bluuebunny/crossref_metadata_2025 huggingface dataset:
Spin polarization of a magnetic electron gas induced by a van Vleck ion
Authors: Palermo L. , da Silva X. A.
Published: July 2007

The mutual polarization of a magnetic electron gas and a van Vleck ion, interacting via exchange, are theoretically investigated using the double‐time Green function method. A pair of equations describing the dynamics of the electron gas and the ion are conveniently decoupled and an analytic expression for the electron gas polarization is obtained which depends on the square of the exchange parameter. Besides a RKKY‐like term, a new term associated to the process of formation of the magnetic moment of the ion appears.
The construction of central‐force interatomic potentials from whisker deformation data
Authors: Bowen D. K. , Vitek V.
Published: July 2007

Stress‐strain curves on single crystal whiskers can extend to the non‐linear region. It is shown that these provide data which can assist in the empirical construction or verification of central force potentials. Potentials constructed in this way are suitable for studying the atomic structure of lattice defects when large deviations from the ideal lattice positions occur in their cores. Furthermore, the effects of high external stresses on crystal defects can be investigated. An example of the latter may be found in the field‐ion microscope and as an application the non‐linear dilatation of the lattice under high hydrostatic tension is calculated, and the possible behaviour of dislocations in FIM specimens is briefly discussed.
Certain Features of Semiconductor Behaviour in Strong Parallel Magnetic and Electric Fields II. High‐Frequency Field Energy Absorption
Authors: Berezhkovskii A. M. , Ovchinnikov A. A.
Published: July 2007

It is shown that if in a semiconductor placed into strong parallel electric and magnetic fields the spin splitting energy in magnetic field is close to the energy of a Wannier‐Stark ladder step in an electric field, the high‐frequency field energy absorption occurs at two frequencies instead of one as in absence of an electric field.
Spin‐Wave Instability Theory in Cubic Single Crystal Magnetic Insulators II. Applications
Authors: Patton C. E.
Published: July 2007

The theory developed in Part I is used to calculate butterfly curves for parallel pumping in [001] and [110] magnetized yttrium‐iron garnet and transverse pumping in [001] magnetized YIG for comparison with available experimental data. The results are in good agreement with available experimental data and resolve several previously unexplained anomalies.
The Density of Unoccupied Electronic States in Tantal
Authors: Boiziau C. , Dose V. , Scheidt H.
Published: July 2007

The density of states in the unoccupied part of the conduction band of polycrystalline tantal is investigated using ultraviolet isochromat spectroscopy and soft X‐ray appearance potential spectroscopy (SXAPS). Density‐of‐state functions deduced from the two different measurements are quite similar and compare favourably to existing theoretical calculations.
Green's Theorem Calculation of Energy of Exciton near Hard Walls
Authors: Gallardo J. , Mattis D.
Published: July 2007

The method of Harper and Hilder is modified to obtain the energy levels of hydrogenic atoms or excitons near hard walls. The method is nonvariational, can be used with two or more bounding surfaces, and gives good results in various cases. It is only required to know the wave functions in the absence of such surfaces and to perform some elementary integrals.
Manetoelectronic Denisty of States for a Model Crystal
Authors: Wannier G. H. , Obermair G. M. , Ray R.
Published: July 2007

As shown previously, a first principles calculation of the quantum states of a crystalline electron in a magnetic field leads to a difference equation. The splitting of one Landau level by a particularly simple crystalline potential has been extensively studied. This paper proceeds further for this model case: All quantum numbers characterizing each individual state are exhaustively defined. One of them is discrete and the other two are contionous phases. It is shown that are ene gy eigenvalues depend only on the sum of the cosines of these phases. These properties permit the calculation of the density of states, which still shows as a function of the magnetic field the well known erratic behaviour. Physically understandable rusults are obtained, however, for the integrated density of states: at two magnetic fields which are numerically nearly equal, the intergrated densities of states of states are numerically almost indistinguishable although very different analytically.
Harrison's Method Electrical Transport Properties of Alkali Metals
Authors: Thakur J. , Srivastava P. L. , Mitra N. R.
Published: July 2007

Electrical resistivity and thermo‐electric power of alkali metals are investigated in the structure of Harrison's first principle method. Slater's value of α in the Xα exchange with unmodified VOPW presents a good picture with Kleinman and Langreth exchange and correlation. The effect of different forms of exchange and correlation is discussed.
X‐Ray Thermal Diffuse Scattering Measurements of the [100] and [111] Phonon Dispersion Curves of Indium Arsenide
Authors: Orlova N. S.
Published: July 2007

Measurements of the thermal diffuse X‐ray scattering are performed from the (100) and (111) planes in indium arsenide single crystals. The intensities of the second‐ and third‐order scattering are calculated using the two‐atom chain model with different masses and four interaction parameters. The first‐order thermal diffuse scattering intensity is analysed and yields the acoustic and optic phonon dispersion curves in the [100] and [111] symmetry directions.
Spectral Distribution and Decay Times of the Luminescence of Mn2+ on Different Lattice Sites in ZnS
Authors: Busse W. , Gumlich H.‐E. , Geoffroy A. , Parrot R.
Published: July 2007

The spectral distribution of both, excitation and emission of luminescence of ZnS:Mn with manganese on different lattice sites is investigated by dye laser spectroscopy. An unambiguous correlation of zero‐phonon lines due to Mn2+ in cubic and different axial lattice sites is given. The relative oscillator strength of the fluorescent transition of Mn2+ in different crystal sites is determined by exciting within the zero‐phonon transitions of the absorption and by registering the zero‐phonon lines in emission, which belong to the same lattice sites, respectively.
Measurement of Mode Grüneisen Parameters in Si
Authors: Prechtel J. , Kalus J. , Lüscher E. , Pintschovius L. , Ghosh R.
Published: July 2007

Stress‐induced changes in energy of the phonons of the lowest acoustic phonon branch in the (q00) direction are investigated in Si by using inelastic neutron scattering. Applying uniaxial stress of 10 kbar causes energy changes in the order of 1%. A change in sign is found approximately in the middle of the Brillouin zone. Depending on the mutual orientation of the stress, the wave vector and the polarization vector of the phonons, different changes in energy are observed. From the observed energy changes general mode Grüneisen parameters are derived.
Ground and Excited States of the Biexciton and the M‐Band Luminescence in Highly Excited ZnO
Authors: Schrey H. , Klingshirn C.
Published: July 2007

Additional measurements on polarization properties and excitation intensity dependence are presented for the resonant two‐photon Raman scattering via biexcitons in highly excited ZnO at low temperatures reported earlier [1]. The binding energy of the biexciton ground state (symmetry Λ1) is determined to (19 ± 0.2) meV. An excited biexciton state (symmetry Λ5) is observed for the first time in II‐VI compounds. Moreover, the excitation spectra of the M‐band and of the bound exciton I9 are given for different polarizations of the exciting light. They indicate that the M‐band can be ascribed to a superposition of extrinsic and intrinsic recombination processes.
Local fields and electric dipole moment of localized impurities in crystals
Authors: Glinchuk M. D. , Deigen M. F. , Rozhko V. S.
Published: July 2007

Dipole transition matrix elements of impurity centres are estimated accounting for the “acting” electric field in the crystal Ea. Ea is shown to depend on the type of wave functions and radii of states among which the transition occurs. Some examples where Ea is essential are considered. It is found, for example, that the dipole moment of the localized electron is expressed via oscillator forces submitted to the sum rule only in the case when the radius of electron state is large enough. A corresponding criterion is given. The examination of matrix elements in the dipole transitions of off‐centre ions shows that in this case the acting field is near to the Lorentz field.
The exciton‐neutral donor and biexciton complexes in polar semiconductors
Authors: Mokross F. , Büttner H.
Published: July 2007

For a system of two different excitons interacting with LO‐phonons an effective Hamiltonian is derived. As a special example the exciton‐neutral donor binding energy is calculated using a variational procedure and discussed as a function of memh for different material parameters. The results are also compared to those of other authors and to experimental data. Corresponding calculations are done for the biexciton complex.
A hopping model for activated charge transport in amorphous silicon
Authors: Grünewald M. , Thomas P.
Published: July 2007

A variable‐range hopping model is examined for transport in a band tail. For an exponential density of states the difference of the activation energies of conductivity and thermoelectric power, ΔE, is essentially determined by the steepness of the tail. ΔE varies slowly with tempera ture and depends little on the localization length α of the wave functions and on the magnitude of the density of states. From comparison of the results with experimental data it is suggested that this model applies to activated transport in n‐type amorphous silicon.
Frequency shift analysis of matrix isolated impurities
Authors: Jodl H. J. , Theysohn G. , Bruno R.
Published: July 2007

A method is presented for calculating the shift of an impurity spectrum in the rare‐gas solids Ar, Kr, and Xe. The intermolecular potentials are approximated by Lennard‐Jones potentials for this purpose, the parameters of which are obtained from specially chosen combination rules. An expression for the lineshift in terms of these parameters, the host geometry, and the polarizability of the species is derived. If certain variables in this expression are appropriately grouped together there exists a linear relationship which is graphically demonstrated for various examples. It is shown that the consideration cannot be extended to neon as a host matrix.
On the phase diagram of itinerant magnets in the presence of unmagnetic electron bands
Authors: Bünger M. , Jelitto R. J.
Published: July 2007

The phase diagrams of itinerant magnets are strongly affected by the presence of additional bands of unmagnetic electrons. This feature is worked out by consideration of a simple model consisting of a single Hubbard band and a free electron band, which works as a source or sink for magnetic electrons.
Force‐correlation function method applied to the Hall coefficient of paramagnetic transition metals
Authors: Schiller W. , Christoph V. , Sellger R.
Published: July 2007

Using the formalism of force‐correlation functions for systems with two groups of charge carriers a formula for the Hall coefficient is derived. The formula is applied to the phonon‐assisted interband and intraband scattering of electrons and holes in paramagnetic transition metals. The experimentally observed maximum of the Hall coefficient at low temperatures can be understood by the different temperature dependences of the interband and intraband scattering terms in the expression for the Hall coefficient.
Polariton structure of interband transitions in semiconductors
Authors: Stahl A.
Published: July 2007

A set of semiclassical wave equations is derived describing the interaction of virtual or real electron‐hole pairs with electromagnetic radiation. The solutions of these equations are the polaritons formed in an interband transition. They describe a renormalized photon surrounded by a cloud of electron hole pairs. The concept of interband polaritons is particularly useful for the understanding of effects that are sensitive to the structure of the electron‐hole cloud. As examples the phenomena of polariton free surface layers (similar to exciton free layers) and electronic Raman scattering in a depletion layer with distorted bands are discussed.
Double quantum external photoelectric effect in crystalline tetracene
Authors: Arnold S. , Pope M. , Hsieh T. K. T.
Published: July 2007

The subject effect (DQEPE) is confirmed in tetracene crystal and attributed to the interaction of a free electron and a trapped charge‐transfer (CT) state. The energy of the trapped CT state is 2.85 eV. The light intensity (I) dependence of the DQEPE yield Y is Y ∝︁ I1.5. Greatly improved experimental techniques for measuring photoemission rates and retardation potentials in a gaseous environment are described.
Temperature dependence of the electrical conductivity and thermoelectric power in MoTe2 single crystals
Authors: Conan A. , Delaunay D. , Bonnet A. , Moustafa A. G. , Spiesser M.
Published: July 2007

Thermoelectric power and de electrical conductivity measurements are made on MoTe2 single crystals in a wide temperature range (77 to 770 K). The results are analyzed on the basis of impurity conduction. It is shown that three processes contribute to the total conductivity: hopping conduction between impurity sites, scattering by optical modes, and, in the intrinsic domain, scattering by impurities. The thermal energy gap (0.99 eV) is approximately the same as the optical one (1.03 eV). The effective mass of electrons in the conduction band is not very different from the mass of the free electron (m* = 0.76m), and the dielectric constant of the medium is K ≈︁ 34. The agreement between theoretical and experimental results is quite good. A comparison with previous results performed on compact polycrystalline samples is given.
On the long‐wavelength behaviour of solids with fluorite structure
Authors: Bedi S. S. , Verma M. P.
Published: July 2007

A rigid‐ion Lundqvist many‐body potential model is used to derive expressions of the second‐order elastic (SOE) constants and the principal Raman frequency for solids crystallizing in the fluorite structure. Application to CaF2, SrF2, and BaF2 clearly demonstrates the inadequacy of the Born potential and the importance of three‐body interactions in this system.
On the nature of U5+ centres in LiF
Authors: Lupei V. , Lupei A.
Published: July 2007

EPR results on eight new U5+ centres of cubic, tetragonal, or lower symmetry obtained by X‐ or γ‐irradiation of LiF‐U6+ single crystals are reported. No trigonal U5+ centres are observed. From the analysis of the EPR spectra tentative models of charge compensation are proposed. The centres consist of uranate (+5) structures and not of uranyl UO complexes. From the structure of the U5+ centres that of the parent U6+ centres is inferred.
Sondheimer oscillation in aluminium single crystals
Authors: Sato H.
Published: July 2007

The Sondheimer oscillation is measured for the magnetic field in the (110) plane. For the field along the [100] and [110] axes, the periods, the phases, and the field dependences of the amplitude were carefully examined. When the field is tilted from the principal axes, a peculiar wave form is observed. Realistic calculations which include the exponentially decaying factor are made on the basis of the 4‐OPW Fermi surface (FS) model. The peculiar wave form and still unexplained field dependence of the oscillation amplitude are well explained when the exponential factor is taken into account.
F‐center photoionization in the semicontinuum model
Authors: Edwards A. H.
Published: July 2007

The L‐band calculation of Page, Strozier, and Hygh (PSH) is re‐examined. Using the semicontinuum model, the dipole strength and photo cross‐section are calculated for ground state to continuum state absorption of light. The results do not agree with those of PSH. The salient difference is that the dipole strength is structureless and leads to a cross‐section similar to that of hydrogen. From this it is concluded that this simple model does not account for the L‐band structure.
Determination of thermodynamical parameters of the EHD in silicon from luminescence data
Authors: Schmid W.
Published: July 2007

The usual lineshape calculation of the EHD emission in silicon by a convolution integral over the densities of states of electrons and holes results in a considerable deviation from the experimental photoluminescence spectrum on the low energy side. As shown in this paper, agreement between the experimentally determined and the calculated lineshape is considerably improved as lifetime broadening of the final states of the recombination is taken into account. As a consequence of this lineshape fit improved parameters for the EHD in Si are obtained (carrier temperature 5 K): carrier density n = (3.5 ± 0.05) × 1018 cm−3, work function Φ = − 9.3 meV. Furthermore, for the first time a lineshape analysis of the TA‐phonon assisted emission is given which allows to check the validity of the composition of the much stronger TO/LO phonon assisted luminescence out of the two components. From the carrier density within the EHD and the lifetime a value of the Coulomb enhancement factor is estimated and compared with published theoretical data.
Dynamical superhyperfine interaction of atomic hydrogen in KCl and RbCl
Authors: Hoentzsch Ch. , Spaeth J. M.
Published: July 2007

With ENDOR the shf and quadrupole interactions of the nearest Cl neighbours of atomic deuterium on cation sites in KCl and RbCl and of first‐ and second‐shell neighbours of interstitial deuterium in RbCl are measured. Comparison with previous data of atomic hydrogen shows a distinct isotope effect indicating a considerable influence of the zero‐point vibration on the shf interactions. Using an orthogonalization model including a Cl3pσ covalency, the vibrational potential is determined from the isotope effect. It is shown that this theoretical model cannot explain the measured spin density at the nearest Cl neighbours.
Inelastic neutron scattering by cerium hydrides. IV. Crystal field
Authors: Vorderwisch P. , Hautecler S. , Suck J. B.
Published: July 2007

The crystal field splitting of the 2F5/2 ground state of the Ce3+ ion in CeDx is investigated by means of the inelastic neutron scattering technique. Polycrystalline samples with 1.94 ≦ x ≦ 2.74 are studied. A consistent description of the results can only be obtained assuming that, over the investigated concentration range, the samples have tetrahedral vacancies. For CeD1.94 an effective charge of −0.30 |e| is derived, both for octahedral and tetrahedral deuteriums. For CeD2.42 an effective charge of −1.44 |e| has to be attributed to the octahedral deuteriums. For CeD2.74 scattered intensity due to crystal field transitions is also observed but as a structureless component of the spectra.
Density of states of expanded liquid mercury an effective medium approach
Authors: John W. , Keller W.
Published: July 2007

Numerical results for the density of electron states for expanded liquid mercury are presented in the density range from 13.55 to 5.0 g cm−3. The calculations are based on a single‐site effective medium approximation for the muffin‐tin model of a liquid metal. The energy‐dependent effective medium is determined from a generalized Wigner‐Seitz condition. The gap between the 6s and 6p states occurs at ∂ = 5.6 g cm−3, in good agreement with nonrelativistic band‐structure calculations for hypothetical forms of crystalline mercury.
Renormalization of polaritons due to virtual formation of biexcitons at high densities of excitation
Authors: May V. , Henneberger K. , Henneberger F.
Published: July 2007

The change of polariton dispersion is investigated theoretically if a large number of polaritons is present at frequencies near half the biexciton energy. A renormalization process takes place for polaritons via virtual formation of biexcitons and stimulated by the intense polariton distribution. Assuming an energetically sharp distribution as is realised e.g. by dye laser excitation the original polariton dispersion is shown to split into two new branches. These effects should manifest if e.g. an additional but weak probe light is used to testify the optical properties of the laser excited crystal. Furthermore the scattering lines of the two‐photon resonance Raman process via biexcitons are characteristically modified due to both selfrenormalization of the incoming polaritons and renormalization of the scattered polaritons. Explicit results are presented for CuCl where agreement with experimental data recently published is found.
Third‐order elastic constants, thermal expansion, and bulk modulus of body‐centered cubic crystals
Authors: Rao R. Ramji , Rajput A.
Published: July 2007

The method of homogeneous deformation is used to derive the expressions for the second‐order elastic and third‐order elastic (TOE) constants of a body‐centered cubic crystal. Two‐ and threebody interactions are taken into account. The TOE constants of the four alkali metals Li, Na, K, and Rb and of the two transition metals Ta and Nb, are then evaluated, and these, in turn, are employed to calculate the low‐temperature limit of thermal expansion for these metals following Thurston and Brugger's equation for the elastic waves propagating in a homogeneously strained crystal. The Anderson‐Grüneisen parameter δ is calculated for each of the metals from its TOE constants, and used to investigate the temperature dependence of the bulk modulus via Anderson's theory.
Impurity‐induced infrared absorption in AgBr
Authors: Ram P. N. , Tiwari M. D.
Published: July 2007

The impurity‐induced infrared lattice absorption is calculated for AgBr crystal in the low‐concentration limit of substitutional impurities. Employing a nearest‐neighbour perturbation model, where along with the change in mass at the impurity site the changes in the central and non‐central impurity‐host crystal interactions are considered, the localized mode frequency and band‐mode additional absorption due to lithium and sodium impurities are discussed. The main features of the experimentally observed absorption are reasonably explained.
Point defects in NaCl crystals doped with divalent ions the NaCl‐NaF:Pb++ crystal
Authors: Pinatti D. M. G. , Zílio S. C. , de Souza M.
Published: July 2007

Fluorine anions in the NaCl lattice act as capture centers for Pb2+ diffusion. ITC measurements show four dielectric relaxation peaks, one due to unperturbed Pb2+ vacancy dipole and the other three at lower temperatures. The perturbed Pb2+ ions also show new optical absorption bands that grow during annealing at RT. The defects produced by quenching crystals from high temperature are found to be strongly dependent on the lattice parameter. The temperature of the maximum of the three new ITC peaks are nearly independent of the divalent ions Eu2+, Sn2+, Zn2+, Pb2+, or Mn2+ that are also used to dope the NaCl‐NaF crystals.
Resonant scattering of light by slowly fluctuating localized electrons
Authors: Hizhnyakov V. , Tehver I.
Published: July 2007

The effect of slow fluctuations on resonant scattering of light by localized electrons is considered in the systems with a central peak. The cases of excitation in the regions of broad phonon wing of the absorption spectrum and of zero‐phonon line are discussed. It is shown that the spectrum of resonant scattering essentially depends on the fluctuation amplitude ≪0 and rate γ as well as on the radiative damping rate γ. In particular, in case γ ≪ γ ≪ ε0 (limit of nearly static fluctuations), at an excitation in the region of zero‐phonon line, the resonant emission line becomes fluctuationally broadened by ε0 (γ/2γ)1/2. In glasses at T → O this broadening mechanism should be dominant as its contribution decreases slowly with temperature (∼ T2).
Resonant Raman scattering in crystals with self‐trapping excitons
Authors: O'Connel‐Bronin A. A. , Plekhanov V. G.
Published: July 2007

The multiphonon resonant Raman scattering spectra in NaI and LiH (LiD) cubic crystals characterized by self‐trapping of excitons are investigated experimentally. The first‐order scattering cross section is established to depend on the wave vector. The frequency dispersion of the secondorder scattering cross section is in agreement with the theory. The 1S and 2S excitons as intermediate states as well as the damping of excitons are taken into account. The temperature dependence of the second‐order intensity in both luminescence and resonant Raman scattering is studied in the region of 2 to 300 K. This dependence is shown to weaken with the shifting of the exciting light frequency from the exciton band bottom towards the red. The temperature behaviour of the frequency dispersion of the second‐order Raman cross section at the temperature change in the crystals with self‐trapping excitons is discussed.
Evidence for a landé‐factor g =2 in n‐type inverted silicon MOSFET surfaces
Authors: Köhler H. , Roos M.
Published: July 2007

The determination of the spin splitting in quantities of the cyclotron energy in quantizing electric and magnetic fields from the application of the “tilted field method” on n‐type inverted silicon MOSFET surfaces is re‐examined under the additional consideration of a finite valley splitting. The procedure shows up to be very complicated since various important parameters and their angular variation are not known exactly. The summary of a series of different experiments yields an almost equal number of differing results for the g‐factor evaluated from the particular cases for M* ==g · mc/(2m0 cos ξ) =1/2 and 1, even after an exact correction of the data. The assumption of an interaction of the two “inner” of the four broadened levels belonging to a Landau quantum number n and a modification of the line shapes in their interspace allows a complete discussion of all data on the basis of a g‐factor equal to 2.
A simple linear method in band theory application to B.C.C. tantalum
Authors: Lopez‐Aguilar F.
Published: July 2007

Using a variational expression generalization for the APW method and radial part wave functions at fixed energies a diagonalizable secular matrix is obtained. A parameter relationship of the variational expression with potential depending variables is established. This method is applied to tantalum for which various band calculations by means of typical methods exist. It is noticed that, if there is some divergency, this is only a small one, and, in any case, can be solved by exchange and correlation potential parametrization.
Electronic polarization (relaxation) effects in the core level spectra of semiconductors II. Application to Ga3d and Si2p levels
Authors: Bechstedt F. , Enderlein R.
Published: July 2007

Core electron binding energies of semiconductors are strongly influenced by core hole polarization (relaxation) effects. Polarization energies are calculated for the Ga3d hole in Ga–V semiconductors and the Si2p hole in Si. The values range between 4 and 7 eV. From the polarization energies, atomic core level energies, and solid state core level shifts theoretical values for the core electron binding energies are derived. Good agreement with experimental results is obtained.
On the hopping mechanism of dispersive transport
Authors: Zvyagin I. P.
Published: July 2007

The hopping mechanism of dispersive transport is discussed starting directly from the hopping transport equation, in contrast to the conventional approach using the continuous‐time random‐walk model. The power‐law decay of the transient current due to the spread of the packet of photo‐injected carriers, is obtained for impurity‐band hopping both in the presence of trapping and without it. The analysis shows that variable‐range hopping generally cannot be ruled out as a probable mechanism of dispersive transport in disordered semiconductors.
Logarithmic derivative reflectance spectra of BaO and SrO
Authors: Rao A. S. , Kearney R. J.
Published: July 2007

Experimental studies of both reflection and wavelength derivation reflection spectra from freshly cleaved SrO and BaO crystals at 55 K are presented. The data are interpreted in a manner consistent with optical data on MgO and CaO crystal surfaces, in terms of Γ‐, X‐, and L‐excitons.
NMR studies of the effect of alloying Cu, Ag, Tl, I, and Ge into amorphous As2Se3
Authors: Ueda S. , Shimizu T.
Published: July 2007

77Se and 205Tl NMR measurements are carried out for a‐As2Se3 with different additives. It is found that the NMR chemical shift and the increase in electrical conductivity with the addition of different elements have a close relationship. The results can be explained by assuming that some fraction of additive elements are positively ionized, resulting in an increase in the density of negatively charged Se dangling bonds and a shift of the Fermi level.
EPR investigation of Gd3+ and Eu2+ in the α‐ and β‐phases of lead phosphate
Authors: Razeghi M. , Buisson J. P. , Houlier B.
Published: July 2007

The X‐band EPR spectra of Gd3+ and Eu2+ diluted in Pb3(PO4)2 crystals are studied. Lead phosphate exhibits a ferroelastic phase transition at 180°C and the EPR spectra obtained in each phase differ from each other. The spectra are very complex because the zero field splitting has the same order of magnitude as the Zeeman term. The spin Hamiltonian parameters and the energy levels are computed. “Forbidden” or “missing” transitions and line intensities can be explained. Very likely Gd3+ substitute a PbI ion. For Gd3+ in β‐phase g ≈︁ 2 and b = 990 × × 10−4 cm−1 (200°C); for Gd3+ in α‐phase at 25°C g ≈︁ 2, b = 930 × 10−4 cm−1 and b = −77 × × 10−4 cm−1.
The effect of oxygen on the properties of evaporated amorphous silicon
Authors: Kubler L. , Jaéglé A. , Koulmann J. J.
Published: July 2007

The influence on the ESR centres of oxygen incorporation into evaporated amorphous Si is studied. The main effect is not the reduction of the spin density NS, which can be only reduced of a factor 10 after an intitial contamination, but a strong enhancement of the spin–lattice relaxation time T1, which makes the signal very saturable and apparently weak for usual microwave powers. This T1‐ rising is correlated with a slight g‐factor decrease, and lineshape changes from Lorentzian to Gaussian. This was also observed by others with alternative treatments for reducing the spin density, such the rise of the substrate and annealing temperature or hydrogen incorporation. This initial spin density reduction seems always to imply the passivation of the same kind of defects, accessible to the gases and located at the internal surfaces of porous samples. But many other defects with T1‐times in the range 10−5 s at 300 K are not suppressed by this way.
On the indirect exciton energy levels in crystals of diamond and sphalerite type with spin–orbit interaction
Authors: Tolpygo K. B. , Zubkova S. M.
Published: July 2007

The theory of direct excitons in crystals of diamond and sphalerite type is generalized to the indirect exciton case. The problem is solved by the variational method taking into account the anisotropy and the nonparabolicity of the electron band. The comparison with the theoretical and experimental data known from literature is carried out.
Effect of temperature on the off‐diagonal alloy properties
Authors: Wysokiński K. I. , Piłat M.
Published: July 2007

A version of the coherent potential approximation is applied to study the density of states and the conductivity of binary and/or ternary alloys. A tight‐binding one‐band Hamiltonian including diagonal thermal and diagonal and off‐diagonal substitutional disorders is used. The superposition of thermal and off‐diagonal disorders produces some effects not observed in the previous one‐band theories. The concentration dependence of the AuxAg1−x alloys density of states is calculated.
Temperature dependence of electron concentration in intrinsic‐like HgTe
Authors: Szlenk K.
Published: July 2007

Measurement of the Hall effect and the conductivity are performed on HgTe sample characterized by an electron mobility of 106 cm2/Vs at 1.6 K. The range of temperatures investigated is 1.6 to 32 K and the external magnetic field covers the region 50 to 800 G. The interpretation of the data based on the conductivity tensor components analysis shows that the sample contains 5 × 1016 cm−3 acceptors. The acceptor ionization energy is estimated to be 2 meV. It is shown that a simple one‐carrier expression for the Hall constant may give wrong results for the electron concentration, differing by a factor of two from the correct value. From the behaviour of conduction electron concentration below 6 K it is found that the conduction–valence band overlap is smaller than 0.3 meV.
Evidence for excitonic absorption in metallic MnAs0.88P0.12
Authors: Bärner K. , Stearns R. , Braunstein R.
Published: July 2007

The derivative spectra of the reflectivity are measured for the double exchange ferromagnet MnAs0.88P0.12 at 300, 100 K and for quantum energies between 1.6 and 5.0 eV. Excitonic absorption is found beyond interband transitions and is tentatively assigned to a triangular resonant state.
Statistical continuum theory of heterogeneous solid media  information theory approach. I. Static field theory
Authors: Pompe W. , Vojta G. , Kreher W.
Published: July 2007

A new approach to the statistical continuum theory of random heterogeneous solids is developed This approach is based on the consistent use of the principles of information theory in an effort to connect observable quantities of real solids with fundamental quantities of solid state theory. The physical entities to be treated are space and time‐dependent generalized stochastic fields leading necessarily to a functional formalism. As an example the static elastic properties of a heterogeneous solid are calculated. Starting from a variational principle for the entropy functional a probability functional for the deformation field and the Hooke tensor field is given from which the expectation values of all relevant quantities can be evaluated. The physical implications of the formulae derived are discussed. The conception of structure temperature is introduced.
Dielectric spectrum of BaTiO3 single crystals
Authors: Turik A. V. , Shevchenko N. B.
Published: July 2007

The dielectric spectra of BaTiO3 single‐domain and polydomain tetragonal crystals with 180° domain structure are investigated over the wide range of frequencies 103 to 1010 Hz. It is shown that the dielectric permittivity ε33 of the single‐domain crystals has three regions of dispersion including two regions of the high‐frequency relaxation. There is no relaxation contribution to the permittivity ε11 of the single‐domain crystals. In polydomain crystals the decrease of the low‐frequency dielectric permittivity and the increase of the high‐frequency one, due to the domain piezoelectric interaction, are observed. The results obtained are discussed from the point of view of the structural disorder in a real crystal. Conclusions are made concerning the ion potential relief character and the phase transition nature in BaTiO3.
X‐ray absorption edge shifts of cobalt and rare earth ions in rare earth–cobalt intermetallic compounds
Authors: Hatwar T. K. , Malik S. K. , Ghatikar M. N. , Padalia B. D.
Published: July 2007

Chemical shifts ΔE of K‐absorption edge of Co and L3‐absorption edge of Gd, Tb, and Dy in GdCo3, TbCo3, and DyCo3 intermetallic compounds are measured and effective charges on the absorbing ions are estimated. It is found that the magnitudes of ΔE and q for Co in RCo3 (R refers to rare earth ions) lie in between those reported for RCo5 and RCo2 compounds. Further, ΔE and q of Co increase with increasing number of rare earth atoms per cobalt atom (RCo5 < RCo3 < RCo2) while the magnetic moment of cobalt in these intermetallic compounds indicates the opposite trend (RCo5 > RCo3 > RCo2).
Quantum‐chemical calculations of electronic and hole centres and surface of NaCl crystals (I)
Authors: Shluger A. , Kotomin E. , Dzelme Yu.
Published: July 2007

A new parametrization scheme for electronic structure calculations of defects in alkali halides by the CNDO semiempirical method is presented and developed. The model of a molecular cluster is used taking into account, for the first time, the non‐point nature of ions outside the cluster, which considerably reduces the surface effect. The derived parameters reflect the ionic nature of the NaCl crystal. An attempt to calculate the F centre properties (first of all the position of its ground state level within the gap and the spin density on the neighbouring ions) indicates the necessity to extend the valence basis set by introducing an additional wave function centered at the vacancy. The parameters of the latter are derived.
Polariton effects in crystalline xenon
Authors: Kink R. , Selg M.
Published: July 2007

The polariton theory is applied to the calculation of reflectivity, absorption, and resonant luminescence spectra in the region of Γ(3/2) exciton band in crystalline xenon. Agreement with experimental curves is achieved when the value of deformation potential C = (1.3 ± 0.2) eV is used. Approximately the same value is independently determined for C by the exciton self‐trapping barrier theory.
On the adhesion theory for two metallic surfaces
Authors: Kobelev A. V. , Kobeleva R. M. , Ukhov V. F.
Published: July 2007

Unlike the foregoing investigations of surface features of two contiguous metals in terms of the density functional formalism the value of the variational parameter characterizing the inverse size of the electron tail is treated as a function of the distance between the metals. Electron distribution, surface energy, adhesion energy, and interaction force at various distances are found both in the jellium model and with consideration of the discreteness of the ion charge.
Angular behaviour of the polar optical phonons in AIIIBVI layered semiconductors
Authors: Allakhverdiev K. R. , Babaev S. S. , Salaev E. Yu. , Tagyev M. M.
Published: July 2007

The behaviour of the infrared‐active lattice vibrations of GaS, GaSe, InSe are studied by measuring the room‐temperature reflectivity in the 100 to 500 cm−1 frequency range when the incident wave vector is inclined at an angle θ to the crystallographic Z‐axis. The experiments are performed in polarized light. The transverse and longitudinal mode frequencies are obtained by Kramers‐Kronig analysis of the reststrahlen bands observed in these crystals with θ between θ and 90°. The experimentally observed changes of the lattice frequencies with an angle θ are interpreted on the basis of the theory of long‐wavelength lattice vibrations in anisotropic crystals. Good agreement between the experimentally observed dependence and the calculated one according to the theory gives the possibility to elucidate the nature of a fine structure observed in the reststrahlen region of the layered GaS, GaSe, InSe crystals.
Density functional approach to point defect properties of copper–germanium alloys
Authors: Gollisch H.
Published: July 2007

Point defect properties of copper–germanium alloys are studied by using a recently developed method which is based on the density functional formalism. The charge density is approximated by superimposing the charge densities of the respective free atoms. The kinetic energy is obtained from a modified Thomas‐Fermi‐v. Weizsäcker expression. To test the reliability of this method, some properties of the perfect copper lattice and of Cu2 and Ge2 molecules are studied. Results on the formation energy of vacancies and divacancies in copper are presented. Further calculations are concerned with the interaction energy of two Ge atoms embedded in Cu and with the influence of two neighbouring Ge impurities on the formation energy of a vacancy in copper.
Thermally activated domain wall motion and coercive field in materials with extremely narrow domain walls
Authors: Fähnle M. , Kronmüller H. , Barbara B.
Published: July 2007

By computer simulation the shapes and energies of displacement nuclei are calculated for thermally activated domain wall motion in materials with so‐called ferroelectric, i.e. monoatomic domain walls. The energies of the critical nuclei decrease with increasing magnetic field, H, however, a simple 1/H‐law is only obtained for very small values of H. Thermally activated domain wall motion only occurs at high values of H. It is shown that the intrinsic coercive field Hc corresponding to the critical field for the movement of domain walls in multi‐domain particles is appreciably lowered by thermal instabilities.
Interatomic potentials from experimental phonon spectra I. Prototypes
Authors: Esterling D. M. , Swaroop A.
Published: July 2007

The systematic procedure developed by Brosens et al. to derive the interatomic pair potential from the experimental phonon spectra is modified and generalized to cover a variety of metals. A set of metals (Na, K, Al, Cu, Fe) is investigated as prototypes for the general class of simple, noble, and transition metals. Comparisons are made between this potential and other potentials including those obtained from liquid metal structure factor data and pseudopotential theory. Reliability of the potentials is argued on the basis of comparison with experimental data, reliable theoretical potentials (where available), and internal consistency. The potentials are not recommended in modeling defects (vacancies, cracks) which introduce large inhomogeneities in the electron density.
Calculation of the magnetization–temperature dependence of rare‐earth and yttrium iron garnet solid solutions
Authors: Falileev G. N.
Published: July 2007

Using the known compensation temperature–concentration dependences in the molecular field theory simple expressions for calculating the temperature dependences of the c‐sublattice magnetization and the saturation magnetization for the iron garnet (IG) solid solutions of the Y3–c‐RcFe5O12 (R = Gd, Tb, Dy, Ho, Er, Tm, Yb) type are obtained. The calculated data agree well with the experiment at T ⪆ 150 K. The discrepancy between the calculated data and the experiment at T ⪅ 150 K is due to the rare‐earth ion interaction. The correction for this interaction shows that in IG″ besides the Kramers‐Anderson exchange between the rare‐earth ions there exists an additional exchange interaction like that of the Ruderman‐Kittel‐Kasuya‐Yosida (RKKY) type playing an important role at temperatures lower than “the low temperature” point. The expression is obtained with a correction for interactions between the rare‐earth ions using the experimental magnetization temperature dependence of “pure” rare‐earth IG of the R3Fe5O12 type, which allows the calculation of the magnetization–temperature dependence of IG under investigation including temperatures ⪅ 150 K.
Temperature dependence of atomic core levels in solids II. Chemical shift and relaxation energy of core levels and their dependence on temperature
Authors: Hübner K. , Bechstedt F.
Published: July 2007

For the first time an explicit investigation is made of the T‐dependence of atomic core levels in non‐metallic solids and shown that this dependence can be of the same order as that one of electronic energy bands. The basis for these investigations are models for the chemical and the relaxation shift of core levels, which lead to absolute core‐level energies and their shifts in agreement with corresponding photoemission data. After separation of the T‐dependence of conduction‐band and core‐exciton energies of GaP, Si, and SiO2 (Part I of this work), which is involved in the total T‐dependence of corresponding core‐level to conduction‐band transitions, it is shown in the present part that multiphonon and hydrostatic processes lead to large and different T‐coefficients of the Si2p level in Si and SiO2, whereas the different contributions to the T‐dependence of the Ga3d level in GaP give an almost vanishing total effect. Furthermore, it is shown that the main contributions to the T‐dependence of atomic core levels in semiconductors and insulators come from the electrostatic interaction of the core electrons with their environment, whereas the T‐dependence of relaxation energies is very small. The results are in agreement with data, which are deduced from corresponding experiments performed by Aspnes et al.
The relation between bonding and g‐shift for TI2+ and Pb3+ in alkali halides
Authors: Moreno M.
Published: July 2007

The experimental g‐shift of TI2+: KCl, TI2+ :KBr and Pb3+ :KCl is theoretically analysed within the [ML6] cluster model (M metal ion; L ligand). Though the calculations include up to third‐order contributions in energy (especially important for TI2+ :KBr) and reproduce the sign and the qualitative trends of the experimental g‐shift of the systems studied, a reasonable quantitative agreement with the observed g‐shift is, however, only reached for TI2+ :KCl.
Interatomic potentials from experimental phonon spectra II. General results
Authors: Swaroop A. , Esterling D. M.
Published: July 2007

A method described in the previous paper to extract interatomic potentials from phonon spectra is applied to a variety of simple, noble, and transition metals. The interatomic pair potentials for Rb, Pb, Ag, Au, Cr, Mo, W, V, Nb, Ta, Ni, Pd, and Pt are reported and discussed in the light of other available potentials.
AC Conductivity and dielectric anomaly in annealed EuSe
Authors: Kuivalainen P. , Kaski K. , Eränen S. , Stubb T. , Stubb H.
Published: July 2007

The complex permittivity of annealed n‐type EuSe is measured in the temperature range T = 4.2 to 295 K for frequencies 103 to 2.4 × 1010 Hz. Using further the results of the crystallographic characterization and the dc conductivity σdc, Hall effect, and optical transmittance measurements one finds that shallow donors associated with Se vacancies and a decrease in their concentration due to annealing can explain the T and annealing dependences of σdc and σac. At T = 295 K where σdc is high, no frequency dispersion of σac is found even at microwave frequencies. When σdc decreases drastically at low temperatures a strong frequency dependence of σac is seen. It follows the relation for ac impurity hopping conduction proposed by Pollak: σac ⋐ ωs with s < 1 at low frequencies and 1 < s ≦ 2 at high frequencies. A maximum hopping frequency τ−1 = 4.0 × 109 s−1 and an average hopping length R = 9 Å at low temperatures is estimated. The annealing dependence of the dielectric constant at low temperatures can be explained by the polarizability αd of neutral Se vacancies. From the measured αd = 3.2 × 106 Å3 the carrier effective mass m* = 0.28 is estimated.
Mode damping by electrostrictive interaction in a uniaxial ferroelectric
Authors: Roy G. K. , Das A. N.
Published: July 2007

An electrostrictive‐type coupling model is used to study mode damping in the paraelectric phase of a uniaxial ferroelectric with an anisotropic soft‐mode dispersion. Mode linewidths are obtained by solving the phonon Green's function equation of motion in the self‐consistent phonon approximation (SPA). The dominant contribution to the damping comes from the scattering of a soft optic phonon with the absorption or emission of an acoustic phonon. The soft‐mode damping γs shows a (T – T0)1/2 behaviour while, in the isotropic limit, it becomes linearly dependent on temperature. For large anisotropy, the longitudinal acoustic attenuation α1 ∝ ω2(T – T0)−1/2X XIn (T – T0)−1/2 or ω2(T – T0)−1/2 according as the soft‐mode is under or over‐damped in the opposite limit, the corresponding dependences are ω2(T – T0)−1/2 and ω2(T – T0)−3/2, respectively. If soft‐mode interaction with other modes (e.g. phonon density fluctuations) are considered, γs becomes nearly temperature‐independent in this case α1 behaves like ω2 (T – T0)−1 which compares favourably with experimental observations on lead germanate where the soft‐mode is overdamped.