bluuebunny/crossref_metadata_2025_split huggingface dataset:
Reduction Symmetry Method in Crystal Field Theory (III)
Authors: Kustov E. F. 
Published: July 2007

AbstractThe commutators of tensor operators of the point groups G are introduced and tensor operators {kk′} are regarded as infinitesimal operators of the unitary groups U[γ]γ. The classification of the eigenfunction and symmetry properties of tensor operators are developed by using the following reduction series U4l+2 ⊃ U × U × … ⊃ (SU2 × U) × (SU2 × U) × … ⊃ (SU2 × G) (SU2 × G) × … ⊃ SU2 × G. The main formulas of this analysis are given for the matrix elements of the tensor operators.
Molecular Exciton Cluster States and Spectra with Application to Benzene
Authors: Hoshen J. , Kopelman R. 
Published: July 2007

AbstractA cluster expansion is introduced for the determination of the optical absorption properties of cluster states in substitutionally disordered molecular crystals. This method is based on the Green's function expansion in terms of the localized cluster states in the energy region of the guest sub‐band. The general features of the method are illustrated by calculations for a simple two‐dimensional molecular crystal. The physical parameters for the calculation are based on the v11 benzene vibrational exciton. Both polarized and unpolarized cluster optical absorptions display strong asymmetries, with respect to the isolated guest spectral line. Calculations are performed on the monomer, dimer, trimer, and tetramers (three kinds) of the benzene crystal ac‐plane.
A Method to Derive Interatomic Potentials in Metals from Experimental Phonon Spectra
Authors: Brosens F. , Cornelis J. , Wallace D. C. , Devreese J. T. 
Published: July 2007

AbstractIn the central potential approximation, the first and second derivative of the two‐body potential for metals in the atom positions can be calculated by inverting the eigenvalue equations for the phonons. The potential is obtained from a straightforward integration procedure. Explicit results for Na are reported, which fully reproduce the experimental phonon spectrum.
On the Excited States of the T1+ Monomer Centre in NaI
Authors: Benci S. , Fontana M. P. , Manfredi M. 
Published: July 2007

AbstractThe Ax emission band in NaI crystals containing the TI monomer centre is studied in the temperature range 25 to 295 K using a lifetime technique after pulsed excitation in A, B, C, and D absorption bands. The results show that the decay signal is complex and consists of three different slopes better resolved with A and D excitation than with B and C excitation. It is possible to conclude that at least two radiative levels contribute to the Ax emission. The measured behaviour of the intensity of the different components is discussed taking into account also the available data of integrated emission.
A Theory of Dislocation Dynamics with Internal Stresses
Authors: Gradowczyk M. H. , Rojas F. A. 
Published: July 2007

AbstractThe present theory completes a previous work by extending Kröner's concept of incompatibility of crystal deformation to dislocation sources. For a continuous distribution of moving dislocations, differential equations are derived that describe crystal flow in terms of the total stress tensor, particle velocity vector, flux‐density vector, a plastic strain tensor produced by the dislocations created by dislocation sources, and the necessary constitutive equations. This new theory allows for the explicit computation of the internal stress field produced by the motion and creation of dislocation fields when some additional assumptions are introduced.
The Electronic Spectra of Charge‐Ordered Crystals
Authors: Ionov S. P. , Lubimov V. S. , Manakova L. A. , Makarov E. F. 
Published: July 2007

AbstractThe electronic excitation spectra for an isotropic charge‐ordered crystal are analysed on the basis of a band model. It is found that the charge‐ordered state is unstable relative to a transition into a state with pair correlation when the intra‐band attraction among the electrons exceeds a certain critical value which is a function of the kinetic energy and interaction parameters. In this situation a state is realized in which the charge order and the pair correlation are coexistent and which is possible at a finite value of the band width.
The Crystal Potential and Summation of Diverging Series
Authors: Tupizin I. I. , Abarenkov I. v. 
Published: July 2007

AbstractAn 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

AbstractA 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

AbstractRaman 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

AbstractThe 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

AbstractThe 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

AbstractVarious 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

AbstractThe 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

AbstractThe 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.
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

AbstractKorringa'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

AbstractThe 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

AbstractMeasurements 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

AbstractExcept 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

AbstractCoherent 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

AbstractThe 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

AbstractThe 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

AbstractUtilizing 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

AbstractNumerical 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

AbstractA 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

AbstractInfrared 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

AbstractThe 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

AbstractL3‐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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractAn 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.
EPR of U5+ in alkali fluorides with a new charge compensation mechanism
Authors: Lupei V. , Lupei A. , Georgescu S. 
Published: July 2007

AbstractA 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

AbstractThe 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.