bluuebunny/crossref_metadata_2025_split huggingface dataset:
Self‐Consistent Phonon Calculation for Hexamethylenetetramine Molecular Crystals
Authors: Wasiutyński T. 
Published: July 2007

AbstractThe self‐consistent phonon approximation is formulated for crystals with orientational degrees of freedom. The lattice dynamics regarded as a function of temperature and pressure is calculated for hexamethylenetetramine in the “6‐exp” atom‐atom approximation. Good agreement of the calculated lattice constant, phonon frequencies, and elastic moduli with experimental data is obtained.
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

AbstractMagnetoreflection 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.
Critical Behaviour of Conductivity and Dielectric Constant near the Metal‐Non‐Metal Transition Threshold
Authors: Efros A. L. , Shklovskii B. I. 
Published: July 2007

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractX‐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

AbstractSecondary 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

AbstractCalculations 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

AbstractThe 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.
Phenomenological theory of coupling of debye relaxations with IR‐active phonons
Authors: Merten L. , Borstel G. 
Published: July 2007

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractAn 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

AbstractZero‐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

AbstractThe 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

AbstractAt 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

AbstractThe 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

AbstractThe 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

AbstractResults 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

AbstractMeasurements 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

AbstractAn 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

AbstractThe 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

AbstractAn 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

AbstractMuch 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

AbstractAnalysis 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

AbstractThe 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

AbstractInclusion 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

AbstractSelf‐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.