bluuebunny/crossref_metadata_2025_split huggingface dataset:
Temperature and hydrostatic pressure measurements of the exciton peaks in BiI3 and SbI3
Authors: Brothers A. D. , Wieliczka D. M. 
Published: July 2007

AbstractExciton peak positions are measured as a function of temperature in the range 90 to 270 K and as a function of hydrostatic pressure to 3500 atm in thin film samples of BiI3 and SbI3. Values for (∂E/∂P)T are −8.6 × 10―4 eV/atm and −32 × 10―4 eV/atm for BiI3 and SbI3, respectively. (∂E/×T)P for BiI3 varies from −0.49 × 10―4 eV/K near 100 K to +0.56 × 10―4 eV/K near 200 K, and for SbI3 the value is found to be +7.3 × 10+4 eV/K. Estimates of the electronphonon coefficient, (∂E/∂T)V, are made. Comments are made concerning the nature of the excitons and the band structure in these materials.
Calculation of nuclear quadrupolar relaxation in liquid metals
Authors: Halder N. C. 
Published: July 2007

AbstractThe nuclear quadrupolar relaxation (NQR) rate W1q in liquid metals, in 69Ga, 115In and 201Hg is calculated by using the Warren theory. In order to do this, the experimental static interference function I(K) is considered and the dynamic interference function I(K, 0) is generated in the zero frequency limit. W1q is calculated in the temperature range 20 to 150 °C for 69Ga, 170 to 650 °C for 115In, and ‐‐‐35 to 80 °C for 201Hg. It is found that in all the three cases, the temperature dependence of the calculated W1q is in very good agreement with that of the experimenta W1q, but numerically the magnitudes of W1q for 115In and 201Hg deviate considerably from the experimental values. It is concluded that the antishielding factors used may be in error and that the present theory is superior to the earlier classical diffusion theory.
On the theory of thermal conductivity in quantum crystals
Authors: Pushkarov D. I. 
Published: July 2007

AbstractThe influence of defects on the specific heat and thermal conductivity of quantum crystals is considered. As a rule, the presence of defects decreases the thermal conductivity due to phonon (or electron) scattering. However, at sufficiently low temperatures the point defects turn into quasi‐particle defectons which can transport heat by themselves. The defecton thermal conductivity may be significant, due to the other temperature dependencies obtained in this work. Defecton statistic effects in the low‐temperature heat capacity and thermal conductivity are considered, and analytic formulae for all interesting cases are obtained.
Reaction field and dipolar interaction in quasi‐onedimensional heisenberg systems. Application to CsNiF3
Authors: Scherer C. , Barjhoux Y. 
Published: July 2007

AbstractAn improved mean field theory, the reaction field approximation, which was previously used for a Heisenberg paramagnet, is extended to include the magnetic dipolar interaction. It is shown that in spin chains, when the intra‐chain interaction is ferromagnetic, the dipolar inter‐chain interaction plays an important role in explaining the phase transition to an antiferromagnetic three‐dimensional order at low temperature. A study of the magnetic susceptibility of CsNiF3 is presented as an explicit example. Values for the intra‐chain J and inter‐chain J' exchange parameters are derived from TN and χ⊥(TN). The temperature dependence of χ⊥ is very well reproduced by the model.
Field‐induced phase transitions in a randomly diluted antiferromagnet. Mean‐field description of classical spin model
Authors: Moreira F. G. Brady , Fittipaldi I. P. , Rezende S. M. , Tahir‐Kheli R. A. , Žekš B. 
Published: July 2007

AbstractWithin a mean‐field virtual crystal approximation, a phase diagram is given in the H–T plane of a randomly diluted, uniaxial Heisenberg antiferromagnet with classical spins and single ion anisotropy. The temperature dependence of the critical fields, separating the antiferromagnet (af), the spin‐flop (sf), and the paramagnetic (pm) phases are analyzed as a function of the dilution. For systems where the anisotropy is of crystal‐field origin and where the non‐magnetic impurity does not perturb the crystal field, the phase boundaries show an interesting dependence on the dilution. On the other hand, in systems such as (Mn)x(Zn)1‐xF2, where the anisotropy is due to two particle interactions, such as dipolar forces, the present theory would predict only a linear scaling of the phase diagram with magnetic dilution. Thus, the study of magnetically diluted systems would seem to offer a possible method of characterizing the anisotropy.
Thermal Reorientation Kinetics of I in RbI
Authors: Delbecq C. J. , Stokes J. P. , Yuster P. H. 
Published: July 2007

AbstractExperimental studies of the thermally activated reorientation of I in RbI: AgI single crystals are carried out by measuring the decay of the optical anisotropy of preferentially oriented I at fixed temperatures. These studies show that in the temperature range investigated, contrary to previous results by others, thermally induced changes in orientation of I in RbI result almost totally from elementary jumps in which the orientation of the I molecule ion changes by 60°. The activation energy for this 60° jump is determined to be (0.306 ± 0.005) eV.
Surface Density of States in a Simple Model of Nickel
Authors: Kolář M. 
Published: July 2007

AbstractA simplified model of a semi‐infinite transition metal crystal is investigated. The changes at the crystal surface are represented only by the reduction of the number of nearest neighbours. The surface density of states is calculated, which includes the deviations from the bulk behaviour in all atomic layers. In the investigated model these deviations can be summed explicitly. The results indicate that also the further layers behind the surface one may have appreciable influence on the surface quantities.
Bond Orbital Model for Tetrahedral Semiconductors on the Base of Molecular Parameters. Covalent Crystals and Perturbative Treatment of Partially Ionic Semiconductors
Authors: Dyachkov P. N. , Levin A. A. 
Published: July 2007

AbstractA new version of the BOM is proposed on the base of the CNDO procedure. The standard values of the molecular integrals describing a wide range of molecular systems are used. The method allows to calculate the valence band structure, some features of the interband transitions, and the cohesive energies of crystals in accordance with the experiment and with no adjustable parameters. Correlations between the bond orders in the isoelectronic series and some properties of the semiconductors such as the bond lengths, cohesive energies, and the hole effective masses are obtained.
Theory of the Excitonic Mechanism of Crystal Defect Formation
Authors: Anselm A. I. , Lang I. G. 
Published: July 2007

AbstractTo look into the factors responsible for a large efficiency of defect formation in alkali halides, an excitonic mechanism with an exciton localized at a single lattice site is discussed. The exciton contains a hole forming a small polaron. The nonradiative annihilation of the exciton necessary for defect formation is shown to be improbable due to the fact that the polaron displacements of atoms are small. The large efficiency of the excitonic mechanism is due to the localization of the hole forming the exciton at two lattice sites, since such a localization gives rise to large equilibrium displacements of atoms (of the order of an interatomic spacing).
Generalized Boltzmann Equation with Field‐Dependent Collision Terms Inclusive of Interband Transitions
Authors: Holzhütter H.‐G. , Mocker M. 
Published: July 2007

AbstractA coupled system of generalized Boltzmann equations with field‐dependent collision terms is derived, taking into account both intraband and interband transitions of the charge carriers. The field dependence of the collision terms is shown to be of importance if the field‐induced energetic uncertainty becomes comparable with the mean energy of the charge carriers. The interband part of the one‐electron density operator is investigated which is associated with the occurrence of an interband current.
Low Temperature Specific Heats of TCNQ Complexes
Authors: Kondow T. , Mizutani U. , Massalski T. B. 
Published: July 2007

AbstractThe low temperature specific heats of NMP(TCNQ), Q(TCNQ)2, DAD(TCNQ)2, TEA(TCNQ)2, Li(TCNQ), and Rb(TCNQ) are measured at temperatures between 1.5 and 4.2 K. The specific heats (C) can be expressed in the form C = γT + αT3 + HT−2. Both γ and H originate from lattice defects and/or impurities in the specimens and they can be discussed in terms of their concentration as determined from the magnetic susceptibilities of the same specimens. With increasing impurity concentration, both γ and H become measurable at an impurity concentration exceeding 2.2 × 1021/mol TCNQ and increase at a rate which keeps the ratio H/γ almost constant. These behaviors are consistent with a scheme of randomly distributed impurities. The Debye temperature for each specimen is also determined from the α‐coefficient. The velocity of sound, and hence the rigidity of the lattice, can be estimated from the Debye temperature.
Carrier‐Assisted Radiation Pumping of Spin Waves in Ferromagnetic Semiconductors under a Strong Magnetic Field
Authors: Pereira P. R. R. , Amato M. A. , Miranda L. C. M. 
Published: July 2007

AbstractThe carrier contribution to the generation of spin waves in ferromagnetic semiconductors under the simultaneous presence of a laser field and a strong dc magnetic field is discussed. It is shown that for degenerate semiconductors this electronic contribution predicts a threshold pumping power within the experimental reach. Also, this carrier‐assisted mechanism excites spin waves in a broad range of relatively high valves of q and may be useful as a new tool for investigating the s–d exchange constant.
The EPR Fine Structure Spectrum of Dislocations in Silicon
Authors: Bartelsen L. 
Published: July 2007

AbstractThe EPR fine structure spectrum of dislocations in plastically deformed silicon single crystals is measured using pure, boron, and phosphorus doped silicon. The EPR transitions of chains of coupled spins Si = 1/2 are calculated using the following spin Hamiltonian:

The results reveal that the fine structure spectrum can be explained as caused by transitions taking place in chains of 2, 3, 4, and 5 coupled spins of unpaired electrons in the dislocation core. In heavily boron doped, plastically deformed crystals the EPR signal of boron atoms located in the stress field of the dislocations is found. The main axis of the stress field is nearly parallel to the Burgers vector of the primary glide system.
Orthogonalized Augmented Plane‐Wave Method for the Solution of the Energy Band Problem in Solids
Authors: Chatterjee S. , Sinha P. 
Published: July 2007

AbstractThe advantages of the OPW and the APW methods are combined to formulate a method for the solution of the energy band problem in solids. Here the valence and the conduction electrons described by the APW's are orthogonalized to the core states constructed by the tight‐binding approximation. The method is applied to wurtzite ZnS and used to obtain the energy values at the highest‐symmetry points – Γ and A – inside the Brillouin zone. The preliminary results appear to indicate that wurtzite ZnS is a semiconductor. The various band gaps calculated from these band energy data are compared with experiments and other theoretical calculations.
Hyperfine Field on the Nucleus of an Atom of a Transition d Element (Metal), Magnetie Moment and Electron Structure of a Ferromagnetic Metal Atom
Authors: Galperin F. M. 
Published: July 2007

AbstractSome fundamental characteristics of the unfilled d shell of a transition d element are calculated at first time and then a number of the most specific properties of the d substance (an isotope, a free atom (ion), and the transition d metal atom) are calculated with those characteristics taken into consideration.
Impurities and Positronium Formation in Molecular Crystals
Authors: Goworek T. , Rybka C. 
Published: July 2007

AbstractThe influence of impurities on the positronium formation in molecular crystals is investigated. The intensity of the narrow component in the momentum spectrum is measured as a function of the admixture mole fraction and discussed in the framework of a simple two‐process model. The specific positronium formation times for various admixtures are found to be 3 × 10−13 to 10−11 s.
The Effect of Non‐Transition Metals on the Magnetic Susceptibility of Aluminium Solid Solutions
Authors: Lakner J. 
Published: July 2007

AbstractNon‐transition metals, the effect of which is analysed in binary and ternary solid solutions of aluminium, decrease the magnetic susceptibility of aluminium to a higher degree than to be expected on the basis of spin susceptibility of conduction electrons in metals generally accepted. Using the pseudopotential method and a nearly free‐electron approximation, the present model explains the experimental results as the change in the Landau diamagnetic susceptibility resulting from the electrons and holes on the Fermi surface of the third zone, having the two potential parameters W(111) = 0.018 Ryd and W(200) = 0.053 Ryd.
Correlations and Plasmons in Quasi‐One‐Dimensional Metals
Authors: Campos V. B. , Hipolito O. , Lobo R. 
Published: July 2007

AbstractThe longitudinal dielectric function for a quasi‐one‐dimensional electron gas is obtained in a self‐consistent field approximation (SCFA) scheme. Short range correlations arising from both, Coulomb and exchange effects through a local field correction depending on the pair‐correlation function are included. The structure factor and the dielectric function are determined in a self‐consistent scheme. Numerical results are presented for the structure factor, the static pair‐correlation function, the plasmon dispersion and the compressibility. It is found that in the high density limit the results for the structure factor and the pair‐correlation function are the same as given by Girardeau's exact expression. The plasmon dispersion curves indicate the onset of a minimum for xs ⪆ 7, in contrast with the RPA results. For values of xs in this region, the compressibility values become negative, indicating a possible instability of the system. In both RPA and SCFA results the plasmon modes are not Landau damped in contrast to isotropic metals.
Contribution of the d‐Electron Screening to the Properties of Impurities in Transition Metals
Authors: Lannoo M. , Allan G. 
Published: July 2007

AbstractThe screening around impurities in narrow band systems is analyzed in a linearized approximation, leading to a dielectric constant formalism. The response matrix is determined, first using a Green's function technique and then a method of moments. An application is made to the case of d‐electrons in transition metals. Near the centre of the series it is found that most of the screening occurs on the impurity site. Finally the results are used to calculate the dissolution energy of the impurity.
On the Bose‐Einstein Condensation of Excitons
Authors: Kotelnikov Yu. E. , Kochelaev B. I. 
Published: July 2007

AbstractThe Bose‐Einstein condensation of Frenkel excitons in molecular crystals is investigated by means of the Green's function method without the use of the effective mass approximation, the kinematic interaction between excitons being taken into account accurately. The spectrum of elementary excitations at low temperatures in the case of dipole–dipole interaction between molecules is obtained. The temperature of the phase transition and its dependence on the concentration of excitons are estimated.
Magnetic and Thermodynaniic Properties of Heisenberg Open Finite Chains II. Specific Heat
Authors: Rakitin Tu. V. , Kalinnikov V. T. , Hatfield W. E. 
Published: July 2007

AbstractCalculations are made of spin specific heat, C'M,, as a function of reduced temperature, kT/J, for Heisenberg open finite chains containing up to 11 ions (N ≦ 11) with spins Si of 1/2 to 5/2 on the basis of the model described in a preceding communication. Cha‐acteristic features of the C'M(kT/J) curves corresponding to various Si‐ and N‐values are discussed. These can be used in interpretation of experimental temperature dependences of C'M for the respective ferro‐ and antiferromagnetic chains.
Weak‐Field Thermomagnetic Coefficients of a Trigonal Bismuth Sample
Authors: Hansen O. P. 
Published: July 2007

AbstractThe weak‐field thermomagnetic coefficients of bismuth with heat current in the trigonal direction have been measured in the range of temperatures from 10 to 65 K. The data are analysed in terms of the symmetry restrictions imposed by the 3m point group. The temperature dependences of the coefficients point to a diffusion thermoelectric power down to temperatures which depend on the coefficient in question, the highest temperature being around 40 K.
The Influence of Antiferromagnetic Order on the Evaluation of Spin Disorder Resistivity
Authors: Gratz E. , Poldy C. A. 
Published: July 2007

AbstractIt is known that the parameters ϱ spd (spin disorder rosistivity) and θp can be used for estimating the effective mass m* of the conduction electrons and exchange constant G. ϱspd is the temperature independent contribution to the electrical resistivity in the paramagnetic temperature range whereby this contribution is due to the disordered localized spins, and θp is the paramagnetic Curie temperature. The above procedure is appeared in the literature for the rare earth melalv (R) and their interinetallic compounds not only for ferromagnetic (Ral2) but also for antiferromagnetic (Rau2, Rag2, Rcu2) systems. It is tested whether such a procedure for estimating m* and G is valid irrespective of whether the ordering is ferromagnetic or antifcrromagnctic. This investigation is carried out for the system Gd(Ni, CU)2 and it is shown that the above procedure is principally inapplicable for θspdi values obtained from antiferromagnctic samples. In addition a discussion is given of the validity of the free electron approximation in the Gd(Ni, Cu)2 and the GdZn2 systems, whereby it is concluded that the s‐character deteriorates with increasing electron concentration.
Ultrasonic Absorption in the Intermediate State of Gallium and Indium
Authors: Singh O. 
Published: July 2007

AbstractThe absorption is stndied of longitudinal ultrasound in the intermediate state of indium and gallium single crystal cylinders at 1.17 and 0.39 K, respectively, for frequencies ranging between 10 and 70 MHz. The results are analysed with a theory developed by Andreev which is extended to allow for the arbitrary values of ql. The width of the normal regions calculated from the results is in reasonable agreement with that calculated using Landau's unbranched model with surface energy parameters equal to 3.5 × 10‐5 and 8 × 10−5 cm for indium and gallium, respectively. The nonlinear variation of the high‐frequency‐limit attenuation is perhaps due to the curvature of the N‐S interfaces near the surface of the specimen.
Temperature Dependence of the Grüneisen Constant of Si and Ge
Authors: Soma T. 
Published: July 2007

AbstractThe Grüneisen parameters of the individual modes of phonons and the mean Grüneisen parameters are obtained for Si and Ge in the higher‐order perturbation method. Calculations are made for normal modes propagating in the [l00]‐, [110]‐ and [111] Indirections.
Magnetic Circular Dichroism of Neutron‐Irradiated MgO and CaO
Authors: Vance E. R. , Manson N. B. 
Published: July 2007

AbstractMagnetic circular dichroism (MCD) measurements are made on neutron‐irradiated MgO to examine the sharp 524, 642, and 649 nm optical absorption lines, which are known to be A ↔ E transitions within trigonal centres. The MCD of the 524 nm line is strong at low temperatures, as expected from previous Faraday rotation measurements, and vibrational sidebands are observed in the MCD measurements. The MCD signals from the 642 and 649 nm lines are very weak, but temperature‐dependent: assuming the centres are paramagnetic the magnitudes of the spin‐orbit splittings of the 2E‐states are calculated as ⪅ 0.1 cm−l from the MCD data. In neutron‐irradiated CaO some sharp, strong temperature‐dependent MCD signals are observed after the samples are annealed at moderate temperatures; the corresponding absorption lines at 465, 573, 602, 672. and 678 nm are not previously reported. The centres responsible for these lines appear to be paramapnctic and to possess at least axial symmetry.
Magnetic Properties of a' Random Binary System
Authors: Hattori K. 
Published: July 2007

AbstractThe quenched random system composed of A and B magnetic atoms is theoretically investigated. The exchange integrals between A‐A and B‐B atoms, JA, JB, are assumed to be positive, bnt no restriction is imposed on the exchange integral between A‐B atoms, JAB. By using effective Hamiltonians, the transition temperature and specific heat above it are calculated in the Heisen‐berg and Ising models. It is found that the transition temperature depends on the sign of JAB in the Heisenberg model, while it does not in the Ising model. The expression of the specific heat obtained is applicable to any paramagnetic phase, without restrictions on the magnitude and sign of JA, JB, and JAB.
Elastic Interaction between Crowdions and Extended Atomic Defects in Isotropic Media
Authors: Scheu W. , Frank W. , Kronmüller H. 
Published: July 2007

AbstractThe interactions between the partners of crowdion‐vacancy, crowdion‐crowdion, and crowdion‐dumbbell pairs in face‐centred cubic crystal sare calculated within the framework of the isotropic linear elasticity theory, taking into account the finite extensions of these defects. It is found that among all types of pairs there are configurations which can form bound states at small defect separations. For a number of crowdion‐vacancy and particularly of crowdion‐crowdion configurations, pairs of neighbouring bound states exist which are capable of giving rise to relaxation phenomena.
Phase Transitions in Spin‐One Ferromagnetic Systems
Authors: Kumar V. , Sharma K. C. 
Published: July 2007

AbstractA theory is given for the spin‐one ferromagnetic system containing all the major interactions. The anisotropy term is evaluated exactly whereas the bilinear and the biquadratic exchange terms are evaluated in RPA. A closed form expression for the critical temperature Tc at which the quadrupolar to paramagnetic phase transition occurs, is given. The variation of Tc vs. the biquadratic exchange parameter is presented and is compared with the earlier results.
Mössbauer Effect of γ‐Irradiated CaF2 Doped with 57Co
Authors: Sato Y. 
Published: July 2007

AbstractMössbauer spectra of 57Co doped into CaF2 single crystals are decomposed into a single line and a quadrupole doublet. The former is ascribed to Fe2+ ions on Ca2+ sites with cubic symmetry, and the latter to Fe2+ ions on axially symmetric interstitial sites associated with an adjacent cation vacancy, γ‐irradiation further produces a new line which arises from Fe2+ ions on interstitial sites with cubic symmetry. When the irradiated crystals are quenched from 500°0, only two lines appear which arise from Fe2+ ions on interstitial sites associated with two neighbouring cation vacancies.