bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐Lying Collective Electronic Excitations in Semiconducting Charge‐Ordered Crystals
Authors: Lubimov V. S. , Ionova G. V. , Pachev O. M. , Manakova L. A. , Makarov E. F. , Ionov S. P.
Published: July 2007

The properties of the excited states of a charge‐ordered crystal with arbitrary band width are analysed. It is shown that the collective excitations in such systems are within the energy gap of the one‐particle spectrum, if the intra‐site electron correlation corresponds to the attraction and prevails over the inter‐site repulsion. For two kinds of collective excitations, the dispersion relations are found and the lower spectral boundaries are determined.
Magnetization and Susceptibility in the Truncated Anderson Model
Authors: Debnath G. , Rudra P.
Published: July 2007

Thermodynamic Green's functional method is used to obtain the expression for the spin polarization of the conduction electrons and the d‐electrons in the presence of an external magnetic field for systems described by the truncated Anderson Hamiltonian. The behaviour of the susceptibilities χc and χd at low and high temperatures is determined.
The Initial Susceptibility of Cobalt after Electron Irradiation at 4.2 K
Authors: Schaefer H. E. , Dander W.
Published: July 2007

After electron irradiation at 4.2 K the initial ferromagnetic susceptibility of cobalt shows a marked decrease (“susceptibility dip”) of about 50% when the specimen is annealed between 20 and 30 K and regains its initial value in recovery stage Ic of the electrical resistivity between 40 and 50 K. Similar, yet smaller “susceptibility dips” are observed at 5, 55, 70, and 150 K. The 30 K decrease of the initial susceptibility is tentatively ascribed to the pinning of domain walls by interstitial atoms of hexagonal symmetry in the [0001]‐dumbbell configuration which, however, may be stable only under the influence of close Frenkel pairs in their neighbourhood. This model is in good agreement with simultaneous electrical resistivity and magnetic after‐effect measurements reported elsewhere which indicate that the anisotropy axis of the freely migrating interstitial atom has a component parallel to the basal plane.
Quantitative Analysis of Interference Structures in the Transmission Spectrum of Very Thin CdS Platelets
Authors: Voigt J. , Senoner M. , Rückmann I.
Published: July 2007

A quantitative analysis is given of interference structures appearing in very thin CdS single crystals of high perfection at 1.8K on both sides of the A ground‐state exciton. It is shown that the structures are due to Fabry‐Perot interferences of polaritons of the lower and upper branch, respectively. From the analysis of the interferences the energy of the transversal exciton is determined with high precision to be Et = 2.5528 eV, shifted from the transmission minimum by 0.8 meV in the low energy direction. This is to be expected in the polariton model.
Mössbauer Spectrum near the Critical Point
Authors: Selyutin G. Ya.
Published: July 2007

It is shown theoretically that the exchange narrowing of the Mössbauer line is induced by the magnetic phase transition in crystals. The discontinuity of the shift, the intensity of the Mössbauer spectrum, and the spin correlation function 〈SiSj〉 near the critical point are determined.
Far‐Infrared Single Crystal Studies of TTF–TCNQ
Authors: Coleman L. B. , Fincher C. R. , Garito A. F. , Heeger A. J.
Published: July 2007

Polarized and unpolarized far‐infrared (12 to 100 cm−1) reflectance data are presented for TTF‐TCNQ. The results confirm the existence of the energy gap, in the conducting state at 100 K, and in the dielectric regime at 4.2 K and provide a quantitative measure of the residual conductivity in the gap region. The data are discussed in terms of the Peierls‐Fröhlich model. Analysis of the results yields an estimate for the Frohlich mass of approximately 1500 times the band effective mass and a value for the pinning frequency in the dielectric regime, ωF ≈ 2 cm−1.
The Influence of the Hyperfine Range Magnetic Field on Charge Transfer States at the Metal–Organic Semiconductor Interface
Authors: Kaulach I. S. , Silinsh E. A.
Published: July 2007

Hyperfine range (HFR) magnetic field (H < 300 Oe) induced increase of photoconductivity and decrease of dark conductivity are observed in thin‐layer sandwich type Au‐tetrathiotetracene‐Al systems. The experimental results are interpreted in terms of HFR magnetic field influence on single‐triplet transition rate of charge transfer (CT) states at metal (Me)‐organic semiconductor (OS) interface in the framework of Merrifield's et al. hyperfine interaction model. It is suggested that in the case of photoconductivity, triplet 3CT states are initially formed by molecular triplet exciton dissociation at the Me‐OS interface, whereas in the case of dark conductivity, singlet 1CT states are initially formed in the process of non‐equilibrium hole injection from the Me electrode. The HFR magnetic field reduces the 1CT–3CT transition rate and is thus assumed to cause the observed increase of photo‐ and decrease of dark conductivity. The possible role of the inhomogeneous local magnetic field caused by the charge carriers trapped near the Me‐OS interface on the rate of 1CT–3CT transition is also briefly discussed.
Computation of the Two‐Centre Molecular Integrals Involved in the Exchange Interaction Between Ligand and Metal Electrons for the [VF6]4− Cluster in KMgF3
Authors: Emery J. , Fayet J. C.
Published: July 2007

In order to make an advance in the analysis of recent Spectroscopie studies on the [VF6]4− cluster the matrix elements are computed involved in the exchange interaction between the electrons of the metal and the electrons of the ligand, when a molecular orbital model is used. In comparison with [NiF6]4− and apart differences arising from different electronic configuration, the widely spread atomic orbitals of V2+ result in a noticeably larger interaction.
Volume Plasma Faraday Effect in n‐Type GaAs
Authors: Rheinländer B.
Published: July 2007

The infrared Faraday rotation of transmitted light is measured on highly Sn‐doped, n‐type GaAs epitaxial layers in the spectral region of the plasma resonance at temperatures of 300 and 130 K. The experimental results are explained applying the Drude theory. The statistical effect of the energy distribution of the electrons in the band is taken into consideration. The damping influence on the rotation is discussed in detail. The availability of the plasma rotation method for the determination of volume properties of epitaxial layers is pointed out.
Impurity Scattering in n‐Ge in Quantizing Magnetic Fields
Authors: Babich V. M. , Baranskii P. I. , Demidenko L. S. , Ilchishin V. A. , Korolyuk L. S. , Samoilovich A. G.
Published: July 2007

The magnetic field dependence of the magnetoresistance in nondegenerate n‐Ge is experimentally and theoretically investigated. The conductivity component connected with the impurity scattering (σ) is selected. Its magnetic field dependence in quantizing magnetic field is in accordance with theory.
Bound Exciton–Neutral Acceptor Complex in ZnTe
Authors: Wardzyński W. , Pataj K.
Published: July 2007

The splitting of I1 (2.3758 eV) and I2 (2.3590 eV) lines observed in luminescence at liquid helium temperature due to uniaxial stress is investigated. The I1 line splits into three components, all of them allowed for both polarizations. The I2 line splits into two components, one allowed for ℰ⟂p and the second allowed for both polarizations. The I1 and I2 lines are interpreted as due to radiative annihilation of excitons bound to neutral acceptor and donor, respectively.
Reaction Field Approximation for Heisenberg Paramagnets and the Magnetic Specific Heat
Authors: Scherer C. , Aveline I.
Published: July 2007

The concept of “reaction field” introduced by Onsager is applied to the case of a Heisenberg system in the paramagnetic phase. The procedure to obtain the static q‐dependent susceptibility and pair correlation functions is reviewed and extended to calculate the magnetic specific heat. The result of this calculation is compared with experiment.
Plasmon–Phonon‐Coupled Mode Sideband of Excitons
Authors: Ueba H. , Ichimura S.
Published: July 2007

The plasmon–LO‐phonon coupled mode sidebands of the exciton absorption spectra are investigated by means of the dielectric function formalism. Exciton‐coupled mode field coupling strength is calculated against the free carrier concentration, 1016 to 1018 cm−3. At any concentrations it is found that the coupling strength is very small. A possibility of the observation of this sideband is discussed.
Elastic Quasi‐Isotropy Normal to the Basal Plane in the Hexagonal System
Authors: Kirchner H. O. K. , Bluemel K. H.
Published: July 2007

Within the framework of the integral theory of anisotropic elasticity the Green tensor for point forces and the prelogarithmic tensor for dislocation energies are derived for the direction normal to the basal plane of a hexagonal crystal. They can be written in closed form and no numerical integration is necessary. The stress and displacement fields of a dislocation running in that direction are derived. Since a hexagonal crystal is transversely iso‐tropic in the basal plane, the most important dislocation interaction problem in a hexagonal crystal, the interaction between a glide dislocation in the basal plane and a dislocation tree normal to it, can be treated with the same ease as in isotropy.
Triplet Exciton Decay Processes in Crystalline Tetracene
Authors: Arden W. , Kotant M. , Peter L. M.
Published: July 2007

The decay of triplet excitons in crystalline tetracene is investigated. The monomolecular decay is characterised by a rate constant βt ≈ 5 × 103 s−1 and the bimolecular annihilation of triplet excitons is characterised by a rate constant γtt ≈ 2 × 10−11 cm3 s−1. The interaction between mobile triplet excitons and trapped holes is characterised by a rate constant γth ≈ 1 × 10−11 cm3 s−1. These values differ by two orders of magnitude from previous estimates. Some possible reasons for this are discussed.
X‐Ray Scattering from Crystals with Intracellular Disorder
Authors: de Almeida A. M. Ozorio , Ellis S. Caticha
Published: July 2007

X‐Ray scattering from disordered crystals has been the subject of numerous investigations. In this paper an analysis made of the distribution of intensity scattered by crystals with static and dynamic disorder inside each cell resembles a general treatment for fluids. It is shown that only a two‐particle probability distribution function is needed in the theoretical treatment to account for correlated motions of the atoms. This is in contrast with the simpler case of independent motions described by a single‐particle probability distribution, which leads to a generalized Debye‐Waller factor. As an outcome of this approach it is demonstrated that static and dynamic disorder effects on the scattered intensity are indistinguishable for the infinite crystal. On the contrary, very small crystals would be expected to show differences in both cases. Finally the results are applied to the phase transition of NaN02.
Formal Results on the Susceptibility of Dilute Alloys. I. Green's Function Approach
Authors: Bisch P. M. , Gomes A. A.
Published: July 2007

The susceptibility of conduction electrons of dilute alloys is discussed including both electron–electron interactions and one‐electron impurity potentials. Adopting a two band model (s–d bands) a formal result is obtained for the partial susceptibilities, using the Green's function formalism and the Hartree‐Fock approximation. This solution is given in terms of a power series involving the Coulomb interaction and impurity parameters. An approximation is proposed, corresponding to a maximum exchange enhancement, for summing the power series, and an effective Coulomb scattering is obtained (for the fluctuations in the host) mediating the impurity scattering process.
Electron Nuclear Double Resonance of Mn2+ in SrF2
Authors: Falin M. L. , Meiklyar V. P. , Ulanov V. A. , Zaripov M. M.
Published: July 2007

The superhyperfine interaction (SHFI) of Mn2+ with four nearest coordination spheres of F− environment in SrF2 crystal is studied by means of electron nuclear double resonance (ENDOR). The SHFI constants are determined. The covalency parameters in the [MnF8]6−complex are determined for the fluorite type crystals.
Magnetism in S = 3/2 Models Magnetization Jumps and Reorientations, Heat Magnetization, and Trieritical Points
Authors: Sivardiére J.
Published: July 2007

We have studied in the molecular field approximation the statistical mechanical properties of a S = 3/2 ferromagnetic Hamiltonian with single‐ion uniaxial and biaxial anisotropy, anisotropic exchange, and a magnetic field. The phase diagrams exhibit various features such as: trieritical points and lines, magnetization jumps and reorientations, and heat magnetization.
Free‐to‐Bound Transitions in Lithium‐Doped Zinc Telluride
Authors: El Akkad F.
Published: July 2007

The two no‐phonon emission lines at 2.336 eV ± 4 meV (A) and 2.362 eV ± 8 meV (B) characteristic of Li‐doped ZnTe at 4.2 K are studied as a function of temperature (4.2 < T < 300 K) and of impurity concentration (5 × 1016 to 5 × 1018cm−3). It is found that the energy of the B transition decreases linearly with the pone; 1/3 of the donor concentration. It is shown that this law is expected theoretically for free‐to‐bound transitions in a compensated p‐type semiconductor when the screening length is determined by the concentration of ionized impurities (Debye and Conwell model). The effect of doping on the temperature dependence of the intensity can be explained in terms of the previously proposed interpretation: conduction band‐to‐acceptor transition for the A‐line and donor‐to‐valence band transition for the B‐line.
Calculation of Hot Electron Mobility and Electron Temperature in n‐Type InSb under the Assumption of Maxwellian Distribution
Authors: Komissarov V. S.
Published: July 2007

Formulae with matrix elements regarding screened polar optical (PO), deformation acoustic (DA), and impurity (I) scattering, nonparabolicity, and s‐p interaction of the conduction band for nondegenerate and degenerate cases are used for the calculation of the mean energy 〈ϵ〉 of hot electrons in n‐type InSb semiconductors. The distribution function (DF) taken as an unexpended diplaced one, due to the lower values of Te, results in an electron‐gas “cooling” in nondegenerate n‐type InSb below To = 80 K, as well as in a reduction of the electron mobility in comparison with the calculated one for expanded DF. Taking into account the interaction of C‐ and V‐zones limits the runaway of electrons. A strong increase of the electric field increases the mean energy of carriers, being more effective for high‐resistance specimens due to the increase of the frequency of inter‐electron collisions. The mean energy of carriers calculated is compared with data of experiments performed in a wide range of electron concentrations.
Spatial and Spectral Spin Diffusion in Dilute Spin Systems
Authors: Vugmeister B. E.
Published: July 2007

The character of spatial and spectral spin diffusion Processes in disordered spin systems is studied. The analysis is based on averaging of the rate equations, valid for fixed spin positions, over different particle configurations. New Expressions for the spatial and spectral diffusion constants are derived and compared with earlier theories.
Evidence for a Partial F.C.C.—H.C.P. Phase Transition in Solid Neon from Raman Scattering
Authors: Schuberth E. , Creuzburg M. , Müller‐Lierheim W.
Published: July 2007

Raman spectra from crystalline neon are investigated that show a sharp peak at 25.2 cm−1 identified as first order. Its energy is very close to that of the TA phonon at the L‐point of the f.c.c. Brillouin zone. A first‐order Raman effect is expected for the h.c.p. structure with the Raman‐active TO phonon almost degenerate with the TA phonon at the L‐point. From the haracteristic temperature dependence of the intensity of the peak, from its sharpness, and from its energetic position a partial phase transition at 10.5 K is deduced.
Green's Function Approach for High‐Temperature Ferromagnetic Systems in the Presence of Uniaxial Anisotropy
Authors: Kumar V. , Sharma K. C.
Published: July 2007

The effect of uniaxial anisotropy on the transition temperature for a ferromagnetic spinone system is investigated using the double time Green's function technique. The calculations are carried out up to the second order in the exchange coupling and exactly in the anisotropy term. The localized and the static correlations used in the formulation are calculated selfconsistently. The calculation is compared with corresponding results for an ordered system, which predicts an effective reduction in Tc with anisotropy in high temperature crystalline magnetic systems.
k‐q Functions and the Landau Theory of Second‐Order Phase Transitions
Authors: Litvin D. B.
Published: July 2007

The use of k‐q functions is introduced into the Landau theory of second order phase transitions. The expansion of an arbitrary density function in a crystal is given in terms of basis functions of irreducible representations of a space group which are linear combinations of k‐q functions. This leads to a new condition on the irreducible representations which can be associated with second order phase transitions.
Lattice Dynamics of Gold on an Extended de Launay Model
Authors: Cavalheiro R. , Shukla M. M.
Published: July 2007

The phonon dispersion relations along the three principal symmetry directions as well as the (θ‐T) curve of gold are calculated on the basis of the extended de Launay model. The computed results are found to give a good account of the experimental observations.
Piezoresistance of Uniaxially Deformed n‐Si
Authors: Mitin V. V. , Tolpygo E. I.
Published: July 2007

Piezoresistance of nondegenerate n‐Si is calculated for the case when the directions of the current and compression axis coincide with the 〈100〉 axis. Low temperatures are considered (T<100 K), when electrons are scattered by acoustic phonons, ionized impurities, and electrons of its own and other valleys. The piezoresistance is shown to depend not only on the impurity concentration but also on the scattering intensity of the electrons of one valley by those of the other valleys. The latter may be determined by comparing the experimental and theoretical deformation resistance dependences under the conditions when the acoustic scattering is inessential due to low temperatures or high doping levels.
Surface Plasmon and Polariton Dispersion at Rough Boundaries
Authors: Kröger E. , Kretschmann E.
Published: July 2007

The influence of surface roughness on electromagnetic surface modes is investigated theoretically. A frequency or wave vector shift is found and a damping of the modes depending on the autocorrelation of the surface roughness. This could be described by an explicit analytic expression. There is good agreement with experimental results on sinus‐odially modulated silver surfaces.
Optical and Electrical Spectroscopy of Zinc Oxide Crystals Simultaneously Doped with Copper and Donors
Authors: Muller G.
Published: July 2007

In Cu‐doped ZnO crystals the Fermi level is varied by additional donor doping (In or H) in the energy range 0 to 250 meV below the bottom of the conduction band. The results of optical absorption and electrical conductivity measurements carried out in these crystals can be explained by using the model in which Cu acts as an acceptor with an energy spacing of ECA = 190 meV from the bottom of the conduction band. Moreover oscillator strengths of inner transitions of the Cu2+‐ion are presented.
Determination of Optical Constants: Au
Authors: Jungk G. , Gründler R.
Published: July 2007

The dielectric function ε = ε1 + iε2, of Au films is measured at room temperature in the spectral range 1.6 ≦ E ≦ 4.6 eV with an ellipsometric null method. Numerical consideration of the free‐electron like part allows the computation of the interband dielectric function and its energy derivatives above 2 eV. The mathematical lineshape analysis of the derivative spectra of εl and ε2 leads to critical points at 2.54 ± 0.02, 3.28 ± 0.01, 3.64 ± 0.01, and roughly 4.5 eV of M0, M2, M1, and M2 type, respectively, and a rough estimate of the relative oscillator strengths.
Exciton Band Width versus Peak Shift in NH4Br in Phases II and III as Measured by Thermoreflectance
Authors: Rieflin E. , Stühmer G. , Schwab E.
Published: July 2007

Experiments on the temperature dependent band widths in comparison to peak energies of the low energy exciton doublet of NH4Br are performed between room temperature and liquid nitrogen temperature. The band width is established as a linear function of the peak shift following the order parameter in phase III as pointed out earlier. The discussion includes this underlying temperature dependence of the peak energies.
A Generalization of the Sondheimer‐Wilson Formula for the Averaged Density Matrix and Magnetic Susceptibility of Conduction Electrons in a Model of Disordered Semiconductors
Authors: Bezák V. , Banský J.
Published: July 2007

A model of disordered non‐degenerate semiconductors is investigated in which the bands are randomly undulated. A compact formula is derived for the averaged canonical density matrix of the conduction electrons in the presence of a magnetic field. A generalization of the formula of Sondheimer and Wilson is thus achieved. The magnetic susceptibility due to the conduction electrons is calculated. The enhancement of the diamagnetism with increasing disorder is discussed and interpreted as a consequence of the increase in the average density of the conduction electrons.
Surface Specifie Heat of Isotropic Solids
Authors: Grechko L. G. , Falko G. L. , Fedorchenko A. M.
Published: July 2007

The correlation functions of thermal fluctuations in a semi‐infinite isotropic elastic solid are obtained and the contribution of the surface to the specific heat is evaluated on the basis of the fluctuation‐dissipation theorem. The calculated specific heat coincides better with the experiments than the results of previous papers. The theory presented may be generalized for solids of complicated geometry and symmetry.
Band Structure Changes in Interealates of Niobium Diselenide
Authors: Birks A. R. , Hind S. P. , Lee P. M.
Published: July 2007

The KKR method has been used to calculate the energy bands in both pure NbSe2 and a simple model of its intercalates. The results show a complicated dependence of the upper valence band states on changes in the lattice parameter and charge transfer. These effects are estimated on the basis of data from optical and NMR measurements. Changes in the density of states afford an explanation of the changes in superconducting transition temperature as observed experimentally.
Two‐Photon Absorption of Anisotropic Excitons in β‐AgI
Authors: Dinges R. , Fröuhlich D. , Uihlein Ch.
Published: July 2007

Exciton fine structure is measured in hexagonal AgI by two‐photon absorption. A detailed assignment of the different P‐exciton states is possible using the appropriate polarization configurations. The experimental results can be explained quantitatively taking into account anisotropy as the only perturbation on the hydrogenic series. Using an effective mass Hamiltonian for degenerate valence bands it is shown that the Hd‐contributions to the hole kinetic energy are responsible for the mass anisotropy of the A and B series. An expression for the two‐photon transition probability in a three‐band model using the abovementioned Hamiltonian is derived.
Influence of Phonon Damping on the Electrical Resistivity of Open Electron‐Phonon Model Systems
Authors: Röpke G. , Christoph V. , Vojta G.
Published: July 2007

A direct calculation of the electrical resistivity of an electron‐phonon system is made by means of the force‐force correlation function method. The coupling of the system considered to a bath is essential for the correct physical and mathematical formulation of the theory. It is assumed that the conduction electrons interact with the longitudinal phonons (only normal processes are taken into account). The interaction of the longitudinal phonons with the bath is given by a relaxation term with a wave‐vector dependent relaxation time. The corrections to the electrical resistivity caused by the finite coupling strength between phonons and bath are derived in lowest order and discussed; they are connected with the phonon drag.
Lattice Vibrations of V2O5. Determination of TO and LO Frequencies from Infrared Reflection and Transmission
Authors: Clauws P. , Vennik J.
Published: July 2007

The reflectance of V2O5, single crystals is measured between 20 and 2000 cm−1. Eleven resonances are observed: six with E ∥ a, two with E ∥ b, and three with E ∥ c. Values of ϵ0 are obtained separately from thin crystal channel spectra: ϵ0 = 37.2 with E ∥ a and 20.1 with E ∥ c. The reflectance data are analysed by Kramers‐Kronig integration, classical dispersion analysis, and four‐parameter dispersion analysis, all three methods yielding comparable results. Additional transmission measurements of thin single crystals and powders are performed. For the thin single crystal, absorption peaks at 982 and 1038 cm−1 are observed which are assigned to direct absorption of a TO and a LO vibration.
Thermomagnetie Properties of n‐Type and p‐Type HgTe
Authors: Jędrzejczak A. , Dietl T.
Published: July 2007

The electric conductivity, Hall coefficient, thermoelectric power, longitudinal and transverse Nernst‐Ettingshausen effects are measured between 100 and 300 K on heavily doped n‐type and p‐type HgTe samples. The temperature coefficient of the band gap, d|Eo|/dT = −5 × 10−4 eV K−1, is obtained by fitting the electronic effective‐mass data with a band model based upon the full Kane theory with higher‐band corrections. The phonon‐drag contribution to the thermoelectric power of p‐type samples is observed. The heavy‐hole effective mass m*/m0 = 0.45 ± 0.04 and one of the deformation potential constants, a = (2.7 ± 0.3) eV, are determined.
The Model Hamiltonian and Some Properties of Magnetic Resonance of Triplet Excitons. I. Non‐Localized Exciton Approximation
Authors: Berim G. O. , Kessel A. R.
Published: July 2007

A theoretical investigation is made of the magnetic resonance of triplet excitons (TE) inchains of molecules, each of them containing two paramagnetic spins S = 1/2 coupled by a strong exchange interaction (I) and by exchange (K ≪ I) with spins of neighbouring molecules. A Hamiltonian describing the appearance and interaction of unlocalized TE is proposed. The zone structure and wave functions of single‐exciton and bi‐exciton states are defined. It is shown that the considered model describes well the effects in electron paramagnetic resonance (EPR) spectrum which do not depend on temperature (nonexistence of hyperfine structure and of fine structure, caused by inequivalent positions of the molecules in the crystal). Qualitative estimations of the temperature behaviour of the EPR lines are made from the analysis of the bi‐excitonic states.
An Analytically Tractable Structure Model for Disordered Systems. II. Complicated Structures and Application to Amorphous Selenium
Authors: Unger H.‐J.
Published: July 2007

A previously proposed structure model is generalized to the case of a complicated structure where the corresponding crystalline unit cell contains more than one atom. An analytical expression for the radial distribution function is obtained depending on two adjustable parameters. The model is applicable for the description of amorphous structures containing a significant fraction of larger distorted clusters. Numerical calculations are performed for amorphous selenium. A comparison with the experiment shows that the monoclinic form predominates over the trigonal form in a‐Se.
Mobility of holes in p‐InSb crystals
Authors: Filipchenko A. S. , Bolshakov L. P.
Published: July 2007

Results are given of an investigation of the temperature and concentration dependences of the Hall mobility of holes in p‐InSb crystals. The theoretical foundation of the results obtained is based on a strict account of the real structure of the valence band in indium antimonide crystals. It is shown that the experimentally obtained dependences of the Hall mobility of holes on concentration and temperature in p‐InSb crystals are well explained if one assumes that the holes are scattered by acoustical phonons and ionized impurities. The scattering of holes on optical phonons in the investigated interval of concentrations and temperatures can be neglected.
The Kondo effect in a magnetic field
Authors: Litvinov V. I.
Published: July 2007

The integral equation method for the vertex part proposed by Cheung and Mettuck is generalized for the case of a magnetic field. The s‐d part of the diluted magnetic alloy resistance in the linearized parquet approximation is calculated. The agreement with the results obtained previously is discussed.
Roth's method and moment conservation of the spectral weight function for the neutral Hubbard model
Authors: Heiner E.
Published: July 2007

The method of Roth for the calculation of a (2 × 2 ) Green's function matrix is equivalent to the moment technique for the spectral weight function up to the fourth order. The method can be extended in order to derive a set of orthogonalized operators. With this method a selfconsistent calculation of the single‐particle Green's function and all important correlation functions of nearest neighbour sites is possible for the neutral Hubbard model. The results agree for a sc lattice in the strong correlation region at arbitrary temperatures with those of Shiba and Pincus for finite Hubbard systems. An antiferromagnetic coupling of neighboured lattice sites is increasing with a decrease of the temperature, a metalinsulator transition does not exist in this two‐pole approximation.
Derivation and calculation of a sequence of lower‐bound results for lattice thermal conductivity
Authors: Srivastava G. P.
Published: July 2007

Better variational lower bounds are derived as approximate solutions of a linearized Boltzmann equation for phonons. The role of the off‐diagonal part of the linearized phonon collision operator is considered in the frame of B displaced Planck distribution for phonons. A result derived here can be expressed in terms of the Debye term and the Ziman‐limit conductivity result‐showing that the Debye term is the “zeroth approximation” of that variational lower bound.
Spectral dependence of the photocurrent in semiconductors
Authors: Holzhütter H.‐G. , Mocker M.
Published: July 2007

The distribution function of optically excited carriers is calculated by solving the Boltzmann equation with the help of a weak‐field approximation method. A general expression for the photocurrent as a function of the excitation energy is derived. For low excitation energies the structure of the photocurrent strongly depends on the mutual influence of elastic scattering and recombination. For sufficiently high excitation energies impact ionization takes place. If the carriers are injected closely below the ionization threshold a steep dip in the photocurrent is predicted. An expression for the quantum efficiency is presented which enables the determination of the ionization probability from optical measurements.
Magnetic circular dichroism of indirect excitonic transitions in GaP and In1‐xGaxP
Authors: Donecker J. , Kluge J.
Published: July 2007

Measurements of the magnetic circular dichroism (MCD) of the indirect absorption edge of intrinsic GaP and In1‐xGaxP (x ≧ 0.77) at temperatures from 2.5 to 300 K and at magnetic fields up to 52 kOe are reported. The fine structure of the indirect absorption edge is well resolved. The MCD peaks corresponding to LA and TA assisted indirect transitions have the same shape as the corresponding peaks in the wavelength modulated spectra, but the latter have different signs. That means that the thresholds of the indirect absorption edge are rigidly shifted by the magnetic field and the direction of the shift is connected with the type of the phonons. Additionally an MCD peak appears in mixed crystals which corresponds to the no‐phonon transition. It has the same sign as the LA‐MCD peak. An attempt is made to interprete the experimental results by group theory within an exciton model. Further theoretical effort is necessary for a satisfactory explanation.
The electronic structure of AgCl. Energy bands, densities of states, and charge densities
Authors: Wang J. Shy‐Yih , Schlüter M. , Cohen M. L.
Published: July 2007

The electronic band structures of silver chloride is calculated using the empirical pseudopotential method. The density of states is compared with photoemission experiments. The charge distributions are calculated in the (100) plane.
The importance of backscattering in high‐energy electron diffraction calculations
Authors: van Dyck D.
Published: July 2007

The Schrödinger equation, describing the motion of a high‐energy electron in a crystal foil, can be solved exactly with appropriate boundary conditions. The formal analytical solution in matrix form however is not suitable for numerical calculations but can be expanded in a series of terms proportional to subsequent powers of the electron wavelength. In the case of fast electron scattering, the electron wavelength decreases so that, to a good approximation, the series can be cut off after the low order terms, yielding a relatively simple expression. This expression reveals two corrections to the dynamical solution of Howie and Whelan, which was obtained by neglecting second order derivative terms in the Schrüdinger equation. From an analysis of these corrections, an investigation of the validity of the Howie‐Whelan solntion could be performed as function of the cqwtal thickness and an estimation for the thickness limit could be derived.
Deformation potential theory for optical and acoustic phonons in PbSe‐Like semiconductors
Authors: Szymański J.
Published: July 2007

The matrix elements of electron‐phonon nonpolar interaction in PbSe‐like band structures is given for optical and acoustic phonons in the deformation potential approximation. Nonparabolicity and nonsphericity of conduction and valence bands (two‐band Kane model) are taken into account giving nonvanishing electron‐optical‐phonon nonpolar interaction and changing considerably the standard electron‐acoustic‐phonon matrix element.
Energiebetrachtung zum Peierls‐Übergang
Authors: Salchow R. , Liebmann R. , Appel J.
Published: July 2007

In einem einfachen Modell einer linearen Kette von dreidimensionalen Atomen mit stark gebundenen Elektronen wird die Änderung der Gesamtenergie beim Peierls‐Übergang berechnet. Die Wechselwirkung zwischen den Elektronen wird in der Hartreenäherung berücksichtigt. Die Ergebnisse werden für ein halbvolles Band angegeben. Dabei zeigt sich, wie die Änderung der Elektron‐Ion‐Energie und der kinetischen Energie den Peierls‐Über‐gang begünstigt, während die Änderung der Elektron‐Elektron‐Energie und der Ion‐Ion‐ Energie dem Übergang entgegenwirkt. Die Gesamtenergie wird abgesenkt. Austausch‐ und Korrelationseffekte werden diskutiert.
The electron‐phonon enhancement factor on the Fermi surface of copper
Authors: Bosacchi B. , Franzosi P.
Published: July 2007

The Fourier series representation technique is extended to deal with the study of the electron‐phonon enhancement factor λ on the Fermi surface. The scheme is applied to the case of copper end a detailed map of λ(k) at any point of the surface is obtained. Finally the results are compared with the predictions of microscopic calculations of λ(k) and the meaning of discrepancies and agreement are briefly discussed.
The damping of Excitons by phonons
Authors: Klähn D. , Kilimann K.
Published: July 2007

A theory of the exciton ionization by phonons is developed. A non static effective electron‐hole interaction is derived from the Bethe‐Salpeter equation in the ladder approximation. The imaginary part of the effective potential is related to the life time and to the level width of the excitons. An application of the theory to Ge shows, that the ionization of excitons by LO‐phonons in comparison with the recombination is the dominating damping process, where KBT is or order of the effective binding energy of the excitons.
On the theory of lattice vibrations in mixed crystals. The connection between the REI model, ATA, and CPA
Authors: Böttger H.
Published: July 2007

For an isotopically disordered one‐dimensional model of a mixed crystal a hierarchy of equations for conditionally averaged Green functions is derived. A definite single‐site approximation leads to the random element isodisplacement model (REI‐model). This approximation is shown to be equivalent to a somewhat modified average t‐matrix approximation (ATA) and, for a self‐consistent formulation, to a somewhat modified coherent potential approximation (CPA).
The critical field for the nucleation of n‐domains in type I superconductors
Authors: Kronmüller H. , Riedel H.
Published: July 2007

In type I superconductors of ellipsoidal shape and large demagnetization factors the maximum diamagnetic magnetization in general is larger than the value (‐)HC/4π. Furthermore the magnetic field where the intermediate state starts is shifted to higher values. These deviations from the macroscopic theory may be attributed to the effect of the s‐n‐wall energy as well as to the interaction energy of domain walls with lattice imperfections. An investigation is made of the combined effect of these two energy contributions on the mag‐netization curve of type I superconductors.
Calculation of the electrostatic energy of the laves phases C14, C15, and C36
Authors: Rennert P. , Radwan A. M.
Published: July 2007

The electrostatic energies of point ions in a uniform compensating background are evaluated for a number of crystal structures using a modified form of the Ewald‐Fuchs method. These are the first calculations for the Laves phases C15 and C36.
Noncritical fluctuations and the temperature dependence of the order parameter
Authors: Franck S. , Kühnei A. , Nattermann T. , Wendt S.
Published: July 2007

An effective Hamiltonian with anharmonic terms of fourth and sixth order is investigated by means of perturbation theory. Noncritical fluctuations of the order parameter lead to a temperature dependence of the coefficient of the quartic term. It turns out that this coefficient can vanish at a certain temperature T1. Depending on the parameter of the substance a first‐order phase transition, a tricritical point, or a second‐order phase transition may appear. The model describes the situation in ferroelectric TSCC.
Non‐empirical calculation of structural properties and valence force fields: Application to Se and Te
Authors: Bullett D. W.
Published: July 2007

A local orbital method is used to describe the electronic structure and the static and lattice dynamical properties of a covalent solid on an equal footing, with present application to trigonal Se and Te. Special point sampling is utilised to provide accurate values for the total energy as a function of atomic positions. With the introduction of a single parameter to describe the van der Waals interaction between non‐bonded atoms good agreement is obtained with the observed crystal structures and with empirical valence force fields.
Local mode of H− substitutional ion in KCN
Authors: Castro J. C. , Basso H. C. , de Souza M.
Published: July 2007

The local mode of a substitutional H− ion in KCN single crystals, the U‐center, is measured in the three phases of the crystal. In the cubic phase the local mode peaks at 510 cm−1 with a linewidth of 120 cm−1. The higher energy phonon side band in the lower temperature phase is found to be displaced 80 cm−1 from the local mode peak. The low‐temperature linewidth of the H local mode is approximately twice that of the D localized vibration. Also in opposition to the behavior in the alkali halides the linewidth keeps a high value even a t temperatures as low as 2 K.
The state of dispersion of Pb2+ in highly doped KCl crystals
Authors: Marculescu L. , Popescu M. , Baltog I. , Ghita C.
Published: July 2007

The state of dispersion of Pb2+ in highly doped KCl crystals after different heat treatments is investigated. Optical and X‐ray diffraction measurements show that the new phase formed in KCl crystals by precipitation of the impurity is imperfectly ordered and composed of agglomerations of cation‐vacancy dipoles, different from the case of Pb‐doped NaCl crystals where a PbCl2, phase is obtained. X‐ray diffraction and luminescence data show that after both, strong precipitation and dissolution, the impurity evolves towards the same equilibrium state characterized by partial aggregation of Pb2+ cation‐vacancy dipoles.
Direct identification of the L′3 → L1 transition in Ge
Authors: Humlíček J. , Lukeš F.
Published: July 2007

The electroreflectance (ER) spectra of n‐type Ge with free carrier concentrations of 6.2 × 1018 cm−3 are studied within the energy range of the E1 and E1 + ΔI transitions. A detailed analysis of the spectra shows in addition to the Franz‐Keldysh (FK) effect an electron distribution (ED) effect, too. An appropriate electrode potential decreases the intensity of the electric field and increases the probability of the occupation of the bottom of the conduction band at the same time so that the ED effect predominates over the FK effect in the ER spectrum. The lineshapes of both the effects agree well with the theoretical curves. Therefore the transition L′3 → L1 clearly manifests itself in the E1, E1 + δ1 structure.
A model of the electrical transport phenomena in imperfect crystals of copper phthalocyanine IV. Concentration of charge carriers and current—voltage characteristics
Authors: Hamann C. , Tantzscher Ch. , Lehmann G.
Published: July 2007

Using the results of former papers the effective concentrations of carriers, which mainly determine the dependence of the conductivity on temperature, will be found theoretically for copper phthalocyanine crystals as well as for thin films having different real structure. The relations between the trap distribution parameter Tc as a function of temperature and the assumed trap spectra agree well with experimentally determined ones.
Hyperfine fields acting on diamagnetic impurities and the anisotropic exchange via the s‐ and d‐electrons in the rare‐earth metals
Authors: Godovikov S. K. , Kozin M. G. , Turovtsev V. V. , Shpinel V. S.
Published: July 2007

Using the Mössbauer effect the hyperfine fields for the tin impurity in the dysprosium host is measured over the temperature range 6 to 293 K. Two hyperfine fields are found in the ferromagnetic region, which differ one from another by ≈20%. In passing through the Curie point the shape of the spectrum drastically changes by smearing because of the superposition of many hyperfine fields in the antiferromagnetic region. The existence of a temperature anomaly appears in a marked distinction of the hyperfine field curve from that for magnetization in the antiferromagnetic region. These results cannot be explained within the framework of the isotropic s—f exchange interaction model and suggest the significant role of the d‐electron exchange and anisotropic effects.
Many‐body effects on the third‐order elastic constants and pressure derivatives of the second‐order elastic constants of CsCl structure solids
Authors: Puri D. S. , Garg V. K. , Verma M. P.
Published: July 2007

Lundqvist's potential for ionic solids is used to derive expressions for the second‐order elastic (SOE) constants and the third‐order elastic (TOE) constants. It is observed that the Cauchy relations among these constants do not hold even at 0 K and the discrepancies in this relation are directly expressible as function of the many‐body term in the crystal potential. It is demonstrated that all parameters except one appearing in the expressions for the TOE constants and pressure derivatives of the SOE constants can be calculated from the values of the SOE constants, the equilibrium condition, and an approximate assumption concerning the overlap repulsion. Determining the remaining parameter (a2∂2ƒ/∂r2)0 from the expression of the pressure derivative of the bulk modulus all the six TOE constants and the remaining pressure derivatives of the SOE constants can be calculated. The values so obtained approach quite close to the measured values as against those of other theoretical workers in all the cases in which experimental results are available.
The magnetic after‐effect spectrum in cobalt between 20 and 350 K after electron irradiation at 4.2 K
Authors: Schaefer H. E. , Dander W.
Published: July 2007

After electron irradiation of cobalt single crystals a spectrum of magnetic after‐effect processes is observed where nearly all the relaxation peaks anneal out rapidly during the measurement. For the discussion of the experimental results a survey is given of the configurations of interstitial atoms and close Frenkel pairs which can be expected for the h.c.p. lattice. Taking into account resistivity annealing experiments the relaxation process at 325 K can be attributed to a diffusion after‐effect which is due to a freely migrating defect.
Dielectric function of hexagonal metals III. Magnesium
Authors: Taut M. , Ziesche P.
Published: July 2007

The longitudinal dielectric function ϵ(q, ω) of magnesium is calculated using model wave functions and realistic energy band values. Comparison with experiment is performed in the optical properties (including anisotropy), plasmon dispersion relation, and the half‐width of the plasmon peak.
Quantum chemical investigations on vacancy diffusion and vacancy lattices in crystallites
Authors: Müller H. , Opitz Ch.
Published: July 2007

Problems of activation of vacancy diffusion in the volume and on the surface of nickel and iron are discussed with the two‐dimensional cluster model of the solid by means of semi‐empirical molecular‐orbital methods (EHT, HMO). In addition it is shown with the model of the two‐dimensional crystallite that, besides the predominant trend to vacancy condensation, a vacancy repulsion acts, which accounts for the existence of relatively stable vacancy lattices in metal crystals.
On the theory of electron localization in amorphous systems
Authors: Schreiber J. , John W.
Published: July 2007

For a generalized tight binding model of amorphous one‐component systems, where the energy levels and the surrounding overlap and hopping integrals are linearly coupled, the localization theory of Economou and Cohen is applied. In terms of several approximations the localization function L(E) is discussed. Thereby it is confirmed that the off‐diagonal randomness has a delocalization effect. An Anderson transition can only take place if diagonal disorder is strong enough. As an example, the influence of diagonal spin disorder due to electron correlation, treated in the alloy analogy for the Hubbard model, is studied.
Anharmonicity in the surface and size effects in the specific heat of linear chains
Authors: Ma S. K. S. , de Wette F. W. , Alldredge G. P.
Published: July 2007

Contributions of anharmonicity to the size and surface effects in the specific heat of the monatomic linear chain are investigated with anharmonic perturbation theory for small to intermediate number of atoms and for temperatures from 0 K up to θ0, the zero‐temperature Debye temperature. It is shown that the surface specific heat CS can reliably be extracted for chains of intermediate particle number, although size effects are still quite appreciable for chains of as many as 50 atoms. The main effects of anharmonicity in the surface specific heat (as compared to that of a harmonic chain) are a small peak at T ≈ 0.09θ0 and a linear increase with T for T ⪆ 0.7θ0.