bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of the Quantum Theory of Diffusion to H and D in Niobium
Authors: Sussmann J. A. , Weissman Y.
Published: July 2007

A detailed description is presented of the quantum mechanical diffusion mechanism for the case of strong coupling between lattice phonons and the migrating particle. In this mechanism the particle is excited from a localized ground state to time‐dependent Wannier states in several accessible bands, from where it falls down into neighbouring or non‐neighbouring sites. This mechanism leads to a classical‐like expression for the jumping probability, W = ν0 e‐E/kT, where E is the excitation energy to the band and ν0 is related to the width of the band. This theory is shown to account for the experimental data on hydrogen diffusion in niobium, which exhibits two sets of activation energies and pre‐exponential factors, and a highly non‐classical isotope effect. Available diffusion data in V and Ta are also found compatible with the same model.
Recombination in Semiconductors by Excitation of Plasmons
Authors: Tussing P. , Rosenthal W. , Haug A.
Published: July 2007

In a highly doped semiconductor excitation of plasmons is shown to be an effective mechanism for electron‐hole recombination. If the carrier concentration is sufficiently high the corresponding recombination rate overwhelms that for the radiative process by orders of magnitude.
Theory of Disordered Systems III. Single‐Site Theory of Transport in Systems with Randomly Distributed Scatterers
Authors: Fischbeck H. J.
Published: July 2007

In case of systems with randomly distributed scatterers the self‐consistent single‐site approximation of Schwartz and Ehrenreich [1] leads to a simple expression for the kernel of the Bethe‐Salpeter equation in terms of the local self‐energies. Thus, a consistent single‐site theory of linear response of such systems is given. It is applied to the disordered Kronig‐Penney model which permits exact solutions of the Schwartz‐Ehrenreich and the corresponding Bethe‐Salpeter equation. Numerical results of the density of states and the mobility of the electrons are given. The existence of a mobility edge is established.
Fluctuation‐Dissipation Theory of Polariton Raman Scattering in the Vicinity of Phonon Resonance
Authors: Strizhevskii V. L. , Obukhovskii V. V.
Published: July 2007

Quantum theory of stimulated Raman scattering (SRS) by polaritons is developed which is valid both far from phonon resonance and in the vicinity of it. The theory is based on treating polariton SRS as an interaction process of pumping radiation with fluctuation (Langevin's) sources of an active medium. The principal peculiarity of a resonance region is the necessity to take into account non‐equilibrium (dependent on pumping field) fluctuations of polarizations. Therefore, the correspondent generalization of the fluctuation‐dissipation theorem was made, the necessary correlators of Langevin forces and fluctuation sources were found up to the second order with respect to the pumping field. On the base of fluctuation‐dissipation theory of scattering the intensity of polariton SRS was determined as a function of macroparameters of the system. The temperature dependence of gain factor and scattering intensity is discussed.
Resonant Absorption of γ‐Rays by a Deformed Crystal
Authors: Srivastava B. P. , Sharma H. N. K. , Bhattacharya D. L.
Published: July 2007

The exponent of the Debye‐Waller factor and the energy shift for γ‐rays resonantly absorbed by a deformed crystal (deformation introduced along a line) have been calculated using the technique of double time Green's function. It has been found that due to deformation there exists an observable broadening of the Mössbauer line and an additional energy shift also appears which however may not be well observed. A quadrupole splitting of the line has been predicted on the basis of deformation of the lattice.
On the Interaction of Electrons and Holes in a Molecular Crystal
Authors: Jurgis A. , Silinsh E. A.
Published: July 2007

A self‐consistent polarization field (SCPF) method is used in the framework of a classical electrostatic model to compute single charge carrier polarization energies and electron‐hole interaction energies in naphthalene and anthracene crystals. The crystal polarization energy Pe‐h of an electron‐hole pair localized on specific sites and the energy of the respective charge transfer (CT) states ECT have been calculated for nearest‐neighbour states in naphthalene and five specific sites in anthracene. The estimated polarization energy of a single charge carrier is 1.27 eV for naphthalene, 0.07 eV below the mean reported experimental value, and 1.52 eV for anthracene, 0.06 eV below the mean experimental value. The energy of the nearest‐neighbour CT state is estimated to be 4.29 eV for naphthalene, 0.09 eV below the reported experimental value, and 3.19 eV for anthracene, 0.26 eV below the reported experimental value. The energy of the first conductivity level for electrons E'C is estimated to be 3.96 eV for anthracene, 0.06 eV above the reported threshold for intrinsic photoconductivity. The validity and perspectives of the SCPF method are discussed.
Transport Relaxation Time for Phonon‐Impurity Scattering
Authors: Altukhov V. I. , Zavt G. S.
Published: July 2007

Starting from the Kubo formula the thermal conductivity is expressed by the two‐phonon Green's function. The latter is calculated for a cubic lattice containing a small fraction of impurities with taking into account the change of force constants. It is shown that the procedure of the configurational average introduces a correlated motion of two phonons which is described by the vertex correction. This correction appears to be essential if the impurity generates at least two resonant modes with close frequencies but with opposite parity. In this case the scattering cross‐section is decreased by the vertex correction. The correlation in the phonon motion is expected to be essential in KI: Ag.
Use of Green's Function to s–d Interaction in Ferromagnetism
Authors: Nicoarǎ I.
Published: July 2007

The influence of the s‐d interaction on the ferromagnet with S = 1/2 with nearest‐neighbour exchange interaction has been considered using the double‐time Green's functions. The equations of motion have been obtained for two kinds of Green's functions G(−) and G(+), which are derived using commutator and anticommutator, respectively. Expressions are given for the energy shift and damping caused by the s–d interaction, using the decoupling approximation proposed by Katsura and Horiguchi.
The Influence of Quantum Effects on the Low‐Temperature Creep of Zinc Crystals
Authors: Natsik V. D. , Osetskii A. I. , Soldatov V. P. , Startsev V. I.
Published: July 2007

The creep of single crystals of Zn deformed by simple shear along the basis plane (0001) has been investigated in the temperature range 1.5 to 80 °K. It is shown that the influence of temperature on the creep in the investigated temperature range is weaker than that predicted by the classical theory of thermally activated creep. A peculiar temperature run of parameters describing the creep process has been discovered and proved anomalous as compared with the classical run. A mechanism for the influence of zero‐point vibrations of the dislocation line on its motion through an aggregate of local obstacles is suggested and analyzed. The results obtained in this way show a satisfactory agreement with the experimental data.
On the Theory of Nonradiative Transitions in Polar Media I. Processes without “Mixing” of Quantum and Classical Degrees of Freedom
Authors: Dogonadze R. R. , Kuznetsov A. M. , Vorotyntsev M. A.
Published: July 2007

The probability of radiationless transitions between two multi‐dimensional parabolic terms is calculated. The dependence of transition probability on temperature and heat of the processes is investigated. It is shown that all the degrees of freedom of the system may be divided into “classical” and “quantum” degrees of freedom in dependence of their behaviour during process. The activation dependence of the transition probability on the temperature is associated with the motion along the classical degrees of freedom only. The change of states of quantum degrees of freedom leads to the appearance of an exponential non‐temperature factor in the transition probability. The transition is shown to be of activationless or barrierless character at large values (modulo) of the heat of process for the processes without “mixing” of classical and quantum degrees of freedom.
On the Elasticity of Ionic Compounds under Hydrostatic Pressure
Authors: Vuxcevich M. R.
Published: July 2007

The pressure derivatives of the elastic constants of CsCl, NaCl, and ZnS lattices were calculated as functions of the difference in sizes of ions and the range of the repulsive forces. General conditions for the negative pressure dependence of the shear constants are examined. The agreement with the experimental results is reasonable, which indicates that the ion‐size difference will have to be taken into account in all future considerations of the elastic properties of compounds.
An Analytical Discrete Lattice Calculation of Vacancy Decay in Dilute Alloys
Authors: Macewen S. R. , Koiwa M.
Published: July 2007

A discrete lattice calculation of vacancy annealing in dilute alloys has been performed, and the results are compared with the Damask and Dienes chemical reaction rate formulation. It is shown that the reaction rate scheme overestimates the initial transient vacancy decay rate. However, at long times, after the attainment of chemical equilibrium between solute‐vacancy complexes and f pee vacancies, the discrete lattice and rate theory formulations become identical. The rate of solute‐vacancy complex formation has been calculated using the theory of stochastic processes to determine the expected time required for a vacancy to migrate from a given position in the lattice to a first nearest neighbour position of a solute atom. Good agreement is found with the transient rate constants found from the discrete lattice annealing calculations.
Non‐Linear Theory of Charged Dislocations and Their Surrounding Charge Clouds
Authors: Whitworth R. W.
Published: July 2007

The thermal equilibrium charge on a dislocation is calculated on the assumption that the dislocation is charged by a number of identical point charges at sites on its core. The surrounding charge cloud is treated as a continuum interacting with the dislocation by electrostatic forces only. The non‐linear differential equation of this cloud is solved numerically for cases appropriate to ionic crystals and semiconductors in both the intrinsic and extrinsic regions. The effect on the free energy of electrostatic interactions between nearest‐neighbour charges on the core is taken into account. The force to pull a dislocation from its charge cloud is calculated.
EPR Optical Detection of F Centre Pairs in Alkali Halides I. Pumping Cycle Kinetics and Characteristics of the Resonances
Authors: Ruedin Y. , Schnegg P.‐A. , Jaccard C. , Aegerter M. A.
Published: July 2007

The electron paramagnetic resonance of F centres in the ground and excited states has been optically detected in the following alkali halide crystals: NaCl, KF, KCl, KBr, KI, RbBr, and RbI. A decrease of the radiative quantum efficiency of the F centre luminescence is observed when microwave transitions are induced between the spin levels. The mechanism responsible for this effect is an electronic tunnelling through the crystal field potential; the electron in the relaxed excited state of an F centre (F̃*) is transfered non radiatively to another nearby F centre in its ground state (F0), and leads to the momentary formation of an α and an F′ centre. Such a process is a function of the total spin of the F̃*‐F0 pair. The role played by the paired centres is confirmed by measurements at different F centre concentration. Moreover at high optical excitation pumping rate the population of the intermediate complexes (F′‐α) is large enough to allow an estimation of the rate of the reverse process F′ + α → F0 + F0.
Metal–Insulator Transition and Lifshits Instability in the Hubbard Model
Authors: Weller W.
Published: July 2007

The metal–insulator transition (Mott transition) in the Hubbard model for a half‐filled narrow band is considered by means of the functional integral method. The functional integral is calculated in the static approximation. Higher‐order fluctuations in the exponent of the functional integral are neglected, and a self‐consistent approximation scheme analogous to the coherent potential approximation is used. An instability in the free energy leading to a first order transition is obtained. This instability corresponds to the instability studied by Lifshits in connection with the passing of an energy branch through the Fermi energy.
Raman Effect in Selenium–Tellurium Mixed Crystals
Authors: Geick R. , Steigmeier E. F. , Auderset H.
Published: July 2007

The Raman effect in selenium–tellurium mixed crystals has been studied. Four first order Raman lines have been observed and associated with long wavelength lattice modes of the alloy system. The experimental results together with existing infrared data indicate that the long wavelength modes from three of the six optical branches in these trigonal crystals show “multimode behaviour” while the other three branches show “one mode behaviour”. All the experimental data are compared to and discussed in connection with an existing random‐element‐isodisplacement model calculation of the eigenfrequencies of the lattice modes as a function of composition. The calculated frequencies and the predictions of the model for the behaviour of the long wavelength lattice modes are in, at least, qualitative agreement with the experimental data.
NMR Lineshapes of Magnetic Diluted Systems in Solids
Authors: Freude D. , Schmiedel H.
Published: July 2007

There are two well‐known methods for calculating dipolar broadened NMR lineshapes of magnetic diluted spin systems in solids, viz. the “method of moments” and the “statistical theory”. It is shown, that in the case of a two‐dimensional statistical spin distribution the above‐mentioned methods give different results, which can be explained by the arbitrary assumption of a Lorentzian lineshape within the method of moments. An extension of statistical theory by introducing a minimum distance between the spins reveals lineshapes, which converge more rapidly and thus better correspond to real situations. Finally lineshapes of magnetic diluted spin pairs are discussed.
Dependence of Positron Lifetimes on the Temperature and Phase of Some Organic Media
Authors: Thosar B. V. , Laciu R. G. , Kulkarni V. G. , Chandra Girish
Published: July 2007

Measurements of the value τ2 and the intensity I2 of the long component in the life‐time spectra of positrons annihilating in some simple unassociated, polar, and associated organic liquids are reported as a function of temperature. A semi‐empirical free volume model for the formation and quenching of ortho‐positronium atoms is developed for simple molecular materials to explain the correlation between viscosity η, density ϱ, and τ2 observed in unassociated and polar liquids. Such a correlation is not observed for associated liquids in which the presence of hydrogen bonds between molecules apparently influences the annihilation process. The data of positron lifetimes during the solid‐liquid phase transition in some organic systems are discussed in terms of the free volume model.
Dislocation Description of Twin Dynamic Behaviour
Authors: Boiko V. S.
Published: July 2007

Relationships are obtained which allow to describe the characteristics of the twin as functions of time and medium constants for dynamic external loads within the framework of the dislocation theory. The case of large velocities is considered when the drag force arising in the dislocation motion is of the viscous friction character and, yet, the relativistic effects may be neglected. Two problems in the dynamic behaviour of the twin are solved, namely the escape of the elastic twin from an unloaded crystal due to the surface tension force and passage across the crystal by the twin in a uniform stress field. The obtained results are compared with experimental data for iron, calcite, and cadmium. It is shown that in the considered case of large velocities the twin motion velocity should be weakly (at least, not exponentially) dependent on temperature. This result is in agreement with available experimental data.
Charge Compensation of Fe3+ Ions in V2 O5 (ESR Measurements)
Authors: Jansen K. , Sperlich G.
Published: July 2007

The ESR spectrum of iron doped V2O5 single crystals shows two different line groups. Analysis of the angular dependence and intensity measurements prove that, contrary to Cr3+ ions, Fe3+ ions occupy lattice sites and tetrahedral sites as well. This is in agreement with published Mossbauer results. Moreover, the influence of the Fe3+ concentration upon the signal intensity of the vanadium 3d1 electron was investigated.
Properties of Pseudoatoms Derived from the Shaw Model Potential
Authors: Paasch G. , Woittennek H.
Published: July 2007

A smooth pseudoatom potential is derived from the Shaw model potential for simple metals. The screening by a homogeneous conduction electron density yields nearly the same results as the self‐consistent screening. The minimum perturbation in terms of phase shifts is investigated. The pseudoatom is compared with the Mattheiss potential and with the renormalized atom via the logarithmic derivatives.
Determination of Optical Constants: a‐Si
Authors: Jungk G.
Published: July 2007

Optical studies of a‐Si samples are reported. The dielectric function was determined ellipsometrically from 2.0 to 4.0 eV. Different concentrations of slit‐like voids may account for the variation of the optical constants in the investigated films. The depolarization factor L of the voids was estimated to 0.75 5 ≦ L ≦ 0.9. A structure of the optical constants near 3.5 eV indicates an exciton‐like transition. The extrapolation of the hv √ E2eff curves points to a pseudogap of the ideal a‐Si comparable with the indirect gap of the crystalline phase. These pseudogaps increase with increasing void‐concentration.
Temperature Dependence of Half‐Widths and Integral Absorptions of the Impurity Polyatomic Ion Vibration Bands in Alkali‐Halide Crystals
Authors: Tsyashcrenko Yu. P. , Demyanenko V. P. , Pinkevich I. P.
Published: July 2007

A quantitative study has been made of the temperature dependence of the half‐widths Avlp and integral optical densities A of the vibration bands v2 and v3 of the impurity anion NCO− in CsI crystals, of the vibrations v1 and v2 of NCS− in CsI, of components of the v3 triplet of the complex SO. Ba2+ in KCI (in the temperature range 100 to 500 °K); and of the vibrations v2 and vg of the anion NO in CsI (in the temperature range 100 to 700 °K). It is shown that the integral absorption remains practically constant for all the bands over the whole temperature range while the half‐widths increase sharply with increasing T. In the representation Δvl/2(T) = Δv1/2(0) + α Tn at rather high T (T > 300 °K) the effective ñ for different bands are calculated with the purpose of determining a preferable mechanism of anharmonic interaction of corresponding intramolecular vibrations (local vibrations in the crystal‐matrix) with lattice phonons. The results obtained agree with existing theoretical views of the role of dissociation and modulation mechanisms of the local vibration band temperature broadening.
Paramagnetic Resonance of a Cu(II) Ion in a Distorted Tetrahedral Environment
Authors: Voronkova V. K. , Zaripov M. M. , Kogan V. A. , Yablokov Yu. V.
Published: July 2007

Electron paramatic resonance has been studied in polycrystalline undiluted samples of several salicylaldimine derivatives of known crystal structure. In two studied compounds (bis (N‐isopropylsalicylaldiminato) Cu and bis (N‐t‐butylsalicylaldiminato) Cu) the structure of the nearest environment is intermediate between tetrahedral and square coplanar. The obtained values and signs of anisotropy of the g‐factors show the essential difference between the properties of the copper ion in these two compounds and that in the case of the planar complex. On the basis of a theoretical analysis it is found that the electronic properties of the copper (II) ion in bis (N‐isopropylsalicylaldiminato) Cu and bis (N‐t‐butylsalicylaldiminato) Cu can be explained by consideration of the nearest environment symmetry as a elongated tetrahedron with d p ground state and taking into account the admixture of upper 4p and 4s states into the 3d wave functions. The values of the covalency and admixtures of the 4p and 4s states are estimated for bis (N‐isopropylsalicylaldiminato) Cu. A suggestion is made about the variation of these values under the transition to the more distorted structure in bis (N‐t‐butylsalicylaldiminato) Cu.
Tenfold d‐Electron Degeneracy in the Hubbard Model and Transition‐Metal Magnetism. II. The Antiferromagnetic‐Paramagnetic and Antiferromagnetic‐Ferromagnetic Transitions
Authors: Siegel E. J.
Published: July 2007

Intrasite d‐d electron interactions are included in the Hubbard model, replacing the simplified s‐electron picture of Penn [5]. The technique is identical with an earlier paper by Siegel and Kemeny [1] on the ferromagnetic‐paramagnetic transition. Choosing k‐space correlation functions to represent the paramagnetic, ferromagnetic, and antiferromagnetic states, the tenfold degenerate eigenvalue problem is solved and free energies are self consistently calculated for various combinations of the three parameters: d‐band filling, direct interaction Coulomb constant divided by one third the d‐bandwidth, and exchange interaction constant divided by one third the d‐bandwidth. The antiferromagnetic‐paramagnetic and antiferromagnetic‐ferromagnetic phase boundary surfaces are plotted a t near zero and finite temperatures with the third dimension being the exchange interaction constant divided by one third the d‐bandwidth (J/4T). The near zero temperature trend shows that the inclusion of exchange, as expected, enhanced the antiferromagnetic region at the expense of the paramagnetic region. This antiferromagnetic region enhancement over the para‐magnetic region is exactly the opposite of the nondegenerate. t‐matrix result of Caron and Kemeny [7]. Finite temperature calculations for the antiferromagnetic‐paramagnetic transition show the same trend with increasing exchange, but shift the phase boundary so as to decrease the antiferromagnetic region relative to that at near zero temperature for the same value of exchange interaction constant, J. The antiferromagnetic‐ferromagnetic phase boundary shift as a function of increasing d‐d exchange interaction constant, J, is much harder to calculate since two ordered magnetic spin states are in competition. Within the scatter of the calculated free energy values, it is seen that the ferromagnetic region is enhanced at the expense of the antiferromagnetic region, but just barely. This enhancement is much less than that of ferromagnetic over paramagnetic. in [1], or of antiferro‐magnetic over paramagnetic, here, where strongly spin ordered magnetic spin states are compared to a spin disordered state. For the slight ferromagnetic region enhancement over the antiferromagnetic region, again the trend is opposite to the t‐matrix result, which enhances the antiferromagnetic region at the expense of the ferromagnetic region relative to Penn's [5] nondegenerate calculation. But again, since Penn's [5] calculation is good only to direct Coulomb interaction constant K = 0.25, while Caron and Kemeny's [7] is good rigorously down to K = 0.0, nothing can be said abont, how the calculation compares with Penn's [5] for K < 0.25.
Nichtlineare optische Suszeptibilitäten von Zinkblende‐ und Wurtzit‐Kristallen
Authors: Hübner K.
Published: July 2007

Auf der Basis der semiempirischen Theorie für die lineare optische Suszeptibilität von Phillips und Van Vechten und eines Ladungstransportmodells von Tang und Flytzanis wird ein Modell ohne freie Parameter für die Suszeptibilität 2. Ordnung aafgestellt, das außer den Valenzelektronen explizit Rumpf‐d‐Elektronen berücksichtigt. Das Modell liefert für die untersuchten 29 IV–IV–, III–V–, II VI– und I–VIT–Verbindungen mit tetraedrischer Koordination numerische Ergebnisse in guter Übereinstimmung mit experimentellen und theoretischen Werten anderer Autoren und erklärt das anomale nichtlineare Verhalten von ZnO und ähnlichen Verbindungen ohne wesentliche Zusatzannahmen. Es wird eine Beziehung zwischen dem Miller‐Koeffizienten und der effektiven Ionenladung von Szigeti hergestellt.
Low‐Field Transport Properties of a Polar Semiconductor in a Quantizing Parallel Magnetic Field
Authors: Magnusson B.
Published: July 2007

Low field momentum distribution functions and magnetoresistance in a quantizing magnetic field have been calculated numerically for a semiconductor with a parabolic conduction band. The shifting of maxima and minima in the magnetoresistance versus magnetic field curves and the detailed behaviour around the first magnetophonon resonance point are investigated. A second resonance series is noticed for higher lattice temperatures.
Zero‐Phonon Lines in the Luminescence Spectra of Impurity Organic Crystals and Electron–Phonon Interaction
Authors: Sapozhnikov M. N.
Published: July 2007

The temperature dependences of the shapes, widths, positions, and intensities of the zero‐phonon lines in the fluorescence spectra of 1.12‐benzperylene and coronene in a normal hexane matrix are investigated experimentally. The results obtained are discussed in terms of the theory of the electron‐phonon interaction in the impurity crystals. Thestrength of the impurity‐matrix coupling in the systems studied is shown to he very small. At high temperatures the zero‐phonon linewidth is a result of the Raman seattering of the lattice phonons by the impurity molecule. The estimation of the change in normal mode frequencies upon the phototransition in the impurity molecule and the mixing of the initial state normal modes of the crystal show that those values are small. The anharmonicity of the crystal vibrations in the “soft” organic crystals in contrast to the “hard” inorganic crystals is shown to play an important role and to determine considerably the zero‐phonon line characteristics.
Energy Bands of Copper Chloride I. Theory
Authors: Calabrese E. , Fowler W. B.
Published: July 2007

A calculation of the energy bands of CuCl has been performed in which several innovations and refinements were made to previous procedures. In this paper the method used in the calculation is described. The combined plane‐wave tight‐binding (“mixed‐basis”) method of Kunz was adapted to the zinc blende lattice. Tight‐binding functions were truncated so as not to overlap one another, as suggested by Deegan and Twose. Overlap between these functions and the short‐range potential (which was not truncated) of neighboring ions was significant and so was included. A semiempirical approach was devised which involved adding a square‐well potential of adjustable strength to each ionic potential. In order to obtain coefficients of p‐like and d‐like contributions to a given eigenfunction, a technique was devised for projecting these from the plane waves.
Electron Transport in Zincblende Semimetals
Authors: Rode D. L. , Wiley J. D.
Published: July 2007

Iterated solutions of the Boltzmann equation with Fermi statistics are presented for the zero‐gap semimetals α‐Sn, HgSe, and HgTe. Electron scattering operators in the first Born approximation arc derived for the T8c conduction band including piezoelectric and deformation potential scattering by acoustic phonons, polar optical mode Scattering, and Coulomb scattering. It is found that quantum corrections are, in many cases, comparable to RPA corrections but opposite in algebraic sign so that the overall corrections to the classical calculations are much smaller than previously indicated.
Electron Diffraction Study of the Local Atomic Arrangement in Amorphous Tellurium Films
Authors: Ichikawa T.
Published: July 2007

Amorphous thin films of tellurium have been prepared by condensation on the substrate at liquid helium or nitrogen temperature in high vacuum, and a transmission electron diffraction study of these films has been undertaken. Diffraction patterns of such films show diffuse halos, which are characteristic of a highly disordered structure. A radial distribution analysis has been carried out in order to investigate the local atomic arrangement in the films. The result leads to the conclusion that the atoms in the films are arranged in short chains.
X‐Bay Photoelectron Spectra of Palladium–Silver and Nickel–Copper Alloys
Authors: Nemoshkalenko V. V. , Mindlina M. A. , Senkevich A. I. , Aleshin V. G.
Published: July 2007

With a Varian (IEE‐15) electron spectrometer the valence band photoelectron spectra and the binding energies of inner‐core electrons are measured for palladium‐silver alloys. For the nickel–copper alloys the photoelectron spectra known from literature are compared with the results of density‐of‐state calculations. Kβ2,5 emission bands of pure nickel and pure copper and from the alloy Ni–Cu (50 at%Cu)are also reported.
Pressure Dependence of Mossbauer Fraction and Energy Shift for Fe6' in Cu Lattice at 298 and 94 °K
Authors: Nandwani S. S. , Puri S. P.
Published: July 2007

The pressure variations of the Mössbauer fraction and the second order Doppler shift for Fe57 in Cn lattice at 298 and 94 °K have been calculated taking into account the effects of change of mass and force constant due to impurity atoms. Due to the large error in the experimental value of Williamson and Ingalls no conclusion regarding the change of force constant with temperature has been obtained. The pressure dependence of centre shift at the two temperatures enables one to calculate the explicit temperature dependence of isomer shift with pressure. It is shown that at 82 kbar the isomer shift at 298 °K becomes equal to that at 94 °K. This is explained using the band model of lattice.
The electronic structure of sns2 and snse2
Authors: Ch. Schlüter I. , Schlüter M.
Published: July 2007

The energy bands of SnS2 and SnSe2 are calculated using the local empirical pseudopotential method. The form factors of the pseudopotential were adjusted in such a way that the calculations reproduce the experimental values of the fundamental gaps. The space group D of these compounds is studied in detail and a comparison is made between the recent calculation by Fong and Cohen and the present work. Prom the pseudo wavefunctions the charge distributions of electrons and holes are calculated.
Uniaxial stress dependence of the fermi surface of zinc
Authors: Gerstein I. I. , Elbaum C.
Published: July 2007

The effect of uniaxial stress on the areas of several orbits of the zinc Fermi surface have been studied by measuring the phase shifts of the magnetoacoustic quantum oscillations of ultrasonic attenuation. The orbits studied were the two third‐zone electron orbits, i.e., the lens (ζ) and the needles (α), and four hole orbits on the second‐zone monster, i.e., two horizontal arm orbits (ß), with the magnetic field, H, along 〈1120〉 and 〈1010〉, and two junction orbits (Γ), with H along 〈1120〉, and the bow‐tie orbit (σ) with H along 〈1010〉. Calculations of the stress dependence were carried out on the basis of a non‐local pseudopotential model for the zinc Fermi surface and compared with the experimental results obtained. Good numerical agreement was thus obtained in half of the cases studied. Discrepancies of about a factor of two were found in the other cases, while the largest and smallest values of the stress derivatives (both measured and calculated) differed by as much as a factor of 100.
Temperature dependence of the hyperfine structure splittings of XO3 molecule‐ions in single‐crystal calcite
Authors: Serway R. A. , Chan S. S. L. , Marshall S. A.
Published: July 2007

The hyperfine structure splittings of three paramagnetic molecule‐ions formed in γ‐ir‐radiated single‐crystal calcite have been measured from 4.2 to 320 K. These molecule‐ions are CO, PO, and AsO Over this temperature range, they exhibit a decrease in hyperfine structure splitting with increasing temperature of between 1 to 2%. An analysis of the data suggests that the loss of unpaired electron spin‐density at the central atom is approximately proportional to the fractional increase in spin‐density at the oxygen‐orbitals. Furthermore, the fractional change in p‐character is about three times the fractional change in s‐character for these species. In all cases, calculated equilibrium bond‐angles are found to be insensitive to temperature. Estimates of the unpaired electron molecular orbitals are given using the hybridization ratios determined from the data.
Effect of localization on the temperature‐dependent impurity resistance in dilute non‐magnetic alloys
Authors: Dubey S. K. , Dayal B.
Published: July 2007

A previous theory of the temperature dependence of the impurity resistance, is applied to those alloy systems where the constituent atoms have nearly equal masses. An expression is developed for the matrix element describing the coupling of the unperturbed phonons with the impurity electrons. Detailed calculations are made for a silver—cadmium dilute alloy, silver being the host metal. Satisfactory agreement with the experimental values is obtained. The results show that in this alloy system resonance states below the Fermi level are formed. Further it is shown that localized states are formed when the structure is h.c.p. and resonance states when the structure is cubic.
On the possibility of amplification of acoustic waves in the presence of plasma oscillations in strong electric fields in semiconductors Karnatak University, Dharwar, Mysore State
Authors: Krishnamurthy B. S. , Mulimani B. G.
Published: July 2007

A kinetic method is developed of the acoustic wave amplification in a strong electric field in semiconductors with a simple parabolic band. A displaced Maxwellian distribution for electrons is assumed. It is found that the influence of the plasma oscillations on the acoustic wave amplification is significant and in the low electric field region the maximum value of the amplification factor is identical with the published result. In the strong field region the amplification factor takes a negative value because of the direct electric field term although the drift term may also contribute if the electron drift velocity exceeds the sound velocity.
Optical absorption associated with superlattice in silicon carbide crystals
Authors: Dubrovskii G. B. , Lepneva A. A. , Radovanova E. I.
Published: July 2007

Absorption of polarized light in the near IR and visible region has been measured in n‐type silicon carbide crystals of seven modifications: 4H, 6H, 8H, 15R, 27R, 33R, and 21R. All the samples showed clearly resolved polarized absorption bands at wavelengths characteristic for each crystal structure. It is shown that the energy position, temperature behaviour, and selection rules for these bands are in good agreement with those predicted theoretically for interminizone transitions in the Superlattice. The period of the super‐lattice is identified for several groups of SiC structures defined according to the principle of their unit cell formation.
Colour centres near ionic crystal surfaces
Authors: Bartczak W. , Sugier H.
Published: July 2007

Using the methods developed in part I the self‐trapping energy of the VK centre on and below the surface has been calculated for NaCl and KCl crystals. It is found that the energy decreases monotonously by approaching the external surface. Conclusions concerning the migration of VK centres towards the surface of the crystal are discussed.
On the Influence of Electronic Exchange and Correlation on Pseudopotential Calculations. III. Equilibrium Lattice Constants and Bulk Properties
Authors: Hafner J.
Published: July 2007

The equilibrium density, the corresponding binding energy, the equation of state at T = 0°K, the isothermal compressibility, and the pressure derivative of the bulk modulus of 11 simple metals are calculated by pseudopotential theory, using the Xα‐approximation for the exchange and correlation interactions. The agreement with experiment is satisfactory. It is shown that the best possible description of the atomic wave functions within the Xα‐theory is a good approximation for the core wave functions in the metal and that the magnitude of the conduction band‐core exchange interaction is strongly dependent upon the overlap of the conduction‐ and the core electron densities.
Non‐Adiabatie Calculation of the Binding Energy of the Exeiton–Neutral Donor Complex in Semiconductors
Authors: Munschy G. , Cababatos C.
Published: July 2007

A new non‐adiabatic method of calculation is presented and applied t o the case of an exciton + − bound to a neutral donor ⊕ −. The one‐particle equation deduced is analogous to Born's nuclear equation, but non‐diagonal coupling terms are not involved. Moreover, as shown by the numerical results obtained, the ⊕ + − − complex becomes unstable for intermediate values of the electron to hole effective mass ratio.
Magnetic Moment Calculation in Disordered Systems: α‐Manganese
Authors: Röpke G.
Published: July 2007

The possibility to calculate magnetic moments in elemental disordered systems (amorphous state, lattice distortions, surfaces) using the inhomogeneous electron‐gas model is considered. Therefore a partial summation of the gradient expansion of the kinetic energy density functional is performed, which allows to obtain the Hund's rule. This model is applied to the calculation of the atomic magnetic moments in α‐Mn. The results are compared with recently measured values for the magnetic moments in this system.
½ 〈111〉 Stacking Faults in A15 (β‐W) Superconductors
Authors: Essmann U. , Zerweck G.
Published: July 2007

In A15 structures ½ 〈111〉 stacking‐faults break the linear chains of A atoms. This affects the electronic structure resulting e.g. in changes of the superconducting critical temperature Tc. Moreover the large pinning forces observed in these compounds are attributed to the described planar defects.
Dynamical Resonance Phenomena of the Optical JT‐Problem in Trigonal Systems
Authors: Sigmund E. , Wagner M.
Published: July 2007

If a degenerate high‐energy excitation is coupled to a low‐energy one, via the effective splitting of the former dynamical resonances may be expected. We confine us to the treatment of the optical absorption‐function of a so‐called E–e Jahn‐Teller system, appearing in trigonal symmetries. The calculation, which will be done by the use of a nonlinear canonical transformation, confirms our expectations. It is to be seen that each single excitation line displays a resonance behaviour. This also manifests itself in the total structural form of the optical spectra.
Exchange Narrowing in the ESR Spectra of Metallic GdxLa1–xB6 (x = 1 to 0.01)
Authors: Sperlich G. , Janneck K. H. , Buschow K. H. J.
Published: July 2007

Linewidth measurements are reported of the ESR spectra of metallic GdxLa1–xB6 compounds (x = 1.0 to 0.01). This system is remarkable because contributions from the exchange narrowed linewidth ΔHex and the homogeneous linewidth ΔHhom can be estimated. The interpretation of ΔHex leads to values for the “exchange fields” He as a function of the Gd concentration.
Effect of Deuteration on the Davydov Splitting and on the Shape of the Absorption Band of OH Stretching Vibrations in Alcohol Monocrystals
Authors: Mikhailov I. D. , Savelev V. A. , Sokolov N. D. , Bokii N. G.
Published: July 2007

A theoretical investigation has been made of the dependence of the shape and breadth of the absorption band for stretching vibrations vOH in hydrogen bonds of an alcohol crystal on the concentration of an isotopic impurity. It is shown that the structure of the VOH band is caused by Davydov splitting. The local mode frequencies arising on deuteration of an individual alcohol molecule are calculated using the Green's function method and the introuction of such impurity molecules into the crystal is found to lead to an effective disintegration of the hydrogen‐bonded chain to shorter chains. A method is proposed of constructing the absorption vOH band contour. The vOH absorption band contour is constructed for various concentrations of deuterated molecules under the assumption of their random distribution. The splitting of the vOH doublet decreases with increasing concentration of deuterated molecules, owing to the effective decrease of the lengths of the chain formed by hydrogen bonds in an alcohol crystal.
Spin‐Magneto‐Phonon Resonance and Nuclear Polarization by DC in Semiconductors
Authors: Kalashnikov V. P. , Karyagin V. V.
Published: July 2007

The enhancement of steady‐state nuclear magnetization due to the interaction of nuclear spins with hot conduction electrons in a semiconductor is investigated theoretically taking into account the effects of spin‐magneto‐phonon resonance (SMPR). It is shown that the stationary nuclear magnetization oscillates strongly in the vicinity of the SMPR points as a function of the magnetic field strength. The effect of nuclear de polarization in n‐type InSb observed by Clark and Feher [2] is considered as being conditioned by the SMPR transition.
On the Definition of the Current Operator Application to the Hubbard Model
Authors: Cabib D. , Kaplan T. A.
Published: July 2007

A disagreement existing in the literature between various calculations of the electrical conductivity in the single‐band Hubbard model is resolved. The error, which occurs in a paper by Monecke, is pointed out and explained to be due to the assumption that the current operator, defined as the commutator of the Hubbard Hamiltonian with the polarization operator, is invariant under the lattice translations. The existence of ambiguity in the definition of a “good” current operator (which is translationally invariant) is pointed out and resolved in a large class of situations.
Conduction Band Parameters of Bi2See from Shubnikov‐de Haas Investigations
Authors: Köhler H.
Published: July 2007

The oscillatory magnetoresistance (Shubnikov‐de Haas effect) was observed on n‐type Bi2See single crystals between 1.6 and 4.2 K. The electron concentrations ranged from about 4 × 1017 to 4 × 1019 cm−3. Stationary magnetic fields up to 10.5 T were available. The angular dependence of the Shubnikov‐de Haas periods revealed an almost rotational symmetry of the Fermi surface around the kc = 0 a parabolic ε(k⊥) relation is found, parallel to the kc‐axis an increasing nonparabolicity is evaluated. Only above 1019 electrons/cm3 the deviation of the shape of the Fermi surface from that of an ellipsoid of rotational symmetry becomes significant. The cyclotron mass mc perpendicular to the kc‐axis (B ‖c) is determined to be 0.124 m0 in the whole concentration range. Fermi energies are evaluated up to 160 meV. The single‐valley model for the lowest conduction band of Bi2See is confirmed.
A Scanning Electron Diffraction Study of Vapor‐Deposited and Ion Implanted Thin Films of Ge (I)
Authors: Graczyk J. F. , Chaudhari P.
Published: July 2007

The structure of Ge films evaporated by several methods onto room temperature substrates of NaCl and films which have been ion implanted with 3 × 1015, 30 to 40 ke V Ge74 has been investigated via a scanning electron diffraction instrument with electron energy filtering. The radial distribution functions of all films are very similar and exhibit strong correlation peaks at 2.45, 4.00, and (4.85 ± 0.075) Å with coordinations of 4, 12, and ≈ 7, respectively. The remaining major peaks for higher r deviate substantially from the diamond cubic form of Ge. From the RDF analysis a density ratio ϱ0/ϱcryst of 0.86 to 0.95 is determined. Thermal anneal experiments showed that the films may be annealed to a density of ≈0.97 of crystalline density without crystallization. Amorphous films which have been ion implanted with the same energy and dose as the polycrystalline films show a considerable concentration of pores and voids of a maximum diameter of 100 Å. The RDF, however, is identical to the as‐deposited film with the exception of some changes in the correlation for r greater than 11 Å.
Phonon Dispersion and Phonon Densities of NaNO3
Authors: Plihal M.
Published: July 2007

Phonon‐dispersion branches for specific directions of the wavevector, one‐ and two‐phonon densities, and the lattice specific heat Cv have been calculated for NaNO3 using the same lattice dynamical model which was applied recently to trigonal CaCO3. The results of the calculations are compared with available experimental data from inelastic neutron scattering experiments, the specific heat Cp, and the two‐phonon infrared absorption spectrum.
Magnetic Properties of the Ordered and Disordered CrPt3 and CrPt Phases
Authors: Besnus M. J. , Meyer A. J. P.
Published: July 2007

The structural and magnetic properties of cold worked and annealed Pt‐Cr alloys with Cr content 14 < c < 60 at% have been investigated. The annealed alloys show a spontaneous magnetization from 18 to 45 at% Cr with a maximum value for ferrimagnetic CrPt3. Above 17 at% Cr the appearance of a spontaneous magnetization follows the paramagnetic concentration range of Pt‐Cr alloys, and above 41 at% Cr there is a transition from non stoichiometric ferrimagnetic CrPt3 to non stoichiometric antiferromagnetic CrPt resulting in a pseudo critical concentration range. The cold worked alloys have a paramagnetic behaviour which is ascribed to an incoherent distribution of localized moments with prevailing antiferromagnetic exchange.
Pseudopotential Band Structure of ZnO
Authors: Bloom S. , Ortenburger I.
Published: July 2007

The empirical pseudopotential method is used to compute the band structure, density of states, and reflectivity of hexagonal zinc oxide. The local approximation is assumed, on the basis of recent photoemission experiments which showed that the zinc core 3d band lies deep below the valence band. The calculation succeeds in identifying the reflectivity peaks, but gives stronger amplitudes than are observed, and these identifications are shown to fit well into the systematics of the other three zinc chalcogenides. Reasonable agreement is also obtained with the experimental locations of the maxima in the density of states and with the large value of the valence bandwidth.
A Self‐Consistent Approach to the Problem of Bloch Electrons in the Electric Field
Authors: Henneberger K. , Röseler J.
Published: July 2007

Starting from the concept of the “field‐modified model crystal”, a complete set of Bloch functions is self‐consistently defined. The Houston‐type functions constructed with the aid of this set are exact solutions of the time‐dependent Schrödinger equation. The equivalence of the model problem which is defined self‐consistently and the equation given by Wannier is shown. By appropriate choosing the phases of the Bloch functions a splitting of H into two commuting parts is found, the periodic part of which possesses a continuous, but limited spectrum, whereas the Stark‐ladder belongs to the remaining part.
Two‐Photon Excitation Spectra of Trivalent Rare‐Earth Ion Luminescence in Crystals
Authors: Apanasevich P. A. , Gintoft R. I. , Korolkov V. S. , Makhanek A. G. , Skripko G. A.
Published: July 2007

Theoretical and experimental investigations are made of two‐photon absorption spectra of trivalent rare‐earth ions in crystals. Effective cross‐sections, selection rules, and relative intensities of the components are found. The most intensive two‐photon transitions are established. Two‐photon excitation spectra of luminescence of Er3+ and Ho3+ ions in CaF2 are obtained and effective excitation cross‐sections are measured using a tunable dye laser (715 to 850 nm). It is shown that the spectra have a peculiar structure which differs considerably from that of the one‐photon absorption spectra. The experimental results agree with theory. It is established that one of the levels in Ho3+ is the level 5G6 rather than 3K7 usually assumed.