bluuebunny/crossref_metadata_2025 huggingface dataset:
Triplet absorption of crystalline anthracene by high resolution photoconductivity spectroscopy
Authors: Fort A. , Coret A.
Published: July 2007

The photoconductivity spectrum of anthracene crystals at 4 K is studied with a cw dye laser in the spectral region corresponding to triplet exciton creation. Photocarriers are created by the detrapping of charges by excitons. This spectrum is complex but shows the different vibronic excitations which correspond to the purely electronic transition S0 → T1 coupled with different totally symmetrical vibration modes of the molecule. Lattice vibration modes also appear. The spectrum confirms the value of the Davydov splitting for the 0–0 transition and the broadening of the lines for the 0–1 and 0–2 transitions. The presence of satellite lines shows the formation of triplets on disturbed sites of the lattice.
On the dependence of light reflection from metals on adatom characteristics
Authors: Brodskii A. M. , Urbakh M. I.
Published: July 2007

Relationships are derived to interrelate the experimentally observed variation in the reflection coefficient of light from metals during the adsorption and the microscopic characteristics of an adatom. A method is developed, similar to the introduction of pseudo‐potential in solid state physics and molecular physics, to describe the electron properties of adatoms. The theoretical relationships derived are suitable for treating experimental data. The theory proposed shows that optical methods are quite promising for the determination of important microscopic characteristics of adsorption.
Acoustic phonon scattering by electron‐hole droplets in Ge. Deformation potentialsz
Authors: Markiewicz R. S.
Published: July 2007

Deformation potentials are calculated for electron‐hole droplets in Ge and compared to the values for degenerate n‐ or p‐doped Ge. Due to the conduction band anisotropy and the presence of an equal number of electrons and holes, it is found that screening effects are substantially reduced, but that there are strong anisotropies in coupling to various phonon modes.
Electrical resistivity of antiferromagnetic alloys coherent potential approximation
Authors: Borodachev S. M. , Masharov S. I. , Volkov V. A.
Published: July 2007

The electrical resistivity of a binary antiferromagnetic alloy due to the scattering on spin disorder and on the static potential of a random distribution of different atoms over the lattice is calculated in the coherent potential approximation. The concentration dependences of the contributions to the resistivity due to the spin disorder and elastic scattering as well as of the interference term caused by differences of the atomic levels and exchange constants of the components are obtained. If the Fermi energy is far from the spectrum gap arising with the transition to the antiferromagnetic state, the resistivity of alloys decreases while the magnetization of the sublattices increases. A more complicated behaviour of the resistivity versus magnetization takes place in those alloys in which the Fermi energy is near the spectrum gap. In this case an increase of elastic and spin contributions to the resistivity is possible with antiferromagnetic ordering. This increase of some contributions to the resistivity may be a reason for such anomalies as the maximum or minimum of the total resistivity in the region of the Neel point.
Theory of radiation of charged particles channeled in a crystal
Authors: Kumakhov M. A.
Published: July 2007

The theory of a new effect is developed — spontaneous radiation of γ‐quanta in channeling relativistic particles. The radiation characteristics are different from the presently known types of radiation sources. At energies of the order of 1 GeV the radiation is characterized by wavelength ⪅ 10−3 Å. The radiation intensity is about 6 to 9 orders of magnitude higher than obtained from modern synchrotrons. Its monochromaticity is considerably better than bremsstrahlung or synchrotron radiation. The radiation is fairly well polarized. In the quantum treatment, the radiation is caused by transitions between levels, which are formed in channeling in the fields of atomic planes and rows. A classical and quantum calculation of the radiation intensity is presented. At a current of about 100 μA and a target thickness of ≈︁ 1000 μm, about 1018 to 1019 quanta/s are radiated. The spectral density of the radiation is considerably higher than that of any known kind of radiation. It is suggested that the radiation might be used to influence selectively nuclear transitions and to produce a γ‐laser. This radiation can be used to study the crystal lattice potentials, measure the particle energy, and so on. Dispersion is studied. The possibility of enhancing the induced radiation is shown. A number of studies on light channeled particles are analysed.
Coulomb gap in a disordered semiconductor
Authors: Bonch‐Bruevich V. L.
Published: July 2007

Conditions of the Coulomb gap to appear in the energy spectrum of a disordered semiconductor are studied avoiding assumptions too model‐like. In particular the form of the electron‐electron interaction potential is not specified, up to the assumption that it is screened in some way. Low temperature screening, absorption of low‐frequency electromagnetic waves, and some thermodynamic properties are considered for a system containing a “soft” density of states gap.
Parametric action of a strong radiation field on anisotropic semiconductors
Authors: Anh Vo Hong , Dat Nguyen Nhu
Published: July 2007

The process of parametric excitation of electromagnetic eigenwaves in the class of uniaxial semiconductors is studied. The instability threshold field values are estimated numerically for a typical sample of a wurtzite structure crystal of CdS.
Image potential near a dielectric–plasma‐like medium interface
Authors: Kornyshev A. A. , Rubinshtein A. I. , Vorotyntsev M. A.
Published: July 2007

The effect of penetration of an electric field into the metal, electrolyte, solution, or other plasmalike medium (PLM) on the image‐potential energy in a dielectric adjacent to such a PLM is studied. The asymptotics of classical attraction of a test charge to an “impenetrable metal surface” at large distances and of effective interaction with “dielectric‐dielectric interface” at small distances are found. The latter results in a repulsion of a charge from the boundary, when the dielectric constant of the background medium, in which plasma particles are dissolved, is smaller than the permittivity of the dielectric. A minimum of the image‐potential curve is situated between the “repulsive” and “attractive” limiting branches. Thus an adsorption minimum is predicted within the scope of purely electrostatic considerations, the value of the “desorption energy” being estimated.
The critical voltage effect in transmission electron microscopy. IX. The Calculation of Critical Voltages and Experimental Extinction Distances in Complicated Many‐Beam Systems
Authors: David M. , Gevers R.
Published: July 2007

By means of an algorithm based on perturbation theory, the illumination conditions under which a given many‐beam system has a twofold degeneracy, can be calculated quickly and to arbitrary accuracy, provided a good estimate of the critical values is available. The same technique can be used, conversely, to calculate corrections on theoretical extinction distances so as to fit critical voltage measurements.
Calculation of nuclear double‐resonance relaxation rate
Authors: Whitaker M. A. B.
Published: July 2007

A quantity W (Δ), related directly to the relaxation‐rate of the double‐resonance process, is defined. Its properties, and those of its Fourier transform, are discussed. Algebraic calculations of these functions are performed using the Dyson expansion, and numerical results presented.
A many‐body model of dieletric polarisation in solids. II. The universal model
Authors: Jonscher A. K.
Published: July 2007

On the basis of the experimental data previously presented, the case is made for a unifying approach to the interpretation of the common dielectrie response of most solids. This approach starts from the unique property of this response tht the ratio to the energy lost per cycle to the energy stored in the system is independent of frequency. A “screened hopping” model involving many‐body interactions is proposed which has the requied generality and which should account for the commonly observed behaviour. The physical nature of the loss peaks is also interpreted in terms ofthe new model as the result of sequential interplay of the different types of decay processes in the time domain. The relation of the new model to the classical Debye picture is briefly discussed.
Polarizabilities of donors in elemental semiconductors
Authors: Palaniyandi E. , Balasubramanian S.
Published: July 2007

Using the multivalley effective‐mass theory and the variational method the following expression is obtained for the polarizability α of donors in elemental semiconductors: α = 4a4/[(−a/K) + + (1/m*) + 2EDa2], where a is the effective Bohr radius for the donor, K the static dielectric constant of the host semiconductor, m* the conduction electronic effective mass, and ED the donor ionization energy. The estimates of α for various donors in Si and Ge are discussed.
Temperature dependence of knight shift and susceptibility of single crystal molybdenum
Authors: Karcher R. , Köbler U. , Lüders K. , Szücs Z.
Published: July 2007

The temperature dependences of the Knight shift (for 95Mo and 97Mo) and the magnetic susceptibility of single crystals of molybdenum are measured in the temperature region 10 to 300 K. Furthermore the relaxation rates are determined on the same samples. A complete separation of the quantities into the s‐, d‐, and orbital contributions is made using some additional assumptions. For the temperature dependence it results that the orbital contribution predominates.
Influence of an electric field on the Jahn‐Teller split absorption band 4A2g → 4T1g(P) of Co2+ in CdF2 and CaF2
Authors: Krylov V. A. , Ulrici W.
Published: July 2007

The change of the optical absorption of the Jahn‐Teller split and vibration‐induced 4A2g → → 4T1g(P) band of Co2+ in CaF2 and CdF2 induced by external electric fields up to 160 kV cm−1 applied in the three main crystallographic directions is measured with polarized light at 78 K. In all cases an enhancement of the oscillator strength is found increasing quadratically with the electric field. The field‐induced changes of the moments of the absorption band are calculated up to the third moment. However, both the line shape of the electromodulated spectra and the dependence on field direction and polarization cannot be explained within the approximation of the linear interaction with even and odd lattice modes. From the experiments arguments are derived that higher‐order interactions with even and odd modes determine the electromodulated spectra.
Thermopower and the transverse nernst‐ettingshausen effect in semiconductors in a strong electric field
Authors: Babaev M. M. , Gasymov T. M.
Published: July 2007

The thermopower and the transverse Nernst‐Ettingshausen effect of semiconductors in strong (heating) electric and nonquantisized magnetic fields are investigated regarding the phonon darg of electrons and the heating of the phonons. The dependences are obtained of the thermopower and Nernst‐Ettingshausen voltage on the electric field strength E0. It is shown that in the case of strong electron and phonon heating and impurity scattering of the nondegenerate electrons the electronic part of the thermopower increases α E04/3, whereas the phonon part α E010/3, if the phonons are scattered by electrons. But if the phonons are scattered by crystal boundaries the electronic part of the Nernst‐Ettingshausen voltage in weak magnetic fields is α E010/3, whereas the phonon part is α E04.
On the properties of cobalt impurities in zinc selenide crystals. I. Effect of in‐band position of states on photoelectric properties
Authors: Radlinski A. P.
Published: July 2007

The absorption and photoconductivity spectra of ZnSe:Co in the spectral region of the 4A2(4F) → → 4T1(4F) and 4A2(4F) → 4T1(4P) crystal field transitions within the 3d7 shell of the Co2+ ion are presented. The structure of the photocurrent observed here is associated with the degeneracy of the 4T1(4P) and 4T1(4F) excited states of the Co2+ ion with the conduction band of ZnSe. The autoionization times of Co2+ ion via those states to the conduction band are estimated to lie within the range of 10−9 to 10−4 s.
The influence of fast neutron irradiation on the critical current densities in Nb3Sn. II. High neutron doses (Φ > 2 × 1018 neutrons/cm2)
Authors: Fähnle M.
Published: July 2007

By a model calculation within the framework of the Labbé‐Friedel‐van Reuth theory of A15‐superconductors the influence is investigated of radiation induced atomic disorder on the critical temperature Tc, the Fermi level EF, the Ginzburg‐Landau parameter x, the coherence length ξ, the penetration depth λ, the critical fields Hc and Hc2, and the critical current density Jc. The theory describe successfully the experimentally observed depressions of Tc, Hc2, and Jc after high dose neutron irradiation. The assumption of statistically distributed atomic defects turns out to be more appropriate to explain the experimental results than the assumption of a strongly inhomogeneous defect distribution.
Theory of radiation of relativistic channelled particles
Authors: Kumakhov M. A. , Wedell R.
Published: July 2007

A quantum theory of spontaneous γ‐quantum radiation by relativistic channelled particles is developed. Planar channelling of positrons and axial channelling of electrons are considered. The calculation results are in a good agreement with classical estimates. Population of the levels is studied and the conditions of spontaneous radiation are analyzed in detail. It is shown that, when the transitions between the lower levels are considered, one may limit oneself to the solution of the problem for a single potential well since in this case the neighbouring wells fail practically to affect the calculation results. The effect of the potential unharmonism on spectral distribution is estimated.
Instability of the flux‐line lattice in a type‐II superconductor containing planar imperfections
Authors: Brandt E. H.
Published: July 2007

The pinning forces exerte dby a planar imperfection on the flux lines in a type‐II superconductor are calculated from the Ginzburg‐Landau theory. The elastic strains caused by this imperfection in the flux line lattice are found to diverge at large inductions. As a consequence, even rather thin planar inclusions or grain boundaries lead to a breakdown of elastic linearity and to a trapping of flux lines by the imperfection as the induction is increased. This result does not require a vanishing shear modulus of the flux‐line lattice but is due entirely to the nonlocal character of the elastic behaviour of the lattice. It may explain in certain cases the observed peak in the volume pinning force at large inductions.
Polariton effects on the recombination of cold biexcitons
Authors: Ostertag E. , Bivas A. , Grun J. B.
Published: July 2007

If one assumes that, when the biexcitions are created directly by two‐photon absorption, a cold gas of biexcitons with virtually no interaction is formed, the position and shape of the narrow emission lines due to their recombination should be influenced by some salient features of the final state. By adopting a special geometrical configuration, the magnitude of the wave vector of the exciton left in the final state is varied in a range such that this exciton penetrates deeply into the polariton region of its dispersion curve when it is of transverse type. The resulting observations of a shift and broadening of the narrow emission line associated with that final state, as well as of the change in the ratio of the intensities of the two narrow lines, are in good agreement with the theoretical predictions derived from that model.
Thermal conductivity of irradiated RbMgF3
Authors: Hartmann J. B. , Martin J. J.
Published: July 2007

The thermal conductivity of RbMgF3 has been measured from 2.5 to 80 K as a function of radiation damage. When RbMgF3 is irradiated at room temperature with 1.5 MeV electrons, F centers and complementary fluorine interstitials are produced. Irradiation to an F‐center concentration of 4 × 1017 cm−3 causes a reduction in the thermal conductivity at all temperatures below 25 K. No change in thermal conductivity is observed when the F centers are converted to R centers by a 673 K anneal. This result shows that the radiation‐induced phonon scattering is caused by the interstitials. This additional scattering can be described by the sum of frequency‐independent and point defect scattering terms. The observed increase in the frequencyindependent term is consistent with the formation of interstitial clusters during irradiation. Both contributions to the scattering grow linearly with F‐center concentration.
Temperature‐dependent multi‐ion interactions in simple metals
Authors: Takanaka K. , Yamamoto R.
Published: July 2007

An expression for the temperature‐dependent multi‐ion interactions in simple metals is obtained using the temperature Green's function. This expression gives a damping factor, exp (−πTr/vF) for the two‐body interaction, where T, vF, and r are the temperature, Fermi velocity, and the distance between two ions, respectively.
Lattice heat capacity and thermal vibrations in alkali metals
Authors: Vaks V. G. , Zarochentsev E. V. , Kravchuk S. P. , Safronov V. P. , Trefilov A. V.
Published: July 2007

The lattice heat capacity cv(T) and mean‐square displacement of the ion, \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {x^2 } $\end{document}(T) in alkali metals are calculated on the basis of the pseudopotential model described earlier. Agreement with experiment is in general quite good for Debye temperatures θ and not bad for the dependences cv(T). Some conclusions on the phonon spectrum of Cs are drawn from the data on heat capacity. The pseudopotentials giving a good description of the phonon spectra of Na, K, Rb are proposed. Values of \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {x^2 } $\end{document}(T) are discussed in detail and compared with experiment. The sensitivity of all the results to the approximations for the dielectric function ε(q) and for the pseudopotential V(q) and for the pseudopotential V(q) is also discussed.
Polarization effects in mössbauer absorption of a FeCl2 · 4 H2O single crystal in an external magnetic field
Authors: Spiering H. , Hösl S. , Vogel H.
Published: July 2007

An unusual Mössbauer spectrum of a single crystal in external magnetic field is discussed concerning the line positions and the intensity of the transitions. A method is described to attach the line positions to the appropriate transitions. The intensity is in contradiction to a simple intensity sum rule as a result of polarization effects. It is shown that the thickness effect provides additional information of the orientation of the electric field gradient.
Symmetry and analytical expressions of matrix components in the tight‐binding method
Authors: Kuznetsov V. N. , Men A. N.
Published: July 2007

The method of summing over lattice sites in the tight‐binding method which systematically deals with the point symmetry of crystals is taken into consideration. General expressions of matrix components via independent parameters by summing over an arbitrary number of neighbours in arbitrary crystals are deduced. Group‐theoretical calculations are done and simple formulas are obtained for cubic crystals. As an example, the structures of NaCl, zincblende, diamond, and CaF2 are considered.
A computational study of the correlation of attenuated total reflection minima with surface polariton modes
Authors: Aers G. C. , Boardman A. D. , Clark P.
Published: July 2007

A computational study is made of the variation of θmin, the coordinate of the minimum in Rp, the p‐polarised reflected intensity of typical ATR systems, with d i.e. the air/vacuum spacer thickness (GAM system) or the metal film thickness (GMA system). θmin is correlated to detailed numerical solutions of the dispersion equations of these coupled systems using both a free‐electron model of sodium and some experimental magneto‐optical nickel data. For many frequencies the minimum in the sodium GAM reflectivity agrees well with actual solutions of the coupled system dispersion curves but cut‐offs occur below which a minimum in Rp cannot be associated with such solutions. For the sodium GMA reflectivity, θB, the pseudo‐Brewster angle, shifts to smaller values as Ω is increased and when θB is in the neighbourhood of θp, the plasmon angle, somewhat indistinct results can occur as d is decreased. Nevertheless there is a fair correlation of the minima in Rp with surface mode existence for all frequency branches of a (θmin, d) curve which progress towards the origin without a break. Finally, some experimental data for nickel is used to produce (θmin, d) curves over a range of d for which a significant departure from free‐surface behaviour is expected. For GAM the qualitative behaviour for nickel is, in fact, similar to that of sodium but the maximum value of θmin falls well below the 90° region as the frequency increases. For GMA the picture is much more complicated with only the curve for the lowest frequency resembling that obtained for the free‐electron sodium metal.
First‐harmonic electromodulation in uniaxial crystals
Authors: Suisky D. , Boyn R. , Enderlein R. , Biering B.
Published: July 2007

A discussion is given of an electromodulation mechanism which occurs in uniaxial crystals for electric fields perpendicular to the optical axis. This mechanism is based on the electro‐optical Pockels effect, which produces a rotation of the polarization directions of the ordinary and extraordinary light waves propagating in the crystal. In the presence of polarization‐dependent zero‐field transmission of the material, this leads to a change of transmission which is linear in the field. It is shown that this mechanism makes a significant contribution to the F ⟂ c electromodulation spectra of transition metal centres in wurtizite‐type CdS obtained recently, and that on this basis it is possible to understand the spectral shapes of the signals found in this geometry. An analogous mechanism should contribute to the electroreflection of uniaxial crystals.
Long‐range order in nearly one‐ and nearly two‐dimensional Heisenberg ferromagnets. An upper bound for the magnetization
Authors: Treplin W. , Jelitto R. J.
Published: July 2007

A modified Heisenberg model of ferromagnetism is considered with variable dimension, which is suitable for a description of substances with nearly linear and nearly planar magnetic coupling. By application of Mermin and Wagner's method an exact upper bound is derived for the spontaneous magnetization as a function of the lattice dimension. This upper bound breaks down in a qualitatively different way for the transition to one and to two dimensions. For the latter case a residual three‐dimensional admixture to the interaction suffices to produce a practically three‐dimensional behaviour.
A new type of F3‐centre in additively coloured CaF2
Authors: Alcalá R. , Orera V. M. , Beamonte J.
Published: July 2007

Two new absorption bands at 450 and 320 nm appear in additively coloured CaF2 crystals by bleaching in the α‐band (375 nm) at temperatures ≈ 160 K. These two bands have dichroic behaviour, from which is found, that they are due to two different optical transitions in the same centre, which have their electric dipoles perpendicular, and parallel to the 〈110〉 directions, for the 450 nm band; and along the 〈100〉 directions, for the 320 nm band. Thermal reorientation measurements show that the dichroism induced by light with E ∥ [110], disappears at 210 K, while the dichroism induced by light with E ∥ [100] does not disappear until 270 K. This 270 K disorientation is accompanied by a transformation to the linear F3‐centre, which appears with some dichroism. A model for the centre responsible for these two bands, consisting of three F‐centres forming a right‐angled triangle in a (100) plane, gives good agreement with the experimental results. This is a new type of F3‐centre in the fluorite structure, in contrast with the linear F3‐centre found by bleaching in the α‐band at RT.
Static and dynamic critical behaviour of uniaxial ferroelectrics and the phase transition in TGS
Authors: Nattermann Th.
Published: July 2007

Using the Larkin‐Khmelnitskii theory for static critical behaviour it is shown how recent renormalization group calculations on static and dynamic phenomena in uniaxial ferroelectrics can be generalized to the whole critical region. The cross‐over between classical and asymptotic behaviour is described in terms of two additional non‐universal parameters b and t0 which are defined by microscopic quantities. The critical temperature dependence of the elastic and electrostrictive constants, the dynamic susceptibility, and the ultrasonic attenuation are treated as a ramification of the Larkin‐Khmelnitskii theory. The correlation parameter δ and the kinetic coefficient T exhibit no anomalous behaviour close to Tc. The ultrasonic attenuation above Tc is shown to be dominated by energy fluctuation. Contradictions in previous interpretations of the data of ferroelectric TGS are removed. At zero temperature Te0 = 4 × 10−7 s is found for the energy relaxation time and ξ0 = 3.7 and ξ∥o = 3.9 Å for the transverse and longitudinal correlation length, respectively. From b = 0.036 and t0 = 0.04 the size of the critical region of TGS is estimated to t = (T – Tc)/Tc ≈︁ 1.6 × 10−2 to 1.8 × 10−3 depending on the quantity which is considered, but the uncertainties in determing t0 are large.
On the microscopic mechanism of the ferroelectric phase transition of TSCC: Mn2+
Authors: Franck S. , Kühnel A. , Wendt S.
Published: July 2007

In ferroelectric TSCC doped by Mn2+ ions Windsch et al. observed a large rotation of some EPR lines, the limiting rotation angle being 30°. Using Newman's superposition model, the EPR fine structure tensor Dij is calculated via the field gradient tensor Vij in a point charge model. It turns out that rather small (some 10−9 cm) shifts together with a small change in the effective charge of the oxygen ions at the sites of the distorted octahedron around the manganese ions are sufficient to explain the large rotation of the EPR lines in a good agreement with the experimental results.
Temperature‐independent intersystem crossing in anthracene crystals
Authors: Michel‐Beyerle M. E. , Haberkorn R.
Published: July 2007

The temperature dependence of the delayed fluorescence of anthracene crystals observed under the condition of inhomogeneous excitation can be explained by the influence of reabsorption on the exciton distribution. It does not reflect a thermally activated intersystem crossing rate.
Release of stored energy of X‐irradiated KCl and CsBr
Authors: Schrey P. , Balzer R. , Peisl H.
Published: July 2007

The energy release of KCl and CsBr single crystals is measured after X‐irradiation at 10 K. Release maxima are found at 22, 28, and 36 K (KCl), and at 24 K (CsBr). The energy release is correlated with the annealing of irradiation‐induced Frenkel pairs by comparison with the results of optical absorption measurements. Below 50 K the release of stored energy is mainly due to the recombination of α + I Frenkel pairs. A value of 5 eV per recombination is found as an upper limit for the α + I Frenkel pair in KCl.
Dynamical screening of excitons by free carriers
Authors: Haug H. , Thoai D. B. Tran
Published: July 2007

The Bethe‐Salpeter equation for the electron‐hole polarization function is treated in the screened ladder approximation. The dynamically screened Coulomb potential is taken into account in the plasmon pole approximation. An iterative procedure leads to an effective Bethe‐Salpeter equation in which the screening is described by a virtual exchange of plasmons between the electron and hole. The resulting exciton binding energy is considerably larger than that obtained by the statically screened Coulomb potential approximation. Results of the numerical calculation of the binding energy as a function of the free carrier density are given. The calculated binding energy vanishes only asymptotically in the limit of high free‐carrier densities.
Electronic elementary excitation in the far UV‐spectral region core level exciton and core hole polarization
Authors: Enderlein R. , Seifert B. , Bechstedt F.
Published: July 2007

An electronic elementary excitation of a crystal is considered which might be of interest for the interpretation of the structures of far UV reflectance and photoemission spectra associated with transitions from core levels into conduction bands. The primarily generated strongly localized core hole polarizes the electronic subsystem. The polarization is accompanied by the generation of a secondary less strongly localized hole to which the conduction band electron is bound. The excitation energy is lowered by the polarization energy of the core hole as compared to the excitation energy of an exciton. The static polarization energy and polarization charge density are calculated for a core hole of the Ga atom by means of the Thomas‐Fermi approximation. Polarization energies of several eV are obtained which decrease nearly inverse proportionally with hole radius. The influence of the polarization energy on UV reflectance and photoemission spectra is discussed. It is concluded that the core hole polarization energy contributes to the difference between band distances obtained by reflectance and photoemission measurements, supposed the dispersion of the polarization energy is important.
Nonlocal theory of elastic interaction between point defects
Authors: Gairola B. K. D.
Published: July 2007

A weakly nonlocal version of the nonlocal continuum theory is used to calculate the interaction energy between a pair of identical defects in a face‐centred cubic lattice when the defect spacing is large. The lattice is assumed to be the same that was considered by Hardy and Bullough, namely, an idealized harmonic f.c.c. lattice held together by nearest‐neighbour forces which are such that the crystal is elastically isotropic in the long‐wavelength limit. An analytic expression is obtained for the interaction energy which is orientation dependent and varies as the inverse fifth power of defect separation. This expression for the interaction energy is in complete agreement with the result of Hardy and Bullough which they obtained by using Kanzaki's method of lattice statics. In this way it is demonstrated that the nonlocal continuum theory can reproduce the results of the lattice theory.
The optical spectra of rare‐earth cobalticyanides
Authors: Bonnet M. C. , Antic‐Fidancev E. , Porcher P. , Lemaitre‐Blaise M. , Caro P.
Published: July 2007

Rare earth cobalticyanides Ln[Co(CN)6] · n H2O have two cristalline forms, an hexagonal one from La to Nd with n = 5 and an orthorhombic one from Ce to Lu with n = 4. The optical absorption spectra of the Nd3+ (4f3) and the fluorescence spectra of the Eu3+ (4f6) in the pure and doped salts are recorded at low temperatures (4.2 or 77 K) in the polycristalline form. The orthorhombic phase exhibits spectra corresponding to a lanthanide in a C2v site under a weak crystal field (Bq2, Bq4 < 500 cm−1). The spectra for Nd3+ and Eu3+ in the hexagonal lanthanum compounds are compatible with a C3h site symmetry. There is evidence of selection rules for the dipole electric transitions in 4f3, and the 4f6 configuration has levels which are Kramers doublets.
Crystal growth and EPR study of Cr+‐doped MgTe single crystals
Authors: Boulanger D. , Martin M.‐S.
Published: July 2007

EPR measurements are made for X and Ka bands on Cr+ in MgTe single crystals. Growth of these single crystals is described. The spin‐Hamiltonian parameters for Cr+ are determined at liquid nitrogen temperature as follows: g = 2.0038 ± 0.0005; |A| = (13.4 ± 0.4) × 10−4 cm−1; |a| = (9.2 ± 0.5) × 10−4 cm−1; |D| = (28.1 ± 0.3) × 10−4 cm−1 and |a – F| = (11 ± 0.4) X × 10−4 cm−1. The dependence of D on temperature is studied. For Cr+, Mn2+, and Fe3+: phenomenological relationships for the g‐factor are established for various II–VI compounds with blende or wurtzite structure.
Dynamic nuclear polarization in TCNQ salts
Authors: Alizon J. , Berthet G. , Blanc J. P. , Gallice J. , Robert H. , Fabre J. M. , Giral L.
Published: July 2007

Temperature dependence of the dynamic nuclear polarization enhancement AL of the protons in tetrathiafulvalinium‐tetracyanoquinodimethan salt (TTF‐TCNQ) and potassium‐TCNQ salt is reported. In TTF‐TCNQ, AL is temperature‐independent in the range 150 to 300 K; in K‐TCNQ, AL is strongly temperature‐dependent. These results, and the knowledge of the frequency dependence of the relaxation time T1 of the protons allow to determine the cut‐off frequency vC of the ld diffusion mechanism with respect to the nuclear and electronic frequencies vI and vS. vC is larger than 10 GHz in TTF‐TCNQ and vC is of the order of vI in K‐TCNQ. Moreover, because of the large enhancement AL observed in K‐TCNQ, the T1 anisotropy is determined on single crystals.
Calculation of the g‐shift and superhyperfine structure tensor of Mn2+ ion in AIIBVI tetrahedral semiconductors
Authors: Hamera M.
Published: July 2007

Multiple scattering method is used to obtain wave functions and energy levels of the 17‐atom cluster MnB4A12. The influence of all other atoms of the crystal is assumed to produce an additional constant potential only. At the Mn site, the correction to the potential is introduced to give most probable values of the cubic field splitting parameter Δ. For these values of Δ, the g‐shift and superhyperfine structure tensor are calculated and compared with experimental data.
Localized magnons around an impurity in a spin chain with next‐nearest neighbour interaction
Authors: Georgiev G. , Gochev I.
Published: July 2007

Localized magnons around a ferromagnetic impurity spin in a linear Heisenberg ferromagnet with next‐nearest neighbour interaction are discussed. It is shown that the number and existence regions of these localized states are essentially dependent on the next‐nearest neighbour coupling intensity.
Electro‐birefringence from paraelectric defects in cubic crystals
Authors: Lüty F. , Diaz‐Gongora A.
Published: July 2007

The electro‐birefringence effects are discussed and produced in an (electro‐optically inactive) cubic crystal by the presence of electric dipole defects which are alignable by electric fields. Based on the electronic polarizability tensor of the defects and their paraelectric alignment functions, expressions for the field‐induced birefringence (Kerr or Pockels effect) can be derived for all interesting symmetries of dipoles, electric fields, and light‐geometry, and their connections — by Kramers‐Kronig relations — to electro‐dichroism can be given. Various methods for electrobirefringence measurements with dc, ac, dc + ac, and birefringence‐bias techniques producing linear, quadratic, and quartic E‐dependence of the electro‐optical effect are discussed. The derived behavior is illustrated by experimental examples selected from Kerr effect measurements on CN− defects in alkali halides.
Thermal properties and Grüneisen parameters of gold using Toya's first principle approach
Authors: Prasad B. , Srivastava R. S.
Published: July 2007

The phonon frequencies and Grüneisen parameters are calculated for different values of the phonon wave vector in the three principal symmetry directions for gold using Toya's first principle approach. The average value of the Grüneisen parameter γth is also calculated for different temperatures and the results are compared in the light of available experimental and theoretical results.
Large unit cell calculations of the band structure of ionic crystals using the Mulliken‐Ruedenberg approximation
Authors: Evarestov R. A. , Ermoshkin A. N.
Published: July 2007

The Mulliken‐Ruedenberg approximation which has been developed for the molecular calculations is generalized for solids in the framework of the large unit cell approach. In such a model the periodical boundary conditions are taken into account. The calculations of the main features of the band structure for the series of alkali halides are performed using the LUC model in the Mulliken‐Ruedenberg approximation. A comparison is given between the results obtained and both the experimental data and results of band structure calculations using the LCAO basis set. The conclusion is drawn that the application of the Mulliken‐Ruedenberg approximation for the Hamiltonian matrix element estimation allows to obtain quite satisfactory results for ionic crystals without any preliminary calibration of atomic parameters.
Observation of precursory phonons of an F.C.C.‐H.C.P. transition
Authors: Sacchetti F. , Bosi P. , Dupré F. , Frollani G. , Menzinger F. , Spinelli M. C.
Published: July 2007

A study of the phonon dispersion relation in a Co0.65Ni0.35 alloy shows some novel features, which may be connected to the transition from the f.c.c. to the h.c.p. phase, which is expected at lower temperatures. In fact, in the study of the [ζζζ] T branch in the f.c.c. phase the presence of extra phonon peaks is observed at intermediate q‐values, though the static f.c.c. phase cannot allow for the presence of a new branch. It is believed that a softened optical branch of the h.c.p. phase which interacts with the acoustic branch of the f.c.c. phase may cause the observed phonon structure.
A computer simulation of point defects and planar defects in nickel
Authors: Miller K. M. , Bristowe P. D.
Published: July 2007

The properties of point defects and planar defects in nickel are investigated using computer simulation techniques. An empirical pair potential consisting of nine cubic splines is derived for nickel. This potential reproduces the experimental vacancy formation and migration energies, the intrinsic stacking fault energy, and the elastic constants. The displacement fields, formation energies, and formation volumes are calculated for the vacancy, the saddle‐point configuration associated with vacancy migration, and six interstitial configurations. In addition, results are presented for the intrinsic and extrinsic stacking faults and the twin boundary. The vacancy results are in agreement with previous calculations but the interstitial formation energies are significantly larger than those calculated previously. The crowdion is found to be the most stable interstitial configuration with a formation energy of 8.77 eV and formation volume of 0.42 atomic volumes. The energies of the twin, intrinsic, and extrinsic stacking faults are found to be 97, 158, and 199 mJm−2, respectively.
An interatomic potential for copper from the phonon spectra
Authors: van Heugten W. F. W. M.
Published: July 2007

A potential function for copper is presented that is derived from phonon dispersion curves. Also some defect calculations are made. The vacancy formation and migration energy are in good agreement with the experiments. The 〈100〉 split interstitial configuration is found to be the most stable one.
On the no‐phonon structure in mixed crystals with indirect band structure
Authors: Strehlow R.
Published: July 2007

The no‐phonon structure occurring in the optical interband absorption spectra of mixed crystals with indirect band structure is investigated. Starting from a simple model for describing Wannier excitons in mixed crystals this structure is shown to be due to disorder‐induced indirect exciton formation. The calculations yield lineshape and composition dependence which are in good agreement with experiment.
To the theory of the tunnel effect in a semiconductor in constant electric and magnetic fields
Authors: Oleinik V. P. , Sinyak V. A.
Published: July 2007

Interband tunnelling in a semiconductor in constant electric E and magnetic H fields is studied in the framework of the two‐band model. An accurate formula is deduced for the tunnel current on the basis of the quantum transition theory in systems with unstable ground state, which is correct for arbitrary values of magnitude and direction of E and H. Giving an example of the two‐band model, those striking features of the dynamical description of the physical system which are due to the instability of its growth and state concerning the creation of electron–hole pairs are discussed.
Effect of atomic size on the effective polarizability of physisorbed atoms
Authors: Meixner W. C. , Antoniewicz P. R.
Published: July 2007

Calculations of the effective polarizability of atoms adsorbed on metal surfaces have usually represented the atom by a polarizable point dipole. A simple model which includes an atomic size parameter but neglects exchange and correlation effects between the atom and the metal is used to investigate the validity of the polarizable point atom approach. A gradient expansion technique within the density functional formalism is used in this calculation. Results indicate that atomic size can greatly reduce the enhancement of the effective polarizability found from the polarizable point atom approximation.
Temperature behaviour of the dielectric constant of tetragonal BaTiO3
Authors: Lima C. A. S. , Scalabrin A. , Miranda L. C. M. , Vargas H. , Porto S. P. S.
Published: July 2007

A model is developed which assumes the temperature dependence of the dielectric constant of BaTiO3, along the ferroelectric axis in the tetragonal phase to be essentially determined by a relaxation mechanism involving dipole reorientation controled by tunneling across a double well potential barrier. Model parameters are determined from fits to existing data and it is shown that the fitted curves adjust well to the data within experimental error. The results support the hypothesis that the ferro‐to‐paraelectric transition in BaTiO3 is not governed by phonon assisted processes but must be ascribed to an intrinsic type of disorder.
Configuration interaction and intensity of crystal field spectra of the tetrahedral Ni2+ Ion
Authors: Gomathy V. , Basu C. , Ghosh U. S.
Published: July 2007

The tetrahedral crystal field acting on the F‐state ion Ni2+ in ZnO mixes the odd parity states of excited 3d74p and 3d74f configurations with the even parity states of 3d8 configuration and this mixing has some contribution to the intensity of electric dipole transition between crystal field levels. The contribution of such configuration interaction to the oscillator strength for transitions from the lowest crystal field level T1 to the higher ones is estimated within the framework of crystal field theory using the powerful Racah method and Richardson function for the radial integrals. The results indicate that such contribution to the intensity is quite significant; it accounts for an appreciable amount of intensity observed particularly for one of the transitions T1 → T2. Moreover, this contribution to all the three transitions considered, is found to be much larger than the intensity for the corresponding magnetic dipole transitions.
Low temperature polaronic hopping conduction
Authors: Thomas P. , Wuertz D.
Published: July 2007

The de‐hopping conductivity and thermopower is calculated for temperatures below half the Debye temperature θD. A percolation approach with multiphonon hoprates is used. Two sets of energetically overlapping states near the Fermi level are consdered. The coupling to the phonons is assumed to be larger for the lower lying set of states. The resulting thermopower is negative and slowly varying with temperature, but bends down to higher neegative values at low (≈︁ 0.1θD) temperatures. The conductivity essentially follows a In σ ∼ T−1/4 law. The thermopower is much more stable with respect to changes in the density of states as compared to a model without multiphonon effects.
Improvements of the equation of motion technique for disordered systems
Authors: von Heimendahl L.
Published: July 2007

The equation of motion technique for the calculation of dynamical properties of large disordered systems is reconsidered and two improvements are proposed. Several procedures for the numerical integration of systems of second‐order differential equations are discussed and one particular efficient integration formula is selected. A modified cosine transformation for finite integration intervals is presented, which gives a positive, bell‐shaped function as a representation of the δ‐function instead of sin (ωT)/ωT. This allows an easier discussion of highly structured spectra. The results are demonstrated by a simple example.
Resonant Raman scattering in InP
Authors: Sinyukov M. , Trommer R. , Cardona M.
Published: July 2007

Measurements are made of resonant first‐ and second‐order Raman scattering by phonons in InP for photon energies from 1.75 to 3.05 eV. The behaviour of all observed resonances can be well explained by effects due to the E0 + Δ0 and the E1 band gaps. A comparison of the first‐order deformation potential at E1 with the one of GaAs is made and several electron‐two‐phonon deformation potentials are evaluated.
Partial densities of states and electron–phonon interaction in transition metals
Authors: John W. , Hamann D. , Urwank P.
Published: July 2007

For the 3d‐metals Ti, V, Fe, and Cu the electronic part η of the electron–phonon coupling constant is calculated in the rigid muffin‐tin approximation. Moreover for V and Cu the pressure dependence of this quantity is investigated. The results show that the correct evaluation of the angular momentum components of the electron density of states at the Fermi energy is important. Especially the density of p‐ and f‐states exhibit a strong energy dependence within the d‐bands and lead to a variation in η as one crosses the transition metal series. The non‐spherical corrections to the Gaspari‐Gyorffy formula are small.
Parametric excitation in Kane's semiconductors
Authors: Anh Vo Hong , Dat Nguyen Nhu
Published: July 2007

The parametric excitation of electromagnetic vibrations in narrow‐gap semiconductors with nonparabolic energy dispersion defined by Kane's model is investigated using the quantum formalism proposed previously. The numerical estimates performed for the typical sample of n‐InSb crystals in different cases show that the deviation from parabolicity lowers the instability threshold field values by about one order of magnitude.
Hyper‐rayleigh scattering in disordered sodium nitrate
Authors: Vogt H. , Neumann G.
Published: July 2007

Hyper‐Rayleigh scattering is observed in single crystals of NaNO3 when the order‐disorder phase transition point and the nearby melting temperature are approached. Besides the central line at twice the wavenumber of the exciting radiation a wing and a phonon band are found. All spectral features are due to the rotational mobility of the NO3− ions. The temperature dependence of the hyper‐Rayleigh intensity is interpreted in terms of an Ising model.
The phonon spectra of γ‐ and β‐CuBr
Authors: Haxton M. B.
Published: July 2007

A previously paper on CuI, is repeated for CuBr. A rigid ion, second neighbor ionic model proposed by Banerjee and Varshni is used to fit the inelastic neutron scattering data of Prevot et al. for the γ phase of CuBr. The model consists of a long range Coulomb contribution calculated directly and short range parameters calculated for the best fit to the data. The dispersion curves of β‐CuBr are calculated using the same model. The long range Coulomb contributions are again calculated directly. The short range parameters for the ideal wurtzite structure are calculated by Euler transformations of the zinc blende short range interaction matrices. Because of the nonideal structure of β‐CuBr and because of the expansion of the lattice with the increase in temperature, some small corrections are necessary. With these long range and short range contributions, the dispersion curves for β‐CuBr are calculated.