bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagation of Electromagnetic Waves for a Rotation of the Magnetic Field from the Faraday to the Voigt Configuration
Authors: Stegmann R. , Oelgart G.
Published: July 2007

In a semiconducting bismuth‐antimony alloy, the propagation of electromagnetic waves is observed in the frequency range between 35 and 70 GHz and for temperatures of liquid helium in the total angular range between the Voigt and the Faraday configuration. The comparison with the theoretical dispersion behaviour, where a conductivity tensor corresponding to a tripartite ellipsoidal Fermi surface is utilized, allows a qualitative understanding of the wave propagation. The influence of the redistribution effects of the charge carriers among the individual parts of the Fermi surface in a quantizing magnetic field is essential, and the inaccurate knowledge of the energy level structure makes the comparison between experiment and theory difficult.
Nonradiative and Radiative Recombination in Lead Chalcogenides
Authors: Ziep O. , Genzow D. , Mocker M. , Herrmann K. H.
Published: July 2007

In a wide range of temperature and doping level the small signal lifetime for Auger recombination, spontaneous emission of radiation, and for spontaneous emission of plasmons is calculated in the compounds Pb0.78Sno. 22Te and Pb0.91Sno0.09Se. In the calculation degeneracy of the carrier gas and anisotropy of band structure is taken into account. At donor concentrations ND > 1019 cm−3, recombination by emission of plasmons dominates. For ND < 5 ×1016 cm−3 radiative recombination dominates in Pb0.78. Sno22Se up to temperatures TL < 180 K, in Pb0.09Se, however, up to 230 K.
Far‐Infrared and Submillimetre Dielectric Response of (NH4)2BeF4
Authors: Petzelt J. , Volkov A. A. , Kozlov G. V.
Published: July 2007

The far‐infrared spectrum of the complex dielectric constant ε*(ω) of (NH4),BeF4 single crystals is measured by means of precise transmission spectroscopy (5 to 34 cm−1 region) and reflection spectroscopy (34 to 400 cm−1 region) at 90 and 300 K. The temperature dependence of ε″ (30 cm−1) revealed a disorder‐induced absorption in this region, but no other anomaly in the incommensurate phase due to amplitudon or phason absorption, confirming the order‐disorder nature of the incommensurate phase transition.
On the Lattice Dynamics of Alkali Metals
Authors: Soma T.
Published: July 2007

The experimentally observed dispersion curves of alkali metals are reproduced using second‐order perturbation theory and the local Heine‐Abarenkov model potential. The effect of different forms of exchange and correlation on the phonon frequency, the compressibility problem, and the crossing of dispersion curves in [001] direction are investigated.
Dispersion in Ferromagnets Interacting with Phonons
Authors: Suguna A. , Shrivastava K. N.
Published: July 2007

Considering the magnon‐phonon interaction it is found that there is a net reduction of Heisenberg exchange. The reduction is maximum for the magnon wave vector given by K0 = v/(2cD) where v is the sound velocity and D is just 2SJa with a0 as the lattice constant. Thus the reduction is found to depend inversely on the exchange itself. There is also a new analog of Kohn like discontinuity occuring at the magnon wave vector k8 = ½ (qm + v/cD) with qm as the maximum wave vector of the phonons. In consideration of thermal magnon scattering a kink in the magnon dispersion is thus predicted at ks. The approximate temperature dependence arising in the exchange from the magnon‐phonon coupling is discussed.
Angular and Frequency Dependence of the EPR Linewidth in a 1D Alternating System: K‐TCNQ
Authors: Alizon J. , Berthet G. , Blanc J. P. , Gallice J. , Robert H. , Thibaud C.
Published: July 2007

A theoretical expression of the angular and frequency dependence of the EPR linewidth of K‐TCNQ is given using the alternating 1D Heisenberg model characterized by the exchange integrals J and γJ ≫ hωe with ≪ Measurements of the EPR linewidth are performed on a single crystal at high fields (0.3500 and 1.2500 T) and also at low field (0.0020 T) on crystals aligned along their c‐axes. The results obtained in these two extreme cases allow to determine the order of magnitude of the cut‐off frequency of the intrachain diffusion mechanism of electronic spins. Moreover, the results at high fields, at the two frequencies 9600 and 35000 MHz show that the observed spectral densities ψ(ω) are constant in the frequency range 9600 to 70000 MHz.
Energy Loss of Charged Particles in Crystals
Authors: Burenkov A. F. , Komarov F. F. , Kumakhov M. A.
Published: July 2007

The theory of charged particle energy loss by collisions as a function of impact parameter in a crystal is developed on the basis of the quantum dielectric formalism. Simple and convenient formulae taking into account the energy zone structure of crystals are obtained for nonrelativistic and relativistic particles. The relation between energy loss and local electron density is studied. For the first time it is shown strictly that the hyperchanneled particle energy loss in the axial channels is less than that in corresponding plane channels. Comparison with main experiments is presented.
Theory of Optical Absorption of Hg1–xCdxTe Solid Solutions with Account of the Contribution of Quasilocal Acceptor Levels
Authors: Mekhtiev M. A. , Ismailov T. G.
Published: July 2007

The calculation of the optical absorption coefficient in the presence of quasilocal acceptor levels is given. The general formula obtained in the dipole approximation is applied to Hg1‐xCdxTe solid solutions. The semimetal and semiconductor cases are considered. A comparison with the experiment for the semiconductor case is given and the values of Fermi energy, the concentration, and energy of acceptors are found.
On the Structural Transition from NaCl to CsCl Type in Alkali Hydrides
Authors: Bashkin I. O. , Dymova T. N. , Ponyatovskii E. G.
Published: July 2007

The solid sphere ion model is used to compare specific volumes of NaCl and CsCl phases of alkali hydrides. The transition of the NaCl to CsCl type is shown to be thermodynamically possible in K, Rb, and Cs hydrides. A phase transition is detected in KH at pressures about 40 kbar, and the T—P diagram of KH is studied. On the basis of the analysis performed, the NaCl to CsCl transition in LiH is unlikely, unless the electron configurations of the ions are changed.
Temperature Dependence and Critical Behaviour of the Resistivity Anisotropy in Ferromagnetic Metals
Authors: Ky Vudinh
Published: July 2007

The resistivity anisotropy in ferromagnetic metals due to spin—orbit interaction is considered with spin correlation taken into account. The obtained temperature dependence of Δρ shows a round peak in accord with experiment. In the critical region d(Δρ)/dt diverges as (—t)1—(α + φ) (t = (T — TC)/TC, magnetic field h = 0), where φ is the cross‐over exponent.
Electronic Structure of Fe3Si‐Type Alloys I. X‐Ray Spectra of Mn3Si, Fe3Si, and Fe3Al and Comparison with Band Structure Calculations
Authors: Himsel A. , Blau W. , Merz G. , Niederlag W. , Querin U. , Weisbach J. , Kleinstück K.
Published: July 2007

Al‐ and Si Kβx emission spectra as well as Mn‐ and Fe L2,3 emission and self‐absorption spectra of the alloys are measured using a primary Bragg‐Soller spectrometer. Within error limits, the energetical positions of features in the spectra agree with those from calculated band structures. It follows that the APW method is applicable also to magnetic alloys if the band model of ferromagnetism is used. In non‐SCF calculations the transition metal configuration 3dn‐14s1 gives better results than 3dn‐24s2, as it is concluded from the relative position of s‐ and d‐bands.
Piezo Hall Effect in p‐Germanium
Authors: Gorbachuk N. T. , Mitin V. V. , Tkhorik Yu. A. , Shvarts Yu. M.
Published: July 2007

The piezo Hall effect is investigated on p‐Ge samples of low (ρ300 K = 0.02 Ωcm) and high (ρ300 K = 16 Ωcm) resistivity in the temperature range 77 to 300 K for the case of current i and uniaxial stress x being parallel to the 〈100〉 and 〈111〉 crystal axes. Taking into account the dispersion law anisotropy for light and heavy holes allows to explain the experimental temperature dependence of the piezoresistivity coefficients for samples of low and high resistivity and orientational dependence of this effect. The deformation potential constants b and d are obtained which vary in the range b = — (2.2 to 2.6) eV, d = — (4.4 to 5.1) eV depending on the values of the band parameters A, B, and C given in literature. These values of b, d are considerably different from those obtained by means of the same experimental data, but without taking into account the dispersion law anisotropy.
A Test of Convergence of the Expansion of the Screening Charge Density for Impurity Ions in Semiconductors
Authors: Mohammad S. Noor
Published: July 2007

In the development of the theory of potential of an impurity ion in semiconductors the Fermi‐Dirac integral F1/2 appearing in the density of the screening charge ϱ(r), is expanded in powers of the potential. In the solution of the Poisson equation with ϱ(r) always the problem arises about how many terms in the expansion of F1/2 one should account for to get a meaningful result. It depends on the convergence of the series. A critical study is made of the convergence of the series. For this new formulae for the Fermi‐Dirac integral are proposed, and it is observed that the said convergence is more pronounced for the degenerate sample than for the non‐degenerate one.
Room Temperature Investigation of the Photostimulated Afterglow in KCl:Eu2+ Crystals
Authors: Vassilev Y. , Karamikhailova M. , Mladenova M. , Georgiev M.
Published: July 2007

KCl:Eu2+, γ‐irradiated at room temperature, is known to luminesce on photoexcitation in the F band with the emission spectrum of the impurity ion. Several experiments are made now: firstly, the emission—excitation spectrum is found to exhibit a peak at about 600 nm. Secondly, the samples are excited with the SH of a mode‐locked Nd glass laser. The maximum emission response occurs delayed at about 400 ns relative to the laser‐pulse peak. The luminescent quantum yield is estimated to be about 1%. The decay kinetics of the phosphorescent tail is also followed. A tentative model is discussed in which the luminescence results from a two‐step electron tunneling between excited F and Eu levels.
Peculiar Properties of Surface Magnetism in Thin Film Alloys
Authors: Zagórski A. , Nazarewicz W. , Ferchmin A. R. , Sukiennicki A.
Published: July 2007

Concentration and exchange dependent Curie temperature is discussed within the framework of the Ising model as a function of film thickness. Phase diagrams include a magnetic phase induced by the surfaces which is most likely to appear in surface‐enriched films at the percolation threshold.
Dielectric Behaviour of Non‐Crystalline Model Systems Employment of an Embedding‐Field Method
Authors: Lenk R. , Ziegenhals G.
Published: July 2007

For a disordered system composed of identic spherically symmetric multipoles the relationships between averaged local and corresponding macroscopic field quantities are explored for static electric fields. This dielectric problem its analogous to the problem of multiple scattering of waves in a disordered muffin‐tin system. Therefore it is treated by an adaption of an embeddingfield method formerly developed for multiple scattering. The two different versions of the embedding scheme both yield the well‐known Lorentz formula for the local electric field. On the other hand, different results are obtained for the derivative of the field.
Magnetic Interactions in α‐MnSeO4, α‐FeSO4, and β‐FeSO4
Authors: Wegener W. , Hautecler S. , Legrand E. , Will G.
Published: July 2007

The energy distributions of neutrons scattered by α‐MnSeO4, α‐FeSO4, and β‐FeSO4 in their paramagnetic state are measured by means of a time‐of‐flight technique. The values of two exchange parameters can be estimated from a simultaneous analysis of the second moment of these energy distributions and of the Néel temperature computed in the Green's function approximation. Various two‐parameter analyses are performed and checked by comparing calculated and experimental values for θ and χ(TN). For α‐FeSO4 the dominant interactions occur between the first and second neighbours. For β‐FeSO4 two‐parameter analyses do not lead to a satisfactory description of all data; however, one can conclude that the interactions between third neighbours are important, in agreement with the stability conditions for the observed spin configuration.
Impact Ionization and Magneto‐Transport in InSb in High Crossed Electric and Magnetic Fields
Authors: Ašmontas S. , Dargys A. , Subačius L.
Published: July 2007

High‐field microwave conductivity and impact ionization due to hot carriers are studied experimentally in n‐ and p‐type InSb at liquid nitrogen temperature in magnetic fields B up to 2.3 T. The employment of high microwave fields instead of dc electric fields allowed to overcome the problem of an uncontrollable breakdown caused by the Hall field at the precontact regions in high‐mobility semiconductors. It is found that the threshold field of impact ionization ET increases with B in n‐type as well as in p‐type InSb. In n‐InSb at high magnetic fields (μB » 2π) the ratio ET/B is found to be constant and close to that calculated recently by Bruhns and Hübner. From the measurements of microwave conductivity in prebreakdown fields it is possible to deduce an electron Hall drift velocity vy. It is found that vy is a function of the ratio E/B only, although due to the hot electron effects a systematic deviation of vy from the simple relation vy = E/B is observed.
First‐Order Raman Scattering in NiS2—xSex
Authors: Lemos V. , Gualberto G. M. , Salzberg J. B. , Cerdeira F.
Published: July 2007

The Raman spectra of the system NiS2—xSex are investigated for x = 0.00, 0.10, 0.20, 0.30, and 0.55, at room temperature and at temperatures below the point where the x = 0.55 sample suffers a resistivity change from semiconducting (T ≥ 120 K) to metallic (T ≥ 120 K). No qualitative or anomalous quantitative changes in the spectrum of the sample are found that could be associated with the anomaly in its transport properties. A mode of Ag symmetry appears in the alloys which is absent from the spectrum of pure NiS2. This mode is successfully explained in terms of vibrations of S—Se pairs. Other modes decrease in frequency with increasing selenium concentration, a trend easily understood considering the increase in the unit cell volume with increasing x.
Compositional Dependences of Critical Point Phonons in Mixed Zincblende Crystals
Authors: Jahne E. , Ulrici B.
Published: July 2007

The compositional dependences of long‐ and short‐wavelength critical point phonons in mixed zincblende semiconductors are studied within the framework of an interpolation scheme between the end‐member compound phonons which is founded by an “averaged Green's function” technique. The derived dependences on composition agree with those expected from a model with three virtual atoms per unit‐cell (pseudo‐unit cell model). Explicit results are presented for the compositional dependences of the Γ, X, and L point phonons in In1−xGaxP and Gap1−xAsx and compared with experimentally derived phonon frequencies.
Experimental and Theoretical Investigations on the Picosecond Deactivation Process of Excited F‐Centres in KCl
Authors: Schubebt M. , Vogler K.
Published: July 2007

The deactivation behaviour of optically excited F‐centres in KCl is investigated for the first time by picosecond laser spectroscopy. It is found, that a certain portion of them returns to the ground state during 20 to 150 ps. This fast deactivation process, which depends on the F‐center concentration and the coloration method, can be explained by a model based on energy transfer between excited F‐centres, whereby an inhomogeneous spatial distribution can be taken into account. The critical transfer distance is observed to be R0 = (106 ± 15) Å. Quantitative characteristic results about the inhomogeneous spatial distribution of the F‐centres are presented.
Optical Nuclear Polarization in the 1:1 CT‐Crystal Anthracene/Tetracyanobenzene. Consequences of the Phase Transition at 206 K
Authors: Auch W. , von Schütz J. U.
Published: July 2007

In the 1:1 CT‐complex anthracene/tetracyanobenzene the optical excitation of triplet states causes a nuclear polarization (ONP), which is two orders of magnitude larger than the Boltzmann magnetization. In single crystals this ONP is investigated as a function of the temperature (1.6 K ≦ ≦ T ≦ 300 K), of the magnetic field (0 < Bp ≦ I T) and of the angle between Bp, and the molecular axes. From the extrema of the ONP curves the identity and orientation of the triplet state inducing the ONP may be determined. In addition from cross‐relaxation experiments a quadrupole coupling constant A = (1.0 ± 0.1) MHz and an asymmetry factor η = 0.08 ± 0.02 for the 14N nuclei of TCNB is found. Below the phase transition temperature at Te = ± 206 K the anthracene molecules occupy two magnetically inequivalent positions. For the ONP process between 205 and 40 K this magnetic inequivalence is released by the delocalisation of the triplet state as an exciton.
The Linear Transverse Ising Model with Four‐Spin Interactions
Authors: Morgan M. J.
Published: July 2007

A systematic approximant expansion due to Suzuki is applied to a simple quantum spin system; namely the linear transverse Ising model with four‐spin interactions (lattice coupling) described by the Hamiltonian . The free energy per spin and specific heat of this quantum model is calculated for the first three approximants to the partition function, and the results are discussed within the framework of the findings of other authors.
Temperature Dependence of Raman Scattering and Partial Disorder in AgI Crystals
Authors: Mariotto G. , Fontana A. , Cazzanelli E. , Fontana M. P.
Published: July 2007

A detailed study is made of vibrational dynamics in silver iodide by the temperature dependence of Raman scattering from liquid helium temperature up to melting. A complete set of data like temperature dependence of intensity of all features, of the degree of polarization of the spectra, of peak position and halfwidth clarifies the complex nature of dynamics of β‐AgI. It is shown that the anomalous features of Raman spectra are very likely due to the one‐phonon density of states connected with the onset of partial disorder in the β‐phase. In α‐phase an anomalous decrease of Raman cross section is found with increasing temperature: this effect is connected with the possibility of a continuous partial order‐disorder transition at ≈︁ 430 °C.
Investigation of the Electronic Structure of Uranium Compounds UX2 (X Pnigogen) and UAsY (Y Chalcogen) by Positron Annihilation
Authors: Dȩbowska E. , Rozenfeld B.
Published: July 2007

The electron momentum density distributions ϱ(p) in USb2, UP2, and UAsS are calculated for various sets of lattice harmonics Fϱ, using Mijnarends's method. The selection of the adequate set of harmonics is made by a comparison of the anisotropic parts of ϱ(p) with “difference” momentum density distributions ϱr(p) obtained on the basis of the difference angular correlation of positron annihilation quanta curves. From “summed up” distributions ϱs(p) = ϱpoly(p)+ ϱr(p) Fermi momenta along different directions are estimated. The character of the Fermi momenta changes suggests an ellipsoidal shape of Fermi surface, which corresponds to extremely large anisotropy of electric conductivity along [001] and [100] directions of the tetragonal unit cell of these compounds.
Electrical Conductivity of Ammonium Bromide
Authors: Taylor B. E. , Laskar A. L.
Published: July 2007

The electrical conductivity of NH4 Br single crystals is measured by both ac and dc techniques over the temperature range 50 to 130 °C. The dc conductivity as a function of temperature in both, pure and Cd2+ doped NH4Br samples shows excellent Arrhenius behavior. The activation enthalpy for conduction is 1.14 eV. Ac measurements exhibit a pronounced frequency dependent plateau at lower temperatures (≈ 50 to 100 °C). This plateau is believed to be due to a “proton loss”. Due to the existence of the plateau that masks any extrinsic region, the dc technique is more useful in the study of ionic defects in NH4Br. Assuming that transport is due to a vacancy mechanism, the enthalpy of vacancy migration is estimated to be 0.55 eV. The polarity of the vacancy is not uniquely determined.
Single‐Site Theory of Wannier Excitons in Disordered Semiconductors 11. Numerical Results
Authors: Fischbeck H. J. , Strehlow R.
Published: July 2007

The VKE model for semiconducting alloys (mixed crystals) and the point scatterer model with both, a discrete and a continuous distribution of impurity levels as a model for heavily doped and glassy semiconductors, respectively, are considered in a simple parabolic two‐band structure which can be direct and indirect as well. It allows to handle analytically the 𝓀 space integrals occurring in the expression for the dielectric function, which is given in the first part of this paper. In dependence of various parameters 1s exciton lines are calculated in these models mainly in a direct band structure. In the VKE model also indirect bands are studied.
Chemical Bonding, Internal Surfaces, and the Topology of Non‐Crystalline Solids
Authors: Phillips J. C.
Published: July 2007

The rarity of glass‐forming materials requires the existence of topologically specific principles (over and above the randomness implied by the absence of long‐range order) which optimize the glass‐forming tendency (gft). For covalent glassy semiconductors an algebraic condition predicts correctly the composition dependence of the gft. Quite generally substantial medium‐range order associated with quasi‐regular internal surfaces can be found in glasses at distances as large as 1000 Å in suitably selected thin films.
Nonstationary States of Inhomogeneously Heated Charge Carriers
Authors: Bonch‐Bruevich V. L.
Published: July 2007

The behaviour of the charge carriers heated in homogeneously by an intraband light absorption is studied in a hydrodynamical approximation. Conditions are stated under which a stable electron temperature superlattice exists as well as the conditions under which there are no stable stationary states; the electron liquid then moves stochastically, its temperature and velocity being bounded and randomly time‐dependent.
Conductance et magnétorésistance transversale á partir de I'équation de Boltzmann quantique
Authors: Charbonneau M. , Van Vliet K. M.
Published: July 2007

Nous discutons une équation de Boltzmann quantique obtenue récemment. Cette équation est valable pour les cas où la nature quantique du transport est d'une importance fondamentale. Comme première application, nous étudions la conductivité électrique contrölée par la diffusion sur des impuretés. Les résultats sont en accord avec ceux de Chester et Thellung et de Verboven. En deuxième lieu, nous étudions la magnétorésistance transversale dans des champs magnétiques fort et extrëmement fort, Ià où I'équation de Boltzmann elassique est complètement inadeptée. Dans le cas de la diffusion sur des impuretés, on retrouve la formule de Adams e t Holstein qui a été obtenue par une méthode d'opérateur densité ou par la théorie de Kubo. Pour le cas des collisions binaires, Line extension du résultat de Adams et Holstein a été obtenue.
On Energy‐Wave Number Characteristics F(q) in Metals as Derived from Experimental Phonon Spectra
Authors: Brosens F. , Derveese J. T. , Cornelis J. , Nihoul J.
Published: July 2007

A method to determine the energy‐wave number characteristic of metals directly from experimental phonon spectra is discussed with respect to the uniqueness problem, and in connection with the hypothesis of a central interatomie potential.
Luminescence from an Electron‐Hole Liquid in the Indirect Compound Ga(As1−xPx)
Authors: Wilke I. , Schwabe R. , Bindemann R. , Jacobs K. , Seifert W.
Published: July 2007

The radiative recombination from an electron‐hole liquid (EHL) is investigated in highly excited pure Ga(As1−xPx) with indirect band structure. The experimental results reveal an arsenic‐induced radiative no‐phonon (NP) transition from the liquid phase. The comparison of the experimental and calculated lineshape of the EHL luminescence band provides the fundamental parameters of the EHL obtained for a sample with x = 0.88. The equilibrium carrier density and the work function at T = 2 K are determined to be neq = 9 × 1018 cm−3 and Φ = (20 ± 1) me V, respectively. The influence of the arsenic atoms on the energetic properties of the EHL is qualitatively discussed.
On the Influence of a δ‐Correlated Random Field upon the Interband Optical Absorption
Authors: Mironov A. G.
Published: July 2007

The frequency dependence of the light absorption coefficient is calculated in the tail region for disordered semiconductors with the Gaussian Δ‐correlated random field. The optimal fluctuation method is employed. The frequency dependence of the absorption coefficient reproduces the density of states energy dependence (up to some coefficients in the exponent).
Lattice Dynamics of Crystals with Off‐Centre‐Type Impurities
Authors: Plakida N. M. , Ivanov V. V.
Published: July 2007

A model of a defect crystal with substitutional impurities in the potentials with several off‐centre minima is considered. Employing the self‐consistent phonon theory some necessary conditions for the off‐centre displacements of impurities are obtained and the resonant mode frequency is calculated. The importance of impurity fluctuations at the “local phase transition” is stressed. The theoretical results are compared with experiments in defect systems as KBr:Li+.
The Contribution of Reciprocal Lattice Vectors to Processes of Phonon‐Phonon Interaction and Lattice Thermal Conductivity
Authors: Segal A.
Published: July 2007

On the basis of a reformulation of the known wave vector conservation law in interaction processes with three phonons, the Boltzmann equation is solved and an equation of the lattice thermal conductivity is obtained. In this equation the average value of reciprocal lattice vectors occurs, which contribute to the three phonon interaction processes. An application of the equation obtained for different substances would offer information regarding the reciprocal lattice vectors which contribute to these processes.
ENDOR Study of Atomic Hydrogen on Cation Sites in KCl Doped with Sr++
Authors: Studzinski P. , Niklas J. R. , Spaeth J. M.
Published: July 2007

Atomic hydrogen trapped on cation sites (H0s, c‐centres) next to Sr2+ in [110] in KCl is investigated by ENDOR and ENDOR‐induced ESR. The superhyperfine and quadrupole interactions and their relative signs of three types of inequivalent nearest Cl‐nuclei are determined. The transition energy for the first excited vibrational hydrogen state is derived from the superhyperfine temperature dependence. A further H0s, c‐centre is found with Sr2+ probably on a [200] site. The superhyperfine data are discussed considering the local lattice distortion around the cation vacancy.
Landau Model of a Spin Glass with Two Order Parameters: Stability and Derived Physical Properties I. Stability
Authors: Deutges D. , Heber G.
Published: July 2007

A Landau‐type theory of phase transitions with two order parameters is used to describe modificated ferromagnetic and spin glass phases. Phase transitions to “normal” ferromagnetic or paramagnetic states are investigated. We shall be interested mainly in the conditions of stability and the resulting consequences for the choice of coefficients in the Landau free energy. The critical behaviour of the modificated ferromagnetic and spin glass phases against external magnetic fields is shown. Physical properties as susceptibility, magnetization, and specific heat are deduced by chosing suitable linear temperature dependences of the coefficients of the free energy. The competing phases are considered.
Antiferromagnetic Polarization Induced by Localized Moments in Metals
Authors: Oleś A. M. , Chao K. A.
Published: July 2007

The polarization of the conduction electron cloud around the impurity is investigated with a variational trial function for the Wolff model. When there is a local moment, an antiferromagnetic polarization is obtained. The polarization moment on the nearest neighbours of the impurity only partially compensates the impurity moment. That is in agreement with the long‐range nature of the polarization cloud.
Neutron Powder Diffraction Studies of Fluorite and Pyrochlore NdxZr1−xO2−x/2 Solid Solutions with 0.25<x<0.55
Authors: Van Dijk T. , Helmholdt R. B. , Burggraaf A. J.
Published: July 2007

Neutron powder diffraction studies on stoichiometric and non‐stoichiometric pyrochlore phases of the ZrO2‐ Nd2O3 system show remarkable differences in the occupation factor of the 8b site of space group Fd3m to which the pyrochlore phase belongs. Stoichiometric Nd2Zr2O7 can be described as a perfect pyrochlore phase in which the 8b site is either vacant or at least has a low occupation factor. The occupation factors of the 8b sites of non‐stoichiometric NdxZr1−xO2−x/2 pyrochlores reveal that these contain a large degree of disorder in the anion sublattice. Best refinement results on non‐stoichiometric materials are achieved when the overall structure of the non‐stoichiometric pyrochlore phase is described as a hybrid phase consisting of a major volume fraction of the stoichiometric pyrochlore phase and a minor fraction of the defect fluorite phase.
Resonant Secondary Emission Spectroscopy
Authors: Kanehisa A. , Balkanski M.
Published: July 2007

A prospective view is taken of a new field of optical spectroscopy, unifying resonance Raman scattering and luminescence, with main emphasis on conceptual and theoretical aspects. The method consists in explicitly exploiting two independent sets of probes, (e, ω, k) and (e′, ω′, k′), i. e. polarization, frequency, and wave vector of incident and scattered photons, respectively. As compared with absorption spectroscopy, where one has only one set of probes (e, ω, k), the secondary emission spectroscopy gives higher information about the dynamics of the material: interaction in the intermediate states or relaxation, characterized experimentally and theoretically by the secondary emission intensity Is(e, ω, k; e′, ω′, k′), which is a measure of polarization‐, energy‐, and momentum‐correlation of incident and scattered photons.
Spectroellipsometric Investigation of PbI2 Single Crystals
Authors: Ghiţă C. , Ghiţă L. , Baltog I. , Constantinescu M.
Published: July 2007

The complex dielectric and optical constants of PbI2 single crystals are determined in the spectral range 240 to 630 nm by ellipsometric measurements for the first time. The calculated derivatives of the dielectric functions with respect to energy are consistent with the energy band structure of PbI2 reported in the literature.
Estimation of Covalency Effects in the Optical Spectra of Crystals Containing Iron Group Ions (I)
Authors: Kustov E. F. , Uskova L. I.
Published: July 2007

In the limits of a semi‐empirical theory, the influence of covalency effects on the optical spectra of crystals, containing iron group ions is investigated. The general matrices for the calculation of these effects are derived, taking into consideration the configurations of central ion and ligand electrons. Using the unitary group method, ML6, cluster states and states of tensor operators of single and double electron interactions are classified.
Detrapping of Charge Carriers by Singlet Excitons in Naphthalene Single Crystals
Authors: Nakagawa K. , Kotani M. , Tanaka H.
Published: July 2007

The detrapping of charge carriers by singlet excitons in naphthalene crystals is investigated at 300 K. Singlet excitons are generated homogeneously in the bulk of the crystal by two‐photon absorption and the detrapped charges ere observed as pulsed photocurrent. From the wave form and the effect of the pulse to pulse interval the amount and the spatial distribution of trapped, as well as detrapped, charges are determined. The bimolecular rate constant for the detrapping is found to be (8 ± 6) × 10−10 cm3 s−l. The result indicates that, in naphthalene crystal, the probability of the detrapping reaction is high, close to the diffusion limit, whereas that of the singlet‐singlet annihilation is lower.
Acceptor Pair‐Bound Exciton Complexes in Semiconductors
Authors: Molva E. , Magna N.
Published: July 2007

The evolution of the principal bound exciton (PBE) line, for bound exciton‐neutral acceptor(s) complex(es), between low (≦ 1016 cm−3) and high (>1017cm−3 acceptor concentrations in semiconductors, is studied. Photoluminescence experiments are performed in ZnTe, for the five wellknown acceptors Li, Na, Ag, Cu, Au and for several doping levels. High concentrations provide an asymmetric broadening with undulations and a shift to lower energies of the PBE line. Semiempirical calculations are made for the neutral acceptor pair‐bound exciton complex. In ZnTe good agreement is obtained with experiment. The calculations demonstrate that the empirical relation ΔE(R) = E0, exp (−R/R0)3 is a good and simple analytical approximation of the bound exciton localization energy shift, when the exciton is bound on a neutral acceptor pair, with a pair separation R. These calculations permit to evaluate the energy shift of the PBE line maximum for acceptors in highly doped semiconductors.
Variational Treatment of Anisotropic Heisenberg Antiferromagnets
Authors: Figueiredo W. , Gitahy V. H. , Salinas S. R.
Published: July 2007

A simplified version of Falk's variational treatment of the isotropic Heisenberg antiferromagnet is extended by taking into account both, exchange and single‐ion uniaxial and orthorhombic anisotropic interactions. Expressions are obtained for the low‐temperature canted‐paramagnetic phase boundary. For uniaxial anisotropies the expression is of the form Ã – B̃T3/2 – C̃T5/2 – …, and for orthorhombic anisotropies it is of the form \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \alpha \limits^ \sim $\end{document} – \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \beta \limits^ \sim $\end{document}T2 – …. The structure coefficients Ã, B̃, \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \alpha \limits^ \sim $\end{document}, and \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \beta \limits^ \sim $\end{document} agree with previous spin‐wave calculations.
Electronic Properties of Amorphous SixGe1−x:H Films
Authors: Hauschildt D. , Fischer R. , Fuhs W.
Published: July 2007

Amorphous SixGe1−x:H films are prepared by decomposition of SiH4/GeH4 mixtures in a rf glow discharge. With increasing Ge‐content the optical gap as well as the energy and the half width of the luminescence peak decrease linearly. It is concluded that by admixture of Ge the tail state distribution becomes steeper and that the concentration of defects is considerably enhanced. Already small amounts of Ge lead to a drastic decrease of the photoconductivity (ημτ‐product).
Theory of Tunneling into Size‐Quantised Films with Application to Lead
Authors: Cottey A. A.
Published: July 2007

Electrons tunneling through an oxide layer into a sufficiently thin metal or semiconductor film can resonate with the discrete “particle‐in‐a‐box” energy levels in the film. The theory of this effect is given in the case that the electrons couple with only one “standing Bloch wave”. It is possible to deduce from experiment five parameters of the relevant group of electrons. These are the energy, velocity, effective mass, surface scattering phase shift η, and dη/dk (𝒽𝓀 crystal momentum). Values of the energy, velocity, and phase shift for Pb are presented, deduced from experimental data of Jaklevie and Lambe, and an approximate value of the effective mass. (The value of phase shift published earlier by the present author is revised).
On the Statistical Distribution Functions of the Anharmonic Crystal
Authors: Olkhovskii† I. I.
Published: July 2007

The solution technique of the BBGKI equation set for symmetric distribution functions of equilibrium systems is considered. The solution of the integral equations of the first and second approximations with respect to temperature is demonstrated. The equations of state generalizing the corresponding equations in the thermodynamic perturbation theory are obtained. It is also shown that the major contribution to the equations is correlated with the distribution function coefficients which are independent of the distances between crystal lattice points. The coefficients dependent on the distance between the crystal lattice points, and giving appreciable corrections in the equations of state, are obtained.
Modified Fielek Schemes and Phonon Dispersion in Noble Metals
Authors: Agrawal R. M. , Rathore R. P. S.
Published: July 2007

Relations for phonon dispersion in noble metals are derived on the basis of three simplified and improved Fielek schemes. These schemes assume the core‐core interaction to be central pairwise. First of these schemes assumes the interaction among the neighbouring d‐shells to be angular, while the second scheme describes these as purely radial. The third scheme incorporates the volume interactions in the Fielek scheme on the lines of the modified Bhatia scheme. First and second models are further made to incorporate the equilibrium condition on simple and realistic lines. The dispersion relation in noble metals predicted by these schemes are compared (a) mutually and (b) with the experimental data to arrive a t the conclusions regarding the nature and range of the interactions coupling the various constituents of the non‐simple metals, like noble metals.
Interband Absorption in the Coloured Intermetallic Compounds PdIn
Authors: Nomerovannaya L. V. , Kirillova M. M. , Shaikin A. B.
Published: July 2007

The refractive, 𝓃, and absorption, 𝓀, indexes of six intermetallic PdIn compounds are measured in the spectral range of 0.248 to 17 μm by Beattie's polarimetric method, the indium concentra‐tion being 42, 44, 46, 48, 50, and 52 at%. All the compounds are single crystals. It is shown that as the indium concentration increases from 42 to 52 at%, the interband absorption peaks found at 0.15 to 0.17, 0.6 to 0.7, 2.2 to 2.65, and 2.6 to 3.2 eV shift towards lower energies, while that at 4.6 eV does not change its position with temperature and indium concentration. Comparison of the high‐frequency conductivity σ(ω) dispersion and the energy bands of equiatomic β′‐PdIn shows that the interband absorption at 𝒽ω < 3 eV is caused by the electronic transitions from the Fermi level to vacant sp conductive states. The value of the spin‐orbit band splitting along the T‐direction of the Brillouin zone is estimated to be 0.15 to 0.17 eV.
The Influence of the Environment of the Absorbing Atom on the Photoelectric Yield Spectra of Beryllium Compounds in the Soft X‐Ray Region
Authors: Blokhin M. A. , Orlova E. G. , Schweitzer I. G.
Published: July 2007

The photoelectric yield spectra (PYS) of beryllium compounds are investigated experimentally. The BeK absorption edge energy position is shown to be affected by the composition not only of the first coordination sphere, but also of the second one. It is found, that the fine structure of the BeK PYS of beryllium fluoride, hydroxide, and acetate is determined by the first coordination sphere and can be interpreted on the basis of electronic structure calculations for the corresponding structure‐forming tetrahedra, taking into account their distortions in the real crystal lattice. An essential difference is revealed in the fine structure of the initial region of the BeK photoelectric yield spectra of the compounds Na2BeF4, K2BeF4, BeO, BeSO4 · 4 H2O, BeSiO3, and Be3(PO4)2 as compared with the spectra of BeF2, Be(OOCCH3)2 and Be(OH)2. This difference is displayed in the presence of a second line group at a distance of 4 to 5 eV from the first near‐the‐edge group and is due to a strong influence of the second coordination sphere atoms on the formation of fine structure near the beryllium K absorption edge.
Selection Rules and Absorption Lineshapes for Electron Transitions Between Minizones in a Superlattice
Authors: Dubrovskii G. B. , Belyavskii V. I.
Published: July 2007

Theoretical analysis of the selection rules and absorption line shapes is presented for inter‐minizone transitions of electrons in a superlattice. Impurity absorption associated with electron transitions from the donor level is also considered. The results show good agreement with the experimental data on the optical absorption in SiC crystals known as natural electronic superlattices.
Simple Two‐Band s–d Model for Ferromagnetic Semiconductors with Hybridization. II. Further Development and Optical Properties
Authors: Szőcs V. , Čápek V. , Velický B.
Published: July 2007

The two‐band model of electronic states proposed previously for the conduction band of some types of ferromagnetic semiconductors like chromium chalkospinels is discussed and used to prove the significance of the higher order spin‐flip processes. On the basis of this model a simple valence‐to‐conduction band absorption is investigated at a level of the self‐consistent CPA. The inadequacy of such mechanism for the explanation of the absorption edge at least in CdCr2Se4 is shown. The lowest order mean‐field approach to the d‐states is illustrated to yield a spurious agreement with the experiment.
The Effect of Rare‐Earth Impurities on the Electronic Transition in Ce
Authors: Korshunov A. N. , Fedorov V. M. , Sharshakov I. M. , Grokhotov V. T.
Published: July 2007

A model of the α‐γ transition in Ce1−xRx alloys where R is a rare earth is presented which is based on the Ramirez‐Falikov (RF) zone model. It is shown that the decrease in the temperature region of the α‐γ transition in cerium when impurities are added depends in particular on the spin degeneracy (2J + 1) of the impurity element ion. The amount of the impurity added affects the conditions under which the α‐γ transition of type I in pure Ce proves to be a transition of type II in Ce1−xRx alloys. A comparison with experimental results is given.
Isochromat and SXAPS Measurements on Hafnium, Tantalum, and Tungsten
Authors: Dose V. , Scheidt H.
Published: July 2007

The density of unoccupied states in the conduction band of hafnium, tantalum, and tungsten is measured employing soft X‐ray appearance potential (SXAPS) and bremsstrahlung‐isochromat spectroscopy (BIS). The MV core levels of Hf, Ta, and W with binding energies of 1662, 1735, and 1809 eV, respectively, are used for the SXAP measurements. Since the MV spectra of these elements exhibit rather poor signal to noise ratios, a novel self‐tuning SXAP spectrometer which provides the necessary stability for the required long term measurements, is employed. Isochromat spectra are taken using the very same SXAP spectrometer with an additional aluminium absorption foil in front of the X‐ray photodetector. As described previously this setup provides a BI‐spectrometer with a monochromator energy equal to the K‐absorption energy of 1560 eV of the aluminium foil. Experimentally obtained density of state (DOS) functions from BIS and deconvoluted SXAP spectra compare favourably with each other and with theoretical calculations.
Multiple Scattering in Disordered Materials III. Compact Representation of the Wave Number and Introduction of Local Currents
Authors: Lenk R.
Published: July 2007

Some general conclusions are drawn from the basic mathematical structure of a theory developed in a preceding paper. Employing a proper normalization of the strength coefficients which are introduced there together with a reference function to get a general representation of the incident fields, these coefficients as well as the reference function get an enhanced physical meaning. With the normalized strength coefficients a compact representation for the wave number of the medium is derived. Especially the attenuation term Im k2 shows a close correspondence to the kinematic expression on the one hand and to the local loss currents, on the other hand. Via the introduction of these currents a deeper understanding of the loss mechanism and the possibility to analyse the angular distribution of these currents is achieved. The fields for a half‐space and a slab are determined. The results exhibit the shortcomings of a macroscopical‐optical analysis in the case of anisotropic scattering at single scatterers.
Theoretical Investigation of Current Amplitudes of Niobium–Lead Tunnel Junctions
Authors: Meyer H.‐G. , Seidel P.
Published: July 2007

The current amplitudes in Werthamer's general expression for the current in Josephson tunnel junctions are investigated with the help of the proximity‐effect model developed for superconducting niobium tunnel junctions. Numerical calculations of these amplitudes as functions of frequency and temperature are presented for different systems forming the proximity electrode. In agreement with experiments marked differences can be stated between the proximity model and the commonly used standard BCS model also in the ac properties of the tunnel junction.
57Fe Absorption and Emission Mössbauer Spectroscopy in Co (III) Acetylacetonate
Authors: Sanchez J. P. , Friedt J. M. , de Jesus Filho M. F.
Published: July 2007

The results of a comparative investigation of the electronic behavior of 57Fe impurities diluted in Co (III) acetylacetonate is reported for conditions in which these impurities are introduced either chemically or by 57Co electron capture decay. The 57Fe absorber experiments provide evidence for a ± 1/2 electronic ground state in this host. The 57Co source experiments show that a fraction of the EC decay products stabilizes as substitutional Fe3+ sites, while another part occupies defect sites both, as Fe3+ and aliovalent Fe2+ ions. The anomalous Fe3+ component does not reveal the expected paramagnetic relaxation effects. This behavior is attributed to the creation of paramagnetic radicals as a consequence of the self‐radiolysis associated with the Auger deexcitation. A comparative discussion of the after‐effects of the 57Co decay in different chemical environments is presented.
Isotropic EPR Spectra of Vanadium in Alkali Chlorides
Authors: Van Steen F. , Vanhaelst M. , Matthys P. , Boesman E. , Schoemaker D.
Published: July 2007

Some new EPR spectra of alkali chloride containing vanadium are reported. The paramagnetic complex V2+–vacancy, which dominates in quenched crystals, decomposes under X and γ irradiation at room temperature (in NaCl and KCl) and under high uniaxial pressure (in KCl and RbCl). The study of the resulting isotropic EPR spectra proves that the new paramagnetic centre consists of a V2+ ion in a cubic environment. The conversion under pressure is due to running dislocations, which absorb the charge compensating vacancy. With X irradiation, the conversion is one order faster in KCl than in NaCl.
Virtual Surface States of Thin Films
Authors: Jerrard R. J. , Ueba H. , Davison S. G.
Published: July 2007

The surface Green function for a two‐interface system, obtained by the factorization method, is used to study the virtual surface states, which arise when a linear field is applied to a semiconductor thin film. The energies of the surface states are evaluated as a function of the film thickness. The surface density‐of‐states calculation gives a means of distinguishing the surface states according to the surface on which they are localised. It is also shown that the interaction between the states restricts their existence to certain regions of the energy gap.
Anisotropy of Exciton Polaritons and Polariton–Phonon Interaction in 2H‐PbI2 Crystals I. Structure of the Fundamental Excitonic Emission
Authors: Biellmann J. , Meyer B. , Schwab C.
Published: July 2007

A procedure is described which allows to prepare optical faces parallel to the c‐axis by processing bulk 2H‐PbI2 crystals grown from the melt. Its effectiveness is checked by a simultaneous investigation of the reflection and photoluminescence spectra, whose main features compare well to former results obtained on cleaved or natural faces.