bluuebunny/crossref_metadata_2025 huggingface dataset:
Free Carrier Magneto‐Optical Effects in the Reststrahlen Region of n‐PbTe
Authors: Burehard H. , Bauer G. , Grosse P. , Lopez‐Otero A.
Published: July 2007

Magnetoreflection spectra of n‐PbTe in the wave‐number region 20 to 200 cm−l are measured with magnetic fields up to 16 T. The PbTe expitaxial films grown on BaF2 and NaCl substrates have rarrier concentrations between 6 × 1016 and 5 × 1017 cm−3. For the PbTe a model‐dielectric function according to Wallace is used, including frequency and magnetic field dependences of the carrier damping and the effective masses. For small magnetic fields the nonparabolicity of the conduction band can be described by a two‐band Kane model. At higher magnetic fields the influence of polaron coupling is observed.
Theory of Multiphonon Vibrational Relaxation of a Guest Molecule in a Host Crystal
Authors: Yakhot V.
Published: July 2007

The quantitative theory of multiphonon vibrational relaxation of a molecule in a host crystalline matrix is developed. A simple closed formula for the relaxation rate is obtained. Interpretations of several experimental results based on this formula are discussed.
Critical Behaviour of Conductivity and Dielectric Constant near the Metal‐Non‐Metal Transition Threshold
Authors: Efros A. L. , Shklovskii B. I.
Published: July 2007

A system consisting of randomly distributed metallic and dielectric regions is considered. The metal‐non‐metal transition takes place when the volume fraction of the metallic phase approaches the percolation threshold. It is shown that the static dielectric constant diverges near the threshold. Critical indexes are introduced which describe the behaviour of the conductivity and the dielectric constant near the threshold as functions of the volume fraction and frequency. The case of non‐zero dc conductivity of dielectric regions is considered also. It is shown that all indexes describing the critical behaviour of complex conductivity can be expressed by two indexes which are known from computer and model experiments. The results of computer calculations of Webman et al. are analysed.
Ferroelectric Phase Transitions in Systems with Broad Electronic Bands Caused by Interband Electron‐Phonon Interaction
Authors: Konsin P.
Published: July 2007

The possibility of arising of ferroelectric phase transitions and its properties in broad band systems (semimetals, semiconductors) are studied within the frames of the vibronic theory. The quasi‐one‐dimensional, quasi‐two‐dimensional and isotropic three‐dimensional cases are considered in the mean field approximation. The chain and layer structures reveal in this approximation the greatest tendency to ferroelectric phase transitions. The spontaneous lattice distortion (polarization), dielectric susceptibility, forbidden gap, etc. are calculated. The possibility of arising of the ferroelectric semimetal‐dielectric phase transition in all cases is studied. In the case of the isotropic three‐dimensional electronic spectra of the active bands the interband electron‐phonon interaction must be strong.
Photocarrier Generation in a One‐Dimensional solid: Polydiacetylene‐Bis(Toluenesulfonate)
Authors: Lochner K. , Reimer B. , Bässler H.
Published: July 2007

The dependence of the absolute quantum yield for intrinsic photocarrier production in PDA‐TS single crystals on temperature, electric field applied parallel and perpendicular to the polymer chain axis, nad photon energy is measured between 1.6 to 3.0 eV with polarized light. Below 190 K photon absorption leads to formation of a thermalized pair of charge carriers at mutual distance α = (116 ± 5) Å a primary quantum yield of 0.2 at 3.0 eV. Above 190 K, a(T) decreases due to te temperature‐activted scattering. Dissociation of the carrier pair occurs only along the polymer chain and can be described by described by Onsager's theory formulated for one dimension. The spectral response indicates that the dominant 2 eV transition is of excitonic origin nd that it must be superimposed to a second strongly ionizing trasition.
X‐Ray Photoelectron Valence Band Studies on Phthalocyanine Compounds
Authors: Höchst H. , Goldmann A. , Hüfner S. , Malter H.
Published: July 2007

X‐ray photoelectron (XPS or ESCA) spectra of the valence bands are measured from the monomeric β‐modification of H2Pc (H2C32H16N8), FePc, CoPc, NiPc, CuPc, and the monomeric α‐modification of PtPe. In the valence bands contributions from d‐like central metal orbitals can be distinguished from the more strongly bound 2pπ and 2pσ electrons of the outer rings; the 2p‐electron spectra appear to be a superposition of the spectra of benzene and pyrrole, the constituents of phthalocyanines.
Investigation of the Band Structure of Diamond Using Low‐Energy Electrons
Authors: Maguire H. G.
Published: July 2007

Secondary electron emission measurements performed in a conventional UHV LEED/Auger chamber are used to investigate the band structural characteristics of type IA diamond. The Auger spectrum obtained reflects structure in the valence band and compliments the techniques of X‐ray photoemission spectroscopy and soft X‐ray emission. Characteristic loss spectra reveals greater detail than hitherto and confirms some of the earlier optical measurements. K‐shell ionisation spectra confirms theoretical predictions of two conduction bands in diamond separated by a well defined energy gap, and reveals the structure associated with these bands. Low‐energy secondary emission resulting from interband transitions, and stimulated by incident electrons of energy below the K‐shell ionization energy, is observed to correlate with band structure calculations, and also confirms the presence of an energy gap separating two conduction bands. In addition, it provides a means for investigating the energy eigenvalues at various k‐points within an elementary region of the Brillouin zone.
The Conductivity of Semiconductors with Superlattice
Authors: Shmelev G. M. , Chaikovskii I. A. , Shon Chan Min
Published: July 2007

Calculations are given of the conductivity tensor σij for semiconductor structures with one‐dimensional superlattice taking into account the interaction of electrons of one minizone with acoustic vibrations and impurities. Kubo‐Mannary method is used, which allows to leave the limits of the Boltzmann kinetic equation. The dependence of σij on the SL parameters is obtained.
Lattice dynamics of the phases I, II, and III of sodium hyperoxide NaO2
Authors: Brüesch P. , Bösch M. , Känzig W. , Ziegler M. , Bührer W.
Published: July 2007

The lattice vibrations of NaO2, single crystals in the disordered pyrite, the ordered pyrite, and the marcasite phase are studied using far‐infrared reflection spectroscopy and inelastic neutron scattering. The far‐infrared spectra of all the different phases exhibit a high‐frequency group of bands centred at about 240 cm−1 and a low‐frequency group centred at about 100 cm−1. This similarity can be explained by the fact that the phase transitions primarily involve a mere change in the orientationsl arrangement of the O molecule ions. This leads to a relation between certain zone centre and zone boundary phonons in the corresponding Brillouin zones. The high‐frequency group corresponds in essence to the NaCl reststrahlen bands whereas the low‐frequency bands arise from the particular orientation pattern of the molecule ions demanding a primitive cell that is larger than the primitive NaCl‐type cell. The occurrence of the low‐frequency band in the disordered pyrite phase can be interpreted as a precursor effect of the transition into the ordered pyrite phase. The Raman‐ and infrared‐active phonons of the marcasite structure are analyzed on the basis of a simple short‐range force model.
Dielectric function of hexagonal metals: I. General considerations
Authors: Taut M.
Published: July 2007

A calculation of the longitudinal dielectric function ϵ(q,ω) for simple hexagonal metals is described. The RPA‐expression with realistic energy bands and wave functions is used and damping is included phenomenologically.
Phenomenological theory of coupling of debye relaxations with IR‐active phonons
Authors: Merten L. , Borstel G.
Published: July 2007

A phenomenological theory of the coupling of dipole relaxations with infrared active long optical phonons in ferroelectric crystals is given. It is shown that whenever there exists a relaxation process in the crystal the phase transition is driven by a soft relaxation state which in an optical spectrum should appear as a central peak. The occurrence of a soft relaxation state is also responsible for observed discrepancies between values of the static dielectric constant measured by different experimental techniques.
Exciton‐phonon interaction in CdS
Authors: Spiegelberg F. , Gutsche E. , Voigt J.
Published: July 2007

The absorption coefficient of CdS is measured over the whole region of the fundamental absorption edge, between liquid helium and room temperature. The experimental results are analysed and discussed in the framework of known theoretical work on exciton‐phonon interaction. In this connection the excitonic self‐energy in dependence on energy and temperature is derived from the experimental data on the one hand and from theory on the other. It is shown that the absorption behaviour is determined by different mechanisms of interaction, namely i) scattering of excitons optically generated at k ≈ 0 and ii) many‐phonon proceeses and phonon‐assisted transitions. Each mechanism is subjected to a special temperature dependence, reflecting the share of the interaction with acoustic and longitudinal optical phonons, respectively.
Systematic variation of the mean positron lifetime and Gaussian fraction in annealed metals and semiconductors
Authors: Welch D. O. , Lynn K. G.
Published: July 2007

The systematic variation among the elemental solids of mean positron lifetime and the fraction of the annihilation spectrum which is approximated as a Gaussian are discussed in the context of a simple model where the ion cores are treated as repulsive square wells within which the electron density is constant. The calculated results exhibit the observed periodic behavior and are in reasonable quantitative agreement with experimental data.
A calculation of Debye‐Waller factors for F.C.C. metals
Authors: Bertolo L. A. , Shukla M. M.
Published: July 2007

The temperature variations of the Debye‐Waller factor exponents for nine face centered cubic metals, copper, silver, gold, aluminium, nickel, palladium, platinum, lead, and thorium, are calculated on the basis of modified Bhatia's model. The theoretical results are compared with the experimental findings of Y, θM, and 〈u2〉, namely the Debye‐Waller factor temperature parameter, effective X‐ray characteristic temperature, and the mean square displacement. For five metals, copper, silver, gold, nickel, and palladium, where experimental data exist, reasonable agreement is obtained between theory and experiment. For the other metals, such calculations are given for an information to experimentalists.
Theoretical analysis of the charge‐ordered state in a quasi‐one‐dimensional and a finite cyclic system with the Ab‐initio LCAO‐MO method
Authors: Ionov S. P. , Amelin I. I. , Lubimov V. S. , Ionova G. V. , Makarov E. F.
Published: July 2007

A calculation is made of finite cyclic systems, imitating infinite quasi‐one‐dimensional chains using the LCAO‐MO method. Electron correlation is taken into account by artificially reducing the degeneracy, this is equivalent to the introduction of an ordering parameter on atomic centres or chemical bonds. The mutual of the charge‐ordered and Peierls states is investigated. Especial attention is paid to systems with electron attraction on an atomic centre.
Ionicity and structure of solids
Authors: Hübner K. , Bashenov V. K.
Published: July 2007

The average bonding‐antibonding gap of the electronic band structure leading to the definition of the ionicity of the chemical bond of solids is calculated in terms of effective pseudopotential form factors on the Fermi surface and of relevant structure factors. The reformulation of the ionicity parameter with the help of these form factors enables to extract from Phillips' ionicity scale its dependence on the lattice structure considered. This leads to a theoretical explanation of some empirical relations between bonding parameters and the structure of crystals. Numerical results are given for the zincblende, the rocksalt, and the wurtzite structure. The relative stability of the latter is investigated analytically. The usefulness of the method is demonstrated by the investigation of structural disorder in amorphous semiconductors as well as of positional disorder in mixed crystals. It is found that the amorphous phase of III‐V and diamond‐type semiconductors may be considered as a mixing of the zincblende and the wurtzite structure. The energy gap bowing in AlxGa1‐x As is explained in quantitative agreement with experimental results by a distortion of the bonding angles.
Dispersion of phonons in metal carbides (UC)
Authors: Goyal S. C. , Goyal A. K.
Published: July 2007

An attempt is made to formulate the theory of the dispersion of phonons in metal carbides. The three‐body‐force shell model (TSM) is used suitably extended to include the second neighbour short range interactions and the absence of LST splitting to study the dispersion curve of UC. The calculated frequencies of phonons are in good agreement with the experimental frequencies and are much better than those determined by the pseudo‐potential calculations.
Zero‐field splitting of 6S(d5) ions in tetragonal and rhombic symmetry: I. Complete d‐electron calculation of the crystal‐field dependence of D and E
Authors: König E. , Schnakig R.
Published: July 2007

Zero‐field splitting parameters D and E are determined on the basis of a complete crystal field and spin‐orbit interaction calculation for the 3d5 configuration. In tetragonal symmetry, D is presented in graphical form as function of the parameters Dq, Dt, B, and ζ, the dependence on Ds being negligible. In rhombic symmetry, E is chracterized essentially by Dv, the dependence on Du being insignificant. For cubic symmetry, the zero‐field splitting 3a is determined directly by Dq.
Photosensitivity of the EPR spectrum of the F+ center in ZnO
Authors: Soriano V. , Galland D.
Published: July 2007

The F+ charge state of the oxygen vacancy in ZnO is created by means of an electron irradiation performed at an energy lower than the energy threshold for zinc displacements. Charge state changes of this defect observed by EPR under illumination are also described.
On the momentum distribution of electrons in polar semiconductors for high electric field
Authors: Devreese J. T. , Evrard R.
Published: July 2007

At zero temperature and low electron concentration the only important effect limiting the electric conductivity in a perfect polar semiconductor is the emission of real longitudinal optical phonons. An analytical method based on Boltzmann's equation is developed to obtain the electron momentum distribution in this situation. Resulting plots of the distribution function with parameters corresponding to InSb andE= 12 and 120 V/cm are given.
Directional dispersion of polar phonons measured by infrared reflection
Authors: Falge H. J. , Schuller E. , Borstel G.
Published: July 2007

The dispersion of all polar optical phonons may be measured by the attenuated total reflection method. It is shown how the directional dispersion of the extraordinary polar phonons in uniaxial crystals is obtained in a similar way. In the (ω, k) diagram the areas of high reflectivity for transverse magnetic polarized incident radiation are determined by curves which have for k → 0 the same frequencies as extraordinary phonons propagating in an infinite crystal for k → ∞. At arbitrary crystal cuts these curves are boundaries for the existence of surface phonon‐polaritons. The method allows measuring the complete directional dispersion of t—t and l—l branches of Raman‐inactive polar phonons, too.
EPR study on amorphous germanium oxygen, deposition angle, annealing, and substrate temperature effects
Authors: Kubler L. , Gewinner G. , Koulmann J. J. , Jaéglé A.
Published: July 2007

The influence of various deposition parameters, especially oxygen pressure and deposition angle, on the a‐Ge EPR spectra is studied. In most the cases the results corroborate those already obtained by other authors with other methods (such as conductivity). Two different mechanisms are discussed to explain the spin density decrease: saturation of defects by introducing oxygen and decrease of the defect concentration by structural rearrangements with annealing or substrate temperature increase. At suitable annealing conditions a spin density increase suggests the formation of defects such as multivacancies. The comparison of the EPR line thus obtained with the initial one shows that the defects formed during the evaporation of the purest samples are the same. The EPR line is inhomogeneous.
Localized non‐magnetic impurities in metals II. Application to 3d‐transition metal impurities in aluminium
Authors: Mrosan E. , Lehmann G.
Published: July 2007

Results of calculations of residual resistivity, thermoelectric power, Dingle‐temperature, changes in electronic specific heat, and superconducting critical temperature due to 3d‐transition metal impurities in Al are presented. The impurities are described in terms of muffin‐tin potentials, calculated with the Mattheiss construction and the Friedel sum rule. The results are compared with experimental data and show a qualitative agreement.
Acoustic and optical phonons in (SN)x
Authors: Stolz H. J. , Wendel H. , Otto A. , Pintschovius L. , Kahlert H.
Published: July 2007

Measurements are made of both polarized and unpolarized Raman spectra and IR reflection and transmission spectra of epitaxial films of polysulfur nitride (SN)x on mylar substrates as well as of single crystals. The dispersion of acoustic phonons of (SN)x is measured for small wave vectors parallel and in two directions perpendicular to the chain axis by means of inelastic neutron scattering. Force constants are estimated from a simple model.
Neutron investigation of magnons in the Cu2MnAl Heusler alloy
Authors: Kraśnicki S.
Published: July 2007

An investigation is made of inelastic magnon scattering of neutrons in a single crystal of the Heusler Cu2MnAl alloy. The dispersion relations for three principal symmetry directions up to the energy of about 50 meV are established. The exchange parameters may be qualitatively well explained by the RKKY or Caroli‐Blandin type models, although it is possible that the agreement is accidental and another mechanism is responsible for the n.n and n.n.n. interactions.
Lattice dynamics of BaO
Authors: Pandey A. N. , Upadhyaya K. S.
Published: July 2007

The lattice dynamics of BaO is studied by means of a three‐body force shell model. Phonon dispersion curves and specific heat variations with temperature computed in the present study and those obtained by fitting a rigid shell model are compared with available experimental data. Observed values of the second‐order Raman scattering spectra are also interpreted by using the combined density of states and the critical point analysis approach. An excellent agreement is obtained between theory and experiment.
Energy transfer in CaF2Gd, Mn crystals
Authors: Bozhevolnov V. E. , Ivanov L. N. , Kozlov V. K. , Voronov Yu. V. , Timofeev Yu. P. , Karelin V. V.
Published: July 2007

An investigation is made of the energy transfer between the Gd3+ and Mn2+ ions in CaF2Gd, Mn crystals under photoexcitation. The studies performed at varied concentrations and with measurement of the lifetimes of the excited states reveal that the process is intracentric and that only the noncubic luminescent centres of gadolinium seem to be involved in the energy transfer from the Gd3+ ions to the Mn2+ ions.
Conduction current and generalized einstein relations for degenerate semiconductors and metals
Authors: van Vliet K. M. , Marshak A. H.
Published: July 2007

Much confusion exists for conduction in degenerate materials. The complete current expressions are derived from the Boltzmann transport equation. Spatial dependence of all effect, including the variation of band edge, quasi‐Fermi level, chemical potential, and temperature is taken into account. Two different forms of diffusion current D∇n and ∇(D′n), used in recent work on semiconductors, are obtained and the relationship between them is discussed. Expressions for D, D′, as well as other transport coefficients, are given in terms of band integrals for an arbitrary band structure. Several forms of a generalized Einstein relation are derived. Effects of a magnetic field are included when the effective mass approximation applies. Discrepancies in detail with the literature are frequently noted.
Temperature dependence of the EPR and optical spectra in V++‐doped alkali halides
Authors: Vanhaelst M. , Matthys P. , Boesman E.
Published: July 2007

Analysis of EPR spectra shows that RbCl as well as NaCl and KCl contains V++ centres with a cation vacancy along the [100] axis. The temperature dependence of the spin‐Hamiltonian parameters is reported for the three chlorides. The fine structure parameter B decreases with temperature. A theoretical explanation based on crystal field theory is presented and correlated with the optical absorption spectrum in the NaCl case.
Spontaneous emission due to exciton—electron scattering in semiconductors
Authors: Hönerlage B. , Klingshirn C. , Grun J. B.
Published: July 2007

The temperature dependent lineshape of luminescence lines which are due to inelastic scattering of electrons with free excitons is calculated. The results are compared with experimental findings in ZnO, CdS, CdSe, the copper halides, and, especially, with new experiments on CuBr. For the latter substances approximate values of electron and hole masses are given: me = 0.5m0, mh = 2.0m0 for CuCl, me = 0.28m0, mh = 1.4m0 for CuBr, and me = 0.3m0, mh = 2.4m0 for CuI. In CuBr a new line is found which may be due to exciton—hole or exciton—exciton intraband scattering.
Anomalous Hall effect of n‐InSb at high magnetic fields
Authors: Biernat H. , Kriechbaum M.
Published: July 2007

Inclusion of the vectorpotential in the k · p‐perturbation method leads to additional terms in the current operator and scattering probabilities for conduction electrons. These are calculated for high magnetic fields (where nondegenerate statistics prevail) for δ‐function‐like impurity potentials. A reversal of sign in the Hall coefficient is obtained.
Self‐diffusion in nickel at low temperatures
Authors: Maier K. , Mehrer H. , Lessmann E. , Schüle W.
Published: July 2007

Self‐diffusion in nickel single crystals is measured between 813 and 1193 K using ion‐beam sputtering as a microsectioning technique. Gaussian activity‐depth profiles are observed over about three orders of magnitude in the activity decrease. The bulk diffusivities obtained from these profiles are analysed in conjunction with Bakker's high temperature data. The monovacancy contribution to the lattice diffusivity is given by D = 0.92 exp (−2.88 eV/kT) cm2/s. Preliminary isotope effect experiments are also performed at two temperatures in the high‐temperature region. The results support the view that a divacancy contribution to self‐diffusion exists near the melting point.
X‐ray spectroscopic investigation of beryllium by polarized K‐emission valence bands
Authors: Dräger G. , Brümmer O.
Published: July 2007

A theory of polarized K‐emission bands is given and a special method of measuring the spectra by means of a grating spectrometer is described. In this way polarized components of the radiation emitted by a monocrystalline sample are recorded. The spectra of optically anisotropic crystals measured in such a way possess a higher content of information by additional applicable selection rules for polarized radiation compared with the spectra of polycrystalline samples. In the case of the polarized Be K‐emission valence bands conclusions can be drawn on the existence of bonding charges. A more unambiguous correlation of spectra structures with van Hove singularities of the band structure provides a better experimental check up of several recent band structure calculations for metallic beryllium. This is especially demonstrated for the discontinuities at the points Γ and M.
Mixing of frenkel excitons and ionic excited states of a linear molecular crystal with two molecules in the unit cell. II. Physical consequences
Authors: Petelenz P.
Published: July 2007

The direct generalization of the Merrifield theory for a linear crystal with two molecules in the unit cell, presented in part I of this paper, is applied to a model crystal with nearest‐neighbour interactions. The parameters of resulting Merrifield‐type effective Hamiltonian are calculated for several parameter sets along with the intensities of the transitions induced by the mixing. The electric field dependence of the results is discussed. Differences with the Merrifield theory for a crystal with one molecule in the unit cell are pointed out.
Deformation of the vanadium phonon spectrum due to additions of uranium atoms
Authors: Syrykh G. F. , Zheknov A. P. , Zemlyanov M. G. , Chernoplekov N. A. , Solovev G. I.
Published: July 2007

Measurements are made of inelastic incoherent scattering of neutrons by a V0.97U0.03 alloy using a time‐of‐flight spectrometer with cold neutron source. The presence of quasi‐local vibrations (QLV) in the vibrational spectrum of the V0.97U0.03 alloy is revealed and the variation in the efficient force constants (FC) of the impurity atom‐matrix atom interaction is estimated and compared with the interaction between matrix atoms. It is shown that the revealing of the QLV in the spectra of inelastic incoherent neutron scattering essentially depends on the relation between the total amplitudes of neutron scattering by impurity atoms and matrix atoms.
Submillimeter conductivity of thermal and warm electrons in GaAs and InP
Authors: Philipp A. , Kuchajr F. , Seeger K.
Published: July 2007

Experimental and theoretical results of the transmittance of submillimeter radiation (890 GHz) by n‐GaAs and n‐InP show that the relaxation time obtained from the do mobility can be used for describing high‐frequency effects in this range of the spectrum too. The investigation of heating the conduction electrons by the electric field of radiation (“warm electrons”) yields the energy relaxation time τε in GaAs and InP. The experimental values of τε are larger than those calculated by a simple electron temperature model. This discrepancy can be removed by assuming a “two‐electron‐temperature” model.
Wavelength‐modulated reflectivity spectra of ZnSe and ZnS from 2.5 to 8 eV
Authors: Theis D.
Published: July 2007

Wavelength‐modulated reflectivity spectra of ZnSe and ZnS are presented in the energy range from 2.5 to 8 eV. Around the fundamental transition Eo qualitative evidence for the occurrence of excitonic polariton and exciton‐phonon quasi‐bound complex is given. The structure at higher energies is compared to calculated derivative reflectivity curves of Walter et al. The agreement between theory and experiment is good.
Surface effects in spin‐wave resonance in thin YIG films
Authors: Ramer O. G. , Wilts C. H.
Published: July 2007

Ferromagnetic resonance data from thin YIG films are compared with calculated data using the surface layer and tensorial models of surface pinning. The consistency with which the surface layer model explains the resonance phenomena is considered strong evidence that the boundary conditions are provided by surface regions with magnetic properties different than the bulk.
A new approach to overlap repulsion
Authors: Sharma M. N. , Kant A. , Misra K. D.
Published: July 2007

Introducing the concept of interorbital distance (δi), a new form of overlap repulsion (ΨR = Aδ) for ionic crystals is suggested. Alkali halide crystals are considered for the present purpose. The theory is based on the assumption that the ions possess a structure like the Bohr atom and this assumption along with other basic considerations exhibits a gap effect which leads to the formation of the present potential energy function. To test the validity of the new approach the compressibility (β) and the elastic constant (C‖) of alkali halide crystals are evaluated. The closeness of theoretical and experimental values strongly supports the present approach. The ionic radii of alkali and halogen ions are also evaluated and discussed in detail in the light of Pauling's values.
Random collinear structures of a dilute magnetic alloy
Authors: Medvedev M. V. , Zaborov A. V.
Published: July 2007

It is shown that the internal molecular field distribution in a dilute magnetic alloy with infinite‐ranged exchange interactions is a Gaussian one. Magnetic properties of two random collinear states – an uniaxial spin‐glass state and a random ferromagnetic state – are investigated for an uniaxial model of magnetic alloy.
On flux pinning by point pinning centres in Type‐II superconductors
Authors: Schmucker R. , Brandt E. H.
Published: July 2007

The maximal flux line displacement s(O) caused by a point force is calculated from the correct elastic properties of the flux line lattice, which were derived in a provioous paper. For bx2> (1  b)(x Ginsburg‐Landau parameter, b  B/Hc2 reduced induction) non‐local effects become important and lead to an s(O) larger than the result of local theory of elasticity by a factor \documentclass{article}\pagestyle{empty}\begin{document}$ {x \sqrt {b/2(1 - b)} }$\end{document} or even more. As a consequence, the volume pinning force Pv for a depinning process becomes larger by this factor. In particular, near H<c2, Pv ∼ \documentclass{article}\pagestyle{empty}\begin{document}$ {\sqrt {1 - b} }$\end{document} is obtained rather than Pv ∼(1 − b) as obtained by previous authors.
Influence of electron‐ion spin coupling on the electronic properties of a ferromagnetic semiconductor
Authors: Nolting W.
Published: July 2007

An attempt is made to obtain theoretical predictions concerning ferromagnetic semiconductors, especially the Eu‐chalcogenides, by approximate determination of relevant spectral densities. Of primary interest is the influence of the localized Heisenberg spin system (4f‐levels) on the conduction band. The electron‐magnon interaction is regarded in the sense of emission and absorption of magnons by conduction electrons and the resulting distortion of the electronic dispersion is discussed for an f.c.c. lattice. The magnetic red shift of the optical absorption edge, as it is experimentally observed for the ferromagnetic Eu‐chalcogenides, can be reproduced nearly quantitatively by discussing the dependence on the band width. Some well‐known, exactly calculable limiting cases, e.g. the Richmond‐criterion, are shown to be fulfilled by the approximation.
Optical absorption in Hg Te
Authors: Szuszkiewicz W.
Published: July 2007

Optical absorption in intrinsic HgTe is measured in the wavelength range 2.0 to 25 μm at tem peratures from 10 to 295 K. The Γ6 Γ8 transitions are observed at low temperatures. The ab‐ sorption coefficient is theoretically calculated using Kane's formulae for inverted band structure. The results of the calculations based both on the complete Kane theory and on the approximation of large spin‐orbit splitting are obtained. It is shown that the latter approximation does not describe well experimental absorption data. The agreement between the experimental results and the theoretical curves which are based on the complete Kane theory with the reasonable values of band structure parameters is much better. However, for energies greater than 0.35 eV small discrepancy is observed. This discrepancy increases with the energy.
Large unit cell calculations of solids in the CNDO approximation
Authors: Evarestov R. A. , Lovchikov V. A.
Published: July 2007

Equations are obtained describing in the CNDO approximation the electronic states of solids in the framework of the large unit cell approach. The CNDO approximation is applied to first timefor three‐dimensional crystals (diamond and lithium fluoride). The applicability of the CNDO parametrization suggested for molecules for the calculation of electronic states of solids is discussed. The results obtained are compared with those of band structure calculations using the LCAO approximation.
On the third‐order elastic constants of ionic solids with NaCl structure
Authors: Garg V. K. , Puri D. S. , Verma M. P.
Published: July 2007

Expressions for the third order elastic (TOE) constants of ionic solids, crystallizing in the NaCl structure, derived earlier from the Lundqvist potential by Puri and Verma are modified to take account of the second neighbour short range repulsive interactions. These expressions are used to evaluate the TOE constants and the pressure derivatives of the second order elastic (SOE) constants of alkaline earth oxides and those alkali halides which crystallize in the NaCl structure. The calculated values give fairly good agreement with available experimental results and in this respect are better than the results reported by other theoretical workers and those obtained in earlier calculations where short range repulsion was limited to nearest neighbours only.
Extended X‐ray absorption fine structure (EXAFS) studies of the lanthanides
Authors: Vijayavargiya V. P. , Gupta S. N. , Padalia B. D.
Published: July 2007

Extended X‐ray absorption fine structures (EXAFS) of the L3‐edges of dysprosium, holmium, thulium, ytterbium, and lutetium metals are investigated using a focussing X‐ray crystal spectrograph of transmission type. The present results are interpreted in the light of the EXAFS theories proposed by Hayasi and by Lytle and the interatomic distances in these metals are determined using Lytle's method. It is found that Moseley's law applies to the white lines in the lanthanides. A comparison of the widths of the white lines with the positron annihilation rates in lanthanides reveals interesting correlations.
Etude de la chaleur spécifique de sels doubles de TCNQ à cations non‐radicalaires
Authors: Delhaes P. , Keryer G. , Flandrois S. , Manceau J. P.
Published: July 2007

On étudie la chaleur spécifique de sels doubles de TCNQ à cation diamagnétique du type ammonium quaternaire. Les résultats des mesures effectuées dans certains cas entre 0,2 et 300 K sont comparés aux variations thermiques de susceptibilité magnétique. Deux types de complexes sont examinés: ceux constitués d'un empilement irrégulier des plans de TCNQ et se comportant comme des semi‐conducteurs et ceux constitués d'un empilement régulier possédant une conductivité électrique métallique mais à température ambiante seulement. Ces derniers présentent un terme linéaire de chaleur spécifique et un paramagnétisme non nul aux trés basses températures. Pour en rendre compte, on montre qu'il faut appliquer un modéle de Hubbard “étendu” unidimensionnel. Une analyse théorique de ce modéle où les interactions électron‐électron sont dominantes permet d'interpréter les résultats obtenus.
Luminescence of lead‐activated sodium chloride single crystals
Authors: Marculescu L.
Published: July 2007

The emission spectra are measured for different dispersion states of Pb2+ in NaCl crystals. A model for Pb2+ luminescent centres is proposed, which is able to bring into accord the kinetics revealed by optical and electrical methods.
Long‐wavelength optical phonons in MnxHg1−xTe mixed crystals
Authors: Gȩbicki W. , Nazarewicz W.
Published: July 2007

The infrared reflection spectra of MnxHg1–xTe with 0 ≦ x ≦ 0.69 are measured in the optical region from 50 to 300 cm+1 at 90 and 295 K. Long‐wavelength optical phonon frequencies are obtained by means of the Kramers‐Kronig relations. The behaviour of the phonon spectra is classified as a “two‐mode type behaviour”. The random‐element isodisplacement (REI) model is applied to calculate numerically the long‐wavelength optical phonon frequencies in the mixed crystals as a function of composition. The theoretical results are in good agreement with those obtained experimentally.
Inhomogeneous strain field scattering deformation potential. I. Theory
Authors: Farvacque J. L. , Lenglart P.
Published: July 2007

The influence of a small inhomogeneous deformation strain field on the expression of the crystal Hamiltonian is presented in terms of perturbation of the undeformed crystal Hamiltonian. In the different perturbation terms, leading to the band‐structure shift in semiconductors and which explains the conductivity change in piezoresistance experiments, one identifies an aperiodical term with the scattering potential needed for free carrier mobility calculations.
Inhomogeneous Strain Field Scattering Deformation Potential. II. Free‐Carrier Mobility Associated with Dislocation Strain Fields in Semiconductors
Authors: Farvacque J. L. , Lenglart P.
Published: July 2007

Using the expression of the scattering deformation potential, established in a preceding paper, the reversal mobility of free carriers associated to edge dislocations is calculated and the results are compared to those obtained by using the classical deformation potential of Bardeen and Shockley. Collation with experimental data obtained on edge‐dislocated tellurium, shows quantitative agreement with the present results.
ESR and Conductivity in Amorphous Germanium and Silicon
Authors: Movaghar B. , Schweitzer L.
Published: July 2007

A theory is outlined for the temperature dependence of the ESR linewidth in amorphous germanium and silicon. By connecting the lifetime broadening with the motion of the spins, and assuming a distribution of spin relaxation times, the experimental results may be explained, in particular the observed relationship between temperature dependent linewidth and dc conductivity.
Transition‐Active k = O Representations and the Microscopic Description of Phase Transitions
Authors: Agyei A. K. , Birman J. L.
Published: July 2007

Cell preserving transitions that give rise to localized physical quantities are considered from the standpoint of the phase transformations of the individual crystal sublattices. Such transitions are shown to be associated not with irreducible crystal point‐group representations, but with certain generalized transitive imprimitive representations of the transitive permutation groups of the “transforming” sublattices. These representations allow a microscopic treatment of any such transition, and it is shown, in particular, that the thermodynamic order of a crystal transition depends on the microscopic structures of the transforming sublattices. Further, it is shown on the basis of a Landau‐type crystal free‐energy expansion that at least one linear combination of the components of the (generalized) zero‐field inverse susceptibility tensor, χ+1, of the crystal should go to zero at the transition temperature Tc. The case of structural transitions, for which χ+1 is the elasticity tensor, is considered in some detail. It is shown that if a structural transition is brought about by the spontaneous distortion of a sublattice of site point group H ⊆ D4h, then either one of the shear moduli cs = 1/2 (c11 – c12) and c44 will go to zero at Tc.
Channel Model in Isotropic Exchange Theory
Authors: Eremin M. V. , Rakitin Yu. V.
Published: July 2007

The microscopic theory underlying the exchange channel model is described. It is shown that for a large class of isostructural systems the contributions to the exchange coupling parameter J from separate channels (pairs of interacting half‐filled orbitals) depend only slightly on the number of the electrons in the open d‐subshells of the metals, and the value J in the HDVV Hamiltonian is equal to the sum of such contributions divided by the number of interion exchange channels. The applicability of this approach is examplified by the analysis of a number of exchange systems.
Charge Transfer in Aluminium—Magnesium Alloys
Authors: Neddermeyer H.
Published: July 2007

From Kα1,2 and L2,3 X‐ray emission spectra of Al and Mg in a series of Al—Mg alloys energy shifts of the Al and Mg Is and 2p3/2 core levels are determined. The experimental results indicate a charge transfer from Mg to Al sites, which can consistently be explained by considering the difference in electronegativities of the pure metals.
The Dielectric Tensor Components of Czochralski‐Grown 2:1 Mullite Single Crystals
Authors: Büchner U. , Guse W.
Published: July 2007

Electron energy loss measurements up to 40 eV have been performed in order to characterize the dielectric properties of 2:1 mullite single crystals. The dielectric tensor components were determined by application of the Kramers‐Kronig analysis. A band gap of (7.7 ± 0.2) eV was found. The results show a distinct anisotropic electronic structure, which is related to the crystallographic details of the 2:1 mullite.
Influence of the Surface on the Galvanomagnetic Properties of Tellurium
Authors: Englert Th. , von Klitzing K. , Silbermann R. , Landwehr G.
Published: July 2007

At real tellurium surfaces natural accumulation layers exist which can be changed substantially by different etching processes. For pure samples this surface layer may mask the galvanomagnetic properties of the bulk completely. Employing a new etching process the influence of the surface can be reduced drastically, complete elimination is possible using a field effect arrangement. A comparison of the results obtained from samples with different degrees of accumulation at the surface allows a separation of the galvanomagnetic properties of the surface and the bulk.
Infrared Reflectance and Dielectric Properties of Pyrolytic Graphite for E ∥ c Polarization
Authors: Venghaus H.
Published: July 2007

Normal incidence reflectance measurements with E ∥ c polarization are performed on polished surfaces of pyrolytic graphite for 15 meV < ħω < 500 meV and the dielectric function ϵ∥(ω) is derived from the data by Kramers‐Kronig analysis. The function ϵ∥(ω) is characterized by a background dielectric constant ϵ0∥ = 5.4 and free carrier contributions below 100 meV. The free carrier contributions can be characterized to a good approximation by Drude‐like behaviour with a plasma energy ħωp∥ (E ∥ c) = (101 ± 5) meV, giving rise to a maximum of the energy loss function Im [−1/ϵ∥(ω)] at (37 ± 2) meV due to higher lying interband transitions. The plasma frequency determined in the present experiments is in agreement with theoretical estimates of the free carrier plasma frequency.
Two‐Photon Absorption by Envelope–Hole Coupled Exciton States in Cubic ZnSe. I. Energy Spectrum of the Hd Term and Valence‐Band Parameters
Authors: Sondergeld M.
Published: July 2007

In the exciton spectrum of cubic ZnSe a fourfold energy splitting of the 2P exciton is observed by two‐photon absorption. The distinct fine structure is the first experimental evidence for the coupling between the exciton envelope and the total hole spin. The energy spectrum of the spin Hamiltonian for Wannier excitons is calculated by use of a variational technique. Hydrogenic p‐ and f‐like trial functions are introduced as solutions, and the two equivalent Bohr radii are treated as independent variational parameters. First experimental data of the hole effective‐mass parameters for the Γ8 valence band are deduced from the observed energy splitting.
Optical Spectra, EPR, and Spin–Lattice Relaxation of Yb3+ Ions in Crystals Having Perovskite‐Type Structure
Authors: Antipin A. A. , Vinokurov A. V. , Davydova M. P. , Korableva S. L. , Stolov A. L. , Fedii A. A.
Published: July 2007

The optical and EPR spectra of centres formed by Yb3+ ions substituted for monovalent and divalent cations are observed on KMgF3 and KZnF3 crystals. From the Stark structure and from the principal values of the g‐tensor the crystal field parameters of cubic, trigonal, and tetragonal centres are found. It is shown that as a result of substitution Yb3+ into the Me2+ site (cubic centres) the radius of the nearest coordination sphere increases by 20%. The spin‐lattice relaxation time for Yb3+ cubic centres has an unusual temperature dependence of the form T = 82.3T + 1.73 × 10−5T11.
Peculiarities of the Electron–Phonon Interaction in the Spectra of the Molecular Ions O, S, and NO in Alkali Halide Crystals
Authors: Freiberg A. , Rebane L. A.
Published: July 2007

A systematic low‐temperature study is made of deformations of no‐phonon lineshapes in the luminescence spectra of small molecular ions in alkali halide crystals. It is shown that the main peculiarities of the temperature broadening and shift of lines arc connected with the selective manner of coupling of the electron transition in the centre with low‐frequency pseudolocal vibrations. This conclusion is confirmed also by the study of the change of the total Stokes' losses on crystalline vibrations with temperature. It is found that the homogeneous no‐phonon lineshapes are Lorentzians with a slight low‐frequency asymmetry.
Band Formation in Xenon–Argon Alloys Studied by Photoelectron Spectroscopy
Authors: Nürnberger R. , Himpsel F.‐J. , Koch E. E. , Schwentner N.
Published: July 2007

Photoelectron energy distribution curves for xenon‐argon alloys for concentrations ranging from 0 to 100% are measured by excitation with synchrotron radiation at hv = 13.8, 16.5, and 18.0 eV. With increasing Xe concentration the gradual formation of Xe valence bands starting from the atomic Xe 5p(1/2) and Xe 5p(3/2) states is observed. Similarly with Ar the 3p states are broadened with increasing Ar concentration. Rather high concentrations of Xe or Ar are necessary in order to reach the fully developed Xe or Ar bands, respectively. The results are discussed in terms of a concentration dependent tight binding band structure.
Resonance Scattering of Phonons at Trigonal Jahn‐Teller Centres
Authors: Rueff M. , Sigmund E. , Wagner M.
Published: July 2007

The scattering of degenerate lattice waves by localized degenerate electronic states is considered. The inverse lifetime of phonons of ΓE‐type is calculated by the method of double time thermo‐dynamical Green's functions, the T‐Matrix formalism, and an application of a nonlinear canonical transformation. The behaviour of the lifetime in both the extremal coupling cases is given. The temperature dependence of the lifetime broadening is discussed. The resonance frequency is studied for small and strong interactions.
Photoionization of Triplet Excitons in Tetracene Crystals
Authors: Schlotter P. , Kalinowski J. , Bässler H.
Published: July 2007

Transient photocurrents in crystalline tetracene excited by intense ruby laser pulses are investigated. A cubic relationship between charge carrier density and light intensity is observed. The dominant carrier generation mechanism is photoionization of triplet excitons produced by fission of two‐photon‐excited singlet states. The ionization cross‐section is found to be (5 ± 2) × 10−22 cm−2 and the calculated autoionization efficiency of the excited triplet state is about 10−3. From the time response of the photocurrent pulses a rate constant γeh = (1.0 ± 0.5) × 10−6 cm3 s−1 for electron‐hole recombination follows.
Determination of Localized Magnetic Moments in FeCrAl Alloys and the Electron Structure
Authors: Blau W. , Mager S. , Wieser E.
Published: July 2007

The localized magnetic moments of Fe and Cr are determined by combination of saturation magnetization measurements and magnetic diffuse scattering. Power series characterizing the interactions between the different kinds of atoms in the alloys are chosen to describe the concentration dependence of the magnetic moments. The different terms are discussed on the basis of band structure models valid for dilute alloys taking into account their modification by impurity interactions.