bluuebunny/crossref_metadata_2025_split huggingface dataset:
Energy band calculations on helical systems
Authors: Blumen A. , Merkel C. 
Published: July 2007

AbstractThe electronic band structure of systems with screw symmetry is developed using the smallest screw displacement as elementary operation. For atomic and molecular helical systems the resulting Hartree‐Fock SCF‐LCAO formulae are presented. Results for ferroelectric HC1 and HP are reported, obtained with the MUNICH‐CHAIN programme, in which the new formalism is implemented. The connection to the translational symmetry and the numerical results are discussed.
Recombination in Pb0.83Sn0.17Te at high levels of optical excitation
Authors: Hoai T. X. , Herrmann K. H. 
Published: July 2007

AbstractThe photoconductivity and the photomagnetoelectric current in low‐concentration, high‐lifetime Pb0.83Sn0.17Te samples are measured at strong optical excitation using a Q‐switched ruby laser. At quantum flux Q > 2 × 1021 cm−2s−1 nonlinear recombination is observed. From the characteristics measured the coefficient of Auger recombination C = (4.5 ± 1) × 10−27 cm6s−1 is obtained. The results are confirmed by lifetime data obtained at low excitation levels.
Many‐body effects on the second‐order elastic constants of solids with ZnS structure
Authors: Goyal S. C. , Peakash R. 
Published: July 2007

AbstractThe method of homogeneous deformation is used to derive the expressions for the elastic constants of solids crystallising in ZnS structure using the Lundqvist potential for the ionic cohesion. The derived expressions are utilised to explain several physical properties of the large number of semiconductor solids.
Phase transformations in hydrides of rare‐earth metals under pressure
Authors: Bashkin I. O. , Ponyatovskii E. G. , Kost M. E. 
Published: July 2007

AbstractAn analysis of the specific volume dependences on atomic number is carried out for rare earth dihydrides and trihydrides. It is shown that under high pressures the following phase transformations are possible in these compounds: a) structural phase transitions h.c.p.→f.c.c., the ion valency being unchanged, b) phase transitions accompanied by a change of metal ion valency. The dependences of volume versus pressure to 24 kbar and resistivity versus pressure to 20 kbar are measured for NdH2.82 at 20 °C. A reversible resistivity anomaly and an abrupt change of the volume by 1.6% are detected under high pressures. These anomalies are explained to be due to the phase transformation h.c.p.⇄f.c.c.
Modes of cross‐relaxation in nuclear double‐resonance
Authors: Whitaker M. A. B. 
Published: July 2007

AbstractThe modes of cross‐relaxation of a double‐resonance system are discussed, and calculated for a number of simple cases. The action of the dipolar reservoirs in inhibiting the equilibration of the Zeeman reservoirs is described. The behaviour of the system may be interpreted in terms of maximisation of entropy. Numerical results are obtained for the case of double‐resonance in sodium fluoride with the Zeeman field in the (111) direction.
Phonon frequency, electrical resistivity, and inter‐ionic pair potential of Li
Authors: Singh R. N. , Srivastava P. L. , Mitra N. R. 
Published: July 2007

AbstractThe effect of the form of model potential and different forms of exchange and correlation on the phonon frequency, liquid electrical resistivity, and inter‐ionic pair potential is investigated. The metallic properties are found to be sensibly affected with the choice of the form of the model potential. It is concluded that an r‐dependent form of the model potential in the inner region of the ion may be important for a good reproduction of the metallic properties.
The electronic spectra and electromagnetic properties of charge‐ordered systems
Authors: Manakova L. A. , Makarov E. F. , Ionov S. P. 
Published: July 2007

AbstractThe spectrum of collective excitations is found for charge‐ordered systems with a one‐electron gap far less than the band width. The contribution of these excitations to electromagnetic wave absorption is considered. The complex impedance of the charge‐ordered state is calculated.
Three‐phonon scattering relaxation rate and phonon conductivity. Application to Mg2Ge
Authors: Dubey K. S. , Misho R. H. 
Published: July 2007

AbstractThe new expression τ = g(ω) (BN + BUe−θ/αT) Tm is proposed for the three‐phonon scattering relaxation rate taking into consideration normal and umklapp processes. Incorporating the above expression, the phonon conductivity of Mg2Ge is calculated in the entire temperature range 4 to 1000 K and good agreement is found between calculated and experimental values of phonon conductivity of the sample in the entire temperature range of investigation. Analytical expressions are reported to calculate an approximate value of lattice thermal conductivity. The role of four‐phonon processes is also included in the present study. Using the expression for τ proposed by Joshi and Verma and Sharma et al. the phonon conductivity of InSB and Mg2Ge is calculated to compare the results obtained using the proposed expression for the three‐phonon scattering relaxation rate.
Generalization of the heisenberg hamiltonian for the case of an atomic system with a variable atomic spin value
Authors: Popov V. A. , Loginov A. A. 
Published: July 2007

AbstractThe representation invariant relative to spin rotations of the Hamiltonian of electrostatic electron interactions is derived for a dielectric. The temperature dependence of the exciton spectrum and that of the value of the site spin moment, 〈S〉, due to exciton‐magnon interactions are considered.
Spectroscopic investigation of orientational disorder in an acenaphthylene molecular crystal
Authors: Korets A. Ya. , Fedorov V. P. , Korshunov A. V. , Naumov V. A. 
Published: July 2007

AbstractThe electronic absorption spectra of an acenaphthylene single crystal are obtained in polarized light at 4.2 to 300 K. The temperature dependence of band width, maximum and integral intensity of the lowest singlet‐singlet transition at v = 21762 cm−1 are discussed. Both the band broadening of the exciton doublet with va = 21260 cm−1 and vb = 21280 cm−1 and its intensity decrease are shown to be due to a change of orientational disorder, when approaching the orientational phase transition at T = 130 K. The constancy of Davydov splitting Δ = 20 cm−1 with temperature is associated with inter‐ordering of two sets of translationally non‐equivalent acenaphthylene molecules, which does not influence the value of resonance interaction between the pair molecules.
Calculation of a resonant model potential for copper, silver, and gold
Authors: Dagens L. 
Published: July 2007

AbstractThe ionic resonant model potential (RMP) v0(E) = w0(E) + vres(E) for d‐band metals considered previously is slightly generalized in order to take into account the energy dependence and nonlocality of the s and p scattering which were previously neglected. The RMP v0 is written as w0(E) + + vres(E) where w0 is a general Heine‐Abarenkov‐like potential and vres is a finite‐rank operator which acts on d states and is strongly energy‐dependent. The RMP parameters appropriate for the metallic state are calculated from first principles by requiring that the screened metal RMP \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \sum \limits_{ions} {\rm}v_0 (E){\rm +}V_e $\end{document} is equivalent to the true HFS self‐consistent potential. The calculation is done using a Wigner‐Seitz spherical approximation, consistently for the true potential and the model potential. The method is applied to the three noble metals. The results are: (i) the enrgy dependence of the s and p potentials is significant; (ii) the s–p non‐locality is large; (iii) the d potential vres has indeed the resonant form U/(E − ℰ(E)) with ℰ(E) only slightly energy‐dependent. A simpler model potential is then derived (ℰ(E) is replaced by its Fermi level value; the energy dependence of w0(E) is only approximately taken into account through an effective mass approximation) and applied to the calculation of a few illustrative physical quantities. The results are satisfactory, at least for copper and silver, but some physical quantities (such as the elastic constants calculated up to second order according to the long‐wave method) are found to be sensitive to the value of the somewhat arbitrary model radius Rm (the best results are obtained for the largest Rm satisfying the non‐overlap condition).
Absorption and luminescence of V (d3) in II–VI compound semiconductors
Authors: Hoang Le Manh , Baranowski J. M. 
Published: July 2007

AbstractThe absorption and luminescence spectra of the V (d3) in ZnS, ZnSe, CdSe, and CdTe are presented. Values of the crystal‐field parameters Δ, B, and C are derived from the spectra and discussed. The infrared luminescence at about 5000 cm−1, characteristic for the V (d3) impurity in II–VI and III–V compound semiconductors, is shown to be due to the 4A2(F)‐4T2(F) transition.
Spin–orbit‐allowed tunneling splitting in the T × εg Jahn‐Teller problem splitting of the zero‐phonon line
Authors: Ranfagni A. , Viliani G. 
Published: July 2007

AbstractAn orbital triplet state T in a lattice of cubic symmetry (Oh), which by the Jahn‐Teller effect is coupled to tetragonal vibrational modes (εg), is considered. The tunneling splitting in this T × εg, Jahn‐Teller problem with spin‐orbit interaction is computed in Ham's framework and by the WKB approximation in several cases. The results are compared to zero‐phonon line splitting of V2+ impurities in MgO and KMgF3.
Raman spectrum and dielectric relaxation in K4Fe(CN)6 · 3 H2O
Authors: Savatinova I. , Anachkova E. 
Published: July 2007

AbstractThe temperature dependence of the two Raman active modes at 62 and 126 cm−1 corresponding to the symmetry Bg/A″ (z) is investigated. In the vicinity of the phase transition point (TC = 249 K) the mode bandwidths undergo a sharp increase which is assigned to the reduction of the barrier height characterizing the water Brownian motion. A semi‐empirical expression for the temperature dependence of the barrier height is given. Mason's relaxation theory is used to correlate the temperature behaviour of the barrier with the dielectric constant of KFCT as function of temperature and frequency.
Spin–lattice relaxation in Fe3+‐doped andalusite crystals II. The effect of lattice oscillator heating
Authors: Korolev V. D. , Peskovatskii S. A. , Shulga V. M. 
Published: July 2007

AbstractThe phonon bottleneck effect of Fe3+ ions in andalusite crystals is investigated at temperatures T < 0.3 K. In concentrated crystals (c = 0.56%) heating of the phonons within a frequency range exceeding the ESR line width is revealed. An analysis of the influence of the lattice oscillator heating on the spin–lattice relaxation rate is made.
The mean field phase diagrams of YbIG
Authors: Loos J. 
Published: July 2007

AbstractThe existence of the symmetry allowed magnetic phases in ferrimagnetic YbIG is discussed. The continuous and quasi‐continous transformations of these phases are treated using the thermodynamic potential expansions whose coefficients are determined from the well known effective spin Hamiltonian of Yb ions. The special cases with the magnetic field H along the <100>, <111> and <110> symmetry directions are dealt with. The existence of a low symmetry phase connected with the occurrence of a tetracritical point is stated for H ∥ <110>. The collinear‐canted phase transition is of the first order for H ∥ <111> but at high fields quasi‐continuous with a very narrow metastability region. An isolated critical point, in which three first‐order and two second‐order transition lines meet, is found on this first‐order phase boundary. The connection with the Potts model is outlined. The calculated phase diagrams are compared with experimental results.
A Raman and far‐infrared investigation of phonons in the rhombohedral V2–VI3 compounds Bi2Te3, Bi2Se3, Sb2Te3 and Bi2(Te1−xSex)3 (0 < x < 1), (Bi1−ySby)2Te3 (0 < y < 1)
Authors: Richter W. , Becker C. R. 
Published: July 2007

AbstractThe Raman‐active lattice vibrations of Bi2Se3, Bi2Te3, Sb2Te3, and their solid slutions, whose symmetries correspond to the R3m space group, are investigated by Raman scattering. Three of the four expected Raman modes, Eg and A1g, could be determined. The FIR optical properties of Bi2Te3 crystal surfaces of improved quality (E ∥ c and E ⟂ c) and Bi2Se3 (E ⟂ c) are re‐examined near helium and room temperature with a Fourier spectrometer, allowing a determination of the infrared‐active mode frequencies with higher accuracy. The results for Bi2Te3 are compared to the predicted frequencies from the lattice dynamical model, given by Jenkins et al. The frequncy shifts of the Raman‐active modes in the mixed crystals show single‐mode and two‐mode behaviour, which is in agreement with simple models for the substitution of antimony and selenium atoms for bismuth and tellurium, respectively, in Bi2Te3.
Drag effects in a heated electron–phonon system. N‐germanium and n‐silicon
Authors: Kocevar P. 
Published: July 2007

AbstractA linearized version of a recently developed theory of phonon amplification by hot electrons in high‐mobility semiconductors is used to estimate mutual electron–phonon drag and heating effects in materials of lower mobility with dominant acoustic deformation potential and ionized impurity scattering. Accordingly a weakly drifting heated Maxwell distribution is assumed for carriers with quadratic and isotropic energy dispersion. Simple closed expressions are found for the steady‐state mobility, showing explicitly how mutual drag effects (increasing the mobility) dominate at lower electric fields, whereas the effects of phonon heating (lowering the mobility) dominate at fields for which the electron temperature is more than twice the lattice temperature. The results are applied to n‐germanium.
Vibration modes of split screw dislocations in b.c.c. metals
Authors: Kroupa F. 
Published: July 2007

AbstractInternal vibrations of the core of a crew dislocation in a b.c.c. lattice are studied using a simplified model of dislocation splitting on three planes into four partials. While the eigenfrequency of the optical mode is very high, of the order of 1012 s−1, the eigenfrequency of the acoustic mode strongly decreases with decreasing Burgers vector of the central partial. It is proposed that the acoustic mode can be detected from internal friction measurements or from the influence of ultrasound on the yield point of b.c.c. metals at low temperatures.
On the heat capacity of Fe(HCOO)2·2 H2O
Authors: Gochev I. G. , Grozdev K. I. 
Published: July 2007

AbstractThe anomalous long high temperature tail of the magnetic part of the heat capacity of ferrous formate dihydrate is considered. A theoretical model for the explanation of such heat capacity behaviour is proposed. Quantitative consistence with experimental data is obtained.
Concentration of “impurity” ion clusters in a two‐dimensional host lattice
Authors: Trutia A. , Iova F. , Vodâ M. 
Published: July 2007

AbstractThe concentrations of magnetically active ions in clusters occurring in a diamagnetic two‐dimensional host lattice are statistically calculated. This is done for an initial, total active ion concentration ranging from 1 to 100 mol% and for all geometrically possible cluster types from singlets to septets, 449 in all. The model taken into consideration is a real Z(M)X2‐type crystal, where it is presumed that a direct exchange intra‐layer is the only important interaction (no superexchange). These calculations are made in order to provide a basis for understanding the nature of the concentration‐sensitive optical absorption bands and magnetic behaviour of some crystals containing transition metal ions.
Electron energy spectrum of a finite crystals in a uniform electric field
Authors: Berezhkovskii A. M. , Ovchinnikov A. A. 
Published: July 2007

AbstractAn explicit expression for the shift of levels of Wannier‐Stark ladders, which appears due to finite crystal dimensions, is derived. The surface states are analyzed. It is concluded that Wannier‐Stark ladders do exist if crystal dimensions are sufficiently large.
Density of states and itinerant ferromagnetism in amorphous systems with dense random packed hard sphere structure
Authors: Richter J. , Kobe S. 
Published: July 2007

AbstractItinerant ferromagnetism in amorphous systems is investigated. Both, the lattice model and the model of dense random packing of hard spheres (DRPHS) are used to simulate the amorphous structure, for the last one the conncective topology is discussed. The density of states of electrons is calculated using a continued fraction method. The influence of off‐diagonal disorder on density of states and stability of ferromagnetism is considered, where qualitatively similar tendencies are obtained for lattice model and DRPHS model.
Angular dependence of the reflection spectra and directional dispersion of the anisotropic exciton polaritons in ZnO
Authors: Hümmer K. , Gebhardt P. 
Published: July 2007

AbstractThe reflection spectra of the ordinary and extraordinary polaritons near the A1‐, B1‐exciton resonances of ZnO are presented for two geometries: normal reflection of planes inclined to the c‐axis at varying degrees, and oblique reflection of as‐grown (10I10) faces. The experimental and theoretical spectra are compared. The calculations are based on a) the dispersion relations derived from Fresnel's equation taking account of spatial dispersion, b) the Pekar additional boundary conditions combined with an exciton free surface layer. There are indications of the correlation between the thickness of this layer and decreasing oscillator strength of the polaritons.
Lattice vibrations of Bi2Te3
Authors: Wagner V. , Dolling G. , Powell B. M. , Landweher G. 
Published: July 2007

AbstractThe dispersion curves for phonons propagating along the trigonal axis of the layer‐structure emiconductor Bi2Te3 at 77 K have been determined by coherent inelastic neutron scattering techniques. Analysis of these results in terms of a simple interplanar force model shows a) that the forces between adjacent planes of Te atoms are comparable to those between nearest neighbour planes of Bi and Te and b) that foreces between next‐nearest‐neighbour planes have to be considered. In addition, a limited number of phonons were measured at 300 K and some low‐frequency modes propagating along the bisectrix within the planes were determind. The results are compared, where possible, with previous Raman scattering, infrared reflection, and elastic constant measurements.
Photoelectron measurements of α‐phase alloys CuZn, CuGa, and CuGe
Authors: Norris C. , Williams G. P. 
Published: July 2007

AbstractPhotoelectron measurements using 21.2 and 40.8 eV photon energies with α‐phase Cu0.83Ga0.17 and Cu0.87Ge0.13 alloys and several α‐phase CuZn alloys are reported. No clear evidence of a virtual bound or split‐off impurity level at the bottom of the matrix band is found. Comparison is made between the results of published calculations and the measured valence spectrum of CuZn. Core level shifts as determined with AlKα (1486.6 eV) radiation indicate a transfer of charge to the Cu sites takes place as multivalent impurity atoms are added.
Scattering and decay of hot excitons interacting with longitudinal optical phonons
Authors: Aristova K. A. , Giner C. Trallero , Lang I. G. , Pavlov S. T. 
Published: July 2007

AbstractScattering and decay probabilities are obtained for a „hot”︁ Wannier‐Mott exciton of arbitrary kinetic energy which interacts with the longitudinal polarizational lattice vibrations (LO‐phonons). All computations are performed for three different values of the ratio m2/m1, where m1(m2) is the electron (hole) effective mass. It is shown for the case of quite different electron and hole effective masses that the scattering and decay probabilities are essentially comparable. These results show a theoretical evidence in favour of the existence of hot excitons.
Multiple phase transitions in disordering systems
Authors: Kharkats Yu. I. 
Published: July 2007

AbstractThe possibility of realization of multiple first‐order phase transitions in disordering systems is studied. It is shown that for a rather loosely defined law of attraction interaction, pairs of genetically related transitions, or bitransitions, should occur in both symmetrical and asymmetrical disordering systems. Critical conditions for bi‐transition origination and possible schemes of multiple phase transition occurrence are studied. The widely used law of short‐range pair interaction, f(x) = x2/2 (where x represents a long‐range parameter), is shown to feature a specific degeneration owing to which there are no multiple phase transitions in symmetrical systems with this law.
Raman scattering in some III‐VI layer single crystals
Authors: Faradev F. E. , Gasanly N. M. , Mavrin B. N. , Melnik N. N. 
Published: July 2007

AbstractPolarized Raman spectra of InSe and InS crystals are obtained, the assignment of frequencies is given, and atomic displacements are found at all normal vibrations. In the InSe crystal, force constants of interaction of In‐Se and In‐In atomic planes are evaluated. It is shown that the Raman spectrum of InS has a doublet nature typical for layer crystals. The frequencies of rigid‐layer modes forbidden in the Raman spectrum are calculated in the InS crystal. It is found that interlayer force constants in the InS crystal are only 5 to 10 times smaller than the intralayer covalentbond force constants.
Temperature shift of the maximum of neutron critical scattering in the second born approximation
Authors: Mrygoń B. , Urbaniak A. 
Published: July 2007

AbstractTwo approximate forms of the cross‐section obtained in the second Born approximation are discussed with respect to the temperature shift of the main maximum of critically scattered neutrons in ferromagnets. The disscusion is given for the correlation of the Ornstein‐Zernike type and the correlation function reflecting the lattice symmetry of a simple cubic crystal. It is shown that the temperature shift depends on the momentum transfer vector q, the diffusion coefficient D, and the initial wave vector k0.
On the electronic absorption and amplification of sound in semiconductors at “intermediate” frequencies
Authors: Gurlyaev Yu. V. , Ivanov S. N. , Kozorezov A. G. 
Published: July 2007

AbstractOn the basis of the general theory developed earlier of electronic absorption (EAb) and amplification (EAm) of sound (acoustic waves), which is valid, in particular, at the so‐called „intermediate”︁ frequencies ql ≈︁ 1 (q wave number, l mean free path of electrons), explicit expressions are obtained for the coefficients of EAb and EAm of sound for various mechanisms of electron momentum scattering and for arbitrary degree of degeneracy of the electron gas. With the help of these expressions the large number of published experimental data are analysed concerning the measurements of EAb and EAm of sound in the wide frequency range including intermediate frequencies. In all cases quite satisfactory agreement between theory and experiment is reached without using any „matching”︁ parameters.
On the dynamics of electrons in the strong‐coupling Hubbard chain
Authors: Jelitto R. J. , Heise M. 
Published: July 2007

AbstractThe dynamics of electrons is investigated in a Hubbard chain in the strong‐coupling limit. This problem may be figured as the motion of a double‐occupancy (consisting of one spin‐up and one spin‐down electron fixed at the same lattice site) on a linear chain, all other sites of which are occupied by single electrons of arbitrary spin orientation. The spectrum of this problem is derived exactly and from it the partition function is calculated. It agrees with the result derived by Beni and coworkers formerly with the aid of thermodynamic perturbation theory.