bluuebunny/crossref_metadata_2025 huggingface dataset:
The Crystal Potential and Summation of Diverging Series
Authors: Tupizin I. I. , Abarenkov I. v.
Published: July 2007

An infinite series for the potential of an infinite crystal lattice is in general divergent. It is shown that a method of summation can be found to make the series convergent to a value representing the potential inside a large finite crystal of particular shape and structure of its boundaries. This can be considered to be the definition of the infinite crystal lattice potential. For such defined potential a generalized Ewald expression which contains two additional constant terms, exhibiting the dependence of the crystal potential on shape and structure of the crystal boundaries, is developed.
Evaluation of Some Important Lattice Sums on NaCl and CsCl Structures
Authors: Garg V. K. , Verma M. P.
Published: July 2007

A large number of lattice sums appearing in studies on different physical properties of ionic solids are evaluated using a modified Ewald method by splitting them into sums of cubic Bravais lattices. The calculated values of the sums for the CsCl and the NaCl structure are reported in a dimensionless form. It is demonstrated that the sums of the fluorite and zincblende structures can be obtained from these sums by suitable linear combinations.
Raman Spectroscopic Study of the Structural Phase Transitions in K2SnCl6
Authors: Pelzl J. , Engels P. , Florian R.
Published: July 2007

Raman spectra of K2SnC16 are investigated in the temperature range between 50 and 300 K. The observed low temperature spectra are compatible with the proposed monoclinic structure. Two bands that exhibit a soft behaviour are assigned to a zone center and zone boundary rotational mode, respectively. It is supposed that these modes drive the structural phase transitions at T1 = 263 K and T2 = 255 K.
Mn2+ EPR Study of Cu‐As‐Se, As‐Se‐I, and Cu‐As‐Se‐I Glass Systems
Authors: Chepeleva I. V. , Zhilinskaya E. A. , Lazukin V. N. , Chernov A. P. , Olkhovskii V. I.
Published: July 2007

The Cu‐As‐Se and As‐Se‐I ternary systems and the Cu‐As‐Se‐I fourfold system are investigated with Mn2+ EPR at 3 cm band under 300 and 77 K. The obtained results show that I is the third component which most actively transforms the initial As2Se3 glass structure. The EPR spectra of the Mn2+ ions for Cu‐As‐Se and As2Se3 glass systems differ greatly from those for Cu‐Ss‐Se‐I and As‐Se‐I systems. Well resolved HFS on the g = 2 EPR line is observed for some samples containing I or CuI; this line is probably due to the unstable MnI2 compound.
The Linear Ising Chain in the Three‐Dimensional Phonon Field
Authors: Mijatović M. , Milošević S.
Published: July 2007

The model of the linear Ising chain embedded in a three‐dimensional compressible lattice is investigated. Firstly it is supposed that there is only one chain in the lattice and afterwards the problem is generalized to the system of the Ising chains. The specific heat is calculated and the initial susceptibility of the effective magnetic Hamiltonian, including the second‐order contributions of the induced four‐spin interaction treated as a perturbation. This method gives plausible results up to the relatively low temperature region and for lattices which are not highly compressible. Thus the studied model and the applied theoretical method provide a new direction for further investigations of the quasi‐one‐dimensional systems, like CoCI2. 2NC5H5.
X‐Ray Diffraction Study of Magnetostriction and Phase Transitions in a Field in Rare Earth Metals and Some Alloys
Authors: Vorobev V. V. , Belovol V. S. , Finkel V. A.
Published: July 2007

Various aspects of an X‐ray study of magnetoelastic effects in rare earth metals and their alloys are presented. The observation of a phase transition in a magnetic field, the forced magnetostriction of rare earth metals with different magnetic structures, and the influence of the magnetic effects are discussed.
The Phase Transformation in Ferreous Fluosilicate Hexahydrate
Authors: Spiering H. , Vogel H. , Irler W.
Published: July 2007

The structural change of FeSiF6, . 6 H2O at T, = 225 K (P21/C to P3ml above Te) is well known from X‐ray measurements. The temperature dependence of the Mössbauer parameters in the neighbourhood of the phase transformation (200 K ≦T≦ 250 K) is studied. At Te, the quadru‐pole splitting ΔEq changes with increasing temperature by (+0.009 ± 0.002) mm/s, the Lamb factor f slightly decreases by δf = −0.011 ±0.004, while the anisotropy of f changes remarkably: δ(f∥ ‐ f⊥) = −0.18. From the relaxation broadening of the absorption lines below and above Te, it is concluded that the non‐axial terms of the ligand field at the Fe2+ ion do not vanish above the transition temperature.
Spin‐Lattice Relaxation in Fe3+‐Doped Andalusite Crystals I. Relaxation Mechanisms
Authors: Korolev V. D. , Peskovatskii S. A. , Shulga V. M.
Published: July 2007

The temperature and concentration dependences of the spin‐lattice relaxation of Fe3+ ions in natural andalusite crystals are investigated experimentally within the temperature range 0.1 to 4.2 K and concentration range 0.03% to 0.56% at a frequency of 9.2 GHz. Relaxation mechanisms are determined. At very low temperatures the effects of phonon bottleneck are observed.
Spin‐Dependent Wave Functions and Band Parameters at the r Point of InSb
Authors: Gorczyca I.
Published: July 2007

The empirical pseudopotential method is used within the frame of the OPW formalism to calculate wave functions and band parameters for InSb at the Γ point. A comparison with other theoretical and experimental data is made.
Multiple Scattering of Electrons by an Infinite Lattice Plane I. Derivation of the Basic Equations Using a Scattering Amplitude Differential Operator
Authors: Lenk R. , Solbrig H. , Wolf H. , Sommer K.‐H.
Published: July 2007

Korringa's representation of the scattering problem is developed further to obtain a compact scattering formula which is applied to the multiple scattering by an infinite lattice plane. The well‐known results are derived very directly. Prom the computational point of view the calculation of the planar scattering matrix by matrix inversion is found to be more advantageous than by perturbation methods. If the lattice sums in the calculation of the structure matrix are extended to a wide lattice region, the half‐line summation method is more suitable than the ring summation method.
Comments on the Magnetic Resonance Absorption Spectrum of the F2‐, Molecule‐Ion in Single Crystals of LiF, NaF, and KF
Authors: Marshall S. A.
Published: July 2007

The magnetic resonance absorption spectrum of the diatomic fluorine molecule‐ion, F‐2, in single crystals of lithium fluoride, sodium fluoride, and potassium fluoride is reinvestigated. For each of these structures it is found that the spectrum of this molecule‐ion species exhibits the D2h, symmetry of its environment. Comparisons between observed and predicted spectral line field positions and spectral parameters derived from data taken at T = 4.2 K are given.
Phenomenological Interpretation of Positron Lifetimes in Metals
Authors: Tanigawa S. , Hinode K. , Doyama M.
Published: July 2007

Measurements are made of the angular correlation curves of positron annihilation and the positron lifetimes for fully annealed specimens of Mg, Zn, Cd, Al, In, Sn, Pb, Cu, Ag and Ni to clarify the relationship between Γv (annihilation rate of a positron with conduction electrons) and their density Γo. By use of the usual separation method of the total rate Γ into ΓV and ΓC (with core electrons), the experimental relationship between ΓV and ϱ0is obtained. It is found that (1) the metals are sorted into two groups (A and B groups) according to the dependence of ΓV on ϱ0 (2) A‐group metals (Li, Na, K, Rb, Cs and Mg) obey the theoretical relationship, and (3) however, B‐group metals (Zn, Cd, Al, In, Sn, Pb, Cu, Ag and Ni) do not obey. For this difference between both the groups, a possible explanation is presented and the origin of the difference is attributed to the existence of the spaces in the lattice large enough for a positron to become more or less correlated with a single electron in a quasi‐bound state at the low electronic density limit.
Coincidence Site Lattices in Non‐Cubic Lattices
Authors: Fortes M. A.
Published: July 2007

Except for trivial cases, which are not relevant in coincidence site lattice (c.s.l.) models of grain boundary structure, c.s.l. rotations can only exist in C‐lattices, i.e. those which contain an orthorhombic cell with edges o1 o2, o3 so that o12: o22: o32: = n1: n2: n3, the ni being integers. It is also shown that there is a one‐to‐one correspondence between the c.s.l. rotations of a C‐lattice and those of a cubic lattice. However, the reciprocal fraction of coincident points, Σ, tends to be larger in non‐cubic lattices. The implications, for the c.s.l. model of grain boundaries, of the weak incidence of low Σ c.s.l. orientations in non‐cubic crystals are discussed.
Coherent Radiation of the Channelling Positron (Electron)
Authors: Baryshevskii V. G. , Dubovskaya I.Ya.
Published: July 2007

Coherent radiation of a relativistic positron (electron) in a crystal is considered in the case of planar channelling. The wave function of the particle is found, the cross‐section of the process is calculated, and the evaluation of the radiation intensity is given. The coherent X‐ray radiation spectrum is analysed and the observation possibility of the complex and anomalous Doppler effect in this condition is shoun.
Impurity Diffusion with Complex Formation
Authors: Sh. Malkovicii R. , Pokoeva V. A.
Published: July 2007

The non‐linear problem of the diffusion with complex formation is investigated, both in the presence and absence of local equilibrium, in half‐space with constant surface concentration of the free impurity. The background impurity is assumed to be immobile and uniformly distributed in the volume. The complexes are also considered to be immobile. For the solution of this problem an approximate method of successive intervals is suggested based on dividing the whole interval of the diffusion time into sufficiently small intervals where the free and bound impurity concentration values in the terms describing the complex formation and dissociation are supposed to be time independent and equal to the values at the end of the previous interval. In addition, for the local equilibrium region a second method is suggested based on the reduction of diffusion equations for complex formation and dissociation to the equation of the concentration‐dependent diffusion type. The general features of the numerical solutions for free and bound impurity concentration are investigated for various values of the parameters. The results obtained by these methods agree well with analytic ones found in several limiting cases.
The Optical Activity of Tellurium
Authors: Stolze H. , Lutz M. , Grosse P.
Published: July 2007

The optical activity (o. a.) of tellurium samples with carrier concentrations from 10′4 to 3 x × 1018 cm‐3 is measured between 4 and 300 K. A comparison with the circular dichroism of the band edge shows that the 0.a. is mainly related to interband transitions near the energy gap. The experimental results are interpreted using a quantum‐mechanical model, which allows a calculation of the o.a. from the energy bands and matrix elements. The undoped samples show a characteristic temperature dependence of the o. a., which is caused by the temperature dependence of the energy gap. At low temperatures in degenerate samples depopulation effects in the o.a. are observed.
Charge Screening in an Electron Gas of Arbitrary Degeneracy
Authors: Alfred L. C. R.
Published: July 2007

Utilizing the dielectric response formulation, analytic expressions are obtained for the screening field around a localized point charge in a non‐interacting Fermi‐Dirac electron gas of arbitrary degeneracy. Screening field features suggested by the analytic forms are discussed. In the non‐degenerate case, the field is a modified Brooks‐Herring one; the predominant form of the screening factor is inverse exponential with a transition to Gaussian in the range kBTr2≪1.
Many‐Body Effects on the Fourth‐Order Elastic Constants of Ionic Solids II. Applications
Authors: Garg V. K. , Puri D. S. , Verma M. P.
Published: July 2007

Numerical calculations are made for the fourth‐order elastic constants of ionic solids crystallizing in the NaC1 and the CsCl structures from expressions derived in an earlier paper on the basis of the Lundqvist potential. The results are compared with the experimental ones by calculating the second‐order pressure derivatives of the second‐order elastic constants which can be expressed in terms of fourth‐ and lower‐ order elastic constants. The results agree well with available experimental data.
An Interatomic Potential for Molybdenum from Phonon Spectra
Authors: Heugten W. F. W. M. VAN
Published: July 2007

A potential function for molybdenum is presented that is derived from the phonon dispersion curves. On the basis of this potential function some defect calculations are made. The interstitial formation energy (4.4 eV) is in good agreement with the experiment. The first neighbour divacancy configuration is found to be the most stable one.
Infrared Studies of the Layered Compound ZnIn2S4
Authors: Herrmann H.
Published: July 2007

Infrared transmission and reflectance measurements are made on single crystals of the layered compound ZnIn2S4 with C3v5 structure in the range between 4000 and 33 cm‐1,. According to group‐theoretical analysis there are altogether 4A1 + 6E zone centre optical modes which are both ir‐ and Raman‐active. 4A1 + 4E modes should be intralayer modes, the remaining 2A1 + 2E ones should be rigid layer modes with distinctly lower frequencies. The number of observed one‐phonon frequencies is found to be by far larger than expected. 16 frequencies of ZnIn2S4 are found to coincide very well with frequencies of certain critical‐point phonons of cubic ZnS and with certain phonon frequencies of β‐In2S4 respectively. The “additional” frequencies related to the number of frequencies predicted by group theory, which occur in the ir spectrum, are believed to arise from broken translational symmetry of the lattice.
On the Theory of Laser Action in Dense Exeiton Systems
Authors: Haug H. , Koch S.
Published: July 2007

The rate equations for the exciton‐exciton (ex‐ex), exciton‐electron (ex‐e) and exciton‐hole (ex‐h) recombination processes are derived by taking into account polariton effects. The solutions of the stationary rate equations yield the spectra of the spontaneous emission and of the gain. Their temperature‐dependent shifts are calculated by taking into account self‐energy corrections which improve the agreement with experiment. Furthermore, the various threshold pump rates are calculated. Due to these results it is expected that the laser action should start at low temperatures for the ex‐ex process, at intermediate temperatures for the ex‐LO phonon process (which has been treated previously), and at high temperatures for the ex‐e process. The temperature ranges in which the nonlinear recombination processes (ex‐ex, ex‐e) have the lowest threshold increase with decreasing resonator quality. Numerical results arc given for example for CdS.
Extended X‐Ray Absorption Fine Structures and Chemical Shifts for the Heavy Rare‐Earth Oxides
Authors: Gupta S. N. , Vijayavargiya V. P. , Padalia B. D. , Tripathi B. C. , Ghatikar M. N.
Published: July 2007

L3‐absorption edges and the associated extended X‐ray absorption fine structures (EXAFS) for dysprosium, holmmm, thulium, ytterbium, and lutetium in pure oxide form are investigated using a 600 mm curved crystal spectrograph. A forbidden transition corresponding to an electron transition, 2p3/2 ‐→ 4f (unoccupied), is detected for Dy2.,O3, Ho203, and Tm2O3 with open 4f‐shell. Chemical shifts are measured and an attempt made to correlate these with the estimated effective charges on the ions indicates a new trend not reported by others. A prominent absorption peak, A (white line), is also recorded for the oxides. The reduced width of these white lines for oxides indicates the narrowing of the 5d band. The metal‐oxygen distances determined using EXAFS methods agree with the crystallographie data. Shapes of the EXAFS for oxides resemble with each other while they differ from those recorded for the corresponding metals.
A Study on the Magnetic Properties of Compressed D4h Clusters
Authors: Moreno M.
Published: July 2007

A new mechanism for understanding the gzz ‐ g0 value in compressed D4h clusters formed by d9 ions is studied. This mechanism is essentially determined by the A2g non‐bonding level, the ligand spin‐orbit coefficient playing a decisive role. The value of this contribution strongly depends on the degree of hybridization of the equatorial ligands in the A1g ground level. In cases with “true” D4h symmetry (and specially in Cu2+: NH4Br) the new mechanism can well account for the experimental gzz ‐ g0 value without considering the influence of the zero‐point motion. In contrast, in Ni+ :LiF, it is shown that the latter is mainly responsible for the observed gzz ‐ g0 value.
Surface Plasmons in Thin Metal Films, Circular Cylinders, and Spherical Voids
Authors: Maksimenko V. V. , Simosnov A. J. , Lushnikov A. A.
Published: July 2007

The influence of the size effect on the frequencies of the surface plasmons (SP) in thin films, cylinders, and spherical voids is studied within the random phase approximation. The cross sections for excitation of SP by fast electrons and X‐rays and the photoabsorption cross sections are calculated.
Luminescence Band Shape of Free Excitons in AgBr Crystals at High Exciton Density
Authors: Lelyakov A. V.
Published: July 2007

The luminescence band shape of degenerate “triplet” excitons and nondegenerate “singlet” excitons in AgBr crystals at T ≠ 0 K is considered. The quantum‐mechanical transition probability of “hot” singlet excitons with photon and phonon emission is calculated in second‐order perturbation theory. The behaviour of triplet excitons in an external magnetic field is discussed and the theory agrees well with experiments for AgBr crystal.
Distortion of ionic crystals by vacancies
Authors: Augst G. R.
Published: July 2007

An expression for the energy of vacancy formation in ionic crystals with regard to the lattice relaxation is obtained in the model of Kellermann's rigid ions. For the purpose of finding the electrostatic energy the Ewald method is used. The expansion in terms of the normal coordinates allows to express the displacements and relaxation energy in terms of dynamic characteristics of the crystal.
Correlation effects in hopping conduction
Authors: Zvyagin I. P.
Published: July 2007

The rate equation for electron hopping between localized states is generalized to take account of the electron‐electron interaction in the Hubbard model. The generalized rate equation is used to discuss the effect of electron correlations, both repulsive and attractive, on the temperature dependence of hopping conductivity and thermopower.
EPR of U5+ in alkali fluorides with a new charge compensation mechanism
Authors: Lupei V. , Lupei A. , Georgescu S.
Published: July 2007

A new type of U5+ centre is found in uranium‐doped alkali fluorides (KF, NaF, LiF) irradiated with X or γ rays. The EPR spectra recorded at 77 K show near tetragonal symmetry and super‐hyperfine structure of three lines with an intensity ratio of 1:2:1. The U235 hyperfine spectrum, characterized by nuclear electric quadrupole effects larger than that of the magnetic hyperfine ones is analyzed. A tentative model of the centre, with a water molecule in the vicinity of uranium, is discussed.
Phase diagram of electron‐hole liquids in polar semiconductors
Authors: Röslee M. , Zimmeemann R.
Published: July 2007

The phase diagram of electron‐hole drop formation in excited direct band gap semiconductors is calculated including the coupling to LO phonons. Results are given for CdS, CdSe, ZnS, GaAs, and CdTe. Critical temperatures as high as 64 K. (CdS) are obtained. In the self‐energy calculation the single‐plasmon pole approximation with the full RPA treatment is compared critically.
Analytic treatment of stochastic models of crystal growth
Authors: Pfeiffer H. , Haubenreisser W. , Klupsch Th.
Published: July 2007

The kinetic properties of the solid‐fluid interfaces for one‐component systems are examined by means of the kinetic Ising model in “solid‐on‐solid” restriction. Two different dynamic models corresponding to two different possibilities of interpretation of the Ising model are studied which should be appropriate in describing the growth process from the melt and vapour, respectively. Considering rough interfaces, simple analytic expressions for the growth rate and interface thickness are obtained which are exact within the mean‐field and continuum approximation. The used method and its range of validity are discussed in comparison of available Monte‐Carlo results.
On born‐huang invariance conditions
Authors: Sarkar S. K. , Sengupta S.
Published: July 2007

The Born‐Huang (B‐H) invariance conditions have been criticised on the ground of insufficiency of rotational invariance by several workers who have obtained additional conditions. This point is examined and it is shown that the B‐H invariance conditions are complete. The equilibrium conditions for vanishing of stress should be separately considered. Then the additional conditions mentioned above are nothing new.
Positron Annihilation in Hydrides of Transition Metals
Authors: Dekhtyar I. Ya. , Shevchenko V. I.
Published: July 2007

An analysis of the degree of positron localization on alloy components is developed in application to palladium, tantalum, and yttrium hydrides. The method is based on the account of positron‐ionic core electron interaction. From the results a conclusion is made about a principal positron localization on metallic components. The degree of the localization depends on the nature of the transition metals. The electronic structure of a hydrogen ion can be changed from the partial “proton” state (PdH) up to nearly the complete “proton” state (YH). The anionic hydrogen state is excluded.
Far‐infrared absorption in small metallic particles
Authors: Maksimenko V. V. , Simokov A. J. , Lushnikov A. A.
Published: July 2007

The cross‐section for far‐infrared absorption of small metal particles are calculated in the dipole approximation. Considerable deviations are found from the corresponding classical results at small particle size a/l0⪅1 (a is the radius of the particle, l0 the mean free path of an electron in bulk metal). The results obtained give a possible explanation to the size dependence of the absorption coefficient of small metal particles observed in recent experiments.
Two‐photon resonance Raman scattering by excitons via an intermediate excitonie molecule. II Polarization properties and polarition effects
Authors: Henneberger F. , Hennererger K. , Voigt J.
Published: July 2007

Taking into account the longitudinal and transverse character of the excitons as well as their strong coupling with the radiation field (polariton effect) the polarization and angle dependences of the two‐photon resonance Raman‐scattering spectrum of the excitonic molecule are investigated theoretically in cubic and uniaxial crystals. Numerical calculations are carried out for CuCl and CdS in different scattering configurations. Especially the different behaviour of scattering spectrum near the MT‐ and ML,‐band is studied. Well agreement with the experimental observations in CuCl can be stated.
The effective screening function in insulating solids
Authors: Duggan G. , Morgan G. J. , Shahtahmasebi N. , Lettington A.
Published: July 2007

Numerical calculations are presented of an effective dielectric matrix which exactly defines a constituent screened ion in a perfect solid within the Hartree approximation. The accuracy of the numerical method is estimated using previous analytical calculations for a simple model. The nature of the screening charge is then considered in detail for Ge and reasonable values for the magnitude of the “forbidden reflections” are obtained. The behaviour of the screening function for a tight‐binding situation is also investigated.
A many‐body model of dielectric polarisation in solids. I. Experimental data and existing interpretations
Authors: Jonscher A. K.
Published: July 2007

A detailed survey of the dielectric properties of a wide range of solids shows that their response functions in frequency or in time depart strongly from the Debye response and fall into a remarkably common or ‘universal’ pattern. The frequency response is similar for systems with permanent dipoles and with hopping charge carriers of electronic or ionic nature. It is also largely independent of the type of physical structure and chemical bonding in the materials. The existing approaches to the interpretation of this behaviour are shown to suffer from two basic drawbacks: they have to make rather arbitrary assumptions and they are unable to explain the essential similarity of the behaviour of many very different systems.
Emission spectra of NaBr: Tl+ excited in the A‐band
Authors: Casalboni M. , Grassano U. M. , Scacco A. , Tanga A.
Published: July 2007

Emission spectra of NaBr:Tl+ excited in the A‐band are measured at various temperatures. The main emission band, called AT, shows a well resolved doublet, whose splitting increases as T1/2. The time resolved emission spectroscopy indicates the presence of a slow emission in the region of the high energy component of the AT doublet. This behaviour is somewhat different from that of the typical thallium phosphors and may be tentatively explained with a Jahn‐Teller splitting of the emission level.
The EPR linewidth of triplet excitons in single‐crystal anthracene: A re‐examination
Authors: Bizzaro W. , Rosenthal J. , Berk N. F. , Yarmus L.
Published: July 2007

The magnetic resonance linewidths of triplet excitions in single‐crystal anthracene are measured in room temperature experiments performed at 24 GHz. The motivations for this study are the recent appearance of Reineker's linewidth theory, which is based on an extension of the Haken‐Strobl model of exciton transport, and the inability of this theory to account satisfactorily for earlier published results on anthracene. The data presented here, which differ distinctly from the previous work, are fit in both magnitude and angular dependence by the theory. This supports the theoretical assumption that hopping between differently oriented molecules in the unit cell is the dominant spin relaxation mechanism for triplet excitons in molecular crystals similar to anthracene.
Specific heat of TbPO4
Authors: Schwab M. , Kahle H. G.
Published: July 2007

The specific heat of TbPO4 single crystals is measured in the temperature range from 1.2 to 6 K. To avoid any effect of strain, the sample is mounted completely free from external stress. The thermal contact inside the sample holder is established by 3He exchange gas. In contrast to earlier measurements the experiment yields two anomalies close together at T = (2.15 ± 0.02) and (2.28 ± 0.02) K.
Investigation of the highest valence band in (Bi1‐xSbx)2Te3 crystals
Authors: Köhler H. , Freudenberger A.
Published: July 2007

Making use of the Shubnikov‐de Haas effect an investigation is made of the electronic parameters of p‐type (Bi1−xSbx)2Te3 crystals (0 < x < 1) prepared by a vertical Bridgman technique. Magnetic fields up to 10 T flux density are available, the crystal temperature at experiment is between 1.6 to 4.2 K. In the whole range, up to 90% antmony content, p‐type crystals can be prepared, allowing the observation of magnetoresistance oscillations in quantizing magnetic fields at least for B ∥︁ c (c trigonal axis). Up to x = 0.6 the results may be explained in the Bi2Te3 ellipsoidal non‐parabolic model for the constant energy surfaces, where the tilt angle of the principal axes increases monotonously with Sb content while the principal mass parameters remain constant. Simultaneously the g‐tensor components vary monotonously with the composition. For x > 0.6 the obtained data on highly doped samples are no longer complete in order to draw reliable conclusions, thouth it is obvious that the Fermi surfaces change their shape. The energy gap between both different valence band maxima, which are occupied at high hole concentrations, is approx imately constant in the whole range of the mixed crystal system.
Theory of the influence of exciton motion on proton relaxation in orgaic solids
Authors: Schwarzer E. , Haken H.
Published: July 2007

The influence of the motion of triplet excitons on the relaxation of protons in organic solids is investigated. The dependence of the longitudinal relaxation rate T1−1 on the Larmor frequency, i.e. on the external magnetic field, is strongly different for different dimensionalities and types of exciton motion. The latter may be coherent (Bloch wave type) or incoherent (diffusive) or in a manner in between as described by the Haken‐Strobl model which is used in this paper. Using this theory the components of the exciton diffusion tensor and the exciton band width can easily be obtained from proton relaxation time measurements.
Elastic response of the fluxoid lattice with planar pinning forces
Authors: Brandt E. H.
Published: July 2007

The elastic displacements of the hexagonal flux line lattice in a type II superconductor are calculated for planar pinning forces. The nonlocal elastic behaviour, derived in a previous paper from the Ginzburg‐Landau theory, leads to strains which may become larger than the homogeneous strain of the local theory by factors up to 2x2b2/(1 − b)2, where b = B/Bc2 is the reduced induction and x is the GL parameter. At b > 0.5 the displacement of the n‐th lattice plane due to a pair of planar forces at n = 0 and at n ≫ 1 is proportional to (1 − b)−2 (−0.7)|n|. This means that the displacements are oscillatory and diverge as b → 1. The elastic response is extremely anisotropic: the maximum strain caused by the same planar force applied in two different directions may differ by a factor of 23. This result is in conflict with the homogeneous and isotropic response predicted by local elasticity theory.
NMR investigations of the melting behaviour of mesogen compounds
Authors: Limmer S. , Grande S. , Lösche A.
Published: July 2007

Proton NMR spectra of mesogen compounds in the solid phase are recorded. Between 20 and 40 K and 4 and 12 K below the melting point they exhibit a very narrow central line with a structure caused by the chemical shift, which is superimposed to the broad structureless line normally expected from polycrystalline solid phases. It is conclulded that it originates from “quasi‐liquid” molecules whose intensity grows drastically with increasing temperature and involves a maximum of about 2.5% of the whole intensity of the spectrum. In the region close to the melting point appears another lineshape, the so‐called “suprer‐Lorentzian” line, whose intensity increases still stronger than that of the narrow line. It can be shown that it is due to the existence of mesomorphic clusters the directors of which are statistically distributed in the sample. The impurity dependence of these effects is investigated and and an attempt is made to explain them.
Interaction of atomic order with itinerant ferromagnetism
Authors: Billard L. , Villemain P. , Chamberod A.
Published: July 2007

The simultaneous occuring of atomic ordering and ferromagnetic ordering is described in a mean field approximation for an alloy where magnetic properties are due to the itinerant character of electrons: thus a generalization of the Stoner‐Wohlfarth model is proposed for the case of nondisordered ferromagnetic alloys. Treating first the non‐magnetic case, when the intra‐atomic correlation parameter U is not too high, a higher ordering temperature is obtained than in the Bragg‐Williams case; considering then the (weak) ferromagnetic case, also an increase of the Curie temperature is found. However, different situations can occur, according to the choice of the model density of states.
The effect of internal strain and electrical field of an impurity on electron–phonon interaction in n‐type germanium and silicon
Authors: Singh M. Dhwaj
Published: July 2007

Considering the effect of internal strains on bound electron‐phonon interaction, one can get an excellent agreement between theory and experiment for n‐type germanium. It is also established that the large decrease in low‐temperature conductivity on n‐type germanium and silicon, caused by an introduction of impurities with concentration ≦ 1017, cm−3, may be explained be explained by scattering of phonons by charge carriers in the effective electric field of the impurity centres for q < 2kF and by a more consistent consideration of the screening of the phonon‐electron interaction in the region of the long‐wavelength phonons for Ziman scattering for q ≦ 2kF.
Energy bands and electron correlation effects in manganese pnictides
Authors: Bärner K.
Published: July 2007

The optical spectra of MnV compounds (V = P, As, Sb, Bi) do not yield unambiguous evidence on the absorption mechanisms beyond that expected from one‐electron excitations in a spin‐polarized energy band description, although the scattering of the conduction electrons indicates strong d‐electron correlations; in particular, since the scattering is comparable to that of s‐electrons on open 3d and 4f shells a decomposition of the 3d4 cation state into extended and quasi‐localized electron states is suggested.
CPA treatment of the extended Hubbard model
Authors: Mizia J.
Published: July 2007

The Hamiltonian for a completely degenerate d‐band including intra‐atomic Coulomb and exchange interactions is studied using the two‐sublattice alloy analogy method. The density of states and the criterion for the appearance of antiferromagnetism are calculated by this method. At the transition from paramagnetism to antiferromagnetism the energy gap appears. The interaction constant required to produce antiferromagnetism tends to zero at the maxima of the state density function.
Chemical binding and energy band structure of vanadium monocarbide
Authors: Mokhracheva L. P. , Tskhai V. A. , Geld P. V.
Published: July 2007

The band structure of vanadium monocarbide is calculated by the tight‐binding method. A set of ten Boch of the 3d, 4s and 2s, 2p states of the free V and C atoms is used as the basis functions. The secular equation is solved at the 89 points of the 1/48th of the Brillouin zone. The denisty of states and partial density of states histograms are obtained. The populations of the orbitals show that about 0.5 electrons transfer from the carbon to the vanadium, orbitals. However, the distribution of the electron charge density points out that there are strong directional metalnon‐mental bonds, in which carbon cannot be considered as a positive ion.
Anisotropic emission of the X‐ray K‐emission band of nitrogen in hexagonal boron nitride
Authors: Tegeler E. , Kosuch N. , Wiech G. , Faessler A.
Published: July 2007

The intensity distribution of the N K‐emission band of hexagonal boron nitride samples with partially oriented crystallites is found to be strongly dependent upon the take‐off angle of the emitted radiation. The obserbed emission bands can be separated unambiguously into a σ‐ and a ‐subband. On the basis of the directional characteristic of radiating dipoles within the layers (σ‐bondings) and perpendicular to the layers (π‐bonding) the angular dependence of the intensity of the subbands is quantitatively explained. In addition, the degree of orientation of the crystallites in the sample can be determined. The intensity distributions of the emission bands to be expected for single crystals and for samples without any texture are determined; in the latter case the results are found to be in good agreement with experiment.
Jahn‐Teller effect on the 3T1(3F) state and resonance of the 3T1(3P) state with the conduction band in CdTe:Ti2+
Authors: Gardavský J. , Barvík I. , Zvára M.
Published: July 2007

Optical absorption spectra of CdTe:Ti2+ with shallow acceptor levels due to cadmium vacancies, exhibit a well resolved triplet structure of the 3T1(3F) band with temperature behaviour suggesting a dynamical Jahn‐Teller effect in the presence of spin–orbit coupling. The photoionization background and the antiresonance band shape of the 3T1(3P) band provide evidence for its location with in the conduction band.
Direct evidence for the singlet‐triplet exciton annihilation in anthracene crystals
Authors: Michel‐Beyerle M. E. , Haberkorn R. , Kinder J. , Seidlitz H.
Published: July 2007

Two‐photon excited singlet excitons are quenched by triplet excitons generated via the monophoton absorption of ruby laser emission. This causes a linear increase of the first‐order decay rate of the fluorescence yielding a singlet—triplet annihilation rate equal to 3 × 10−8 cm3s−1.
Theoretical study of the forbidden ESR transitions of Cr3+ and Fe3+ in alums
Authors: Fårcaş S. I. , Darabont A. , Nicula A.
Published: July 2007

The forbidden fine structure transitions of Cr3+ and Fe3+ ions in alum is explained, using the general theory of the ESR transitions. The calculated transition probabilities are in good agreement with the previously communicated experimental data. The theoretical angular dependence of both, the allowed and forbidden transition probabilities are also presented.
Effects of correlation interaction of electrons with deep holes in X‐ray spectra of metals
Authors: Grebennikov V. I. , Sokolov O. B. , Turov E. A.
Published: July 2007

Absorption spectra and spectra of emission excited by the secondary method are calculated. Two modes of emission excitation are considered: (i) the energy of exciting quanta is great with respect to the inner‐level ionization threshold, (ii) the energy of exciting quanta is continuously filling a small interval (about the width of the valence band) near the threshold. The calculations are carried out for a simple metal model by a one‐site approximation using the formalism of an effective non‐hermitian Hamiltonian which enables one to involve in a natural way into the theory the readjustment effects of a valence system in the field of the emerging hole as well as the finite lifetime of that hole. In the whole frequency interval of the observed values of the spectra the calculation results obtained by the non‐hermitian Hamiltonian method are compared with those obtained on the basis of other methods known earlier. The effect of overlap of the absorption and emission spectra for the threshold mode of excitation is made depending on the relation between the hole lifetime and the conduction electron relaxation time.
Electron localization and statistical mechanics of a zero‐component classical field
Authors: Kumar N. , Heinrichs J.
Published: July 2007

The formal similarity between the exact analytical expressions for the configuration‐averaged propagator for an electron in a random lattice with a Gaussian site energy distribution, and the statistical‐mechanical two point correlation function for a zero component classical field with a quartic self‐interaction is investigated. This enables one to identify the critical point with the mobility edge and thus to obtain the latter as a function of disorder from the recursion relations of the renormalization group. Two distinct situations corresponding to the random potential being real, or imaginary arise naturally in the treatment. The latter case describes the possibility of random absorption or emission of the particle, e.g., by recombination—generation processes. Explicit calculations for the case of an imaginary potential show that the mobility edge moves inward from the ordered band edge and that an Anderson‐like transition occurs when the disorder exceeds a certain value. When the random site potential is real, the mobility edge is not displayed by the treatment, which leads instead to an unphysical fixed point.
Superconductivity originating from quasi‐orbital electrons I. The model of quasi‐orbital conduction electrons
Authors: Krüger E.
Published: July 2007

It will be shown that an adequate description of superconductivity can be given, similar to ferromagnetism, in terms of „quasi‐orbital”︁ conduction electrons, i.e., of conduction electrons occupying repeatedly for a short time localized orbitals and so performing a kind of hopping motion through the crystal. The present theory is based on the observation that the bands of superconducting metals, and only these, yield a distinct type of localized functions. It is shown that quasi‐orbital electrons cannot be described in the framework of the adiabatic approximation when they are assigned to the narrowest bands of the metals. The non‐adiabatic system requires an extension of the operator of the Coulomb interaction. This non‐adiabatic interaction leads to a spin‐phonon interaction for the bands existing in superconductors.
Surface polariton effects in spontaneous Emission II. Effects of spatial dispersion
Authors: Agarwal G. S. , Vollmer H. D.
Published: July 2007

The radiation characteristics of atoms or molecules placed close to the dielectric surface are studied with full account of the spatial dispersion of the dielectric. Both the cases of plasmon polariton and the exciton polaritons are treated. The appropriate response functions are calculated and used to derive an analytic expression for the widtn of the excited state. The numerical results show that spatial dispersion reduces considerably the width from that one in the absence of spatial dispersion. It is found that spatial dispersion effects become more pronounced as the distance of the atom from the surface decreases.
Inelastic incoherent neutron scattering spectra at different temperatures and computer experiment for external phonon modes of naphthalene crystals
Authors: Bokhenkov E. L. , Rodina E. M. , Sheka E. F. , Natkaniec I.
Published: July 2007

Inelastic incoherent neutron scattering spectra are given from naphthalene crystals at 4.7, 80, and 296 K in the region of external phonon modes. The formulation of a computer experiment to obtain information involved in experimental spectra is analyzed and the advantages of the direct spectral problem as a method of solution are shown. The solving of the problem is carried out in the harmonic and quasi‐harmonic approximations. The results are used to explain the observed temperature changes in the scattering spectra. Computer calculation sensitivity to structural data and parameters of interatomic potential is revealed. Parameter dependence of the crystal phonon density of states on temperature is established.
A new concept of the model for the paramagnetic E′‐centre in the SiO2−x system
Authors: Shendrik A. V. , Yudin D. M.
Published: July 2007

The main paramagnetic centres radiation‐induced in vitreous and crystalline silica are the E′‐centres. Essential basis for this model is the assumption that the unpaired spin is located on the sp3 hybrid orbital of a silicon atom oriented to the O‐vacancy. This assumption is based on the detection of hyperfine structure line doublet with a splitting of 400 G. The intense spectrum of this doublet is observed on samples of silica unenriched with isotope 29Si, but containing hydrogen. From the dependence of the E′signal on the preparation conditions and the samples treatment an association of the doublet not with the 29Si nuclei but with protons is suggested. A model for the E′‐centre is proposed, where the unpaired spin is located on the molecular orbital bonding three ligand oxygens, but not on the sp3 hybrid orbital of silicon.
Electronic ordering in crystals
Authors: Ionova G. V. , Makarov E. F. , Pachev O. M. , Ionov S. P.
Published: July 2007

The charge and orbital ordering are studied in crystals with doubly degenerate or quasi‐degenerate orbitals. A model Hamiltonian is used including the intercentre electron transfer, the intracentre and intercentre Coulomb, and exchange interactions. To obtain the eigenvalues of the Hamiltonian the Zubarev double time Green function technique is applied. In the disordered state a mixing of bands takes place owing to electron—hole correlations. The conditions for the occurrence of orbital and charge ordering are obtained.
Icosahedral microclusters a possible structural unit in amorphous metals
Authors: Briant C. L. , Burton J. J.
Published: July 2007

A model for amorphous metals is proposed. The model is based on the 13 atom icosahedron which is the minimum energy structure for a 13 atom cluster. An amorphous metal is considered as containing a high density of these structural units. The material surrounding the icosahedra is partially ordered. The spaces between the ordered regions may be disordered. This model successfully accounts for the experimental interference functions in amorphous materials. It is explained, why, physically, such a structure should be expected to form. It is also shown that structures such as that proposed are observed when, in a computer experiment, a liquid is quenched very rapidly into the solid phase.
Non‐linear optical properties of the band charge carrier gas
Authors: Fomin V. M. , Pokatilov E. P.
Published: July 2007

A theory of non‐linear optical properties of a band charge carrier gas in semiconductors due to the influnece of a strong electromagnetic radiation on the probability of the carrier scattering by phonons or ionized impurities leading to intra‐band transitions is developed. A general expression for the polarization created by the carrier response in a laser field in the presence of a magnetic field and other electromagnetic fields (in particular, of a probe wave) is obtained at arbitrary temperatures using the path‐integral version of the density matrix method. The polarization spectrum is investigated. A self‐consistent set of wave equations is obtained to describe the propagation of electromagnetic waves, both, of those ones whose frequencies are determined by external sources and of those ones which are generated in the non‐linear medium under consideration. The explicit analytical expressions for a generalized dielectric permeability tensor and a generalized generation tensor are obtained for all principal scattering mechanisms and an arbitrary laser field intensity. The macroscopic (observable) manifestations of the polarization non‐linearity — the effects of the dependence of the absorption coefficient an the refractive index the intensity — are studied. The generation of harmonics and mixed frequencies is described; the influence of the generation on the laser wave propagation is investigated and found to be significant.
Excitons in gallium sulphide
Authors: Belenkii G. L. , Godzhaev M. O.
Published: July 2007

Strong evidences for existence of indirect excitons in GaS are obtained as a result of an analysis of differential absorption and photoluminescence spectra registered in the 4.2 to 100 K temperature range in polarized light. The location of the exciton band in gallium sulphide Eexc = (2594 ± 2) meV and the energy of phonons which take part in indirect transitions are determined. Lines which are interpreted as a result of non‐phonon transitions are observed in the absorption and emission spectra. Lines which arise from the decomposition of bound excitons are found in emission spectra as well. It is shown that polarization pecularities of GaS absorption and luminescence spectra are explained in terms of the band scheme of gallium selenide which is structurally analogous to GaS. The possible nature of the anisotropy of excitons in GaS is discussed.
Superconductivity originating from quasi‐orbital electrons II. The superconducting ground state of quasi‐orbital conduction electrons
Authors: Krüger E.
Published: July 2007

It is shown that the electrons of a certain narrow half‐filled band couple to Cooper pairs below a transition temperature Tc if these electrons behave similar to orbital electrons. This transition is nearly the same as that described by the BCS theory. However, the coupling of the electrons to Cooper pairs is constrained in a new manner by quasi‐classical constraining forces. A comparison of band structure calculations with superconducting properties of metals suggests that quasiclassical constraining forces must exist in any superconducting state. As a result, only those metals become superconducting which have a narrow half‐filled band of a certain symmetry type in their band structure.