bluuebunny/crossref_metadata_2025 huggingface dataset:
Infinite Range Ising Models with a Tricritical Point
Authors: Lee B. S.
Published: July 2007

Two infinite range Ising models are solved exactly using a microcanonical ensemble in the longrange order parameter. Classical tricritical points exist for both models. The first model involving two‐spin and four‐spin terms is relevant to the compressible Ising model. The other model is the infinite range version of the Blume‐Capel spin‐one Ising model. In both the cases the exact free‐energy functions calculated are examples of exact classical Landau free‐energy functions with tricritical point behaviour.
Symmetries of Luminescent Centers in NaCl:Pb2+ Excited in the A‐Absorption Band
Authors: Benci S. , Fermi F. , Reverberi R.
Published: July 2007

Polarized luminescence measurements are used to study the luminescent characteristics of NaCl:Pb2+ excited into the A‐absorption band. For the E1 = 4.01 eV emission it is found that the luminescent centers have tetragonal and trigonal symmetries due to the Jahn‐Teller effect that allows the coexistence of tetragonal and trigonal minima on the 3T1u APES while the E2 = 3.29 eV emission band is due to aggregates of impurities in the {111} planes with trigonal symmetry. The charge compensating vacancy associated to the impurity gives a meaningless effect on the luminescence polarization and its position cannot be ascertained.
Theory of Non‐Radiative Multiphonon Capture Processes. Solution of Old Controversies
Authors: Peuker K. , Enderlein R. , Schenk A. , Gutsche E.
Published: July 2007

The solution is given for long persisting controversial problems in the theory of non‐radiative multiphonon capture of free carriers. Taking the adiabatic wave functions as basic states for the perturbational treatment of the transition probability particular attention is focused on the correct determination of the transition matrix elements. In this framework it is shown that to first order in the non‐diagonal matrix element of the electron‐lattice interaction all approaches developed so far (Condon approximation, non‐Condon approximation, static approximation, etc.) lead to the same result, in contradiction to statements in the literature. In particular, the reason is found why earlier results for the transition rate within the adiabatic and Condon approximation are much too small. The static approximation is shown to be the lowest approximation to the adiabatic approach.
Shallow Acceptors in Cadmium Telluride
Authors: Molva F. , Chamonal J. P. , Pautrat J. L.
Published: July 2007

The photoluminescence spectroscopy of undoped CdTe crystals provides information on two acceptors which are the main contaminants of as‐grown crystals. These acceptors called y and z have their respective ionization energies at 147 and 108 meV. A complete series of two hole replicas is observed for y and only one for z. These acceptors are believed to be due to Cu and Ag impurities. Backdoping experiments with Li and Na give rise to new transitions. From the conduction band–acceptor level transitions, the ionization energies are respectively obtained to 57.8 and 58.8 meV. In the case of Li, some extra‐lines are tentatively identified as phonon interacting two‐hole transitions.
Derivation of the Scattering Model for Josephson Effects from the Ginzburg‐Landau Equations
Authors: Orlowski M. C. L.
Published: July 2007

The scattering model for the Josephson effects as proposed some years ago is rigorously derived from the Ginzburg‐Landau equations. Along these lines the validity of the scattering model and new aspects of the present derivation are discussed. The derivation exhibits the relevance for the fermionic character of the Cooper electrons in the weak‐link situation. Attention is called to the possible application of this model in field theories for dual string.
Micromagnetic Theory of Phase Transitions in Inhomogeneous Ferromagnets. V. Magnetic Phase Transition of Amorphous Fe10Ni70B16P4
Authors: Meyer R. , Kronmüller H.
Published: July 2007

The static magnetization curves and the dc susceptibility of amorphous Fe10Ni70B16P4 are measured in the low temperature region and in the vicinity of the critical region up to fields of μ0H = 1 T. The behaviour near the Curie temperature is found to obey critical material laws with critical indices: β = 0.44 ± 0.02; γ = 1.63 ± 0.07; δ = 4.82 ± 0.20, and the ferromagnetic Curie temperature TC = (191.1 ± 0.1) K. The critical indices satisfy the static scaling law γ = β(δ – 1). The exponent γ is found to apply over a wide temperature range of T – TC ± 100 K. The existence of this extended critical region is attributed to the correlation between temperature dependent spin clusters and the static fluctuations of the exchange interactions.
Cohesion, Phase Transition, Elastic and Dielectric Properties of Group IV Semiconductors
Authors: Ghosh A. , Banerjee L. K. , Basu A. N.
Published: July 2007

Based on the energetics of bond formation and a statistical model analysis of the electronic charge density deformation in a covalent crystal the existence of an effective three‐body interaction is suggested between the particles in a lattice. In addition to describing the different properties of group IV semiconductors the present model together with the idea of the structure dependence of potential parameters provides a satisfactory description of the polymorphic phase transition of these substances.
Relaxation and Optical Processes in Impurity Centres with Two‐Well Adiabatic Potential
Authors: Kulagin S. A. , Osadko I. S.
Published: July 2007

The theory of interwell and intrawell relaxation is developed. Numerical calculation of appropriate transition propabilities is realized. It shows relaxation dependence on temperature and two‐well system parameters. On the basis of the same theory two specific experimental effects (the appearance of “multiplets” in quasi‐line spectra and “hole‐burning”) are analyzed.
Equation of State of SiGe Solid Solution
Authors: Soma T. , Iwanami H. , Matsuo H.
Published: July 2007

The equation of state of the SiGe solid solution is studied using the previous electronic theory based on pseudopotentials. The calculated results for the Si1−xGex solid solution are not monotonous as function of x, and show the softening of the crystal under compression in comparison with those of pure Ge.
Isoelectronic Impurity Te in CdS1−xSex Mixed Crystals
Authors: Goede O. , Heimbrodt W.
Published: July 2007

In hexagonal CdS1−xSex amalgamation‐type mixed crystals the isoelectronic impurity Te is studied as a function of the composition x in the range 0 ≦ x ≦ 1 by emission and excitation measurements at 4 and 77 K. The binding energies of excitons bound to isolated Te impurities and nearest‐neighbour Te pairs are found to decrease strongly with increasing x. For x ⪆ 0.85 isolated Te cannot bind an exciton and, therefore, in CdSe only Te pairs lead to bound impurity states. The measured relaxation energies and halfwidths of the corresponding emission and excitation bands yield information on the x‐dependence of the electron‐phonon interaction for both types of impurities. The experimental results are discussed on the basis of a simple potential‐well model using variational calculations.
Effect of Size Quantization on the Electronic Heat Capacity in Ultrathin Films of Non‐Parabolic Semiconductors
Authors: Ghatak K. P. , Ghosh K. K. , Mukherjee H. M. , Chakravarti A. N.
Published: July 2007

An attempt is made to investigate the effect of size quantization on the electronic heat capacity in ultrathin films of Kane‐type non‐parabolic semiconductors. It is found, taking n‐InSb as an example, that the heat capacity corresponding to a given carrier concentration per unit area increases considerably with decreasing film thickness. The heat capacity corresponding to a given film thickness is also observed to increase with increasing carrier concentration. Besides, humps are seen to appear in the variation of the heat capacity with film thickness as a direct consequence of energy quantization along the direction normal to the film.
Thermal Dissociation Energy of F‐Centers in Alkali Halides
Authors: Dalal M. L. , Sivaraman S. , Murti Y. V. G. S.
Published: July 2007

The electrolytic coloration process is employed to obtain the mobility of F‐centers in KI, KBr, KCl, and NaCl crystals by measuring the drift velocity of the color cloud, during its withdrawal as function of temperature. The motion of the color cloud is timed between two previously marked lines and the field was also measured between them. An optical arrangement is used to project the crystal on a screen. The thermal dissociation energy EF is deduced for all these crystals which comes out to be 2.33, 2.66, 2.88, and 2.45 eV for KI, KBr, KCl, and NaCl crystals, respectively. The variation of the ratio of the fields (Uun/Ucol) in the colored and uncolored regions is also studied with temperature. A Mollwo‐Ivey type of relation is also tried between EF and various physical properties of the crystals, from the extrapolation of which the hS value for NaI is deduced which agrees very well with the reported value, where hS is the Schottky defect formation energy.
On the Lineshape of the Conduction Electron Spin Resonance in Thick Metallic Samples under the Anomalous Skin‐Effect Conditions
Authors: Ustinov V. V. , Khusainov D. Z.
Published: July 2007

The influence is investigated of quantum effects caused by the comparability of the effective de Broglie wavelength of the electron and skin depth, on the conduction electron spin resonance lineshape in thick metallic samples under anomalous skin‐effect conditions. The dependence of the asymmetry parameter of the line of derivative of the power absorbed with respect to the magnetic field (A/B ratio) on the value of the parameter A equal to the ratio of effective de Broglie wavelength of the electron and double skin depth is calculated.
Covalent Contribution to the Bond Energy of Phases with Ll2 Structure
Authors: Bogdanov V. I. , Ruban A. V. , Fuks D. L.
Published: July 2007

The Brillouin‐Wigner pseudopotential perturbation theory is used for calculating multi‐ion interaction effects contributing to the mixing energy and the total bond energy of phases with Ll2 structure. It is shown that these contributions are of the same order as the mixing energy itself and they must be taken into account in theoretical investigations of the order–disorder phase transition temperature. Numerical results for some alloys with Ll2 structure are presented and the phase transition temperatures for Cu3Au and Au3Cu are discussed.
Calculated Low Frequency Phonon Dispersion in Anthracene‐d10
Authors: Chaplot S. L. , Pawley G. S. , Dorner B. , Jindal V. K. , Kalus J. , Natkaniec I.
Published: July 2007

A model is presented for the low frequency lattice vibrations in anthracene‐d10, which involve the molecular external vibrations and certain internal distortions, namely the butterfly and twisting motions. The calculated phonon dispersion relation is compared with experimental results from neutron coherent inelastic scattering, Raman scattering, and infrared absorption.
Influence of Impurities on the Wannier‐Stark Ladder in a Semiconductor in a Strong Electric Field
Authors: Berezhkovskii A. M. , Ovchinnikov A. A.
Published: July 2007

Limitations on the impurity concentration are obtained which, when fulfilled, preserve the Wannier‐Stark ladder even in the presence of impurities.
The Γ‐Point Band Structure Parameters of CdxHg1−xSe (x ≦ 0.1) Mixed Crystals from Thermomagnetic Measurements
Authors: Iwanowski R. J.
Published: July 2007

Measurements of thermoelectric power as a function of magnetic field, α(H), at 30 and 100 K in undoped n‐CdxHg1−xSe (x = 0.05; 0.10) mixed crystals are performed. Electron effective mass at the Fermi level vs. carrier concentration is determined. The fitting of theory to the experimental m(n) dependences leads to the verification of certain band structure parameters. In particular, a modified E0, (x, T) dependence for CdxHg1−xSe in the range T ≦ 100 K is proposed. Additional argument for the modified E0(x, T) dependence is given on the basis of the longitudinal Nernst‐Ettingshausen effect (Δα∞) measurements at 30 K. The theory, which includes the most important scattering mechanisms, gives much better agreement with the experiment when applying the new values of disorder scattering parameters V and W in calculations.
Composition Dependence of the Electron Fermi Surfaces Tilt in Bi1−xSbx Alloys (0 ≦ x ⪅ 0.22)
Authors: Braune W. , Fellmuth B. , Kubicki N. , Herrmann R.
Published: July 2007

The composition dependence of the Fermi surfaces tilt out of the basal plane in Bi1−xSbx alloys (0 ≤ x ⪅ 0.22) is determined by investigating the anisotropy of the propagation of magnetoplasma waves in the Voigt configuration B ⊥ C2 ∥ k. In the semimetallic phase (0 ≤ x ⪅ 0.07) no change is observed whereas in the semiconducting phase (0.07 ⪅ x ⪅ 0.22) a strong composition dependence is obtained, which is marked distinctly especially near the semimetal‐semiconductor phase transitions (at x ≈ 0.07 and x ≈ 0.22) and in the composition region 0.15 < x < 0.18. The dispersion relation of the charge carriers, which is based on the deformation theory, connects the characteristics of the Fermi surfaces with the sub‐lattice displacement, the rhombohedral shear, and the spin–orbit coupling. A first analysis indicates that those composition dependences of these quantities which may cause the complex change of the charge carrier spectrum found are more complicated than assumed till now.
On Superconductivity in Narrow‐Band Alloys
Authors: Beckmann A. , Kolley E. , Kolley W.
Published: July 2007

Two different approaches to disordered superconductors are examined to derive a random contact model on the basis of nonlocal electron‐phonon coupling. The formalism, aimed at transition metal alloys in the weak‐coupling regime, takes into account both, the finite‐range pairing interaction, and the intraatomic Coulomb repulsion of the tightly bound electrons. The superconducting transition temperature of AcB1−c alloys is calculated for the nonmagnetic and magnetic cases.
A Double Chain Approximation to the Ising Model
Authors: Plascak J. A. , Silva N. P.
Published: July 2007

To calculate thermodynamic properties of the anisotropic Ising model, a double chain approximation (DCA), based on Bogolyubov's variational procedure, is presented. The critical temperatures are compared to those obtained by the usual mean field approximation (MFA), the linear chain approximation (LCA), and the exact or series results, when existing. A fitting of the spontaneous polarization and the dielectric constant data for the quasi‐one‐dimensional ferroelectric crystal PMDP (PbDPO4) is also made.
Effect of Surface Polarization on the Absorption Spectrum of an F‐Center
Authors: Mijangos R. R. , Marin J. L. , Clark A. , Navarro A. , Mejía C. Ruíz
Published: July 2007

The effect of surface polarization on the ground state of an Fs center in NaCl and KCl is studied. The energy of this state is calculated by perturbative and variational methods. The F band position of this center is obtained also, for some alkali halides, by first order perturbation. It is found that these effects are, in general, small.
Temperature Dependence of the Phonon Spectra of β‐AgI
Authors: Kostadinov I. Z. , Binh Vu Thi
Published: July 2007

The temperature dependence of the far infrared absorption of β‐AgI is calculated. The increasing occupation of higher energy minima by Ag+ ions leads to the appearance to two low energy phonon lines with frequencies of 38 cm−1 and 85 cm−1, respectively, discussed lately by Brafman and Vardeny. The current calculations taking into account the hopping kinetics for the pseudospin variables lead to an infrared conductivity similar to that observed by Funke and Jost. The activation energy of 75 K is in good agreement with the temperature dependence of the infrared conductivity discussed by Burns, Dacol, and Shafer.
Spin Glass and Superantiferromagnet in the Site Diluted Ising Model on the Frustrated Square and F.C.C. Lattices with First‐ and Second‐Neighbor Interactions
Authors: Katsura S. , Fujiki S. , Suenaga T. , Nagahara I.
Published: July 2007

The diluted Ising model of the frustrated square and face‐centred cubic lattices with first‐ and second‐neighbor interactions are considered by the square‐cluster approximation. Phase boundaries between the paramagnetic and ordered (ferromagnetic, superantiferromagnetic (antiferromagnetic states of the second kind in f.c.c.) and spin glass) phases are obtained. The result explains qualitatively the experiment on MnpCd1–pTe.
f‐Level Green's Function for an Intermediate‐Valence Impurity
Authors: Schlottmann P.
Published: July 2007

Mori's technique is used to rederive the f‐level Green's function of a mixed valence impurity with two arbitrary ionic configurations. The result agrees with the Brillouin‐Wigner resummation of Goldstone diagrams by Grewe and Keiter. The leading Brillouin‐Wigner approximation leads to an unphysical gap in the excitation spectrum of the impurity, which is closed in next leading order. The drastic change in the excitation spectrum is related to a violation of the fluctuationdissipation theorem. The consequences of a gap or continuous excitation spectrum are discussed. The advantage of the method is that only two commutators need to be evaluated in order to rederive the Brillouin‐Wigner results.
Optically Induced Potential Fluctuations in a‐Si:H Films
Authors: Hauschildt D. , Fuhs W. , Mell H.
Published: July 2007

The influence is studied of prolonged optical exposure on the properties of the current path in glow discharge deposited a‐Si:H films. Information on these properties is obtained from concomitant investigations of the conductivity σ and thermoelectric power S as a function of temperature. It is found that the activation energies of both quantities, Eσ and ES, increase appreciably upon light exposure. At the same time the difference Eσ — ES = EQ rises from ≈ 0.09 to 0.21 eV. The quantity EQ, which is an apparent mobility activation energy, is associated with spatial fluctuations of the mobility edge. The increase of EQ is attributed to the enhancement of potential fluctuations which is caused by inhomogeneously distributed charged centers generated during optical exposure.
Magnetization, Electrical Resistivity, Thermopower, and Neutron Diffraction in HoCu2
Authors: Gratz E. , Sechovsky V. , Sima V. , Smetana Z. , Ström‐Olson J. O.
Published: July 2007

HoCu2 shows a Curie‐Weiss susceptibility above 15 K. The paramagnetic moment and the paramagnetic Curie temperture is (10.1 ± 0.1) μB/H0 and (4 ± 2) K, respectively. Two cusps (at about 7 and 10 K) are seen in the magnetization versus temperature curve. These cusps reflect a change of the magnetic structure and the Néel temperature. The influence of conduction electron scattering on the localized 4f moments is estimated for the resistivity and the thermopower. Neutron diffraction measurements show that the antiferromagnetic structure is incommensurate with the crystal lattice.
Ultrasonic Investigation of the Acceptor Ground State of Si(B) I. The Longitudinal and Transverse Relaxation Rates of the Strain‐Split Two‐Level System
Authors: Zeile H. , Harten U. , Lassmann K.
Published: July 2007

The “spin”‐lattice relaxation of the two‐level system of the strain‐split acceptor ground state of Si(B) is determined from the temperature dependence of the ultrasonic relaxation attenuation, around 10 K. It is found that Raman relaxation alone does not fit the results but that, in addition, an Arrhenius‐type of relaxation, possibly due to a Jahn‐Teller effect, is involved. The transverse relaxation is obtained from intensity‐dependent and hole‐burning measurements of the resonant attenuation. At small acceptor concentrations it is determined by “spin”‐lattice relaxation alone; for higher concentrations an additional temperature‐independent interaction term is found much smaller than expected for elastic dipole or exchange interaction.
Host‐Lattice Effects in the EPR Spectra of Gd3+ Doped Hydrated Single Crystals of Rare‐Earth Chlorides, Nitrates, and Sulphates
Authors: Misra S. K. , Mikolajczak P. , Jalochowski M.
Published: July 2007

Large amount of X‐band EPR data, for external field orientations in both, the zx and xy planes, are obtained for Gd3+ ion doped rare‐earth trichloride hexahydrate (RCH), rare‐earth trisulfate octahydrate (RSO), and rare‐earth trinitrate hexahydrate (RNH) single crystals. A rigorous least‐squares fitting procedure is used to evaluate the spin Hamiltonian parameters g‖, g⊥ and b (including those with negative m values). Approximate linear dependences upon the host rare‐earth ion radius is found on both, the parameters b and b within each family of the isostructural hosts investigated.
Calibration of the Isomer Shift for the 67.03 and 13.3 keV Transitions of 73Ge
Authors: Antoncik E. , Bing‐lin Gu
Published: July 2007

The usual procedure using free‐atom/ion wave functions to calibrate the Mössbauer isomer shifts is modified in order to include solid state and band structure effects properly. To estimate the electron densities at the nucleus, self‐consistent‐field Dirac‐Fock‐Slater wave functions are calculated for several electronic configurations of germanium atoms and ions, taking into account the finite space allowed to these atoms (ions) in solids. Using experimental isomer shift (IS) values, the calibration constants for 73Ge both for the 67.03 and 13.3 keV transitions are calculated, giving the relative change of the nuclear‐charge radius ΔR/R = 0.5 × 10−3 and ΔR/R = 1.2 × 10−3, respectively. The accuracy of the calibration procedure is discussed.
The Bond‐Bending Force Model (BBFM) for Phonons in β‐SiC
Authors: Kushwaha M. S.
Published: July 2007

The normal modes of vibrations for cubic SiC are investigated on the basis of an eight‐parameter bond‐bending force model (BBFM). The model consists of short‐range (central + bond‐bending) forces and long‐range Coulombic interactions including up to second nearest neighbours. The results for phonon dispersion relations in the three highest symmetry directions [100], [110] and [111], phonon density of states, and Debye‐charcteristic temperatures are shown. The theoretical results compare well with available experimental data.
Influence of Electron Heating on Reflection and Attenuation Coefficients of Electromagnetic Waves in Semiconductors with Superlattice
Authors: Dykman I. M. , Prokhnitskii L. A.
Published: July 2007

The electromagnetic wave reflection from the surface of a semiconductor with a superlattice in a heating electric field is studied. The superlattice is formed by the periodic distribution of the electron density. Frequency dependences of the reflection and attenuation coefficients in the vicinity of the Bragg resonance are calculated, taking into account electron interaction with lattice vibrations. These dependences are shown to have maxima at the resonance frequency ωR. The shape and magnitude of the maxima depend significantly on the ratio |ξ| ωR/v0, where the amplitude of electron density modulation |ξ| and the effective collision frequency v0 are functions of the heating field. The field dependences of |ξ| and v0 show the possibility of controlling the Bragg reflection effects, which can be used, for example, to investigate v0.
Pseudo‐Exciton Effect in the Schottky‐Barrier Electroreflectance
Authors: Kiselev V. A.
Published: July 2007

An account of the nonhomogeneity of the electric field in the space‐charge region of a semiconductor is presented. It is shown that the nonuniform field can lead to a sharp feature in the reflection spectra in the vicinity of a direct gap. The behaviour of this new lineshape with varying surface potential reproduces closely the behaviour of the exciton lineshape in reflection for which the phenomenon of “rotation” is well known. This effect is determined by the nonuniform character of the joint density of states involved in the band‐to‐band transition in the direction normal to the surface. The modulated electroreflectance spectra obtained employing the theory that neglects exciton effects are in good agreement with literature results of measurements on GaAs with donor concentration of 4.5 × 1016 cm−3 at room temperature. A possible application of the observed phenomenon, as well as that of the exciton lineshape rotation, to investigate the energy and time characteristics of the space‐charge region and surface (interface) states is discussed.
On the Long‐Wavelength Plasmon in the Alkali Metals A Nearly‐Free‐Electron Pseudopotential Calculation
Authors: Oliveira L. E.
Published: July 2007

The nearly‐free‐electron pseudopotential theory is used to calculate the macroscopic dielectric function ε(O, ω) for Li, Na, and K in the long‐wavelength limit within the self‐consistent‐field approximation. The plasmon energies are obtained from the peak position of the energy loss function Im [–1/ε(O, ω)]. The results are compared with the experiment and the agreement is good provided that the effects of ion‐core polarization in the long‐wavelength plasmon energy are taken into account.
Raman Scattering from Nonfundamental States in Ammonium Chloride Crystals at Low Temperatures
Authors: Umarov B. S. , Gorelik V. S. , Sushinskii M. M. , Vetelino J. F.
Published: July 2007

The Raman spectra of the δ‐phase of ammonium chloride crystals at low temperatures are carefully investigated. Particular attention is focused on the Raman spectra in the region of the internal vibrations of the ammonium group. At liquid helium temperatures intense sharp lines of definite symmetry type are observed. These lines are identified as two particle bound states (biphonons) in agreement with theoretical predictions.
Electron–Molecular Vibration Coupling in Trimerized Organic Ion‐Radical Semiconductors
Authors: Yartsev V. M.
Published: July 2007

The totally‐symmetric (ag) internal molecular vibrational excitations in a trimer are considered. The mode corresponding to the charge transfer (CT) between non‐centric molecules is shown to be responsible for the indirect IR activity. The trimer MO calculations in the site occupation representation are simplified and the symmetry of the basis states is taken into account. Eigenstates and eigenvalues of the molecular trimer with two radical electrons are calculated as a function of the transfer integral, t, and the Coulomb „on‐site”︁ repulsion, U. The following interpretation of the absorption spectrum of the cesium salt of tetracyanoquinodimethane Cs2(TCNQ)3 is given: the 1.4 eV peak is attributed to intratrimer charge transfer excitation, the less than 0.6 eV peak corresponds to the electronic transition between neighbouring trimers. The electron–molecular vibration coupling constants and Hubbard parameters U and t are evaluated for Cs2(TCNQ)3.
The Theory of the Strain Effects in the Crystals Described by the s (d)‐f Model
Authors: Stasyuk I. V. , Grigorchuk R. A.
Published: July 2007

In the framework of the s(d)‐f model taking into account the hybridization of localized and band states, the relative shift of the band and its broadening at strain, the possibility of the jump‐like change of unit‐cell volume and the localized state filling under external pressure is investigated. The phase diagrams in the pressure‐averaged total electron number plane are obtained. The behaviour of the crystal compliance at the phase transition induced by pressure is studied.
Metamagnetoelastic and Metamagnetoelectric Behaviour of the Jahn‐Teller Antiferroelastics
Authors: Kaplan M. D.
Published: July 2007

It is shown, that the magnetic field induced structural phase transition from the antiferrodistortional to the ferrodistortional phase is possible in Jahn‐Teller antiferroelastics. The phase transition under consideration is simultaneously the crystal transition from the ferroelectric into the antiferroelectric state. The anomalously sharp change of the crystal strain and polarization is realized in the range of the critical magnetic fields. Such a behaviour of crystal strain and polarization is named metamagnetoelasticity and metamagnetoelectricity, respectively. The properties of the ESR spectra of the system under consideration are analysed. The possibilities of experimental observation of the predicted effects are discussed.
An Interatomic Pair Potential for Copper Calculation of Stacking Fault Energy
Authors: Rautioaho R. H.
Published: July 2007

Two potential functions, V2(R) and Veff(R) are derived for Cu from phonon dispersion curves. V2(R) is fitted to the phonon frequencies without considering possible multi‐ion effects, whereas Veff (R) is constructed by including dominant multi‐ion interactions in the potential. V2(R) is in relatively close agreement with more fundamental potentials at short distances, whereas Veff(R) appears to give a somewhat better representation of the long‐range interactions. Both of these potentials are used in a calculation of the energies of intrinsic and extrinsic stacking faults. This calculation gives reasonable results, 30 mJ/m2 for γintr and 35 mJ/m2 for γextr.
Correlation of Transition Frequencies in Impurity Spectra of Inhomogeneous Solids
Authors: Kikas J. , Rätsep M.
Published: July 2007

A statistical theory is developed to obtain the joint distribution function for frequencies of different transitions in impurities affected by matrix imperfections. The results are specified in the case of an inhomogeneous broadening caused by an elastic interaction of impurities with point defects and applied to axially symmetric impurities in cubic crystals to study the correspondence between ransition frequencies in different orientations of the impurity.
Magnetic Relaxation of Ideal Superparamagnets Studied by Monte‐Carlo‐Simulation
Authors: Kostinek J. , Markum H. , Rauch H.
Published: July 2007

The local and total magnetization of small magnetic particles are calculated using a Monte‐Carlo‐procedure and assuming a Heisenberg nearest‐neighbor interaction. The ideal magnetic system relaxes from a non‐equilibrium to an equilibrium state and the observed time constant is obtained in units of Monte‐Carlo steps for various temperatures. Further calculations deal with a system having an antiferromagnetic coupling for the spins within the surface layer. Such systems may simulate superparamagnetic particles with an oxide layer on their surfaces.
Influence of Secondary Excitons in Intensity and Decay of Phosphorescence in Doped Benzophenone Single Crystals
Authors: Melnik V. I. , Nelipovich K. I. , Faidysh A. N. , Shpak M. T. , Yankovskaya L. B.
Published: July 2007

Spectroscopic and kinetics data for the phosphorescence of benzophenone crystals doped with deuteronaphthalene, naphthalene, and α‐bromonaphthalene are analyzed. It is established that the duration of the delayed excitonic phosphorescence depends substantially on τ of the impurity triplet states at T > 170 K. The results obtained are treated in terms of the thermal detrapping of impurity molecule excitation to the triplet excitonic band of the host.
Thermo‐ and Photostimulated Recombinations of F–HA and α–IA Centres in KBr with Large Na Concentration
Authors: Dauletbekova A. , Akilbekov A. , Elango A.
Published: July 2007

In KBr:Na (1 mol%), containing only FHA and α–IA pairs of radiation defects when X‐rayed at 80 K, the thermo‐ and photostimulated (by illuminating with light corresponding to the maxima of F or HA absorption bands) recombinations of these pairs are investigated. The recombination of α and IA centres takes place without emission. In the course of the recombination of HA centres with F centres excited by light there occurs an emission with the maximum at about 2.8 eV. It arises from the relaxation of a tunnelling‐created (HA + e)* centre. The luminescence, accompanying the recombinations of F and H centres (occurring during HA illumination) is very weak and no so‐called π‐emission (at 2.3 eV) is observed. It is concluded that the creation of a self‐trapped exciton by F–H recombination is not efficient.
Far‐Infrared Reflection Spectra, Optical and Dielectric Constants, Effective Charges, and Lattice Dynamics of the Skutterudites CoP3, CoAs3, and CoSb3
Authors: Lutz H. D. , Kliche G.
Published: July 2007

From the far‐infrared reflection spectra of the skutterudite type compounds CoP3, CoAs3, and CoSb3 the TO and LO phonon frequencies at wave vector |q| ≈ 0 are determined by both Kramers‐Kronig analysis and classical oscillator fit method. In the case of CoP3 large free carrier contributions (plasma resonance frequency ω′p ⩽ Ω+ ≈ 800 cm−1) with strong plasmon–phonon interactions are observed. Attempts to calculate the decoupled phonon frequencies of CoP3 using a three and a four parameter model of the dielectric function failed. Lattice dynamical calculations are done by setting up a valence force field consisting of M–X, two X–X and X4–X4, and M–M stretching constants. The obtained force constants, e.g. f(CoSb): 85, f(SbSb): 87 and 59 N m−1, respectively, and also the calculated effective dynamical charges (from TO/LO splittings) show the mainly covalent character of the skutterudites under consideration. The lattice modes of CoSb3 are found to be both mainly internal vibrations of the planary Sb4‐units, v4 and v5, at 174 and 144 cm−1, respectively, and external ones, v1–v3 and v6–v7, at higher and lower wavenumbers than v4 and v5.
On the Electric Subbands in n‐Inversion Layers on (111) Ge
Authors: Germanova K. , Marinova K. , Trayanov A.
Published: July 2007

Self‐consistent results on the subband energy structure of n‐inversion layers on (111) Ge in the effective‐mass approximation at T = 4.2 and 77 K are presented. The nonequilibrium case with a bias voltage, applied between the inversion layer and semiconductor bulk, is described.
Superionic Conductivity versus Ionicity
Authors: Ogorelec Z.
Published: July 2007

Some arguments given suggest that a group of ANB8–N solid electrolytes with monovalent cations may have a superionic phase only if its permittivities satisfy the equation (ε ∞ ‐ l)/(l/ε ∞ – l/ε0)2 ≈ 1000. There are indications of a wider validity of this equation.
Heat Pulse Diffusion in TEA(TCNQ)2
Authors: Grassi H. , Brau A. , Farges J. P.
Published: July 2007

Measurements of the thermal diffusivityDon crystals of the highly anisotropic organic semiconductor TEA(TCNQ)2are reported. These measurements are made using a heat pulse method. For the crystal axis 1 parallel to the TCNQ columns and associated with the largest component σ1of the electrical conductivity,D1is 1.6×10−3cm2s−1at 300 K increasing to 2.7×10−3cm2s−1at 200 K. The thermal conductivityk1is evaluated to 4 mW cm−1K−1at 300 K with an estimated electronic contribution of only 1%. The phonon mean free path at 300 K is extremely small l1≈ 2 Å, of the order of the shortest distance between the TCNQ molecules in a column. It is thus expected to be almost constant at this temperature, a result which is also suggested from the observedTdependence ofD1. For the crystal axis 2 associated with the intermediate component σ2of the electrical conductivity,D2is 2.3×10−3cm2s−1at 200 K. From these results it appears that the thermal diffusivityDis not markedly anisotropic in TEA(TCNQ)2, as it may be expected from a phonon contribution only.
First Principles Calculation of the Energy Band Structure of Diamond
Authors: Persson A. , Jones R.
Published: July 2007

A first principles calculation of the energy band structure of diamond is performed using a muffin tin potential constructed from atomic functions and including the parametrized exchange term due to Slater. The crystal wave functions are expanded in 2s and 2p like functions obtained by integrating the Schrödinger equation for a single atom with a potential equal to the muffin tin potential inside the muffin tin radius and a constant elsewhere. The method gives a good valence band but a poorer conduction band. Excluding three centre integrals, varying the parameter in the exchange term and altering the electron configuration of the carbon atom does not appreciably change the band structure. Finally, including non muffin tin parts of the crystal potential improves the conduction band.
On the Theory of a Heisenberg Impurity Ferromagnet
Authors: Dyakin V. V. , Karpenko B. V.
Published: July 2007

Using the resolvent method a complete solution of the Wolfram‐Callaway problem for a Heisenberg ferromagnet with a single magnetic impurity atom is obtained. The spectrum of the system is determined and the exact wave functions of the continuous and discrete spectra of the system with a unit spin deviation are found. The theorem of completeness for eigenfunctions is proved. The treatment is performed with reference to the example of an infinite crystal with a simple cubic lattice.
A Pseudopotential Calculation of Phonons in Copper
Authors: Kumar B. , Sharma K. S.
Published: July 2007

The pseudopotential theory of d‐band metals is used for the calculation of phonons in copper by employing a linear form of the model pseudopotential for representing the electron—ion interaction and Moriarty's form for the overlap interaction of d‐bands. The computed frequencies compare well with the experimental data. The validity of the approach is further confirmed through a satisfactory representation of binding energy, compressibility, and elastic constants.
Magnetic Properties and Resonance Study of Gd (CoxAl1−x)3 Compounds
Authors: Burzo E. , Seitabla D. , Chiparǎ M.
Published: July 2007

Gd(CoxAl1–x)3 compounds with x ≧ 0.2 are ferrimagnetic. The mean cobalt moments decrease as the aluminium content is larger. The reciprocal susceptibility for ferrimagnetic compounds follows a hyperbolic law of Néeltype. The electron paramagnetic resonance measurements on GdAl3 show that g = 1.959 and the slope of the linewidth is DH/T = 2.8 G/K. For ferrimagnetic compounds the g and DH/T values are temperature dependent. The data are analysed in a model which uses the two—sublattices molecular field approximation. Finally, the magnetic behaviour of cobalt atoms in Gd(CoxAl1–x)3 compounds is analysed.
Magnetic Relaxation Effects in the Mössbauer Spectrum of Fe2+ in Gillespite
Authors: Srivastava K. K. P. , Mishra S. N.
Published: July 2007

The Mössbauer spectrum of paramagnetic 57Fe2+(3d6,5D) ions of the mineral gillespite (BaFeSi4O10) is found to show the effect of magnetic relaxation (i.e. relatively slow electronic spin relaxation rate) from 4.2 to 300 K. It is widely believed that ferrous ions are strongly coupled to lattice vibrations and consequently any spin relaxation effect on their Mössbauer spectra may be observed at very low temperatures only when the spin—lattice relaxation rates will be relatively small. In view of this the observed electronic spin relaxation behaviour of Fe2+ in gillespite is very unusual and rather surprising. A theoretical analysis is made to identify the dominant relaxation mechanism. A detailed analysis of the electronic eigenstates and their relaxation properties shows that in the case of gillespite the spin—spin relaxation is more dominant than the spin—lattice relaxation, which is rather unusual for Fe2+ ions. On this basis the observed magnetic relaxation behaviour of the Mössbauer spectra is consistently explained. It appears that the spin—lattice relaxation rate depends as much on the magnitude of the crystal field splitting as on the symmetry of the crystal field interaction itself as earlier suggested from the study of the low temperature Mössbauer spectra of Fe2+ in various carbonates.
UV Absorption and Electro‐Optical Alignment Behavior of Off‐Center AG+ in RbI
Authors: Kapphan S. E.
Published: July 2007

The polarization of four uv‐absorption bands due to Ag+ in RbI can be determined from the sign of the zero‐moment absorption change under applied electric field. The oscillator strength of these bands shows a temperature dependence very much like that of Ag+ in RbCl and RbBr, with a minimum around 150 K. Assuming fully polarized uv‐transitions the electro‐optical behavior can be fitted to a dipole alignment model with electric dipole moment values (uncorrected for local fields) of p = 1.6 or 2 eÅ. Two models, suggested earlier by Jimenez et al., a 〈111〉 dipole model and a restricted 〈110〉 model with 90° transitions only, allow a quantitative fit. Polarization and form of the uv‐spectra point to the 〈111〉 model.
Intensity Distributions of the K Emission Spectra of Iron
Authors: Sørum H.
Published: July 2007

A curved crystal spectrometry is applied for the study of the electron excited (30 kV) FeK emission spectra. No fine structure is directly seen in the α1,2 and β1,3 lines, but the lines are strongly asymmetric. The asymmetry is due to relatively strong low energy satellites, probably caused by the radiative Auger effects K → LV, K Deutsche Forschungsgemeinschaft MV, in addition to interactions between the final state hole and the unfilled 3d level for β1,3; and to unresolved splittings of the LIII and LII levels for α1 and α2. Weak high energy satellites are also present for the same lines and most likely due to a spectator vacancy in the M level and/or in the valence band V. The Kβ5 band is corrected for background, continuous radiation, self‐absorption, and overlap with β1,3. The intensity maximum of Kβ5 is (2.56 ± 0.1) eV below the Fermi energy. The FWHM value for α1 is (2.60 ± 0.05), for α2 (3.02 ± 0.05), for β1,3 (4.04 ± 0.05), and for β2,5 (5.56 ± 0.05) eV. The index of asymmetry for α1 is (1.57 ± 0.05), for α2 (1.26 ± 0.05), and for β1,3 (1.77 ± 0.05). The spin—Orbit splitting between the 3p3/2 and the 3p1/2 levels obtained from the Lorentzian fit for β1,3 is (1.22 ± 0.05) eV, whereas Hartree‐Fock calculations give 1.58 eV.
Excition Transfer Between the Manganese Ions in the Semiconductor Alloy Cd1−xMnxTe With x = 0.51
Authors: Müller E. , Gebhardt W. , Gerhardt V.
Published: July 2007

The 2eV luminescence in Cd0.49Mn0.51Te is studied by use of time resolved spectroscopy. A decay time of 24 μs is derived. Slow radiationless energy transfer between Mn2+ ions is observed. The change of the luminescence bandshape during decay suggests a random distribution of excited 4T1‐states with a width of 760 cm−1. These results are confirmed by experiments with varying excitation frequency. A weak residual 2 eV‐luminescence is observed under subresonant conditions. Its intensity is found to be proportional to the square of the laser power.
The Diffusion in the Exciton System and the Nonconservation of Excitons
Authors: Davidović‐ristovski G. S. , Ristovski Lj. M. , Tošić B. S.
Published: July 2007

The diffusion of excitions is considered in systems in which the number of excitons is not conserved. It is shown that in such a system stationary and nonstationary diffusion processes appear, and the corresponding diffusion cofficients are defined and calculated for a simple cubic crystal in the two level approximation. The results obtained show that in this system a nonstationary diffusion process is dominant, and that both diffusion processes are caused by nonconservation effects.
Luminescence Properties of an MA‐Type Center in KCl:SrCl2 and KCl:CaCl2
Authors: Bosi L. , Nimis M.
Published: July 2007

Lifetime measurements are performed on KCl:SrCl2 and KCl: CaCl2 by light excitation in the spectral region of the Z4‐center absorption band. The main results are as follows: i) confirmation of Briginets' discovery, in KCl:SrCl2, of an MA‐type center (i.e. an M‐center perturbed by a foreign positive ion), whose absorption band strongly overlaps the Z4‐center one; ii) evidence of an analogous MA‐type center also in KCl:CaCl2; iii) explanation of the puzzling results on the Z4‐center luminescence, obtained by Nishimaki, who found two emission bands for this center, in KCl doped with Ca, Sr, Ba; iv) assignment of the highest energy Nishimaki emission band to the MA‐type center and not to the Z4‐center. An analysis of the lifetime values for the unperturbed and for the perturbed M‐center in KCl, taking into account the data previously obtained by the authors for the MA‐center in KCl:NaCl and KCl:LiCl, shows their dependence on a relaxation parameter. A critical examination of the matter is also given: in particular, no contradiction is found with Briginets' hypothesis of a similarity between the Z4‐center and the N1‐center in undoped KCl crystals, although this analogy cannot be proved by measurements presented here.
Dielectric Relaxation in Smoky Quartz at Very Low Temperatures
Authors: Saint‐Paul M. , Frossati G.
Published: July 2007

Audiofrequency dielectric measurements down to 4 mK on γ‐irradiated quartz are reported. At the lowest temperatures, the mean relaxation rate increases with dipole concentration and shows a field dependence in small ac measuring fields. Similar properties are observed in other dielectric systems containing randomly distributed electric dipoles.
On Charge Carrier Photogeneration Mechanisms in Organic Molecular Crystals
Authors: Silinsh E. A. , Kolesnikov V. A. , Muzikante I. J. , Balode D. R.
Published: July 2007

An extended phenomenological model of photogeneration mechanisms in organic molecular crystals is proposed which describes intermediate and final stages of multi‐step photogeneration processes in terms of a modified Onsager approach. It is shown that the intermediate geminate charge pair (CP) states can be populated through two independent competitive pathways: viz. via autoionization and subsequent hot electron thermalization in terms of a ballistic model and via optical charge transfer (CT) transitions. The thermalization processes and their dependence on photon energy, temperature, and electric field are studied in some detail. It is demonstrated that the threshold spectral dependence of intrinsic photoconductivity quantum efficiency β(hv) in anthracene‐type crystals is mainly determined by the spectral dependence of dissociation efficiency Ω(hv) of the intermediate CP‐states and can be approximated by Onsager's formula.
The Isotopic Shift of the Excitonic Absorption Lines in Cu2O Crystals
Authors: Bobrysheva A. I. , Jeru I. I. , Moskalenko S. A.
Published: July 2007

It is shown, that the isotopic shifts of the excitonic series in Cu2O crystals are caused by changes of the intra‐ and interband electron (hole)—phonon scattering, related to the change of phonon amplitude with isotopic mass. Estimates of the energy shifts of the excitonic series are obtained on the base of the Fröhlich electron—phonon interaction.
On the Problem of a Power‐Law Localization of Charge Carriers in a Two‐Dimensional Disordered Semiconductor
Authors: Bonch‐Bruevich V. L.
Published: July 2007

Conditions are studied under which the wave functions of localised charge carriers possess a power‐law asymptotics. It is shown that in a disordered semiconductor conditions may be realised under which such an asymptotics has no place to develop: there are no asymptotic regions. The states, however, remain localised in the sense that the static conductivity tends to zero at T → 0.
Absolute Hydrostatic Deformation Potentials of Tetrahedral Semiconductors
Authors: Vergés J. A. , Glötzel D. , Cardona M. , Andersen O. K.
Published: July 2007

The “absolute” hydrostatic deformation potentials of several band states of germanium and silicon are calculated with the LMTO‐ASA method. It is found that for the top valence band state dE/ d In V ≈︁ −8 eV. The results are compared with experimental data available. They agree with calculations based on empirical pseudopotentials provided one assumes dV0/d In V ≈︁ 0, where V0 is the spatially averaged pseudopotential. In order to obtain agreement with empirical tight binding results one must introduce a volume dependence of the term energies. Using the calculated deformation potentials, the dependence of the lattice constant on doping measured for n‐ and p‐type germanium, silicon, and GaAs is discussed.