bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Halogens on the DC Conductivity and Magnetoresistance of (SN)x
Authors: Philipp A. , Seeger K.
Published: July 2007

Exposure of (SN)x to the vapor of Br2, I2, and ICI leads to the componds (SNBr0.4)x, (SNI0.02)x, and [SN(ICI)0.1]x, respectively. The second and the third one of these compounds have not been reported before. In the temperature range 4.2 to 300 K the de conductivity is increased in (SN Br0.4)x and (SNI0.02)x and lowered in [SN(ICI)0.1]x compared to the pristine (SN)x. The anisotropy of the de conductivity in (SN Br0.4)x is about 10 times higher than in (SN)x. Measurements of the transverse magnetoresistance at 4.2 K in all three materials indicate increased disorder due to doping with the halogens.
Multiple Scattering in Condensed Materials. II. Calculation of the Density of States for Non‐Crystalline Copper and Iron
Authors: Blaudeck P. , Lenk R.
Published: July 2007

Based on a previously developed theoretical scheme for the treatment of multiple scattering in dense disordered media, the dispersion relation for the effective medium is evaluated numerically. The result is employed to determine the electronic density of states for two non‐crystalline transition metals. The theory yields satisfactory results for the width of the d‐band and for the total number of states in the d‐band.
Interband Faraday Ellipticity in Germanium
Authors: Kessler F. R. , Metzdorf J. , Zurek W.
Published: July 2007

The interband Faraday ellipticity (IFE) at the E0 transition in germanium single crystals is measured at T = 300 K and B = 2 T. Even at room temperature the oscillatory IFE is due to transitions to exciton states. This is proved as follows: firstly, the IFE of intrinsic material shows characteristic deviations from a theoretical Landau spectrum. Secondly, the doping dependence of the IFE shows the screening influence of free carriers. The IFE is no longer oscillatory at doping concentrations above 1017 cm−3. Only the effect of the spin‐splitting of the band edge in the magnetic field is to be seen. By measuring the interband Faraday rotation (IFR) and comparing it with the rotation calculated from ellipticity according to a Kramers‐Kronig relation, the IFR at the E0 transition is shown to be due to the same origin as the IFE.
The Exchange Charge Shell Model Analysis of Strain Derivatives of Static Polarizability and Effective Charge Parameter of Alkali Halides
Authors: Jain V. C. , Shanker J.
Published: July 2007

An analysis of the strain derivatives of static polarizability and effective charge parameter of alkali halides under hydrostatic pressure is performed within the framework of the exchange charge shell model. Details of the model. Details of the model and relevant theoretical expressions are given. The magnitudes of exchange charge polarization and its volume dependence are evaluated from the overlap integrals recently computed by Hafemeister and Flygare. It is found that the inclusion of the exchange charge polarization in the formulation of the shell model improves significantly the agreement between theory and experiment.
The Properties of Ni Ions in NaCl Single Crystals. I. Vacuum Ultraviolet, Ionic Conductivity, and X‐Ray Diffraction Studies
Authors: Földvári I. , Voszka R. , Morlin Z.
Published: July 2007

The vacuum ultraviolet (VUV) absorption spectra of NaCl:Ni single crystals are studied, and the absorption band data belonging to the monomer Ni2+ ions are determined at RT and 110 K. A new absorption band is observed at 54200 cm−1 (RT). The previously published spectrum completed with another new band at 55800 cm−1 seems to suggest the presence of NaNiCl3 double salt. The supposition that a NaNiCl3 phase might be present is confirmed by ionic conductivity measurements and X‐ray phase analyses.
Line Broadening in the ESR Absorption Spectra of Fe3+, Cr3+, and Gd3+ in the Oxide YAIG
Authors: Serway R. A. , Yang F. H. , Marshall S. A.
Published: July 2007

The ESR absorption spectra of substitutional Fe3+, Cr3+, and Gd3+ ions, in flux grown singlecrystal yttrium aluminum garnet, are reinvestigated. An anisotropic line broadening is observed for Fe3+ in both octahedral and tetrahedral sites and for Cr3+ in octahedral sites. Various linebroadening mechanisms are considered including the mosaic effect, strain broadening, fluctuations in crystal field parameters and variations in the relative slopes of the Zeeman‐split levels. A reasonable fit to the experimental data is possible using a hybrid model wherein the mosaic effect, strain broadening, and fluctuations in the crystal field parameters are assumed to be the dominant mechanisms.
Electronic Structure of Solids under Laser Irradiation
Authors: Wautelet M. , Laude L. D.
Published: July 2007

The electronic structure of solids under laser irradiation is investigated. It is shown that the effect of intense laser fields is to create gaps in the electron band structure of solids, mainly in regions of ϰ‐space where direct and/or indirect transitions may occur.
Piezo‐Optical Properties of Alkali Halides and Their Temperature Dependence
Authors: Meseguer F. , Sánchez C.
Published: July 2007

The spectral dependence in the visible and vacuum ultraviolet of the piezo‐birefringence coefficients of NaF and KF is measured at room temperature. From these data the shear deformation potentials for the valence band are obtained. A piezo‐dichroic effect is also detected in the piezo‐optical experiments and the corresponding coefficients for KF are now reported. Finally the temperature dependence of the photoelastic coefficient Q44 in KBr is measured between room temperature and 700 K. Q44 remains practically constant in the visible range and shows a strong decrease on increasing temperature for photon energies close to the absorption edge. It can be concluded that there is no significant variation of the deformation potential in that temperature region.
Magnetic and Thermal Properties of Nd3+ in Scheelite‐Type Crystals
Authors: Fakioǧlu S. , Erkoç Ş.
Published: July 2007

Using the effective spin Hamiltonian formalism, crystal field calculations of the temperature dependent susceptibilities and the Schottky specific heat are performed over the temperature range 10 to 300 K besides the dipolar and quadrupolar transition probabilities of Nd3+PbMoO4 and Nd3+CaWO4 using different sets of known parameters. Two different methods are employed for the susceptibility calculations and the Curie‐Weiss law is checked. Emphasis is given of the effects of different sets of crystal field parameters.
On the Surface Electrical Resistivity of Thin Semiconducting and Semimetallic Films
Authors: Dimitrov H. D.
Published: July 2007

In the effective‐mass approximation, taking into account the image forces, the low‐energy electronic states in thin crystalline films, the transport relaxation time of current carriers, and electrical resistivity of the films are calculated when the scattering mechanism is connected with the randomly distributed on their boundary surfaces screened Coulomb and electric dipole impurity centres. It is shown that in some cases of thin solid film materials the image forces influence essentially the electronic states and their surface electrical resistivity.
Exciton Contribution to Oscillatory Magnetophotoconductivity in Ge
Authors: Kessler F. R. , Olsowski W.
Published: July 2007

In the region of direct interband transitions, two types of oscillatory magnetophotoconductivity (MPC) spectra of lightly n‐doped Ge samples of various thickness exist. First, there is a correlative spectral dependence, wherein the MPC maxima correspond to magnetoabsorption peaks, and secondly, anticorrelative spectra, which have mimima at positions of absorption maxima. From this the contribution of excitons to the MPC in Ge is deduced. Further analyses of the temperature dependence of MPC, and investigations of external influences such as additional illumination support the assumption that primarily excitons are generated by the absorption mechanism. In a secondary process these excitons may either dissociate into free carriers or recombine instantaneously. If the recombination overbalances the dissociation there is a selective defect in the quantum efficiency of the MPC at each exciton absorption line. Calculations based on the continuity equation and taking the different generation and reduction processes into consideration confirm the spectral conversion of MPC spectra.
Anderson Transition in FeSi2 Films
Authors: Theiner W. A. , Geserich H. P.
Published: July 2007

Thin films of FeSi2 are prepared by electron beam evaporation. Electron and X‐ray diffraction investigations show that the structure of these films can be varied between an amorphous state and polycrystalline α‐FeSi2, depending on the substrate temperature. As found from the temperature dependence of the dc conductivity and from ir reflectance measurements the structural transition is accompanied by a metal‐insulator transition of the Anderson type with a minimum metallic conductivity of about 400 Ω−1cm−1.
Temperature‐Dependent Mn2+‐6S State Splitting in Scheelites
Authors: Biederbick R. , Hofstaetter A. , Scharmann A.
Published: July 2007

Measurements of the EPR spectrum of seven scheelite structure crystals doped with Mn2+ ions are performed in the temperature range from 4.2 to 350 K. Experimental data of a preceding paper [1] are completed. The temperature dependent zero field splitting is discussed in terms of implicit and explicit contributions. Being the most important one the temperature effect of b is described with an Einstein model where the impurity force constant is related to regular cationic vibrations by a Born‐Mayer potential.
Magnetic Dipole–Dipole Hyperfine Integrals for Slater‐Type Orbitals
Authors: Ammerlaan C. A. J. , Wolfrat J. C.
Published: July 2007

In molecular orbital calculations a linear combination of Slater‐type atomic orbitals is frequently used as a trial wavefunction. To determine hyperfine energies, certain matrix elements, involving the electron orbitals and the hyperfine interaction operator, are required. In the paper explicit expressions are reported, which are valid for Slater‐type orbitals with principal quantum number n = 3, and s‐ or p‐type angular dependence. The expressions allow for a fast computation of the integrals.
Electronic Structure of 4d and 5d Transition Metal Clusters
Authors: Seifert G. , Mrosan E. , Müller H.
Published: July 2007

The electronic structure of Nb6‐, Mo6‐, W6‐ and Ta6‐ clusters is calculated using the scattered wave‐Xα‐method (SW‐Xα). Results for the energy eigenvalues of the clusters and the density‐of‐states are presented. Comparing with results obtained from band structure calculations for the bulk materials the small clusters already a reflect a great part of the bulk electronic structure.
Luminescence from Electron–Hole Drops in Undoped GaP
Authors: Schwabe R. , Thuselt F. , Weinert H. , Bindemann R. , Unger K.
Published: July 2007

The radiative recombination from electron–hole drops (EHD) is investigated in highly excited pure GaP in the temperature range of T = 2 to 77 K. The fundamental parameters of the electronhole condensate are obtained by comparison of the experimental and calculated lineshape of the luminescence band, taking into account the “camel's back” density of states of the lower conduction band near the X‐point. The influence of the real density of states of the lower conduction band near the X‐point. The influence of the real density of states on the lineshape is discussed. The equilibrium carrier density and the work function are determined to be neq = 7.4 × 1018 cm−3 and Φexp 15.5 meV, respectively. The “camel's back” depth is estimated to be δ = (9.1 × 1) meV. It appears that the critical temperature of the EHD is Tc ≧ 70 K. Decay measurements led to the conclusion that the luminescence in the high energy tail of the band is caused by a radiative recombination from a gas‐like plasma.
Aggregation of Point Defects in Highly Doped NaCl:Ca++Crystals
Authors: Dubiel M. , Berg G. , Fröhlich F.
Published: July 2007

The spin‐lattice relaxation time T1 measured by pulsed nuclear magnetic resonance is determined by the concentration of the Me2+‐cation vacancy dipoles alone contrary to the dielectric loss measurements. Therefore this method is used to investigate isothermal anncaling proceses via the dipole concentration also for long times when higher aggregates are formed. It is found that aggregation processes take place in NaCl: Ca2+ crystals in the range up to 200°C. For higher temperatures no change in the dipole concentration can be measured. Whereas for small doping levels (up to 500 ppm) mainly dimers and trimers are formed, at high concentrations (over 1000 ppm) the formation of the Suzuki like phase is shown and described theoretically. The enthalpy for the dipole motion (0.60 eV) and the dimer association enthalpy (0.31 eV) are calculated analysing the experimental curves.
On the Electron–Phonon and Exciton–Phonon Interaction in ZnTe
Authors: Friedrich H. , Ozerov Yu. V. , Strehlow R. , Georgobiani A. N.
Published: July 2007

The λ‐modulated reflectance spectrum of ZnTe is investigated in the spectral region above the energy of the exciton ground state Eex1. Structures are observed near the energy Eex1 + ℏωo (ℏωo is the LO‐phonon energy) and at the energy Eg + 1.3 ℏωo (Eg is the gap energy). In accord‐ance with similar results obtained in a lot of other II‐VI compounds the first of these structures is interpreted from the point of view of the exciton‐phonon complex formation. The second struc‐ture not observed experimentally so far is ascribed to a polaron effect. In order to support this interpretation the existing theory of this polaron effect is extended to the determination of the spectral dependence of the real part of the dielectric function. This allows the determination of the behaviour of the reflectance R and of the derivative dR/dλ. The theoretical results are in quite good agreement with the experimentally observed spectrum.
Model for Trapping Rates for Sensitized Fluorescence in Molecular Crystals
Authors: Kenkre V. M.
Published: July 2007

Expressions for trapping (and detrapping) rates, which describe transitions from (and to) the ϰ‐states of the host crystal to (and from) the trap states, and which serve as inputs in a theory of sensitized fluorescence in molecular crystals, are derived from the point of view of their ϰ‐dependence. They are shown to be consistent with experimental observations.
Theory of Resonant Raman Scattering in Systems with Localized Electron States
Authors: Böttger H. , Bryksin V. V. , Kleinert P.
Published: July 2007

On the basis of a general theory of the resonant Raman scattering ordered and disordered systems with strong electron‐phonon interaction are investigated in systems with localized electron states. For a system with a single narrow band a resonance peak occurs, which is analogue to the optical intraband absorption peak of small polaron systems. For models with two bands a luminescence peak occurs with a large Stokes shift besides an analogous resonant Raman peak.
IR‐absorption of OH and OD centres and OH/OH, OD/OD, and OH/OD complexes in Cu‐doped ZnO single crystals. II. Discussion and quantitative interpretation of models
Authors: Gärtner F. G. , Mollwo E.
Published: July 2007

IR‐absorption line spectra observed with ZnO:Cu:H, ZnO:Cu:D, and ZnO:Cu:H:D are discussed. The main peaks are due to vibrations of OH and OD centres in ZnO:Cu. The side bands assigned to vibrations of complexes of the form OH/OH, OD/OD, and OH/OD are calculated by means of a threedimensional, lattice‐induced model.
Biexcitons, impurity transitions, and electron–hole plasma in highly excited II–VI compounds?
Authors: Schrey H. , Klingshirn C.
Published: July 2007

To determine the origin of the so‐called M‐ and P‐emission bands, observed in highly excited II–VI compounds at low temperature, CdS is investigated at 5 K by means of excitation spectroscopy. The results are discussed in terms of inelastic scattering processes, biexcitons, impurity transitions, and electron–hole plasma (EHP), including in the discussion also results published by other authors. It is shown that in II–VI compounds the high‐excitation luminescence is mainly due to inelastic scattering processes and to impurity transitions at low temperature and that it is only to a smaller extent caused by EHP and biexciton decay.
Isothermal compressibility for various liquid metals by harmonic model potential
Authors: Sahay B. B. , Srivastava P. L.
Published: July 2007

Isothermal compressibility of different groups of liquid metals (Na, Rb, Zn, Cd, Al, Tl, In) is calculated using the harmonic model potential. The computed values are better in agreement with the experimental measurements than other theoretical values.
Low‐temperature renormalization group and symmetry breaking perturbations
Authors: Nattermann Th.
Published: July 2007

A skeleton graph approach in d = 2 + ϵ dimensions is used to calculate the low‐temperature properties of a n‐component classical Heisenberg model. The approach allows a unified description of both Goldstone and critical singularities. The influence of small symmetry breaking fields on these singularities is investigated. The main results are closed form expressions for the correlation functions, the equation of state, and the free energy. If n ≧ 3 these fields are stongly relevant. In particular, a small cubic perturbation leads to a first‐order transition. For n = 2 perturbations with the symmetry of a p‐fold axis may become irrelevant for a certain temperature region.
Analysis of phonon frequencies of alkali metals
Authors: Ramamurthy V. , Satishkumar M.
Published: July 2007

Investigations of the dependence of phonon frequencies of alkali metals on their atomic mass reveal that they satisfy an empirical relation Mγv = D(q), a constant which depends on q, with γ = 0.754 ± 0.019 at all wave vectors and hence homologous. It is shown that the observed cross‐over of [ζ00] branches of lithium is not an anomalous property, but only manifests itself in alkali metals of smaller atomic mass due to small variations of γ. The role played by atomic mass which is closely related to that of electron gas constant, C′, in producing a cross‐over is discussed.
A Green's function investigation of nearly two‐dimensional and nearly one‐dimensional Heisenberg ferromagnets
Authors: Treplin W. , Jelitto R. J.
Published: July 2007

A Heisenberg model of a ferromagnet is considered, which is modified such that one can change continuously from three to two and to one dimensions. The thermodynamic properties of this model are evaluated by application of Green's functions. A special decoupling procedure is needed in order to provide a Stanley transition for the two‐dimensional model. The results of the theory are evaluated both asymptotically and numerically. By these results an assertion drawn from a former paper is substantiated in so far, that already a tiny interaction between different two‐dimensional magnetic layers is sufficient for producing essentially three‐dimensional magnetic behaviour. By consideration of the various nearest neighbour spin correlations one gets some insight into the question of the reliability of the applied decoupling scheme.
Dielectric function in HgSe at 2, 95, and 300 K
Authors: Manabe A. , Nougchi H. , Mitsuishi A.
Published: July 2007

Reflectivity spectra of n‐HgSe with various carrier concentrations are measured in the spectral region 20 to 500 cm−1. Their main features are well explained by the theoretical dielectric function which is constructed as a sum of Γ8–Γ8 interband, intraband, and phonon contributions and is calculated using the available values of band parameters in the full Kane theory. The effect of the interband contribution on the plasmon–LO phonon coupling and on the frequency of LO phonon is discussed. Possible origins for the discrepancy between experimental and calculated spectra are also discussed.
Quasiparticle, charge, and energy conservation in weak‐coupling superconductors
Authors: Mathews W. N.
Published: July 2007

The conservation of quasiparticles, electric charge, and energy in weak‐coupling superconductors is discussed and correlated with the interpretation of the quasiparticles as particle and hole excitations. The formulation employed is that of the Bogolyubov‐de Gennes equations. In the absence of spin‐dependent interactions these equations are written in the form of a time‐dependent Schrödinger equation with a two‐component wave function and a two‐by‐two matrix Hamiltonian. Accordingly, the desired conservation laws are discussed on much the same basis as in ordinary quantum theory. The generalization to permit nonlocal quasiparticle and pair potentials and spin‐dependent interactions is briefly presented.
An estimate of the pressure dependence of the dielectric constant in alkali halides
Authors: Varotsos P. A.
Published: July 2007

The determination of the pressure dependence of the static dielectric constant, ϵS, from the Szigeti relations, demands the knowledge of the volume dependence of the Szigeti charge e*, which unfortunately is unknown. By using the classical Lorentz‐Lorenz relation and connecting the ionic polarizability to the bulk modulus a simple relation for dϵS/dP is derived; although it gives the correct order of magnitude, the calculated values exceed the experimental ones by a factor 1.2 to 2. This difference is possibly due to the increase of e* with pressure. A simple relation for the Gruneisen constant of the transverse optical mode is also given.
Resistivity and thermo‐electric power of monovalent, divalent, and trivalent liquid metals
Authors: Sinha N. N. , Srivastava P. L.
Published: July 2007

Electrical resistivity and thermo‐electric power of monovalent (Li, Na, K, Rb, Cs), divalent (Zn, Cd), and trivalent (In, Tl, Al) liquid metals are studied using the corrected parameters of the Shaw model. The results show improvement over those obtained without the parametric correction. Effect of various forms of exchange and correlation correction on the properties is also investigated.
Thermoluminescence of irradiated RbCl and RbCl:Sn crystals
Authors: Sastry S. B. , Balasubramanyam K.
Published: July 2007

The thermoluminescence (TL) and emission of coloured RbCl and RbCl:Sn crystals are studied in the temperature range 300 to 560 K. The glow peaks responsible for F centres and tin centres in different valence states are identified using optical bleaching studies. The glow peaks are analysed using total curve fitting method and the kinetic parameters are determinet. In moderately γ‐irradiated tin‐ doped RbCl crystals, the TL emission bands observed at 4.0, 3.6, and 2.2 eV are attributed to Sn0, Sn+, and Sn− centers, respectively. A comprehensive energy level diagram depicting the various TL processes in these undoped and tin‐ doped RbCl crystals is proposed.
Optical constants of n‐ and p‐type GaAs between 2.5 and 3.5 eV
Authors: Vigil E. , Rodríguez J. A. , Alvarez R. Pérez
Published: July 2007

The normal incidence reflectance for n‐ and p‐type GaAs is measured at room temperature in the spectral range from 1.2 to 3.8 eV. Using Kramers‐Kronig analysis, the values of the refractive index and absorption coefficient for n‐ and p‐type GaAs are determined from the reflectance data in the spectral range from 2.5 to 3.5 eV. The results obtained show that the optical‐constant spectra depend on impurity concentration in the spectral range investigated.
Fast electron scattering in metals with the excitation of a core electron
Authors: Zhivopistsev F. A. , Hernández F. C.
Published: July 2007

Fast electron scattering in thin metallic films is discussed, considering the excitation of a core electron. The first plasmon satellite is incorporated using Langreth's model, but assuming a dispersion law quadratic with the wave vector. An expression is obtained for the transition rate. Numerical results are shown for Li, with a discussion of spectral structure for different values of the scattering angle.
The role of proteins in electron transport at large distances
Authors: Davydov A. S.
Published: July 2007

The simplest one‐dimensional model of the protein molecule is used to show that the wave functions of donor and acceptor molecules attached to it become different from zero over the entire protein molecule. As a result, the protein molecule facilitates the electron transport between the donor and acceptor molecules attached to it.
Optical absorption of off‐centre Cu+ in CsBr
Authors: Rambabu B. , Ramasastry C. , Chowdari B. V. R.
Published: July 2007

Optical absorption studies of Cu+ in CsBr are made in the temperature range 300 to 10 K. Nine absorption bands at 6.15, 5.99, 5.80, 5.64, 5.44, 5.33, 5.09, 4.95, and 4.87 eV could be resolved at liquid helium temperature. Their intensities are temperature independent. The observed spectrum is in agreement with the calculated spectrum, if the Cu+ ion replaces a Cs+ ion and gets displaced along the 〈111〉 direction. The crystal field at Cu+ ion is predominently cubic (δc = —0.45 eV) with a trigonal component (δt = —0.055 eV) superposed on it.
Spin glass transition. A dynamical instability
Authors: Deutges D. , Freudenhammer A. , Heber G.
Published: July 2007

The instability of a spin glass phase against an external magnetic field is discussed on the basis of a Landau‐type theory of phase transition with two order parameters. There are two different critical magnetic fields, both approaching zero for T → TSG; the principal meaning of these critical fields is considered. The free energy F(H, T) develops different branches as a function of H and T with bifurcations typical for non‐equilibrium phase transitions (or dynamical instabilities).
Electronic structure of FeNi alloys studied by means of X‐ray photoelectron spectroscopy. I. Valence bands
Authors: Berndt K. , Marx U. , Brümmer O.
Published: July 2007

XPS spectra of FeNi alloys are measured. Using the spectroscopic, thermodynamic, and magnetic moment data so far available a consistent model of density of states for the FeNi alloys is proposed. This model is based on the band theory of ferromagnetism and some known properties of the electronic band structure of the first series transition metals. The measured XPS valence band spectra can be explained by means of the model DOS neglecting the influence of the transition probabilities. The CPA calculations so far available for FeNi do not fit the spectra satisfactorily. The model assumes the d‐bands to penetrate each other. The up‐spin bands are shifted against the down‐spin bands by a rigid exchange splitting corresponding to the measured atomic magneton number in the alloy. The model neglects the spin–orbit coupling which would couple states of opposite spin but this influence should be small.
Surface impedance investigations of superconducting niobium
Authors: Rothkirch L. , Herrmann R. , Dwelk H. , Preppernau U.
Published: July 2007

Measurements of the surface resistance derivative dR/dH in dependence on magnetic field strength H are presented as observed at pure superconducting niobium single crystals. At crystals not annealed after preparation procedure the measured impedance spectra do not represent the phase transitions appearing at critical fields indicating thus the existence of surface layers with properties different from the bulk of pure niobium. At annealed crystals the critical magnetic field strengths are marked as characteristic discontinuities in the spectra. The spectra observed at recently annealed crystals being in Meissner state can be interpreted by means of magnetic surface states.
UPS measurements of CO chemisorbed and condensed on Rh (111) using synchrotron radiation
Authors: Braun W. , Neumann M. , Iwan M. , Koch E. E.
Published: July 2007

The photoemission data of the clean Rh(111) surface are strongly dependent upon photon energy in the range 15 to 27.5 eV. Three different chemisorption levels are distinguished for CO chemisorbed on Rh(111) by exploiting the dependence of photoionization cross sections upon photon energy. Exciton–electron and electron–electron scattering dominate the intensity variations of the spectra of CO condensed on Rh(111). The origin of the observed energy level shifts of CO in the three different phases, gas, condensed and chemisorbed, are discussed in terms of relaxation, bonding, and chemical shifts.
Evidence of non‐linear behaviour of soft modes in crystals with KDP structure
Authors: Scarparo M. A. F. , Katiyar R. S. , Srivastava R. , Porto S. P. S.
Published: July 2007

The low‐frequency B2 Raman spectra of KDP, RDP, RDA, and D‐RDA are measured as a function of temperature. The low‐frequency data are fitted to the response function of two coupled harmonic oscillators and of two coupled lattice oscillators. The results show that the overdamped proton tunneling mode softens with temperature as the transition temperature Tc is approached. However, the results show that very near Tc, the soft mode attains a hard‐core frequency. In view of these results the various models proposed in literature to explain the phase transition in these crystals are critically examined. The nature of the coupling between the two modes is also discussed.
Acoustic phonon scattering by electron–hole droplets in Ge. II. Total scattering rate and anisotropy of scattering, as reflected in the shape of a droplet cloud
Authors: Markiewicz R. S.
Published: July 2007

The pair‐phonon scattering rate is calculated for carriers inside an electron–hole drop in Ge. The calculation includes effects due to screening, electron mass anisotropy, and sound velocity anisotropy. The net scattering time is shorter by about a factor of two than found in previous calculations. A very similar integral occurs in calculating the force of the ‘phonon wind’ on the cloud of droplets. Using a simple model, this integral can explain the anisotropic shape of the cloud observed experimentally by Greenstein and Wolfe. The calculation accounts for both the broad lobes observed along 〈111〉 axes and the sharp flares along 〈100〉 axes (the latter due to a focusing of the phonons in Ge).
Infrared Studies of Al Complexes in Zinc Sulphide
Authors: Król A. , Kozielski M. J. , Nazarewicz W.
Published: July 2007

Infrared absorption is studied in zinc sulphide doped with the following impurities: a) Al and Cu, b) Al and Ag, c) Al and Au. Al is introduced during crystal growth and Cu, Ag, and Au by saturation diffusion. The ZnS samples with two dopants have the zincblende structure with a low density of stacking faults. In the spectral range 360 to 650 cm−1 a number of absorption bands are observed that could be ascribed to the localized vibrational modes of unassociated Al and Al in complexes with Cu, Ag or Au. On the basis of symmetry considerations the following defect models are suggested: AlZn, AlZn–CuZn, AlZn–AuZn and AlZn–AgS or AlZn–Agi (where i denotes interstitial location). The results obtained with ZnS are compared with published data for CdTe and ZnSe.
Tunnel measurements on Nb single crystals
Authors: Schöneich B. , Elefant D. , Otschik P. , Schumann J.
Published: July 2007

In order to investigate the mechanism of superconductivity in transition metals, tunnel measurements on niobium single crystals annealed in ultrahigh vacuum are carried out. The residual resistivity ratio of the single crystals is r0 ≧ 20 000. The tunnel junctions yielding most information have the structure NbIAg. The insulator is produced by thermal oxidation of niobium. The value of the energy gap is Δ0 = (1.50 ± 0.05) meV. The interpretation of dU/dI and d2U/dI2 characteristics yields the electron‐phonon coupling constant Λ = 0.62 and the Coulomb pseudopotential μ* = 0.002. From tunnel curves values about the thickness and height of the tunnel barrier are estimated. The normal‐conducting properties of the tunnel junctions are discussed.
Quantitative determination of the valley splitting in n‐type inverted silicon (100) MOSFET surfaces
Authors: Köhler H. , Roos M.
Published: July 2007

Shubnikov‐de Haas oscillations including an abrupt phase shift of the extrema position at a critical magnetic field are reproduced by computer simulation allowing a quantitative evaluation of the spin and valley splittings Δs and Δv with a high accuracy. Varying the gate voltage and tilt angle of a sample in the magnetic field data are obtained over a wide field range. The computer simulation also permits to check the assumptions about the magnetic field dependence of the energy width and the shape of the broadened levels in quantizing magnetic and electric fields. The results obtained for Δs and Δv both depend on the history of even the same sample.
Electron spin resonance absorption spectrum of divalent manganese in single crystal calcite
Authors: Marshall S. A. , Marshall T. , Zager S. A. , Zimmerman D. N.
Published: July 2007

The electron spin resonance absorption spectrum of divalent manganese in single crystal calcite is reinvestigated at X‐band wavelength. Electron‐nucleus double resonance absorption spectroscopy is used to assist in the analysis of the hyperfine structure spectrum. Best fit spectral parameters obtained at T = 4.2 K are listed.
Resonant magneto‐Raman scattering in polar semiconductors
Authors: Sampaio A. J. C. , Luzzi R.
Published: July 2007

Resonant Raman scattering of LO phonons in a magnetic field is studied. Direct gap, small effective mass, n‐doped semiconductors are considered in conditions in which the laser and the scattered frequencies are of the order and larger than the harmonics of the cyclotron frequency. In these resonant and near resonant conditions the lifetime of the intermediate states gives important contribution to the scattered intensity. A calculation is presented of the scattering cross‐section for the magneto‐Raman scattering by LO phonons which includes the electron‐phonon interaction contribution to the electron lifetimes. Numerical application to InSb is done.
The electronic spectra and parametric instabilities in charge‐ordered crystals with intraband electron attraction
Authors: Ionov S. P. , Manakova L. A. , Zemskov B. G.
Published: July 2007

An analysis of the electron properties of the charge‐ordered systems with the intraband attraction is given. Two models of these systems are assumed 1) an intraband attraction takes part in the formation both the charge ordering and pair correlation of the superconductive type; 2) only the interband interactions are responsible for the formation of the charge ordering. For these models the coexistence of the charge ordering and pair correlation problem is considered. The interaction parameter regions in which the coexistence occurs are found. The low‐lying collective excitation energies are calculated for the charge‐ordered state both, with pair correlation and without. The possible types of parametric instabilities of the charge ordered state are obtained.
Extended Hückel molecular orbital calculation of defects in LiH cluster crystals
Authors: Ikeya M. , Schwan L. O.
Published: July 2007

An LCAO‐matrix method (extended Hückel MO) is applied for the calculation of the intrinsic and defect properties of LiH crystals. The ionic clusters of Li+ and H− involving 8, 27, and 64 ions are considered as well as the atomic clusters of Li0 and H0. Defect properties of negative and positive ion vacancies, interstitials H− (H) and H0(H), and substitutional H0 at the anion and cation sites (H and H) are discussed from the result of the calculation taking the experimental results into account.
Linear optical response of highly excited semiconductors electron–hole liquid in CdS
Authors: Müller G. O. , Weber H. H. , Höricke I.
Published: July 2007

A thorough investigation of highly excited direct gap semiconductors requires determination of optical constants within the duration of the nanosecond excitation period. A simultaneous recording of reflection and transmission (and probing) spectra is possible by virtue of the multiple track mode of PAR's OMA. Interfacing to a computer and use of classical optics allow for an on‐line absolute calibration. The absolute values of absorption and refraction spectra of highly excited CdS are indicative of an electron–hole liquid with a chemical potential of about 2.537 eV and a density of about 3 × 1018 cm−3.
Collective vibrations of protons in compounds of the KH2PO4‐type the cluster approximation
Authors: Stasyuk I. V. , Levitskii R. R. , Korinevskii N. A.
Published: July 2007

The dynamics of the proton system of KH2PO4‐type compounds is considered in the cluster approximation taking into account their real structure. The equation for the excitation spectrum is obtained. For the particular case of the four‐sublattice de Gennes model the conclusion is made that the excitation spectrum contains the pseudo‐soft mode. It is shown also, that for KH2PO4 model the mentioned conclusion is correct.
Force–force correlation function approach to the electrical resistivity of disordered transition metal alloys
Authors: Goedsche F. , Richter R. , Vojta G.
Published: July 2007

The method of the force‐force correlation functions is used to derive, beyond the weak scattering approximation, a tractable approximative formula for the electrical resistivity of disordered binary alloys. The starting point is a one‐particle expression for the resistivity formulated in terms of the total scattering operator T. The local approximation is introduced by replacing the T operator by the atomic t matrices in order to treat transition metal alloys described by muffin‐tin single‐particle Hamiltonians. Short‐range multiple‐scattering corrections to the local approximation are evaluated using a non‐selfconsistent single‐site average‐t‐matrix approximation. Explicit results are given and discussed for the resistivity of a one‐dimensional muffin‐tin model for transition‐metal‐like alloys with resonant states.
Scattering of X‐rays near the K absorption edge I. Fluorescence and resonant Raman scattering in transition metals
Authors: Suortti P.
Published: July 2007

Formulas for the resonant cross section of X‐rays are rederived emphasizing the similarity of the resonant Raman scattering (RRS) to the fluorescence, and are given in terms of the oscillator strengths and radiative widths. In both processes only a fraction results in radiation, and for the RRS this is termed the RRS yield, wRRS, in analogy with the fluorescence yield, wK. Measurements are made in the symmetrical reflection geometry from totally absorbing samples. The fluorescence yield is measured CuKα as exciting radiation for Ti, V, Cr, Mn, and Fe, the results being 0.237, 0.265, 0.291, 0.325, 0.356, respectively. The absolute cross section of the (1s, 2p) contribution to the RRS is measured for Cr, Mn, Fe, Ni, and Cu at energies 42 to 940 eV below the K absorption edge. The RRS yields are 0.404, 0.426, 0.422, 0.415 (0.460 in another measurement), and 0.473 (0.468), respectively. Also evidence is found for the existence of the (1s, 3p) contribution to the RRS.
The charge on dislocations in alkali halides for the case of a saturated region
Authors: Mejía C. Ruíz , Araiza L. Cota , Broder F. , Mijangos R. R.
Published: July 2007

The case of a charged dislocation with a saturated region is considered, allowing for the discrete nature of charges and defects on the core of the dislocation. The potential due to the charged dislocation is obtained by combining the results of Kliewer and Koehler with those of Whitworth. The method used in this case to determine the charge on the dislocation, is the same as that of Whitworth.
Short‐range magnetic order in a spin glass with nearest‐neighbour random exchange bonds
Authors: Medvedev M. V.
Published: July 2007

In the molecular field approximation it is shown that in a spin glass with nearest‐neighbour random exchange bonds of different signs (IA > 0 and IB < 0) the ferromagnetic short‐range order will appear below the transition temperature from the paramagnetic state, if the concentration c of antiferromagnetic bonds IB is less than a concentration c★, and the antiferromagnetic short‐range one will appear, if c is more than c★. The Edwards‐Anderson spin‐glass state, in which the short‐range magnetic order is completely absent, is realized only at the concentration c★ that corresponds to zero paramagnetic Curie point.
Observation of infrared active and silent modes in cubic crystals by hyper‐Raman scattering
Authors: Vogt H. , Neumann G.
Published: July 2007

Hyper‐Raman spectra of CsI, CsBr, RbI, and SrTiO3 are measured at room temperature. All optical phonons at the I′‐point of the Brillouin zone can be detected including the longitudinal modes and one silent mode. The results obtained for SrTiO3 are compared with data due to other experimental techniques. Superiority of hyper‐Raman speetroscopy in determining frequencies and linewidths is demonstrated.
Propagation of electromagnetic waves in a direction perpendicular to the magnetic field in semiconductive BiSb alloys I. B ⊥ C3 ∥ k
Authors: Stegmann R. , Oelgart G. , Herrmann R.
Published: July 2007

The propagation of electromagnetic waves is detected in semiconductive Bi1−xSbx alloys in a direction perpendicular to the magnetic field at helium temperatures and in a frequency range between 36 and 140 GHz. For lacking spatial dispersion and for arbitrary orientations of the magnetic field the wave propagation takes place within the trigonal plane. In both, the quasiclassical limiting case and the ultra‐quantum state, it is found that the experimentally and theoretically determined values of wave dispersion are in mutual agreement. The measurements permit statements to be made about the cyclotron masses and it is possible to obtain a spin splitting parameter of G2 = 0.9.
Acceptor photoionization and light‐to‐heavy hole absorption in Cd1−xMnxTe
Authors: Wojtal R. , Golnik A. , Gaj J. A.
Published: July 2007

The observed temperature‐dependent absorption spectrum in the wavelength region 2.2 to 12.5 μm in Cd1−xMnxTe mixed crystals can be explained as originating from acceptor photoionization and light‐to‐heavy hole band transitions. The application of simple models to experimental data for two compositions allows to determine the ionization energy of the acceptor level EI = 0.17 eV and the heavy‐to‐light hole mass ratio m/m = 4.
Scattering of X‐rays near the K absorption edge II. Dispersion factors f′ and f″
Authors: Suortti P.
Published: July 2007

The dispersion factors of the X‐ray scattering amplitude, f′ and f″ are studied in the dipole approximation where they are independent of the scattering angle. The imaginary part is proportional to the absorption coefficient, and below the Kedge the observedf″K is in a close agreement with the value calculated from the resonant Raman cross‐section. The real part is calculated from the tabulated absorption coefficients in a number of cases where comparison with direct determinations of f′ can be made. The agreement is good, particularly when extensive data near the edge are available, while recent theoretical values are consistently low. Calculated values of f′ are tabulated for transition elements at several commonly used X‐ray wavelengths.
Calculation of the binding energies of doubly ionizable centres in II–VI compounds
Authors: Hagston W. E. , Sutherland H. H.
Published: July 2007

The ionization energies of a doubly ionizable centre are calculated on the basis of two different models. One of these is the effective mass approximation (i.e. shallow centre approximation) suitably extended so as to simulate central cell corrections, whilst the other, based on the concept of an electronic polaron, is more appropriate to the deeper centre regime. The results obtained provide a parametrization scheme which can be checked by experiment and thus lead to a possible means of identifying such centres.
A self‐consistent model for the electronic structure of the U‐center in alkali halides
Authors: Koiller B. , Brandi H. S.
Published: July 2007

A simple one‐orbital per site model Hamiltonian for the U center in alkali halides with rock‐salt structure is presented, with correlation effects introduced via an Anderson type Hamiltonian. The Cluster‐Bethe lattice method is used to determine the local density of states, yielding both localized and extended states. A one‐electron approximation is assumed and the problem is solved self consistently in the Hartree‐Fock scheme. The optical excitation energy is in fair agreement with experiment. The present approach is compared with other models previously used to describe this center and the results indicate that it adequately incorporates the relevant features of the system indicating the possibility of its application to other physical situations.
Nonradiative multiphonon relaxation in Y3Al5O12:Nd3+
Authors: Perlin Yu. E. , Kaminskii A. A. , Enakii V. N. , Vylegzhanin D. N.
Published: July 2007

Analytical expressions are derived for the multiphonon decay rate of an isolated impurity level, when interaction with low‐frequency phonons appears as small perturbation. Both, the interaction with high‐frequency optical phonons and the contribution of Jahn‐Teller vibrations are taken into account in the framework of first cumulant approximation. In the temperature range 0 to 700 K the transition probability 4I11/2 → 4I9/2 of the states of Nd3+ ions in Y3Al5O12 crystals are calculated numerically. The results are in agreemnt with data obtained from laser genration kinetics.
Effect of uniaxial stress on free and bismuth‐bound excitions in InP
Authors: Weber G. , Rühle W.
Published: July 2007

The reduction of the shear deformation potentials of holes bound to the isolectronic impurity Bi in InP is determined by piezoluminescence. It is compared with the corresponding reduction for holes bound to the Coulomb‐type acceptors C and Zn. The theory for an effective mass acceptor describes well the cases of C and Zn. However, additional effects as local strain and Stark fields must be involved in the case of Bi leading to an extremely large reduction of the deformation potentials. No change in binding energy with applied stress as well as no exchange splitting of the Bi‐bound exciton can be detected within experimental accuracy. The stress dependence of the free exciton reflectance reveals values for the band deformation potentials and a value of 0.07 meV for the exchange splitting of the free exciton in InP.
Electronic Raman scattering on donor levels in three‐band semiconductors with direct band gaps
Authors: van Hieu Nguyen , Áiviet Nguyen
Published: July 2007

A theory is presented of the electronic Raman scattering on shallow donor levels in three‐band semiconductors with direct band gaps. Explicit expressions of the matrix elements of the transitions from the ground state to the first excited state and to the states of the continuous spectrum are derived. It is shown that the most important contribution to these matrix elements comes from the intermediate states in the higher conduction band and in the valence band.
Conduction at low temperature in ion‐implanted silicon
Authors: Bourgoin J. C. , Frossati G. , Ravex A. , Thoulouze D. , Vandorpe M. , Waksmann B.
Published: July 2007

The electrical conductivity in a 4 × 1014 cm−2, 130 keV phosphorus implanted layer is studied, after annealing, in the temperature range 4.2 K to 6 mK. The annealing (at 600°C) is such that most of the phosphorus ions become electrically active and are only partially compensated by the remaining defects. The analysis of σ versus T indicates that three different regimes of conductivity are operating, depending on the temperature range. Between 4.2 K and ≈︁ 100 mK the results find a reasonable interpretation in terms of three‐dimensional variable range hopping (VRH), which follows Mott's law: σ ∼ exp (T−1/4). When T decreases, between ≈︁ 100 and ≈︁ 50 mK the hopping range becomes larger than the thickness of the implanted layer and a twodimensional VRH regime, in which σ ∼ exp (T−1/3), takes place. At low temperature, below ≈︁ 50 mK, σ varies as exp (T−1/2); a tentative explanation of this behaviour is proposed, in terms of correlation effects.