bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of photoemission from molecular crystals
Authors: Belkind A. I. , Brodskii, V.V.Grechov A. M.
Published: July 2007

On the basis of previous approach and considering non‐direct transitions an expression is obtained for calculating photoemission (PE) current from molecular crystals. The model used for treating optical transitions contains the concept f quasi‐particle (hole) formation simultaneously with generation of an electron. The band, corresponding to this quasi‐particle, has a width approximately equal to polarization energy. For the limiting case of high energies a relationship is obtained by means of which it is possible to find energy dependence of density of states in the hole band, using experimental results. The cubic law is deduced for the near‐threshold region using the above‐mentioned relationship based on the experiment. The general expression for photocurrent is also used to obtain the spectral dependences of photoconductivity in the threshold region.
Many‐body lattice theory of elastic dielectric and its application to solids with NaCl structure
Authors: Goyal S. C. , Prakash R. , Tripathi S. P.
Published: July 2007

A phenomenological lattice theory of elastic dielectric based on the Lundqvist potential of ionic cohesion is developed. In this theory the lattice is considered to be initially in the deformed and polarised state. The theory yields the expressions for the second order elastic constant, third order elastic constants, dielectric constants, and its strain derivatives. The many body lattice theory (MLT) of elastic dielectric so developed is applied to calculate the strain derivatives of the high and low frequency dielectric constants of the alkali halide solids. The calculated values of these constants are better than those obtained in all earlier models and are in excellent agreement with the experimental values.
Temperature dependence of the electrical resistivity and Hall coefficient in a model of concentrated alloy
Authors: Brauwers M. , Bhattachrya A. K.
Published: July 2007

The effect of temperature on the electrical resistivity and Hall coefficient of a two s‐d hybridized band model representing concentrated transition and noble metal alloys is investigated. Depending on the location of the Fermi level, the electrical resistivity and Hall coefficient are found to increse or decrease with temperature; such a behaviour is observed in various concentrated transition and noble metal alloys.
Thermal conductivity of NbTi alloy in the low‐temperature range
Authors: Flachbart K. , Feher A. , Jánoš Š. , Málek Z. , Ryska A.
Published: July 2007

The temperature dependence of the thermal conductivity of a NbTi (50/50) superconductor is measured in the temperature range from 1.3 to 300 K in two different laboratories. From the analysis of the experimental results it follows that the electron processes provide the main contribution to the thermal transport in the range above 0.3Tc while phonon contribution is decisive, due to the defect structure, only below this temperature. The values of energy gap 2Δ = (2.15 to 2.3) kTc are found by three independent methods. Further, the temperature dependence of the thermal diffusivity is presented in the temperature range above 4 K.
Disorder scattering in CdxHg1−xTe mixed crystals
Authors: Dubowski J. J.
Published: July 2007

An experimental and theoretical analysis is given of the electron mobility at 4.2 K and the longitudinal Nernst‐Ettingshausen effect at 30 K of highly‐doped (n ≈ 9 × 1017 cm−3) CdxHg1−xTe crystals. The molar composition x of measured samples ranged from 0.03 to 0.33. In the theoretical analysis of the experimental data the band structure is described by the full Kane equation. The dielectric function anomaly is also taken into account in the semimetallic region. In order to explain the experimental data it is necessary to consider both the charged centre and disorder scattering as the compensation effect.
Analysis of charge—orbital ordering in compounds with mixed valence
Authors: Ionova G. V. , Makarov E. F. , Pachev O. M. , Ionov S. P.
Published: July 2007

Orbital and charge ordering in transition element compounds are considered and a theoretical model is established. Special emphasis is given to the divalent compounds of Cu2+, Ag2+, Au2+ and the trivalent compounds of Pd3+ and Pt3+. It is shown that spatial orbital ordering takes place in compounds with strong intracentre correlation (Cu2+, Cr2+, Mn3+) and that strong intercentre correlation results in charge ordering on orbitals or centres (Ag2+, Au2+, Pd3+, and Pt3+). Real systems and lattice distortions caused by spatial and charge ordering are discussed.
Internal strain and piezoelectricity in zincblende crystals
Authors: Varshney S. C. , Gundjian A. A.
Published: July 2007

The internal strain parameter is calculated from lattice dynamics for zinc blende crystals utilizing the modified rigid ion model. The piezoelectric coefficient is then obtained in terms of the internal strain parameter and the deformation dipole moment in the crystal. A simple form of this dipole moment is proposed and is found to predict the piezoelectric coefficient in II–VI and I–VII zinc blende crystals in very good agreement with the measured values.
Effect of nonlocal potential on the Fermi surface of metals and the phase shift parametrization scheme
Authors: Wang J. Shy‐Yih , Rasolt M.
Published: July 2007

The intrinsic problem of treating a nonlocal potential within the muffin‐tin approximation is studied. Specific attent on is given to the effect of the nonlocal nature of the self‐energy (or quasi‐particle potential) on the Fermi surface of free electron like metals.
Relativistic one‐particle equation for electron states of heavy metals
Authors: Gollisch H. , Fritsche L.
Published: July 2007

A scalar relativistic one‐particle equation for a spherically symmetric potential is derived by approximately solving the associated Dirac equation. In contrast to the so‐called Pauli equation this approximation remains valid if the electron is very close to the nucleus. Results obtained for W demonstrate the improvement over the Pauli approximation.
Magnetothermal oscillation and spin splitting of electrons in dilute Bismuth‐antimony alloys
Authors: Matsuo S. , Nishimoto T. , Hayashi S. , Noguchi S.
Published: July 2007

Magnetothermal oscillations in dilute BiSb alloys are measured with dc methods in fields up to 22 kOe. The period of the oscillation agrees with that one obtained from the de Haas‐van Alphen effect. A splitting of the peak is observed around 15 kOe and considered as the spin splitting of electrons. The spin‐splitting factor 2Δ calculated from the present results shows some deviations from unity. The deviations are interpreted by assuming the effect of the other bands after Baraff. The variation of 2Δ with increasing Sb concentration seems to imply that the relative energies of the other bands at the point L in the Brillouin zone „shrink”︁ with regard to the centre of the gap.
Coherent potential approximation for the electrical resistivity of partially ordered binary magnetic alloys
Authors: Goedsche F. , Richter R. , Vojta G.
Published: July 2007

The dc electrical resistivity is calculated for a b.c.c. based random binary substitutional magnetic alloy with partial long‐range order for a single‐band tight‐binding model with diagonal disorder, two kinds of localized spins, arbitrary concentrations and arbitrary order parameter. The spin‐disorder contribution to the resistivity is treated by means of the sd exchange model. Within the framework of the CPA up to second order in the scattering potential a general closed resistivity formula is given in terms of alloy parameters and spin parameters. The resitivity is discussed for various special cases in dependence on the long‐range order parameter.
Phase transition in the mixed crystals TbpY1−pAsO4
Authors: Schwab M.
Published: July 2007

The energy of the four lowest crystal field levels of Tb3+ in the mixed crystals TbpY1−pAsO4 is investigated in the range of the crystallographic and magnetic phase transition as a function of temperature and Tb‐concentration (0.2 < p ≦ 1). The results show that, in contrast to the magnetic ordering, the interaction causing the crystallographic transition is of long range and hence a simple molecular field theory fits the experimental data. For small Tb‐concentrations, the feature of this system are superimposed by effects of internal strains caused by the substitution of Tb3+ by Y3+ ions.
Raman‐induced Kerr effect with elliptical polarization in optically active and anisotropic crystals
Authors: Bergmann J. , Kneipp K. , Ponath H. E.
Published: July 2007

The Raman‐induced Kerr effect (RIKE) with an elliptically polarized pump laser beam in optically active and anisotropic crystals is discussed theoretically, taking into account the phonon‐photon‐interaction. It is shown that also in more complicated cases in crystals the RIKE spectrum can give information about the relative value of the non‐resonant term of the non‐linear susceptibility. General theoretical results are specified for the 817 cm−1 A(LO) mode in the uniaxial crystal LiIO3 and experimentally tested. The relative value v of the non‐resonant term of the fourth‐rank tensor is found to v≦ 0.1.
Electrical properties of TlTe single crystals
Authors: Ikari T. , Hashimoto K.
Published: July 2007

Single crystals of TlTe are grown from Te‐rich melt at 280°C. Resistivity, Hall coefficient, magnetoresistance, and thermoelectric power are measured. The experimental results are explained by the assumption of an uncompensated semimetal model, where the band overlapping energy between conduction and valence bands E0 is 0.20 eV in the high‐temperature phase above 172 K and 0.10 eV in the low‐temperature phase. The effective masses of electrons and holes are m = 1.0m0 and m = 1.5m0, respectively, in both the temperature phases.
Multiple scattering theory for magnetoresistance of localized impurities in metals
Authors: Mrosan E. , Schober Ch. , Lehmann G.
Published: July 2007

It is shown that starting from a multiple scattering formalism for localized impurities in metals the linearized Boltzmann equation can be solved by the degenerate kernel technique. In this case the Boltzmann equation reduces to an algebraic set of linear equations of finite rank. An expression for the magneto‐resistance, valid for arbitrary strength of external magnetic fields is derived. In the limit of a nearly‐free‐electron‐like host band structure the well‐known results for magnetoresistance and impurity resistivity are reproduced.
ESR From boron in silicon at zero and small external stress I. Line positions and line structure
Authors: Neubrand H.
Published: July 2007

ESR observations at zero external stress of the shallow acceptor centre boron in silicon are reported for the first time. These observations have become possible by the high degree of crystal perfection attainable today in Si‐crystal growth. The spectrum, its angular and uniaxial pressure dependence are interpreted in terms of a spin Hamiltonian given by Bir. Additional terms in this spin Hamiltonian are shown to be necessary to account for the experimental result and to explain the differences between the spin Hamiltonian constants derived by different authors.
Site ordering and magnetostriction in garnet II. Iron
Authors: Muller M. W.
Published: July 2007

The crystal field model of Akselrad and Callen is used to estimate the contribution of Fe ions to the growth‐induced non‐cubic magnetostriction of rare‐earth iron garnets. The effect is predicted to be significantly smaller than the previously calculated non‐cubic magnetostriction due to rare‐earth‐site ordering. The estimate uses a feature of the model which renders the local magnetoelastic tensor symmetric in strain and spin components. A plausible argument is adduced to support the conclusion independently of the model.
Singularities in the paramagnetic susceptibility of random Hubbard alloys
Authors: Kolley E. , Kolley W.
Published: July 2007

The paramagnetic susceptibility of disordered alloys is derived from a microscopic Fermi liquid approach for tightly bound electrons at zero temperature. Particle–particle and particle–hole effective vertices are calculated within the coherent local ladder approximation. The correlationenhanced expressions for component and alloy susceptibilities refer to paramagnetic instabilities. Numerical results are presented for partial and total CPA averages of static susceptibilities, magnetizations, and spin‐dependent densities of states.
Ionized impurity scattering in semiconductors
Authors: Gerlach E. , Rautenberg M.
Published: July 2007

In the Brooks‐Herring‐formula the scattering centres are assumed to be independent. The range of validity of this assumption is investigated. For this purpose the scattering of electrons by a pair of impurities is considered as a function of its internal separation and its orientation with respect to the electronic current. For a separation larger than about the Debye‐Hückel screening length the ionized impurities are found to act as independent scattering centres. For smaller separations the interaction of the scattering centres cannot be neglected.
Two‐phonon infrared absorption in In1−xGaxP mixed crystals
Authors: Ulrici B. , Jahne E.
Published: July 2007

The transmission spectra of In1−xGaxP mixed crystals (0 ≦ x ≦ 1) are measured between 20 and 550 K in the spectral range from 400 to 800 cm−1 in which the optical absorption is due to two‐phonon summation processes. The spectra of the end‐member crystals and that of the mixed crystals consist of two main band groups in which up to seventeen structures are observed and pursued over the entire composition range. As an effect of disorder a broadening of the structures and an increase of absorption in the structureless region of the spectra are observed. The structures in the spectra are assigned to particulart two‐phonon combinations or overtones of the various phonon branches — exploiting existing neutron and Raman data of the end‐member crystals as well as the measured temperature dependence of the infrared spectra of mixed crystals. The compositional dependence of the LO, LA, TO, TA phonons at the × and L point and the LO, TO phonons at the T point are derived. It turns out that all phonon modes assigned display one‐mode behaviour.
Manifestation of k‐linear term effects on the Zeeman splitting of the A1‐exciton emission lines in ZnO
Authors: Hümmer K. , Helbig R. , Baumgärtner M.
Published: July 2007

The magneto‐luminescence spectra of the fourfold n = 1 A‐exciton in ZnO up to H = 5 T are reported. The Zeeman splitting in the configuration H ∥ c, k ⊥ c, E ⊥ c clearly demonstrates the influence of the k‐linear energy terms allowed for the k‐components perpendicular to the c‐axis for T7 valence bands of the wurtzite structure. The appropriate exciton coupling matrix is calculated. From the comparison with the experimental results the A‐valence band slope C = = 2 × 10−10 eV cm is calculated.
Many‐exciton effects in the theory of biexcitons
Authors: Ekardt W. , Sheboul M. I.
Published: July 2007

Starting from a previously derived many‐particle equation describing biexciton states, dynamical screening effects in the irreducible exciton‐exciton interaction are taken into account. As a result, density‐dependent biexciton binding energies are obtained. Numerical results are given for CuCl and CuBr in the region of free‐exciton density from 10−5 ≦ nexa ≦ 5 × 10−2. In qualitative agreement with experimental data only a small shift of the biexciton luminescence line should be observed.
On the RKKY interaction in spin glass systems
Authors: Freudenhammer A.
Published: July 2007

The dependence of the critical temperature Tg of a spin glass on the Fermi‐momentum kF is calculated numerically. A strong dependence of Tg on kF is found. A proposal is made that this dependence should be measured experimentally in order to obtain information about the basic interaction in spin glass systems.
The disorder scattering in zincblende narrow‐gap semiconducting mixed crystals
Authors: Kossut J.
Published: July 2007

A formula for the relaxation time for the disorder scattering mechanism is derived taking into account the complexities of the narrow‐gap zincblende semiconductor band structure. The main features of this formula are discussed showing the importance of the actual form of the electron wave functions appropriate for narrow‐gap semiconductors. Other theoretical approaches to the problem of disorder scattering are reviewed briefly.
The Singularities of the Small Polaron Effective Mass
Authors: Mańka R.
Published: July 2007

Using the Bogolyubov inequality, a more general polaron dispersion relation than known up to now is obtained. The effective polaron mass derived from it for high temperatures and for a strong electron–phonon coupling is the same, but for semiconductors with a suitable wide electron band, the effective mass is discontinuous as a function of the temperature and the electron–phonon coupling constant.
Neutron and Light Scattering from Ferroelectric Fluctuations and Domain Walls of Strontium Barium Niobate
Authors: Prokert F. , Schalge R.
Published: July 2007

The mixed crystal SrxBa1−xNb2O6 (x = 0.70) is investigated in the phase transition region by inelastic neutron scattering. Softening of TO modes is not found but an additional quasi‐elastic neutron scattering (QNS) is detected, which arises mainly from a narrow central peak. This peak is also studied by Rayleigh scattering. Both methods show that the central peak consists of two parts. One part has a broad maximum at the diffuse phase transition (PT) and probably arises from phase fluctuations. The other part below the PT increases with decreasing temperature. It depends on static domain structure and the neutron scattering data can be interpreted by scattering from domain walls.
Photoconductivity in a‐GeSe
Authors: Vescan L. , Stötzel H. , Stoica T. , Kottwitz A.
Published: July 2007

The temperature, illumination, frequency, and photon energy dependences of the photoconductivity in a‐GeSe films are investigated. The conduction activation energy is 0.95 eV, the photoconductivity edge lies at 1.4 eV. The photoconductivity is field‐independent up to 3 × 104 V/cm and independent of the frequency of the chopping light up to at least 103 Hz. The activation energy of the photoconductivity decreases with decreasing temperature from 0.3 eV to almost zero. The experimental curves are consistent with activated mobilities for electrons and holes and recombination by both a direct and an indirect mechanism. A value of 4 × 1013 cm−3 for the density of localized states at the Fermi level is obtained.
Electrical Resistivity of Chromium–Gold Alloys between 77 and 700 K
Authors: Eroglu A. , Arajs S. , Moyer C. A. , Rao K. V.
Published: July 2007

Electrical resistivity is measured as a function of temperature of binary chromium alloys containing 0.27, 0.54, and 0.60 at% gold. Gold decreases the Néel temperature linearly at a rate of 32 K/at% gold. Such a decrease is contrary to the predictions of conventional models for the antiferromagnetism in dilute chromium alloys, but can be explained if the excess nuclear charge on gold impurities is totally screened by the band electrons. Total screening also is consistent with the observed temperature variation of resistivity well above the transition temperature if, as seems reasonable, the scattering of electrons from gold impurities reduces the electronic density of states of the alloy at the Fermi energy.
Spin–Orbit and Crystal Field Splittings of the Valence Band in II3V2 Semiconducting Compounds
Authors: Dowgiałło‐Plenkiewicz B. , Plenkiewicz P.
Published: July 2007

The pseudopotential method is used to calculate the spin–orbit and crystal field splittings of tetragonal II3V2 compounds at the Γ point of the Brillouin Zone. The computed splittings are compared to experimental data of other authors. The resulting structure of the valence bands in these compounds at the Γ point is presented.
Lattice Dynamics of Group IV Elements with the Local Heine‐Abarenkov Model Potential
Authors: Soma T.
Published: July 2007

The lattice dynamics of group IV elements is investigated using the local Heine‐Abarenkov model potential. The phonon curves obtained for Si, Ge, and α‐Sn are in good agreement with the observed data of the neutron scattering, contrary to those using the Ashcroft type.
A Cluster‐Bethe Lattice Treatment for the F‐Center in Alkali Halides
Authors: de Queiroz S. L. A. , Koiller B. , Maffeo B. , Brandi H. S.
Published: July 2007

The electronic structure of the F‐center in alkali halides with the NaCl structure is studied using the cluster‐Bethe lattice method. The central cluster is taken as constituted by the vacancy and the nearest and second neighbours to it, respectively cations and anions. The optical transitions are calculated and compared to experimental data on the location of the peak of the F‐absorption band. The agreement obtained indicates that this method may be used to study properties of this defect in alkali halides.
Interface States in One‐Dimensional Models of Deformed Heterojunctions
Authors: Kandilarov B. D. , Tashkova M. G. , Petrova P. C.
Published: July 2007

The existence of interface states in deformed heterojunctions is discussed on the basis of one‐dimensional models and in terms of the scattering matrix approach. Necessary and sufficient conditions for the existence of interface states are derived and numerical results are obtained in a form which makes possible a comparison with the earlier results for ideal heterojunctions without any deformations.
Nuclear Spin Relaxation by Quadrupolar Interactions. The Diffusion of Substitutional Impurities in Ionic Crystals
Authors: Becker K. D.
Published: July 2007

Spectral densities are calculated for nuclear spin relaxation via quadrupolar interactions caused by the vacancy diffusion of impurities in NaCl‐type ionic crystals. The treatment, which takes correlation effects into account, is based on the encounter model. NMR cw‐studies on the diffusion of Na+ ions in AgBr are analysed in the framework of this model. The impurity diffusion coefficients derived are found to be in good agreement with tracer diffusion data.
Electrical Resistivity and Magnetoresistivity of Amorphous CrxPd82−xGe18 Alloys
Authors: Ilonca Gh.
Published: July 2007

Electrical resistivity and magnetoresistivity measurements are made of amorphous alloys obtained by radio frequency sputtering using argon on fused quartz substrates to thicknesses of about 20 μm. The compositions of these alloys are CrxPd82−xGe18, in which x is the atomic concentration and ranges between 0 to 8. A well‐defined minimum in the resistivity versus temperature curve is observed for these alloys. Below the resistivity‐minimum temperature Tm, the resistivity difference Δ∂ varies as — ln T in agreement with Kondo's theory and tends to level off at lower temperatures. The resistivity‐minimum anomaly is always accompanied by a negative magnetoresistivity.
Distribution Functions and Thermodynamical Properties of KD2PO4 and ND4D2PO4 Type Crystals
Authors: Levitskii R. R. , Korynevskii N. A. , Stasyuk I. V.
Published: July 2007

In the cluster approximation, taking into account long‐range correlations, the distribution functions of deuterated ferroelectric compounds of the orthophosphate type are calculated. The behaviour of the single and pair function near the phase transition point for both the KD2PO4 and ND4D2PO4 type structures is analysed. It is shown that expressions for the longitudinal and transversal components of the dielectric susceptibility tensor are in a qualitative coincidence with the experiment in the phase transition region. Phase transition and some thermodynamical properties of deuterated orthophosphates are studied on the basis of the results obtained. It is shown that the long‐range interaction is necessary to form antipolarization states.
Solitary Excitations in One‐Dimensional Ferromagnets with Biquadratic Exchange
Authors: Pushkarov Kh. I. , Pushkarov D. I. , Vlahov J. P.
Published: July 2007

The existence of solitary excitations (“magnolitons”) in one‐dimensional ferromagnets with biquadratic exchange is shown. Such excitations appear as solutions of a nonlinear „Schrödingerlike”︁ equation with higher nonlinearity.
Influence of Exchange Interaction on the Quantum Transport Phenomena in Hg1−xMnxTe
Authors: Jaczyński M. , Kossut J. , Gałązka R. R.
Published: July 2007

A series of anomalous features of the Shubnikov‐de Haas effect in n‐type Hg1−xMnxTe mixed crystals is detected, e.g., strong temperature dependence of the ShdH peak positions, large values of electronic g‐factors, non‐monotonic behaviour of the amplitude of ShdH oscillations as a function of temperature. These features cannot be explained with the help of usual theoretical models. They are explained within the modified Pidgeon‐Brown model including the exchange interaction between conduction electrons and electrons from 3d shell of Mn ions. The quantum oscillations of the thermoelectric power are also observed. The behaviour of amplitude and peak positions of thermoelectric power are compatible with those of magnetoresistance.
The Semiconductor–Semimetal Transition in Bi1−xSbx Alloys with x ≧ 0.22
Authors: Kraak W. , Oelgart G. , Schneider G. , Herrmann R.
Published: July 2007

In Bi1−xSb, alloys with x = 0.13 to 0.25 the electrical transport properties (electrical resistance, magnetoresistance, and Hall effect) in the temperature interval T = 1.8 to 77 K in weak magnetic fields ( μB ≪ 1) are investigated. In alloys with x = 0.19 and x = 0.23 the dependence of the galvanomagnetic properties on the hydrostatic pressure are also investigated. The rapid increase of the charge‐carrier concentration in alloys with x > 0.22 and the qualitatively different pressure dependence of the gelvanomagnetic properties for this two alloys are explained by the overlapping of the conduction band at L with a valence band at an other point of the Brillouin zone, which begins at x ≈︁ 0.22. A semimetal‐semiconductor transition is detected in Bi1−xSb, alloys with x > 0.22 under high hydrostatic pressure.
Atomic Motions Corresponding to Zone Center Phonons in Paratellurite
Authors: Lemos V. , Cerdeira F. , Cardona M.
Published: July 2007

By using standard group theory techniques the symmetrized atomic motions are obtained corresponding to the various irreducible representations of the factor group of paratellurite (TeO2). This material can be treated as a molecular crystal. Thus the various symmetrized atomic motions of the same symmetry which correspond to molecular vibrations of different symmetry should not mix much if the dynamical matrix for the zone center phonons is diagonalized. Simple rules are developed for the identification of the various modes in the observed Raman and infrared spectra. These rules are based on the magnitude and ratios of the observed frequencies, their infrared strength, and their Raman cross‐sections. Strong Raman cross‐section and infrared strength result only if the corresponding molecular vibrations are allowed and if these vibrations are fully symmetrized over the four molecules per unit cell. In this manner a consistent assignment of all the observed infrared and Raman modes is achieved.
Inelastic Scattering of Electrons by Optic Phonons in InSb‐Type Semiconductors
Authors: Pfeffer P. , Zawadzki W.
Published: July 2007

Inelastic scattering of electrons by optic phonons in III–V semiconducting compounds of InSb type is described, taking into account consistently nonparabolic band structure of these materials both in the ε(k) dependence and the electron wavefunctions. The calculations are based on the Kane band model, including explicitly admixture of p‐like components and spin mixing in the conduction band. A variational procedure is used to derive explicit expressions for electric conductivity in case of optic phonon scattering. It is indicated how to generalize the procedure for an arbitrary spherical energy band. Numerical values of electron mobility in n‐InSb at various temperatures and electron concentrations are presented and compared with mobilities calculated in the relaxation time approximation, thus examining the validity of the latter. It is demonstrated that even at room temperature the nonelastic character of electron–phonon collisions has a pronounced effect on the conductivity.
Oscillator Strength of Excitons in Highly Excited PbI2
Authors: Iwasaki T. , Goto T. , Nishina Y.
Published: July 2007

Absorption and emission spectra are measured in highly excited states of evaporated films of PbI2 at 10 K. The spectra show that the oscillator strength of the 1s exciton decreases with the increase in the excitation intensity. This change is related to the decrease in the exciton binding energy in the highly excited state.
Nonadiabatic Quantum Theory of Interstitial Diffusion in Crystals
Authors: Pavlovich V. N. , Rudko V. N.
Published: July 2007

The influence of the nonadiabaticity of interstitial motion on the quantum underbarrier diffusion is discussed. The wave functions of crystals with an impurity, localized in initial and final positions, are obtained and used for the calculation of the probability of interstitial transitions from one interstitial to another per unit time. The temperature dependence of this transition probability is analysed, taking into account the nonadiabaticity of impurity motion and the deviation from the Condon approximation which is used for the calculation of transition matrix elements.
Electron Paramagnetic Resonance Study of Cu2+ in K2Zn(ZrF6)2 · 6 H2O
Authors: Petrashen V. E. , Yablokov Yu. V. , Davidovich R. L.
Published: July 2007

An EPR study of Cu2+ ions in K2Zn(ZrF6) · 6 H2O in the temperature range 4.2 to 300 K at X and Q bands is carried out on single crystals and powders. The presence of the two conjugated Cu(H2O)62+ complexes is discovered and the angles between respective axes of the rhombic crystalline field are determined. The temperature dependence of the Cu2+ EPR spectrum tensor parameters is discussed in terms of dynamic Jahn‐Teller effect. The additional arguments in favour of the dynamic nature of the observed phenomena are given and the values of the energy splittings between three possible Jahn‐Teller configurations δ1,2 ≈︁ 280 cm−1, δ1,3 ⪆ 1000 cm−1 are obtained. The necessity of the “dynamic” approach to the study of “static” Jahn‐Teller EPR spectra of Cu(H2O)62+ complexes in similar low‐symmetry crystals is shown.
On the Calculation of Superhyperfine Structures
Authors: Gehlhoff W.
Published: July 2007

The calculation of SHF structures is studied, which are caused by the interaction with several ligand nuclei if the simultaneous occurrence of electronic spin transitions with and without nuclear flips must be taken into consideration. The connection is shown between two formulas given in the literature for the calculation of SHF structure produced by ligand nuclei with I = 1/2.
Inelastic Light Scattering of the VK Centers in the Alkali Halides
Authors: Goovaerts E. , Schoemaker D.
Published: July 2007

The Raman spectra of the “self‐trapped hole” or VK center are measured in LiF, KCl, and RbCl. The localized vibrational modes induced by these centers are situated at 437, 241, and 244 cm−1, respectively. The vibrational modes of the VK center in the alkali halides and fluorides are calculated, making use of a semi‐empirical energy function. These calculations are discussed and compared with the experimental data.
Photoconduction in Polydiacetylene Multilayer Structures and Single Crystals. Evidence for Band‐to‐Band Excitation
Authors: Lochner K. , Bässler H. , Tieke B. , Wegner G.
Published: July 2007

Steady state photoconduction action spectra for polydiacetylene multilayer structures and single crystals (TCDU) are reported. Interpretation is given in terms of a valence‐to‐conduction band transition which is buried under the vibronic sidebands of the dominant exciton transition. The associated absorption coefficient follows a (αb ħω) ˜ (ħω − Eg)2 law which indicates either an indirect transition or a direct transition between non‐parabolic bands. The general validity of this relationship is supported by literature data for PTS, DCHD, and polyethylene. Gap energies are (2.5 ± 0.1) eV and (2.6 ± 0.1) eV for two different multilayer forms, (2.6 ± 0.1) eV for TCDU, (2.1 ± 0.1) eV for PTS, and (2.30 ± 0.05) eV for DCHD. The transition is three‐dimensional indicating finite valence and conduction band dispersion perpendicular to the polymer chain direction.
NMP‐TCNQ: A Realization of the Disordered Hubbard Model
Authors: Papatriantafillou C.
Published: July 2007

Several quantitative models have been proposed for the electronic structure of the organic charge transfer salt NMP‐TCNQ. They range from the periodic Hubbard model that emphasizes the role of electron correlation in the material, to models that emphasize the role of its intrinsic static disorder. A detailed study is made of the dielectric properties of the material, and it is found that the intrinsic static disorder depends on the electronic density of states at the Fermi level. Then, considering the system as a realization of the one‐dimensional disordered Hubbard model and using available experimental data, the intrinsic disorder and other band parameters are calculated.
Charge Transfer Spectra of Transition Metal Ions in Cadmium Halide Lattices
Authors: Coutts A. G. , Day P.
Published: July 2007

The near ultraviolet spectra of CdCl2 and CdBr2 crystals doped with Fe2+, Co2+, and Ni2+ at 4.2 K and the magnetic circular dichroism spectra of Fe2+, Co2+, and Ni2+ doped in CdBr2 are measured. The intense bands observèd in the spectra are assigned to charge transfer transitions between the halide ions of the crystal lattice and the 3d orbitals of the guest ion.
Transient Infrared Absorption of Bound Polarons in NaCl
Authors: Carlier F. , Jacobs G.
Published: July 2007

When NaCl crystals containing F and M centres are irradiated in the corresponding absorption bands with laser light pulses a transient induced absorption band is found with a maximum at 0.29 eV. This band, caused by bound polarons, shows a structure due to phonon emission. The average energy separation between the maxima is 0.0297 eV at LHT. This is equal to 0.89ℏωLO. The decay of the induced infrared absorption is logarithmic. The source of the attractive field for the polaron is not known.
Mean Free Paths by Inelastic Interactions, Stopping Powers, and Energy Straggling for Electrons of Energies up to 20 ke V in Various Solids
Authors: Akkerman A. F. , Chernov G. Ya.
Published: July 2007

For the elements C, Be, Mg, Al, Si, K, Ge, Sb, Bi calculations of the mean free paths and stopping powers of low energy electrons are performed. In the computational model Lindhard's formalism of the dielectric response function for pair and plasmon excitation and the classical cross‐section for ionization processes are used. Anomalously high mean free paths and small stopping powers in potassium are found. This behaviour which is connected with low electron concentration in the valence band is expected to hold for all alkali metals. For energies above 10 ke V the calculated dE/dχ agree well with values calculated from the Bethe‐Bloch formula. A Monte‐Carlo method is used for the calculation of the energy loss distribution of electrons passing through thin targets. It is noted that Blunck‐Leisegang's theory fails to render the energy straggling at electron energies below 10 ke V.
Theory of the Anomalous Bulk Photovoltaic Effect in Ferroelectrics
Authors: von Baltz R.
Published: July 2007

A model is presented for the bulk photovoltaic effect (BPVE) in ferroelectrics which is due to an asymmetry in the ionization cross‐section for polarized impurities. This asymmetry is related to a quantum‐mechanical interference effect and explains the dependence of the BPVE on frequency and polarization of light, and it gives reasons for a possible sign change of the current, as it is observed, e. g., in BaTiO3. Numerical results agree nearly perfectly with experimental data for Fe‐doped LiNbO3.
Stimulated Intrinsic Recombination Processes in II–VI Compounds
Authors: Koch S. W. , Haug H. , Schmieder G. , Bohnert W. , Klingshirn C.
Published: July 2007

The intrinsic recombination processes are investigated in highly excited II–VI compound semiconductors which involve the following scattering processes: the exciton‐exciton scattering (P2 line), the exciton‐electron or ‐hole scattering, and the sattering of an exciton by emission of a longitudinal optical phonon. From the stationary rate equations the laser threshold and the maxima of the spontaneous and stimulated emission for the various processes are determined. Experimental investigations of these emission processes are carried out for CdS platelets, ZnO and ZnTe crystals. Good agreement between experimental results and theoretical predictions for the temperature dependences of the laser thresholds, and of the emission maxima is obtained.
Two‐Centre Exchange Interactions between Orbitally Degenerate Ions. I. One Ion in Orbitally Degenerate State
Authors: Eremin M. V. , Kalinenkov V. N. , Rakitin Yu. V.
Published: July 2007

A multielectron theory of exchange interactions in transition metal ion pairs in cubic matrices and ions in orbitally degenerate states is developed. The orbitally dependent part of the exchange Hamiltonian is expressed by Pauli operators determined in the orbital space for the case of an orbital doublet and by fictitious orbital operators L̃ = 1 in the case of a triplet state. It is shown that the anisotropic exchange parameters may be estimated from isotropic ones. The theory described is compared with experiment.
Temperature Dependence of the Resistivity in Semimetals of the Bismuth Type
Authors: Kuka G. , Kraak W. , Gollnest H.‐J. , Herrmann R.
Published: July 2007

The resistivity of semi‐metallic bismuth‐antimony alloys and tellurium‐ or tin‐doped bismuth specimens is studied in a temperature range from 1.8 to 77 K. Typical dependences of the specific resistivity ϱ(T) as a function of Fermi energy are conceivable on the basis of an electron‐phonon scattering theory.
Ultrasonic studies of the structural phase transition in squaric acid
Authors: Rehwald W. , Vonlanthen A.
Published: July 2007

The elastic stiffness functions c11, c66, c33, and c44 are determined by ultrasonic velocity measurements as a function of temperature below and above the phase transition at Tc = 373 K. Their variation below Tc is discussed in terms of critical exponents. From their value of the order of 0.5 no unique decision can be made whether the elastic critical behaviour is determined by the crossover exponent for the stress acting as a “modifying field”, is due to a tricritical point, or is the consequence of a weakly first‐order transition.
Two‐centre exchange interactions between orbitally degenerate ions. II. Two ions in orbitally degenerate states
Authors: Eremin M. V. , Kalinenkov V. N. , Rakitin Yu. V.
Published: July 2007

The semiempiric theory of anisotropic exchange interactions developed earlier within the framework of the strong crystal field scheme is applied to describe pair exchange interactions between orbitally degenerate transition metal ions lying on a fourfold axis and having approximately Oh site symmetries. The effective Hamiltonians, with orbitally dependent parts expressed in terms of ficitious orbital moments, are obtained for all possible combinations of orbitally degenerate states (doublet–doublet, doublet–triplet, and triplet–triplet). These include relatively small numbers of anisotropic exchange parameters which can be estimated from the values of isotropic exchange parameters. The theory described is compared with experiment.
Optical absorption study of iron trifluoride thin films
Authors: Lachter A. , Salardenne J. , Barrière A. S.
Published: July 2007

The optical absorption of crystallized thin films of iron trifluoride is investigated. The energy gap in FeF3 thin films is about 5.96 eV. The six peaks characterizing the different spin transitions between the 3d states of iron and two excitonic peaks are obtained. Furthermore, an optical absorption edge corresponding to an energy of 1.1 eV is observed. It can be interpreted by electronic transitions from localized states in the forbidden band due to a lack of fluorine to the delocalized states of the conduction band.
Auger recombination in PbSnTe‐like semiconductors
Authors: Ziep O. , Mocker M. , Genzow D. , Herrmann K. H.
Published: July 2007

In semiconductors with PbSnTe‐like band structure the Auger coefficient Cn is investigated for most probable eeh transitions taking into account a nonparabolic approximation (NPA). Considering mirror‐symmetric hyperbolic bands, singularities in the so‐called “activation energy” appear which are removed if deviations from the hyperbolicity are included. At high temperatures Tl the value of Cn in NPA has the same order of magnitude as the value C proposed by Emtage and is nearly independent of temperature; if Tl < 77 K the value of Cn strongly decreases and differs by orders of magnitude from C. The obtained results are compared with experimental data.
Optical phonons in PdHxand PdDxby superconducting tunneling measurements
Authors: Dumoulin L. , Nédellec P. , Arzoumanian C. , Burger J. P.
Published: July 2007

Tunneling data on phonon spectra for some superconducting PdHxand PdDxcompounds withx≈ 0.95 are presented. The main observed structures correspond to the optical vibrations of H and D indicating that these phonons are basic for the occurrence of superconductivity in these compounds. The position and width of these structures are discussed and compared: anharmonicity effects are observed which indicate that the H force constant is 14% higher than the D one.
A new off‐center defect system, RbCl:Li+, studied with dielectric techniques under hydrostatic pressure
Authors: Thörmer K. , Lüty F.
Published: July 2007

RbCl crystals with Li+ doping, spanning a concentration range of nearly three orders of magnitude, are systematically studied in their real and imaginary dielectric response under hydrostatic pressure (up to 7 kbar). A dielectric constant anomaly and loss peak around 40 K is found, which can be fitted to a Langevin‐Debye behavior of classical dipoles with an activation energy for reorientation of 510 K. For small Li+ doping (N < 1018 cm−3), this dielectric response scales with the Li+ concentration, yielding (for Lorentz corrected fields) a dipole moment of 〈p〉 = 0.52 e Å. The potential barrier for the Li+ dipole reorientation is found to be drastically reduced under hydrostatic pressure, shifting the observable relaxation to low temperatures. Though this pressure tuning process is interrupted by the NaCl → CsCl structural phase‐transition at 4.6 kbar, it can be extrapolated that for about 8 kbar (i.e., 2% compression) RbCl:Li+ should become a tunneling system similar to KCl:Li+. Consequences of the observed behavior for the theoretical understanding and host‐lattice trends of Li+ off‐center defects and for the behavior of FA(Li) centers are discussed.
Inversion of the Γ and L conduction bands of GaSb under uniaxial stress
Authors: Noack R. A.
Published: July 2007

With photoluminescence experiments under high uniaxial stress in 〈111〉 crystallographic direction it is possible to observe the inversion of the Γ and L conduction bands of GaSb directly in the spectra at 6.5 kbar. From the line shifts the uniaxial deformation potential constant of the L conduction band E2 = (15 ± 1) eV is determined. Also the energy separation between the two conduction bands at zero stress is obtained from the experimental data: ΔE = (93 ± 3) meV. A donor level assosiated with the L conduction band is seen in the spectra at higher stresses. The ionisation energy of this donor is determined to Ei = 33 meV.
Effect of the mixing between Frenkel excitons and charge transfer states on charge carrier transfer in a linear molecular crystal
Authors: Petelenz P.
Published: July 2007

The previously proposed model of the mixing between Frenkel excitons and CT states is generalized for a linear model crystal to include the effect of the CT states from the immediate neighbourhood of the charge carrier in the direction of its propagation. The transfer of the charge carrier to one of the molecules involved in the CT state is taken into account. In contrast to the influence of more distant CT states, the effect yields the reduced value of the effective mass of the charge carrier. Some crude numerical estimates are given, and the possible experimental consequences are pointed out.
Frequency‐ and wave‐vector‐dependent dielectric function of a model semiconductor
Authors: Milchev A.
Published: July 2007

Closed analytical expressions for the real part ϵ1(q, ω) and the imaginary part ϵ2(q, ω) of the frequency‐ and wave‐vector‐dependent dielectric function are obtained including a gap Eg in an appropriate manner in the spirit of the nearly‐free electron approximation, and requiring that these functions will exactly satisfy the Kramers‐Kronig relations and the f‐sum rule. In the long wavelength limit the classical Lorentz oscillator result comes out, whereas for ω = 0 the static screening function is decisively improved with regard to earlier model calculations. The results also yield the plasmon dispersion relations ωL(q) and ωT(q) for Si, Ge, GaAs, and ZnSe in good agreement with previous detailed computations. With the average gap width Eg going to zero the expressions obtained transform into the well‐known RPA (Lindhard) formulae for a free electron gas.
Study of the charge exchange process of iron impurity atoms in GaAs and GaP by Mössbauer spectroscopy
Authors: Seregin P. P. , Nasredinov F. S. , Bakhtiyarov A. Sh.
Published: July 2007

The iron impurity atom state in GaAs and GaP is studied by emission Mössbauer spectroscopy. The iron atoms substitute for gallium atoms and have a tetrahedral surrounding. The electron configuration for a neutral state of these centres (Fe)° is 3d54s0.52p1.56 (in GaAs) or 3d54s0.79p2.37 (in GaP), while in the ionized state the population of the 3d shell of iron is increased (3d6). The process of fast electron exchange via the valence band between the (Fe)° and (Fe)− centres at 295 K is observed in slightly overcompensated GaP. For E(Fe) – E(Co) a value of – (0.035 ± 0.002)eV is obtained where E(Fe) and E(Co) are the iron and cobalt levels in GaP, respectively.
Second‐order Raman spectra of LiH
Authors: Laplaze D.
Published: July 2007

The outlines are summarized of the model for LiD and LiH crystals in which long‐range three‐body forces are included, and applied to the calculation of second‐order Raman scattering which is very strong for these crystals. This first approach made with simplifying hypothesis shows that both the interionic interactions between second‐neighbour hydrogen ions and deformation of H− ions must be taken into account to obtain the principal peaks of experimental Raman spectra.
Burstein‐Moss effect and near‐band‐edge luminescence spectrum of highly doped indium arsenide
Authors: Vilkotskii V. A. , Domanevskii D. S. , Kakanakov R. D. , Krasovskii V. V. , Tkachev V. D.
Published: July 2007

The shape of near‐band‐edge luminescence spectrum is studied and a quantitative analysis of the Burstein‐Moss shift in the three band approximation of Kane's theory is carried out. It is shown experimentally that with increasing electron concentration the spectrum maximum is shifted towards the high‐energy region up to 0.7 eV. At the high‐energy side of spectrum a kink is observed, and at the low‐energy side, an emission maximum at 0.39 to 0.40 eV. The calculations show that the spectrum shape may be described by indirect transitions: band–band and band–shallow acceptor. The narrowing of the optical band gap is accounted for a better agreement between the experimental and theoretical spectra.