐dimensional Hubbard model in the strong‐coupling region U≫t (U intra‐atomic Coulomb energy, –t nearest‐neighbour hopping integral) are calculated as functions of temperature and electron density n. In the non‐half‐filled band case n ≠ 1 the Bari‐Kaplan approximation lim (σ/t2) is shown to yield an un magnified image physical singularity in the dc conductivity at T = 0 which is connected with the discontinuity of the zero‐temperature chemical potential at n = 1. An improved calculation for n ≠ 1 going beyond the Bari‐Kaplan approximation (i.e. taking into account the band terms in μ and σ/t2) yields a finite zero‐temperature conductivity which is proportional to the first power in t. For small deviations from the half‐filled band case the conductivity as a function of increasing temperature subsequently shows a metal‐like behaviour, a semiconductor‐like behaviour, and a repeated decrease at high temperatures.
On the nature of E1‐transitions in germanium
Authors: Humlíček J.
Published: July 2007

The E1 optical structure of Ge is studied by unmodulated, first‐derivative, and low‐field electroreflectance spectra. An extensive investigation of heavily doped samples shows only minor changes of their electron structure for impurity concentrations up to 6 × 1019 cm−3. As the freecarrier plasma screens the electron–hole interaction, exciton effects are reduced owing to the doping. While the exciton interaction is found to modify the electroreflectance lineshapes, its role is not essential in unmodulated and first‐derivative ones. The lineshape analysis of the room temperature spectra shows that the E1‐structure is not well described by a simple parabolic critical point of the joint density of states, except for the low‐field electroreflectance, which is fitted satisfactorily by a two‐dimensional minimum (substituting a rather complex band structure with at least one M1 saddle point).
Bandstruktur von bis(catena‐poly(methylarsen))
Authors: Tyutyulkov N. N. , Petkov J. I. , Schuster P.
Published: July 2007

Es wird gezeigt, daß die Energielücke in BCPMA durch die Delokalisation der 4p‐Elektronen der As‐Atome bestimmt wird. Die Energielücke hat nur Korrelationscharakter. Es werden die beiden Formen von BCPMA – mit und ohne Alternation der As–As‐Bindungen in der Polymerkette untersucht. Es wird die Annahme eines strukturellen Peierls‐Übergangs in BCPMA gemacht.
Spontaneous parametric scattering of light in ammonium bioxalate monohydrate
Authors: Georgiev G. M. , Nikolov J. S.
Published: July 2007

Spectra of spontaneous parametric scattering of light are obtained for the first time in the biaxial crystal ammonium bioxalate monohydrate ((NH4)2C2O4 · H2O). Some peculiarities of the spectra, such as asymmetry, intensity fall, and others are explained. Three zones of many‐particle states are discovered. The dispersion of the polariton wave vectors px(v) and pz(v) are determined in the range 900 to 6500 cm−1. By analyzing the Stokes lines in the spectrum of the spontaneous parametric crystal scattering some vibrations of Bx and Bz symmetry are identified for the first time.
Statistical mechanics of impurity states I. General treatment
Authors: Foglio M. E.
Published: July 2007

Creation and annihilation operators that replace one ion in a crystal by an impurity in one of a set of states are defined. To use statistical mechanics with these operators, an ensemble that is close to the grand canonical ensemble is introduced. From the model Hamiltonian (given at a fixed value of the volume per crystal site) and the empirical knowledge of the pressure, most of the thermodynamic quantities are derived. A simple example is worked out in detail.
First‐order raman scattering of CsF doped with F centres
Authors: Lefrant S. , Buisson J. P. , Ghomi M. , Taurel L.
Published: July 2007

First‐order Raman spectra of CsF (F centres) studied in resonance with the F band (610 and 640 nm) at T = 10 K are described. The acoustic (43, 48, and 76 cm−1) and optical (135 and 137 cm−1) modes are due to electron‐phonon interactions involving the displacements of the first and second shells of the F centre neighbouring ions, respectively. The interpretation of the spectra needs to consider the breaking of the “usual” selection rules due to the spin‐orbit coupling in the first I4− orbital state of the F centre via the electron interactions with the Γ, Γ, and Γ phonons.
Binding‐energy calculations for multiple bound excitons
Authors: Wünsche H.‐J. , Henneberger K. , Khartsiev V. E.
Published: July 2007

The Hohenberg‐Kohn‐Sham density‐functional formalism is used to describe multiple bound exciton complexes (MBE's) in indirect semiconductors. Correlation effects are considered in localdensity approximation (LDA) using an interpolation formula concerning the correlation energy of the non‐neutral homogeneous electron‐hole plasma. Adopting an isotropic model band structure for silicon (me = 0.2588m0, mlh = mhh = 0.2375m0) the localization energies of complexes D0Xm are found to be 2.9 meV (BE), 5.6 meV (m = 2), 6.1 meV (m = 3), and 7.8 meV (m = 4). The high stability of these complexes is due to the degeneracies of band edges enabling to occupy the lowest orbital with 12 (4) electrons (holes) without violating the Pauli principle. As a consequence, adding of one exciton doesn't lead to an remarkable enlargement but to an increasing particle density of the complex and, thus, a correspondingly strong enhancement of correlation effects is produced.
Determination of the effective g‐value of electrons in n‐type InSb by spin‐flip Raman scattering and electric‐dipole‐excited electron spin resonance
Authors: Appold G. , Pascher H. , Ebert R. , Steigenberger U. , von Ortenberg M.
Published: July 2007

The effective g‐value of n‐InSb is determined in magnetic fields up to 65 kG by spin‐flip Raman gain measurements and by electric‐dipole‐excited electron spin resonance. Results are shown for a non‐degenerate n‐InSb sample (8 × 1013 cm−3) and a sample with the electron gas being in a degenerate regime (1.35 × 1015 cm−3). The g‐value of the degenerate sample decreases monotonously from 51 at B = 0 to 41 at B = 60 kG. With the non‐degenerate sample two g‐values are found. They are interpreted as associated with shallow donor states, which have different binding energies for the spin‐up and the spin‐down states. For magnetic fields smaller than 28 kG the two donor state g‐values are smaller than the conduction band g‐value, which is measured in the higher doped sample. For magnetic fields above 28 kG, however, the conduction band g‐value becomes smaller than the larger donor state value.
Experimental phonon frequencies of RbBr at 80, 290, and 370 K
Authors: Nilsson G. G. , Karlsson U. , Raunio G.
Published: July 2007

Phonon frequencies in RbBr are determined at the three temperatures 80, 290, and 370 K in the symmetry directions, Δ, Σ, and A using neutron inelastic scattering. The experimental frequency shifts obtained are compared to theoretical mode Grüneisen parameters calculated by Jex.
Phonon conductivity of InSb and GaAs
Authors: Al‐Edani M. C. , Dubey K. S.
Published: July 2007

The new expression for the three‐phonon scattering relaxation rate τ = (BN + BUe−θ/αT) × × g(w) Tm proposed by Dubey and Misho is used to calculate the lattice thermal conductivity of InSb and GaAs in the entire temperature range 2 to 300 K and 2 to 800 K, respectively and very good agreement is obtained between experimental and calculated values of the phonon conductivity of both the samples, InSb and GaAs, in the entire temperature range investigated. The reduced contribution due to transverse and longitudinal phonons are also studied by calculating the separate contribution due to each mode of phonons. The percentage contributions of three‐phonon normal and three‐phonon umklapp processes are studied. A comparative study of present results with the results obtained by previous workers is also made for both of the samples.
Electromagnetic waves in inhomogeneous semiconductors with non‐equilibrium carriers
Authors: Dykman I. M. , Tomchuk P. M.
Published: July 2007

In the presence of a heating field of carriers, F, the propagation of an electromagnetic wave in an extrinsic semiconductor with periodically distributed impurity centres, according to the law N(z) = = N0 + N1 cos z/L, is considered. At a wave vector k, of the wave inside the semiconductor, near to the condition of the Bragg resonance (k≈︁1/2L), the reflection |R0|2 and the transmission |D0|2 coefficients essentially differ from their values in a homogeneous semiconductor even at a small modulation of the impurity concentration (N1≪N0). The electric field F essentially influences the values of |R0|2 and |D0|2. With an increase of the field F, when k⪅1/2L a substantial bleaching of the material occurs. The reflection coefficient sharply decreases with the field, reaches a maximum value, and then asymptotically approaches to the value |R0|lim2, characteristic for the homogeneous semiconductor. When k⪅1/2L, |R0|2 monotonically decreases with the field down to its limiting value |R0|. As the wave vector k deviates from the resonance value, the above peculiarities become less pronounced.
Instability of the OPW Method in Relation to the Error in Core Eigenvalues
Authors: Reser B. I. , Dyakin V. V.
Published: July 2007

The influence of error in core eigenvalues on valence and conduction eigenvalues calculated by the OPW method is investigated. The general consideration is illustrated by a calculation of a silicon crystal. It is shown that due to the tendency of the determinant of the non‐orthogonality matrix to zero, a small error in core eigenvalues may cause large changes in the valence and conduction eigenvalues. Suggestions are given on how to reduce the influence of this drawback of the OPW method on the energy band calculations.
Theory of Free Carrier Absorption in Mixed Semiconductors with Composition Fluctuations
Authors: Kurowska A.
Published: July 2007

A theory of intraband absorption is derived for the cases of a degenerate and a nondegenerate electron gas in mixed semiconductors with composition fluctuations. In those materials both the band edge and the effective mass are position‐dependent. It is found that for low electron concentration (Ne ≈ 1015 cm−3) absorption coefficients due to composition fluctuations (chemical disorder) and due to ionized impurities are of the same order.
Optical Spectra and Band Structure of As2Se3
Authors: Althaus H. L. , Weiser G. , Nagel S.
Published: July 2007

The optical properties of the layer material As2Se3 at low temperature are presented in the energy range between 1 and 9 eV for polarization of light parallel to the three crystal axes including the stacking direction of the layers. The results are compared with a band structure calculated by means of a semi‐empirical tight‐binding model. It turns out that in As2Se3 no lone‐pair bands exist like for instance in Se. In particular there is no gap between bonding and non bonding p states. The direct gap occurs at the point Γ. Some of the upper valence bands near Γ show considerable splitting due to interlayer interaction. Nevertheless selection rules and the dependence of the optical spectra on the polarization of light are essentially determined by the symmetry of the single layer.
Theoretical and Experimental Investigation of the Electronic Structure of the B2‐Compound FeAl
Authors: Ziesche P. , Wonn H. , Müller Ch. , Nemoshkalenko V. V. , Krivitskii V. P.
Published: July 2007

The band structure and the density of states of the ordered B2‐type intermetallic compound FeAl are calculated with a simplified KKR method. The crystal potential is obtained by the Mattheiss construction, which involves a charge transfer from the Al atoms to the Fe atoms. The self‐consistency of the assumed configuration 3d74s1 for the Fe atoms is discussed. The obtained results are contrasted to the results of Okochi. The soft X‐ray emission spectra Fe M2,3 and Al L2,3 are measured. The comparison of the theoretical results with these and other experimental data shows a reasonable agreement.
On the Vibrational Entropy of a ½ 〈111〉 {110} Edge Dislocation in B.C.C. Iron
Authors: de Hosson J. Th. M.
Published: July 2007

A computer simulation study is set up to obtain the vibrational entropy at low temperatures of an edge dislocation. The anharmonic correction to the vibrational entropy at 70 K appears to be about 10%. The contribution of the total entropy to the total free energy is at most 10% of the activation enthalpy for overcoming the Peierls‐Nabarro barrier in the low‐temperature region. A conclusion is made that the results presented here appear to be in reasonable agreement with the experimental data which demonstrate that there is at present no convincing evidence for any considerable activation entropy.
Dynamical Electrical Conductivity, Dielectric Function, and Plasma Oscillations of the Hubbard Model
Authors: Gasser W.
Published: July 2007

Expressions are derived for the longitudinal dynamical conductivity, the dielectric function and the plasmon dispersion relation for the Hubbard model using approximations which are mainly based on the Hubbard‐I decoupling scheme for Green functions. In the limit of zero wave numbers a result is confirmed for the dc conductivity got by Kubo but later criticized by Lawson and Smith. The plasmon dispersion relation is determined up to the second order in the wave number.
Theory of Angle‐Resolved Photoemission. The Role of Electron Diffraction in the XPS Spectra
Authors: Paasch G.
Published: July 2007

The external photoelectron current can be expressed by the one‐electron Green's function for the crystal with surface. By calculating this GF with the matching formalism a general expression for the bulk contributions to the photoelectron spectra is obtained. As a consequence of the electron diffraction the transmission matrix of the surface enters this expression. The result is analyzed for the conditions of the angle‐resolved XPS measurements. Thereby it is shown what types of approximations or errors are contained in the two models which are used till now frequently for the calculation of angle‐resolved XPS spectra. Further a practicable approximate formula is given for these spectra which takes into account the electron diffraction in a simple manner.
Free Charges and Excitons in Stilbene Single Crystals Excited by Electron Pulses
Authors: Kostin A. K. , Savelev V. V. , Vannikov A. V.
Published: July 2007

The kinetics of relaxation currents, light absorption, and luminescence after excitation of stilbene single crystals by a pulse of accelerated electrons with τ = 40 × 10−9 s is measured. The excitation results in bulk generation of free charges and excitons. The microscopic mobilities are found to μp = 2.2 cm2 V−1 and μ, = 0.2 cm2 V−1 s−1. The radiation yield of the charges is G(p, e) = 0.25 ± 0.02, and of triplet excitons G(T) = 3.6. The optical absorption spectrum of triplet excitons is recorded, and it is found that vmax = 2.22 × 104 cm−1, the absorption coefficient Kvmax = 0.17 cm−1, and the oscillator strength of the T2 ← T1 optical transition band f = 0.20. The probability of creation of charges .
Colloid Centres in AgCl Induced by Heavy Ion Implantation
Authors: Diaine Ch. , Gisclon J. L. , Dupuy J.
Published: July 2007

The effects of ion implantation in AgCl produced mainly by the two types of interaction, electronic energy losses and nucleation energy losses, are essentially in the Ag+ lattice. Defects induced by the first one correspond to the photolysis mechanism and by the second perturbation are identified as Ag clusters. From the Lindhard diagram the conditions are given necessary to realize the two types of defects which are related to the mass and to the ion energy. The ions used are Li+, Na+, K+, Rb+, and Ar+ at RT and LNT. Due to the energy range available (50 to 500 keV) it is possible to produce significant variations of the nuclear energy loss ratio with Na+ ion and to observe seperately or simultaneously ionization defects and collision defects. It is found that the collision defects are colloidal silver connected to the nuclear energy losses.
Mössbauer Study of Dy3+ in CaF2
Authors: Kuo Ch. S. , Schroeer D.
Published: July 2007

The Mössbauer electric quadrupole interaction for Dy3+ in both oxygen‐free and oxygen‐injected CaF2 is measured between 90 and 630 K. Comparison of the experimental data with theoretical calculations confirms that in oxygen‐free CaF2 the stabilization occurs via an interstitial F− ion, while oxygen injection leads to stabilization by oxygen substitutionally replacing F− ions.
Ultrasonic Spectrum in a Multidomain Structure
Authors: Mitin A. V. , Tarasov V. A.
Published: July 2007

The spectrum of sound vibrations excited by a rf field in a magneto‐acoustic resonator which is formed on the basis of the Landau‐Lifshits multidomain structure is investigated. The properties of magneto‐acoustic and acoustic resonators are compared. The numerical evaluation indicates that the spectrum may be observed experimentally.
Resonant Raman Scattering in As2S3 Amorphous Thin Films
Authors: Razzetti C. , Lottici P. P.
Published: July 2007

The RRS cross‐section and the optical constants are investigated in amorphous As2S3 thin films. The results indicate that the cross‐section relative to the 350 cm−1 band is simply related to the imaginary (dissipative, or resonant) part of the dielectric susceptibility. Also an enhancement is found of the width of the 350 cm−1 band, which seems to diverge with increasing exciting energy. An absorption–emission picture of the RRS process could account for the observed results.
Radiative Damping Effect for Ultrarelativistic Channelled Particles
Authors: Wedell R.
Published: July 2007

A theoretical consideration is given to a possibility of focusing ultrarelativistic positrons and electrons under channelling conditions. The decrease of the transverse energy is mainly due to energy losses by a specific radiation considered recently by Kumakhov. This effect appears for electrons at energies of several GeV and for positrons of several dozens of GeV. The mechanism perturbing focusing is multiple scattering. The focusing effect decreases the divergence of the beam penetrating through the target. Target thicknesses which are necessary to exhibit this effect are estimated.
Optical Detection of Nuclear Magnetic Resonance with Distant F‐Centre Pairs in KCl
Authors: Jaccard C. , Ecabert M.
Published: July 2007

In quenched KCl crystals containing more than 1017 cm−3 F‐centres and excited by unpolarized light in the F‐band at 12 K, the NMR spectra of the neighbouring nuclei are detected at fields between 0.5 and 5 kG and radiofrequencies between 0.1 and 10 MHz as negative changes of the F‐luminescence (1%). This is explained by a non‐radiative disexcitation of F centre pairs through an F+F− state. Its probability is a function of the singlet admixture of the electronic triplet states, which is produced by non diagonal terms of the hyperfine coupling, and depends on the nuclear spin states. It is shown that the optical cycle produces a second order polarization of the nuclei, which is then decreased by applying a resonant rf field. The spin‐Hamiltonian parameters corresponding to the measured NMR spectra are the same as from single F‐centres ENDOR.
The Reduced Conductivity of Dielectrics when Both Boundary and Dislocation Scatterings are Important at Low Temperatures
Authors: Gupta I. J. , Trikha S. K.
Published: July 2007

An earlier theoretical result connecting the reduced thermal conductivity and reduced temperature of dielectrics is modified to deal with the cases where boundary as well as dislocation scattering simultaneously limit the phonon lifetime at low temperatures. Comparison with experimental data in case of solid argon and neon give satisfactory agreement for T* < 1.4.
Investigations on the Temperature‐Dependent TPA Linewidth of the Z3‐Exciton in CuCl
Authors: Kalm M. , Uihlein Ch.
Published: July 2007

The two‐photon absorption (TPA) line of the longitudinal Z3(1s) exciton in CuCl is studied in the temperature region between 4.6 and 185 K. The longitudinal exciton component has a Lorentzian‐like lineshape in the quoted temperature interval. The halfwidth of the narrow absorption line, which is approximately 0.14 meV below 10 K, increases rapidly for temperatures above 60 K (14 meV at 185 K). The steep increase is correlated with the number of thermally excited LO phonons. The temperature‐dependent halfwidth, which can be fitted by a Bose function with the energy of the LO phonon mode (26.3 meV), is found to be proportional to the LO phonon density. From this it is concluded that the lifetime of the Z3‐exciton is mainly determined by an intraband scattering process caused by the absorption of a longitudinal optical phonon.
On the Representation of the Non‐Stationary Density Matrix of an Electron–Phonon System by a Functional Integral
Authors: Fomin V. M. , Pokatilov E. P.
Published: July 2007

A non‐stationary density matrix depending on the coordinates of both electron and phonons is obtained for an ensemble of electron–phonon systems with an arbitrary initial distribution. This density matrix is used to describe the non‐linear optical properties of a semiconductor in the presence of a flow of electrons. A generalized non‐linear dielectric permeability tensor and a generation tensor are found for this case. The anisotropy of absorption, the birefrigence, the transformation of type, and change of parameters of the laser wave polarization and the second harmonic generation due to the flow of electrons are discussed.
Effect of Correlation on the X‐Ray Diffuse Scattering
Authors: Semenovskaya S. V.
Published: July 2007

The contribution of the correlation in X‐ray diffuse scattering caused by short‐range order in disordered binary alloys is considered. This consideration results in the development of the Krivoglaz‐Clapp‐Moss theory for the X‐ray diffuse scattering. It is based on the equation for the free energy of a disordered alloy which takes into account the correlation terms as well as arbitrary‐range interaction. The derived equation looks like the Krivoglaz‐Clapp‐Moss equation. However, it contains some effective value Veff (k) instead of the Fourier transforms of interchange energies V(k). It means that the value V(k) found from X‐ray data by means of Krivoglaz‐Clapp‐Moss equation is really Veff(k). Some experimental data for Fe–Al and Fe–Si alloys reveal large difference between Veff(k) and real value of Fourier transforms of interchange energies V(k).
Auger Neutralization of Slow Protons at Solid Surfaces
Authors: Horiguchi S. , Koyama K. , Ohtsuki Y. H.
Published: July 2007

Auger neutralization of slow protons (⪅ a few keV) scattered from solid surfaces is considered. The neutralïzation probability per unit time P(s) is calculated as a function of the proton distance s from the surface to P(s) = A{e−2xs + (Bs + C) e + (Ds2 + Es + F) e}, where a0 is the Bohr radius and A, B, C, D, E, F, and x proper constants depending on the target. At large distances from the surface, the second and third terms of P(s) can be neglected. As the proton approaches the surface, however, P(s) deviates from the simple exponential dependence of s. The neutral fraction φ0 is also calculated. From the obtained φ0, it is shown that Auger neutralization is important, at least in the low energy region.
Neutron Scattering on Anomalous Fluctuations of Phonons
Authors: van Trong Nguyen
Published: July 2007

The process of inelastic neutron scattering in crystal is investigated in the case, where the crystal is suggested to be an electron–phonon system placed in an external radiation field. The cross‐section of one‐phonon scattering process is calculated by using the two‐particle correlation function of phonon. It is shown that near the borderline of parametric instability region the correlation function of phonon and the cross‐section of neutron consequently take anomalously large values.
The Excitonic Surface Potential in Semiconductors
Authors: Balslev I.
Published: July 2007

A perfect semiconductor surface produces a potential associated with the translational motion of excitons. This potential is calculated including contributions from both short and long range forces. The relative motion of electron and hole is studied by a variational method and by asymptotic estimates. The potential is used for a detailed description of the exciton free layer and the macroscopic boundary condition relevant for explaining spectra of dispersive resonances. The agreement with previously reported experiments encourages to further studies of the influence of clean surfaces on excitonic phenomena.
Theory of Second‐Order Resonance Raman Scattering in the Case of Strong Excitonic Effects
Authors: Bechstedt F. , Haus D.
Published: July 2007

Calculations of the integral and differential cross‐section of the two‐phonon resonance Raman scattering are presented. Both the cases of Q‐independent deformation and Q‐dependent Fröhlich electron–one‐phonon coupling are considered. The electronic intermediate states are described in the Wannier‐Mott approximation. The large binding energy limit is assumed. It is shown that the intensity of combination tones are negligible small compared to that of pure overtones if the exciton binding energy is large enough. In contrast to the case of uncorrelated electron‐hole pairs for both the allowed and forbidden scattering a pronounced double peak structure is found in the resonance behaviour of the integral cross‐section. Analogously to the case without electron‐hole Coulomb interaction the lineshape of the two‐phonon scattering depends strongly on the incident photon energy. Only far from electronic transition energies the second‐order Raman spectra can be interpreted in terms of the two‐phonon density of states. Results obtained here are in qualitative agreement with experimental observations in CdS.
Functional Integral Approach for the Hubbard Model with Arbitrary Electron Density
Authors: Richter A. , Röpke G. , Goedsche F.
Published: July 2007

The method of the functional integral is applied to the conventional Hubbard model with an arbitrary filled band. A relation between the number of electrons per atom, n, and the chemical potential μ at T = 0 is calculated. The static magnetic susceptibility and the local magnetic moments are studied. Looking for the dependence of magnetic properties on the electron correlation U and the number of electrons per atom n, a phase diagram is obtained at zero temperature. Besides the paramagnetic phase ferromagnetic and antiferromagnetic phases exist within this diagram.
Band Theory of Size‐Quantised Electron States in Thin Crystalline Films
Authors: Cottey A. A.
Published: July 2007

The discrete electron states in a thin film with an arbitrary three‐dimensional periodic crystal potential are analysed. First the quantum size effect states of a symmetric film are found exactly for Dirichlet (ψ = 0) and Neumann (ψ′ = 0) boundary conditions, and the interpolation between these extremes is studied. Then a surface scattering matrix formalism is set up in the general (unsymmetric) case. This treats the general situation when there may be M up‐ and M down‐travelling Bloch waves with a given energy and surface‐parallel component of crystal momentum modulo surface reciprocal lattice. The equations are solved in the general 1 × 1 and symmetric 2 × 2 cases. The QSE solutions approximate gently undulating parallel surfaces in k‐space. Usually the separation of these surfaces is roughly uniform. An exception occurs in the 2 × 2 case near a band edge which is not at the centre or edge of the Brillouin zone; the solutions are then in nearly degenerate pairs. The results agree with available experimental data.
Hot Electrons in Zero‐Gap Semiconductors with Non‐Parabolic Electronic Band Structure
Authors: Beneslavskii S. D. , Ziep O.
Published: July 2007

Mobility μ and concentration n of electric‐field‐induced hot electrons are investigated in zero‐gap HgCdTe with linear band structure at liquid helium temperatures. Taking into account the admixture of s‐like wave functions, dielectric anomalies, and the renormalization of the heavy‐hole spectrum it is shown that Coulomb scattering is the dominant mechanism for relaxation of momentum and energy at 4.2 K. Using rate equations and an electron temperature model the behaviour of μ and n is calculated up to electric field strength of 1.5 V/cm.
Positron Annihilation Rates in the Metals of Ag and Au Series
Authors: Shrivastava S. B. , Bonde H. P.
Published: July 2007

The effect of electron–electron and electron–positron correlations on the positron annihilation rates in the metals of the Ag and Au series are investigated and discussed with the help of positron annihilation angular correlation results. The correlation effects are found to have little variation for the metals in the same series. However, they decrease for the higher Z series metals. The calculated annihilation rates obtained using the simple proposed model are found to be in good agreement with those observed experimentally.
Faraday Rotation in Presence of Attractive Traps
Authors: Bhattacharya D. P. , Kachlishvili Z. S.
Published: July 2007

The Faraday rotation is investigated in the general case for any mechanism of energy and momentum scattering of hot electrons in the presence of a strong or weak transverse magnetic field, when the free carriers are captured by oppositely charged impurity centres. For some definite scattering mechanisms the investigations are carried out in the presence of any finite magnetic field. It is shown that the capture parameters for deep traps and the dominant scattering mechanism of the hot carriers can be determined from a comparison of the theoretical results with experiments. Calculations are carried out assuming a parabolic dispersion law and a scalar effective mass.
Diffraction of Radiation from Channelled Charged Particles
Authors: Baryshevskii V. G. , Grubich A. O. , Dubovskaya I. Ya.
Published: July 2007

An explicit expression for cross‐section and radiation spectrum at diffraction is calculated. It is shown that photons emitted by channelled particles form a typical diffraction pattern which contains information about the crystal structure. It is also shown that the change of the longitudinal energy of the particle caused by the radiation braking becomes important when the particle energy is increased.
Hot Electrons in Semiconductors with Quasi‐Relativistic Band Structure
Authors: Dykman I. M. , Rosenbaum V. M. , Vasko F. T.
Published: July 2007

A kinetic equation for hot electrons in semiconductors of the lead chalcogenide type is obtained. The effects of heating‐up with a dc or microwave electric field are discussed. A function of the electron distribution at quasielastic scattering of acoustic phonons with equiparted energy is derived in the diffusion approximation. Such parameters as current—voltage characteristics, effective high‐frequency conductivity (depending on the intensity of incoming radiation) and μ‐photoconductivity of the electron gas are examined. The kinetic coefficients are calculated for a highly degenerate gas with accurate estimation of the quasi relativistic band spectrum; for non‐degenerate electrons kinetic coefficients are calculated to first corrections by ϵ/ϵg (ϵ average electron energy in the conductivity band; ϵg width of forbidden gap).
On the Non‐Adiabatic Properties of Tunneling Centres
Authors: Wagner M.
Published: July 2007

A non‐adiabatic formulation of the Hamiltonian for tunneling centres is given. A modified Born‐Oppenheimer basis is employed, which attaches the tunneling particle to a central reference coordinate. The non‐adiabatic dynamical lattice disturbance near the defect is established. New phonon assisted relaxation paths of a direct nature are exhibited. In Li+‐like centres these seem to be dominant, since the conventional ones are of pure non‐adiabatic origin (and of second order).
From Small to Infinite F.C.C. Clusters
Authors: Kadura P. , Künne L.
Published: July 2007

The size and shape dependence of some properties of finite f.c.c. clusters is investigated using the tight binding approximation. Three different sequences of rectangular prismatic f.c.c. crystallites with (100) surfaces (edge length A, B, C) are taken into account, namely cubes (A = B = C), plates (A = B ≫C) and rods (A = B≪ C). Numerical results for the s‐like band structure, binding energy per atom, and Fermi level are presented in dependence on the number of atoms, up to some millions of atoms. The results visualize the convergence of the cluster properties and indicate their shape specifity.
Electronic Transport in Doped a‐Ge and a‐Si Prepared by DC Cathodic Sputtering
Authors: van Dong Nguyen , Hai Tran Quoc
Published: July 2007

Doped a‐Ge and a‐Si films are prepared by the cathodic sputtering technique with Sb and In impurities for a‐Ge, P and B impurities for a‐Si. The electrical conductivity σ and thermopower S of different samples are measured as a function of absolute temperature between 150 and 500 K. The effect of doping is to change not only the activation energies Eσ and Es but also other parameters in the expression of σ and S. The considerable reduction in the pre‐exponential term of σ is found to be much larger for a‐Si than for a‐Ge. The temperature shift of the Fermi level is taken into account to explain the variations of these parameters. The activation energy of conductivity is higher than that of thermopower by about 0.10 eV for a‐Ge and 0.15 eV for a‐Si. Experimental data analysis suggests that in both, a‐Ge and a‐Si, for high doping concentrations the hopping conduction through an impurity band makes a dominant contribution to the current flow below room temperature.
Modulation Spectroscopy on the Group IV and VI Transition‐Metal Dichalcogenides
Authors: Saiki K. , Yoshimi M. , Tanaka S.
Published: July 2007

Piezoreflectance, thermoreflectance, and wavelength modulated reflectance are measured on the group IV and VI compounds of transition‐metal dichalcogenides. From the evaluation of the spectra by different modulation techniques, from the alloying effect which is first investigated, and from the comparison with the band structure calculations, it is most likely that the A and B excitons observed in the absorption edge of the group VI compounds correspond to the d—d transitions, while the A′ and B′ excitons observed in the higher‐energy side correspond to the p—d transitions. For the group IV compounds the modulated reflectance is first measured and new peaks are found. The comparison with the band structure calculations is also carried out.
On the Ward Identities for the Hubbard Model. Particle‐Number Conservation and the Analytical Structure of the Self‐Energy
Authors: Behn U.
Published: July 2007

The Ward identities for the Hubbard model connected with the spin‐rotational invariance and the particle‐number conservation are derived considering the equation of motion for 4‐point Green functions. Exploiting the generalized Ward identity connected with the particle‐number conservation, the self‐energy splits into a static part and a dynamic part. The latter describes the effects due to the hopping of the electrons of the opposite spin. The analytical structure which guarantees the particle‐number conservation and the reproduction of the atomic limit, can explicitely be seen so that approximations may be performed conserving this structure.
Spin‐Wave Resonance Spectrum in Thin Films with Internal Isotropically Coupled Impurity Layer
Authors: Duda A. , Maciejewski W.
Published: July 2007

The spin‐wave energies, amplitudes, and resonance line intensities in thin films with an internal impurity layer are analysed. The influence of the ferromagnetic and antiferromagnetic impurity coupling both on bulk and localized states is discussed. It is shown that the impurity coupling can be calculated from the resonance measurements.
A Thermodynamic Theory of the Improper Antiferroelectric Phase Transition in ADP
Authors: Konwent H. , Lorenc J.
Published: July 2007

The thermodynamic theory of the elastic and dielectric properties of ammonium dihydrogen phosphate is developed on the basis of the improper phase transition. By the representation analysis the transformation properties of the two‐component order parameter are investigated and the thermodynamic potential of the Landau type describing both elastic and dielectric properties (triggered by the order parameter) is derived, which correctly describes the space group symmetry change at the transition point. Its minima and their stability conditions are also studied. The temperature dependence of the spontaneous deformations and material constants is analysed in detail and consistency with the available experimental data is briefly discussed.
High‐Temperature Series in the Quantum‐Classical Heisenberg Model
Authors: Wydro T.
Published: July 2007

The recently introduced quantum‐classical Heisenberg model is investigated in terms of the high‐temperature cumulant expansions. The cumulants of the diagrams up to l = 7 lines and the lattice constants of the diagrams up to l = 6 lines for inhomogeneous p.t. and f.c.c. lattices are calculated. The model parameter y expressing the relative magnitude of two types of coupling constants is taken into account. High‐temperature series for the zero‐field susceptibility are constructed for ten values of parameter y and for both p.t. and f.c.c. lattice structures. The Padé analysis of the series is performed. Critical temperatures and critical indices of susceptibility are estimated.
Statistical Theory of Electrical Resistivity of Magnetic Ternary Alloys with Partial Long‐Range Order
Authors: Richter R. , Goedsche F. , Vojta G.
Published: July 2007

Within the framework of the coherent potential approximation the de electrical resistivity is calculted for random ternary substitutional magnetic alloys with partial atomic long‐range order. The special case of nonmagnetic alloys with potential scattering only is included. The case of a two sublattice system with two long‐range order parameters is studied. The spin‐disorder contribution to the resistivity is treated using the s–d exchange model. The calculations are done for a single‐band tight‐binding model with diagonal disorder, three kinds of localized spins, arbitrary concentrations, and arbitrary order parameters. A general closed resistivity formula in terms of alloy, spin, and order parameters is given in second order of the scattering potential and is discussed for a number of special cases including antiferromagnetic ordering.
Microwave Spectroscopy in Semimetallic Bismuth–Antimony Alloys
Authors: Braune W. , Kuka G. , Gollnest H.‐J. , Herrmann R.
Published: July 2007

Results are given of systematic microwave absorption measurements at 35 and 70 GHz in semimetallic BiSb alloys. The great number of experiments permits to made representative statements on the dependence of the cyclotron masses and the relaxation times of the charge carriers from the Sb concentration. Measurements of the electron‐hole hybrid‐resonance field strengths vs. the antimony concentration (x at% of Sb) show that the band parameters of the holes at T do not substantially vary for x < 6.