bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature Dependence of Conductance of Fibonacci Superlattices
Authors: Roy C. L. , Basu Chandan , Domínguez‐Adame F. , Maciá E.
Published: July 2007

Studies of the dc conductance of Fibonacci superlattices at finite temperature are carried out. Exact solutions of the Schrödinger equation are used for our treatment of the problem. The results show a strong dependence of electrical conduction on the chemical potential, which acts as a probe of the underlying electronic structure. In particular, it is found that the Fibonacci superlattice shows metallic behaviour when the chemical potential lies within allowed bands and it behaves as a semiconductor when the chemical potential lies within gaps.
Parametric Interaction of Electromagnetic and Acoustic Waves in Cubic Semiconductors with Strain‐Dependent Dielectric Constant
Authors: Artemenko O. L. , Sevruk B. B.
Published: July 2007

The phenomenon of parametric excitation of a transverse hypersonic wave by laser and microwave radiation in cubic semiconductor crystals with strain‐dependent dielectric constant and piezoelectric effect induced by a constant transverse electric field is investigated. The numerical analysis of the exact dispersion equation is performed providing resonance dispersion relations of the acoustic wave gain factor for a wide range of electromagnetic wave frequencies and semiconductor plasma parameters. The boundaries of the absolute instability region and electromagnetic field threshold value are set up, as well as the optimal relationship between the collision frequency and the frequencies of electromagnetic and plasma waves governing acoustic wave generation and amplification.
Electronic and Positronic Structure of Diamond under Normal and High Pressure
Authors: Aourag H. , Abderrahmane S. Ait , Amrane Na. , Amrane N. , Bouarissa N.
Published: July 2007

Empirical nonlocal pseudopotentials of diamond which can describe the electronic energy structure over a wide energy range of more than 20 eV from the bottom of the valence band are determined. The nonlocality of the potential is described by the Gaussian model. The optimized nonlocal pseudopotential reproduces the energy band structure within 5%. The valence electron and positron charge densities in diamond are obtained from wave functions derived from this model at normal and under hydrostatic pressure. It is found that the positron density is maximum in the open interstices and is excluded not only as usual, from the ion cores but also to a considerable degree from the valence bonds.
Luminescence Bands and their Proposed Origins In Highly Porous Silicon
Authors: Canham L. T.
Published: July 2007

The striking photoluminescent properties of porous silicon nanostructures have received intense study since 1990. This literature review focuses on the large number of models proposed to account for efficient light emission. Two distinct luminescence bands in the visible spectral range have received most study. Strong evidence now exists that quantum confined carriers in crystalline Si can yield efficient red to green (‘S band’) luminescence. The blue (‘F band’) emission reported to date in oxidised porous Si is, however, unlikely to arise from quantum confinement effects.
From Minutes to Months: Ageing of Porous Silicon Single Layers and Superlattices
Authors: Theiss W. , Arntzen M. , Hilbrich S. , Wernke M. , Arens‐Fischer R. , Bercer M. G.
Published: July 2007

The ageing of porous silicon under ambient conditions must be taken into account in the design of optical devices using this material. The time dependence of the optical properties of porous silicon single layers and superlattices is studied in detail using reflectance spectroscopy. In the infrared the Si–H and Si–O vibrational modes are examined which reflect the state of thc internal surfaces. The optical properties in the visible and ultraviolet spectral range are investigated as well. Spectra are recorded on time scales ranging from minutes to months after sample preparation. The observed changes of the optical properties are discussed. It is verified that the stability of porosity superlattice filters in time can be improved significantly by thermal oxidation.
Temperature Dependent Photoluminescence and Optical Absorption of Wide‐Gap Amorphous Silicon Carbon Alloys
Authors: Rüter D. , Rolf S. , Bauhofer W.
Published: July 2007

The photoluminescence (PL) properties of “porous silicon” prepared by spark erosion and of amorphous silicon carbon alloys. amorphous silicon carbon oxygen alloys and hydrogenated amorphous carbon prepared by CVD methods from liquid precursors are measured. Ellicient blue‐green PL is measured as a function of temperature up to 800 K. Optical absorption data in the high energy range (UV) are obtained from transmission measurements. Very low absorption coellicients in the visible spectral range are determined in light‐guiding thin film structures which are grown as a three‐layer sandwich on a silicon substrate.
The Fraunhofer Diffraction by Large Scale Inhomogeneities
Authors: Skryabina S. N. , Skryabin Yu. N.
Published: July 2007

The small‐angle scattering amplitudes by large scale spherical and nonspherical inhomogeneities are considered both in the refraction and diffraction ranges. In particular, the scattering by vortex lines in superconductors is discussed. In all cases the range is determined where the Fraunhofer diffraction part of the amplitude gives the main contribution to the scattering amplitude. This is important since the Fraunhofer diffraction leads to another dependence on the transfer momentum than the refraction contribution in the refraction range.
The Displacement and Stress Fields due to a Sinusoidal Edge Dislocationa
Authors: Anongba P. N. B.
Published: July 2007

Expressions for the displacement and stress fields of a sinusoidal edge dislocation in an infinitely extended isotropic medium are given. It is suggested that information on the physical properties of non‐planar cracks propagating along 〈110〉 in intermetallics with cubic structures may be obtained by studying particular cracks which consist of continuous distributions of sinusoidal edge dislocations perpendicular to 〈110〉 with infinitesimal Burgers vectors.
High Pressure Properties of the Alkaline‐Earth Sulphides
Authors: van Camp P. E. , van Doren V. E. , Martins J. L.
Published: July 2007

The alkaline‐earth sulphides MgS, CaS, SrS, and BaS naturally occur in the rocksalt (Bl) structure. Under pressure they undergo a structural phase transformation to the cesiumchloride (B2) structure. Ab‐initio local density nonlocal pseudopotential calculations of the total energy of the first three compounds are performed in the B1, B2, and in the zincblende (B3) structure. It is found that the B1 structure is stable at normal conditions while the B3 structure is unstable under all conditions. The B1 to B2 phase transformation occurs at, respectively, 230.7, 49.8, and 20.9 GPa. Recent experimental estimates of these transition pressures are 44 and 37.1 GPa for CaS. The calculated lattice constants, bulk moduli, and their pressure derivatives agree well with the experimental values.
Power Losses of a High‐Temperature Superconducting Slab in the Presence of Alternating Currents
Authors: Saif A. G. , El‐Sabagh M. A.
Published: July 2007

The power loss and its two constituent terms due to dissipated and reactive energies are developed for a system of a normal conducting wire (two wires) carrying ac current parallel to a high‐temperature type‐II superconducting infinite slab (moving slab) of arbitrary thickness. The reactive term is found to be either capacitively or inductively reactive. When the thickness of the slab tends to infinity it is found that the results will be in good agreement with the previous results. It is found that the loss along the versor of the anisotropy axis increases as λ‖ increases, moreover it is greater than the power loss along the crystal symmetry direction a. In the isotropic case a satisfactory agreement with previous results is achieved.
TL and IRSL Emission Spectra of Detrital Feldspars. New Experimental Data
Authors: Rendell H. M. , Townsend P. D. , Wood R. A.
Published: July 2007

Measurements of thermoluminescence (TL) and infrared stimulated luminescence (IRSL) emission spectra are presented for natural samples of potassium‐rich detrital feldspars separated from middle‐late Pleistocene fluvial sediments from central Spain. Although IRSL‐based sediment dating techniques have developed rapidly since late 1980's, the understanding of the underlying physics, and of the relationship between TL and IRSL, remains rudimentary. The natural IRSL spectra for all three detrital feldspar samples show three emission bands at 335,400, and 550 nm, in contrast, the natural TL spectra exhibit not only these emission bands but also an additional one at 290nm. The effects of both preheating and IR stimulation on the TL emission spectra are investigated.
Elastic Interaction between Nanocrystals and a Crystalline Matrix: CuCl in NaCl
Authors: Weber H.‐J. , Lüghausen K. , Haselhoff M. , Siegert H.
Published: July 2007

It is demonstrated that doping of NaCl with CuCl increases the elastic constants of NaCl and modifies their dependence on temperature. The changes are significantly larger than estimated for a statistical distribution of the doping material. The crystallinity of embedded CuCl is considered to be responsible for additional elastic effects. This conclusion is verified by studying different kinds of annealing effects. The elastic interaction between guest and host crystals is interpreted as a relaxation of the NaCl lattice which closes the spatial gap caused by the lattice misfit. This results in a spontaneous strain which modifies the elastic constants due to the contribution of nonlinear terms.
Tunneling through Ultrasmall NIS Junctions in Terms of Andreev Reflection. A Nonlinear Response Approach
Authors: Hädicke A. , Krech W.
Published: July 2007

The Andreev current through an ultrasmall NIS junction is calculated in a systematic way by means of a nonlinear response approach basing on the elementary Hamiltonian of quasiparticle tunneling. The voltage dependence of current and differential conductance as well as the Andreev conductance are derived for low‐ and high‐impedance environments, respectively.
Hole Transport in Di‐p‐Tolyl‐p‐Nitrophenylamine Doped Poly(styrene)
Authors: Borsenberger P. M. , Gruenbaum W. T. , Kaeding J. E. , Magin E. H.
Published: July 2007

Di‐p‐tolyl‐p‐nitrophenylamine (DTNA) is a highly polar donor molecule with a dipole moment of 5.78 D. Hole mobilities have been measured in DTNA doped poly(styrene) over a range of fields, temperatures, and DTNA concentrations. The results are described within the framework of a formalism based on disorder, due to Bässler and coworkers. In the disorder formalism, it is assumed that charge propagation occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. A key parameter of the formalism is the energy width of the hopping site manifold. For DTNA doped poly(styrene), the width is approximately 0.15 eV and independent of the intersite distance, or DTNA concentration. The width is described by a model based on dipolar disorder. The model is premised on the assumption that the width is determined by a dipolar component due to the DTNA molecule and a van der Waals component. The interpretation of the experimental results leads to the conclusion that the van der Waals component increases with increasing intersite distance.
Comparative EPR Studies of Dynamics and Exchange Coupling in Ammonium and Potassium Copper (II) Tutton Salts
Authors: Marciniak M. , Hoffmann S. K. , Augustyniak M. A. , Hilczer W.
Published: July 2007

Single crystal EPR studies on (NH4)2Cu(SO4)2 · 6 H2O and K2Cu(SO4)2 · 6 H2O in the temperature range 4.2 to 360 K show that the vibronic g‐factor averaging is completed in the ammonium salt at 340 K but not in the potassium salt. The vibronic effects are discussed in terms of a thermally activated two‐level model which shows that the barrier between two wells of the adiabatic potential surface increases at low temperatures and tends to zero at high temperatures. Exchange coupling effects are different in the ammonium and the potassium salts. The coupling between inequivalent Cu sites appears below 100 K in the ammonium salt and grows up to J = 0.0023 cm−1 at 4.2 K. This coupling is not observed in potassium salt where, moreover, the coupling between equivalent sites is very small allowing the hyperfine structure to be more resolved at low temperature. A giant line broadening and line shift are observed below 30 K in the potassium salt. The above effects are discussed in relation to the possible superexchange pathways and differences in the space distribution in the electron density of the ground state of Cu(II) in both crystals.
Peculiarities of the Piezooptic Effect in Incommensurate Crystals
Authors: Sveleba S. A. , Polovinko I. I. , Bublyk M. I. , Kapustianik V. B. , Krochuk A. S.
Published: July 2007

The influence of uniaxial mechanical stress on the modulated structure which consists of a superstructure sequence is investigated in [N(CH3)4]2MeCl4 (Me=Cu, Fe, Zn) crystals. The peculiarities of the effective piezooptic coefficients π within the incommensurate phase of these crystals are studied by Senarmont's method. The influence of uniaxial mechanical stress disturbs the conditions of modulated structure interaction with defects what leads to the appearance of the widening of metastable states and to anomalies in the physical characteristics. It causes the change of the region of the localization of the incommensurability wave vector k on the commensurate values of higher order.
Study of Excitonic Molecules in Monoclinic Zinc Diphosphide Crystals
Authors: Gorban I. S. , Biliy M. M. , Dmitruk I. M. , Yeshchenko O. A.
Published: July 2007

The photoluminescence (PL) spectra of the monoclinic modification of the semiconductor crystal ZnP2 (β‐ZnP2) are studied under high, intermediate, and low laser excitation intensities at liquid helium temperature. Under high excitation intensities the inverse hydrogenlike M‐series was observed. It is symmetrical with the hydrogen‐like excitonic series. This M‐series is due to the two‐electron transitions from the excitonic molecule (biexciton) state to the ground, excited, and ionized excitonic states. The binding energy of the biexciton in β‐ZnP2 is determined as Δbex = 14.9 meV (0.32 of exciton Rydberg). Under intermediate excitation intensities another G‐band appears in the PL spectrum of β‐ZnP2. This band is interpreted as a result of two‐photon annihilation of free excitonic molecules in the gas phase. A narrow C‐line appears in the PL spectrum under intermediate excitation intensities, too. The analysis of the temperature dependence of the C‐line intensity allows us to assume this line to be the result of two‐photon annihilation of the biexcitons condensed into a dielectric liquid with the binding energy ΔL ≈︁ 0.5 meV.
Electronic Structures of Tetragonal Sixfold‐Coordinated Iron (III) Complexes with High‐Spin—Intermediate‐Spin Transitions
Authors: Kuang Xiao‐Yu , Morgenstern‐Badarau Irène
Published: July 2007

A 102 × 102 matrix for describing the d5 configuration ions within the tetragonal ligand‐field is constituted taking into account the electron‐electron repulsion, ligand‐field, spin—orbit coupling, and Zeeman interactions in the weak‐field scheme. Two different high‐spin—intermediate‐spin transitions, which are, respectively, associated with stronger and weaker axial ligand fields of tetragonal sixfold‐coordinated iron(III) complexes, are investigated. The diagrams of the energy level positions, magnetic moments, and effective EPR g‐factors are derived and expressed as functions of the parameter Δ/ζ, Δ being the energy gap between the high‐spin state (6A1) and intermediate‐spin states (4A2 or 4E) and ζ being the spin—orbit coupling coefficient.
Exciton and Electron Traps on Structural Defects in Molecular Crystals with Dipolar Molecules
Authors: Izvekov S. V. , Sugakov V. I.
Published: July 2007

The depths of exciton and electron traps arising due to the presence of structural defects in organic crystals consisting of dipolar molecules are calculated. In the model studied a structural defect appears if a molecule is rotated by 180° around the long axis as compared to the bulk molecules. The contribution of the electrostatic interaction to the depth of the trap state on such a defect is studied in detail. For crystals consisting of molecules which have a large dipole moment in the ground state and adopting large values in the excited state this contribution may be dominant. The calculations are carried out for triplet excitons and electrons in benzophenone crystal.
Exchange Integrals in the Helimagnetic Phase of ZnCr2xAl2−2xS4 (0.85 ≦ x ≦ 1)
Authors: Hamedoun M. , Rachadi A. , Hourmatallah A. , El Allam D. , Benyoussef A.
Published: July 2007

The spinel solid solutions ZnCr2xAl2−2xS4 are studied by neutron diffraction experiments. They show a helimagnetic phase in the concentration range 0.85 ≦ x ≦ 1. The helix angle decreases as x is decreased. The three first exchange integrals and their evolution with x are obtained. The disappearance of the long‐range order is shown to be due to the large decrease of the ferromagnetic in‐plane interaction. The results are extended to obtain the fourth exchange integral.
Optical Properties of Titanium‐Doped Sb2Te3Single Crystals
Authors: Drašar Č. , Lošťák P. , Navrátil J. , Černohorský T. , Mach V.
Published: July 2007

The reflectance and the transmittance spectra in the IR region are measured on Ti‐doped Sb2Te3single crystal samples grown by a modified Bridgman technique. The plasma resonance frequency, the optical relaxation time, and the high‐frequency dielectric constant are determined by fitting the Drude‐Zener formulas to the reflectance spectra. It is found that Ti impurities in Sb2Te3behave as donors. The transmittance spectra are used for the determination of the dependence of the absorption coefficientKon the energy of incident photons. The optical width of the energy gap is found to decrease with increasing Ti content. The values of the exponent α from the relation ofK∼ λαfor the long‐wavelength absorption edge range within the interval of 1.8 to 2.2, i.e. the dominant scattering mechanism of free current carriers in Sb2Te3crystals doped with Ti atoms is the scattering by acoustic phonons.
Mechanical Relaxation of Linear Viscoelastic Materials Described by the Modified Anelastic Element
Authors: Povolo F. , Hermida Élida B.
Published: July 2007

The stress relaxation and creep in amorphous materials, the dielectric relaxation in conducting polymers, the spin relaxation in spin‐glasses, are examples of processes described by the mathematical formalism of the theory of linear viscoelasticity. This description, given by a spectrum or distribution function, allows to express the temporal evolution of the relaxation through integral transformations. In many cases, however, this evolution is given directly using the empirical expression exp [—;(t/τ)γ], known as fractional exponential behaviour, where τ is a characteristic relaxation time and γ is a constant (0 < γ ≦ 1). It is shown that this empirical expression can be derived from the modified anelastic element (MAE) whose relaxation time depends on the time of the quasistatic test. From this dependence the spectrum for the MAE is calculated and correlated with the log—normal distribution. A novel procedure to calculate the parameters of the MAE is presented and applied to stress relaxation curves.
Polarized XANES Studies of Cu L3 Edges of Bi‐2212 Superconducting Single Crystals Using Total and Fluorescence Yield Techniques
Authors: Venkatesh S. , Srivastava P. , Saini N. L. , Garg K. B. , Studer F. , Ruyter A. , Menovsky A. , Ohkubo H. , Akinaga M.
Published: July 2007

XANES with polarized synchrotron light is now established to be one of the important techniques for the study of the electronic structure of high‐Tc superconductors in order to find out the symmetry of holes and their implication to the theoretical models proposed to explain superconductivity in these cuprates. The XANES spectra are measured by two methods in the soft X‐ray region, namely, total yield (TY) and fluorescence yield (FY) methods, which give different results with respect to out‐of‐plane Cu 3d(z2) symmetric holes. The reason of this discrepancy is not quite clear. Polarized XANES measurements on Bi‐2212 single crystals at the Cu L3 edge by the TY and FY methods are reported and the results are discussed in detail.
Exchange Integrals and Magnetic Properties of Mixed Chromium Spinel Systems: Zn1−xAxCr2X4 (A = Cd, Hg and X  S, Se)
Authors: Hamedoun M. , Zerhouni A. , Rachadi A. , Slimani M. , Benyoussef A.
Published: July 2007

A lot of magnetic measurements are accomplished in order to improve the phase diagrams of the systems Zn1‐xCdxCr2S4 and Zn1‐xCdxCr2Se4 particularly in the region of intermediate concentrations with mixed structure. The exchange integrals for the first and second coordination spheres (J and K) are calculated in terms of dilution for the three systems Zn1‐xCdxCr2Se4, Zn1‐xCdxCr2Se4, and Zn1‐xHgxCr2Se4. The evolution of the structure and the magnetic properties with the substitution of diamagnetic ions at tetrahedral sites is the result of the competition between the ferromagnetic and antiferromagnetic interactions. Besides, the magnetic properties are compared with the phase diagrams (T, x) of these systems. Because of its complicated antiferromagnetic structure, the sulfur compound seems to have particular characteristics among the mixed spinel systems.
Nanostructure of the Surface and Interface and Mechanisms of Thermal Growth of Ultrathin Films of Si3N4 on Si(001)
Authors: Baumvol I. J. R. , Stedile F. C.
Published: July 2007

The chemical situation of the Si atoms at the surface of Si3N4 ultrathin films thermally grown onto Si(001) substrates 2 × 1 reconstructed in a NH3 atmosphere has been recently studied in detail by core‐level spectroscopy using synchrotron radiation, as well as the chemical situation of the Si atoms at the film/substrate interface. The Si atoms at the surface are assigned to be either bonded to N atoms that are located in the layer just below the surface and having one dangling bond, or terminated by NH2. The Si atoms at the interface present intermediate oxidation states. Also recently high resolution isotopic tracing studies of N and H showed that nitrogeneous species (most probably NHx radicals, x < 3) are mobile during thermal growth of silicon nitride films, while isotopic tracing of Si showed that this species appears to be immobile during growth. This newly built experimental scenario for the nanostructure of the surface and interface, and for the atomic transport processes taking place during nitride growth, led us to develop a model for the mechanism of thermal growth of Si3N4 ultra‐thin films on Si(100), that is capable to explain the self‐limited character which makes them extremely attractive for applications in the VLSI and ULSI technologies for microfabrication of silicon devices.
Metal Oxide Surfaces: Electronic Structure and Molecular Adsorption
Authors: Freund H.‐J.
Published: July 2007

The surface science of oxides has received considerably less attention than the study of metal surfaces. We review some of our work on structural characterization and electronic structure investigations on oxide surfaces which we prepare as thin oxide films on metallic substrates. This preparation method leads to certain experimental advantages concerning the application of electron spectroscopies on such systems. We discuss molecular adsorption on non‐polar and polar surfaces of clean oxides. The influence of metal deposition onto the oxide surfaces and on their properties with respect to adsorption is described. Light induced processes with full quantum state detection of desorbing molecules are addressed. The possibility of studying dynamic processes in adsorbates on oxides via electron spin resonance spectroscopy is evaluated.
Surface Modification by Metal Deposition
Authors: Oster K. , Schmidt M. , Nohlen M. , Radnik J. , Wandelt K.
Published: July 2007

The adsorption of metal films is a promising way to modify the physical and chemical properties of substrate surfaces. In the present work several aspects such as growth structure, electronic properties, surface alloy formation and adsorption behavior of such films is discussed on the basis of LEED, ARUPS, PAX, ΔΦ, and TDS results for the deposition of Cu on different Pt surfaces. In a first part results on the growth of multilayer films of Cu on Pt(111) obtained by dynamical work function measurements are presented. Intermediate coadsorption of oxygen on a previously deposited Cu film of 2 ML leads to oscillatory work function changes during further Cu deposition. This behavior is discussed in terms of a growth model which considers the influence of surface morphology via the Smoluchowski effect. On the reconstructed Pt(100) surface the deposition of Cu at 300 K leads to a lifting of the reconstruction in the Cu‐covered areas, while the Cu‐free part of the surface stays reconstructed. This is in agreement with results of Au/Pt(100) and Ag/Pt(100), but in the case of Cu/Pt(100) strain minimization cannot be the only reason for this restructuring so that additional electronic effects must be taken into account. For multilayer Cu deposited on Pt(100) the segregation of Pt leads to alloying and chemical ordering in the temperature range between 500 and 1000 K. Several phase transitions between disordered and ordered surface alloys can be found. Both, a Cu3Pt(100) and a “Cu5Pt3(100)” surface alloy are Cu terminated as detected by PAX, while after annealing to 1000 K the surface again consists only of Pt atoms in the reconstructed arrangement. As an example for modified chemical properties of metal films with respect to the bulk material adsorption experiments of molecular hydrogen on Cu/Pt(100) at 100 and 300 K are presented. In contrast to bulk Cu they show the capability to dissociate H2 for coverages of up to 10 ML Cu.
Study of Metal Coverage on Alumina from Group‐6 Metal Carbonyl Precursors
Authors: Baibich I. M. , Stedile F. C. , Baumvol I. J. R. , Dos Santos J. H. Z. , Santarosa V. E.
Published: July 2007

The metal loading on alumina surfaces from group‐6 metal (Cr, Mo, W) carbonyls and their derivatives is determined by Rutherford backscattering spectroscopy. The subcarbonyl species are generated by photochemical activation and monitored by Fourier transform‐infrared spectroscopy. No chemical adsorption takes place for the three systems studied. Nevertheless, in the presence of the ligand triphenylphosphine (PPh3) and the assistance of UV radiation, surface species chemically bonded to γ‐Al2O3 can be observed, achieving a maximum metal loading of 0.3% in the case of Cr and 0.4% in the case of W.

Die metallische Sättigung von Alumina‐Oberflächen mit Metall‐(Cr, Mo, W)‐Carbonylen der Gruppe 6 wird mit RBS bestimmt. Die subcarbonylen Arten werden mit UV‐Strahlung erzeugt. Für die drei Systeme findet keine chemische Adsorption statt. Mit Triphenylphosphin als Ligand werden an der Oberfläche jedoch chemisch gebundene Carbonylarten beobachtet. Die maximale Sättigung beträgt 0.3% für Cr und 0.4% für W.
Study of RF Sputtered aSi: H and aGe: H by Photothermal Deflection Spectroscopy
Authors: Rovira P. I. , Alvarez F.
Published: July 2007

The effect of hydrogenation in radio‐frequency (rf) sputtered aSi: H and aGe: H films is studied. Photothermal deflection spectroscopy is used to study the evolution of the sample quality on hydrogen incorporation. The influence of the substrate temperature, target bias, and hydrogen partial pressure on the sub‐gap absorption coefficient and on the hydrogen incorporated in the films is reported.
Influence of the Deposition Conditions on the Properties of Amorphous Germanium Nitrogen Alloys
Authors: Vilcarromero J. , Marques F. C.
Published: July 2007

A study of the band‐gap tailoring of amorphous germanium nitrogen alloys is presented. They have been prepared by the rf reactive sputtering technique, using a c‐Ge target in an Ar + NH3 atmosphere. The alloy composition is found to be strongly dependent on the deposition conditions. The optical gap of the material could be tailored from approximately 1.0 to about 3.0 eV by varying the rf power, substrate temperature, nitrogen and hydrogen partial pressures. The rf power, and not the gas composition, is found to be the most important parameter in determining the nitrogen concentration. The hydrogen concentration, on the other hand, has a small influence on the band gap, while the temperature plays an important role on the determination of the alloy structure.
Stress and Elastic Constants of Amorphous Germanium Nitrogen Alloys
Authors: Marques F. C. , Lacerda R. G. , Lima M. M. , Vilcarromero J.
Published: July 2007

Thermomechanical properties (stress, thermal expansion coefficient, and elastic modulus) of a‐GeNx: H thin films prepared by the rf sputtering technique are reported. Stress measurements are made using a Dektak profilometer to determine the radius of curvature of the film/substrate structure. The majority of the films prepared in this work have compressive stress. A second apparatus based on the deflection of a HeNe laser beam is used to determine the thermal expansion coefficient and the Young's modulus/(1‐Poisson's ratio) by the thermally induced bending method. They depend strongly on the nitrogen concentration.
Fibonacci oscillator
Authors: Chatterjee R.
Published: July 2007

It is shown that the eigenvalues of a q‐deformed harmonic oscillator are the Fibonacci number sequence for the q‐values to be the “golden mean”. The physical significance of this q‐parameter will be explored by correlating the various known potentials with the q‐deformed oscillator potentials.
Transport and optical properties of PbTe1−xSx (x = 0.02 and x = 0.05) mixed crystals
Authors: Miljković J. M. , Romčević N. , Popovic Z. V. , Köunig W. , Nikiforov V. N.
Published: July 2007

Far‐infrared and Raman scattering spectra as well as galvanomagnetic measurements of PbTe1−xSx (x = 0.02 and 0.05) mixed crystals in the temperature range from 10 to 300 K are presented. The far‐infrared reflectivity spectra are analysed using a fitting procedure based on the plasmon‐phonon interaction model. Galvanomagnetic and far‐infrared measurements indicate the existence of a phase transition at about Tc = 60 K for x = 0.05, whereas for samples with x = 0.02 the phase transition is not observed. Also, it is found that the long‐wavelength optical phonon modes of these mixed crystals show a “two‐mode” behaviour.
Surface states in finite CdMnTe/CdTe superlattices
Authors: Diouri J. , Afif K.
Published: July 2007

Analytic equations are derived, giving both the envelope functions and the energies of electronic states for finite CdMnTe/CdTe superlattices with different boundary conditions and variable number of CdTe wells and periods of the superlattice. Calculations are performed in the effective mass approximation and transfer‐matrix formalism. The effects of superlattice dimensions and boundary conditions on the surface states as well as the conditions for their existence are presented and discussed.

Des équations analytiques sont établies, donnant à la fois les fonctions enveloppes et les énergies des états électroniques dans les superréseaux finis CdMnTe/CdTe avec differentes conditions aux limites et pour un nombre variable des puits CdTe et de périodes du superréseau. Les calculs sont menés dans le cadre de I'approximation de la masse effective et du formalisme de matrice de transfert. Les effets des dimensions du superréseau et des conditions aux limites sur les états de surface aussi bien que leurs conditions d'existence sont présentés et discutés.
Inter‐Landau level tunneling in an InGaAs/AlAs/GaAs structure under tilted magnetic field
Authors: Gobato Y. Galvão , Berroir J. M. , Guldner Y. , Vinter B. , Nagle J. , Rivera P. H.
Published: July 2007

Magneto‐tunneling studies in a high‐quality InGaAs/AlAs/GaAs triple well structure in a tilted magnetic field are reported. Well‐defined structures in the 1(V) characteristic in the resonance regime are observed which correspond to tunneling from Landau levels in the prewell into the midwell with non‐conservation of the Landau‐level index tunneling transitions. These tunneling transitions with Δn both positive and negative are observed for the first time in the resonant regime under tilted magnetic field. Finally, the results are shown to be in fair agreement with a coherent magneto‐tunneling model where the effect of the transverse component of the magnetic field is treated as a perturbation.
Exchange disorder band bending of the Cd1−xFexS diluted magnetic semiconductor
Authors: Chou W. C. , Kuo S. S. , Chen F. R. , Twardowski A. , Tworzydlo J. , Chen Y. F.
Published: July 2007

The energy of excitons A, B, and C of Cd1−xFexS (x < 0.02) is studied as a function of Fe concentration at T = 10 K in the absence of a magnetic field. The observed band bending effect (largest for exciton A) is attributed to the s, p‐d exchange interaction correction resulting from the exchange disorder.
Oxygen behavior in NiO
Authors: Kamal M. , Debza Y. , Hugel J.
Published: July 2007

The behavior of the 2p functions is studied in NiO within the local spin density approximation (LSDA) based on an LCAO representation. The ionicity is related to the spatial extension of the 2p orbitals and increases when the oxygen orbitals spread out. The corrected LSDA + U version enhances both the ionicity and the spin moment. As a result the intrasite Coulomb interaction U shrinks the d orbitals and extends the 2p functions.
Ordering in GaInP2 studied by optical spectroscopy
Authors: Ernst P. , Geng C. , Scholz F. , Schweizer H.
Published: July 2007

The optical properties of spontaneously ordered GaInP2 grown by MOVPE are studied by means of photoluminescence, time‐resolved photoluminescence, and polarization dependent photoluminescence excitation spectroscopy. Ordered samples grown on a substrate which is misoriented 6° toward [111]B show clear excitonic features both in luminescence and absorption. In excitation spectra both valence band edges are clearly resolved and are shown to be polarization dependent as predicted by band structure calculations. In these samples an additional non‐excitonic low‐energy photoluminescence emission is observed the peak energy of which depends strongly on excitation density and temperature and which shows extremely long luminescence lifetimes. A density of states model for ordered GaInP2 is proposed which allows to explain the results obtained in the last few years. The density of states model is discussed with respect to the microstructure of the ordered alloy as known from TEM investigations.
Deep UV emissions of ionic Cs2+F− states in Ne and Ar matrices
Authors: Sliwinski G. , Bressler Ch. , Schwentner N.
Published: July 2007

Pulsed electron beam excitation of 0.45 to 0.7% CsF doped in rare gas matrices at 5 K leads to emission bands at 196.5 nm (FWHM 9.6 nm) and 227 nm (FWHM 15 nm) in Ne and at 211.2 nm in Ar. The short‐wavelength bands are assigned to B2 → X2− transitions in the ionic Cs2+F− excimer on the basis of transition energies, lineshapes, and the efficiency of Cs2+F− ionization by energy transfer from the host. The long‐wavelength band in Ne corresponds to a C2II‐A2II transition. The 196.5 nm band provides a large cross section for stimulated emission of 2.5 × 10−16 cm2. The red shifts of 0.4 and 0.8 eV of the B → X emission in Ne and Ar, respectively, are discussed in terms of solvation of the Cs2+F− ion and of its dipole moment. Optimal concentrations are similar to those obtained for the gas phase.
Temperature and excitation dependent photoluminescence of undoped and nitrogen‐doped ZnSe epilayers
Authors: Gurskii A. L. , Gavrilenko A. N. , Lutsenko E. V. , Yablonskii G. P. , Taudt W. , Hamadeh H. , Wachtendorf B. , Söllner J. , Schmoranzer J. , Heuken M.
Published: July 2007

The photoluminescence (PL) properties of ZnSe epitaxial layers grown by metal‐organic vapor phase epitaxy are investigated. A correlation between the electron‐hole plasma (EHP) band position and the PL intensity is found and on this basis the nonequilibrium carrier lifetime τ and the PL efficiency η are estimated in the ZnSe layers: τ = 10−9 to 10−11 s, η = 0.1 to 0.001%. It is shown that these parameters decrease with decreasing layer thickness and with increasing concentration of contaminations and native defects. The influence of the excitation level and temperature on the intensity and peak position of the band at 472 nm in ZnSe: N and the band at 620 nm in undoped ZnSe epilayers at 78 K grown at temperatures of TD > 500 °C are studied. It is supposed that these bands are due to recombinations described in the configurational coordinate model.
Ag‐substituted AlCuFe icosahedral phases
Authors: Manaila R. , Macovei D. , Jianu A. , Popescu R. , Nicula R. , Zavaliche F. , Devenyi A. , Xie Y. , Hu T.
Published: July 2007

The gradual substitution of Cu by Ag in the Al62Cu25.5−xAgxFe12.5 icosahedral phase (x ≤ 10) is investigated by X‐ray diffraction from the viewpoint of the induced structural changes. The phonon and random phason disorders, estimated from diffraction linewidths, increase with x, the latter showing, however, an induction range of Ag concentrations. EXAFS results at the CuK and FeK edges show, for low Ag contents, and enhanced disorder in the first coordination sphere, partly relaxed for higher x values. The 6D hypercubic lattice parameter is found to be independent of the Ag content, despite the large diameter of monovalent Ag. The latter facts suggest a d‐sp hybridization of the Ag bonds to the surrounding Al, taking place above a certain x value.
Molecular motions and phase transitions in solid tris(n‐propylammonium) hexabromobismuthate (III)
Authors: Pislewski N. , Tritt‐Goc J. , Jakubas R.
Published: July 2007

The proton NMR second moment and spin‐lattice relaxation time for polycrystalline tris(n‐propylammonium) hexabromobismuthate (n‐C3H7NH3)3BiBr6 (nPBB) are measured from 70 to 290 K. The results are interpreted in terms of the methyl and ammonium group reorientations for which the activation parameters are determined. In addition linear thermal expansion measurements are carried out in the temperature range 270 to 295K. A distinct anomaly in the thermal dilation occurs at 281 K indicating a first‐order phase transition.
Lattice dynamics of bromotrifluoromethane
Authors: Zeng W. Y. , Anderson A.
Published: July 2007

The vibrations of crystalline bromotrifluoromethane at low temperatures are represented by a simple force constant model. Sixteen adjustable parameters (ten intermolecular and six intramolecular) are used to calculate the frequencies of twenty‐one lattice modes and thirty‐six components of internal modes, and good agreement with recent spectroscopic results is obtained. Principal intermolecular interactions, primarily between various pairs of halogen atoms, are identified and appear to be of the standard van der Waals type. As a result, intramolecular force constants are only slightly changed from their gas phase values.
The amorphous transverse spin‐3/2 ising model
Authors: El‐Atri A. , Ainane A. , Saber M.
Published: July 2007

The amorphization of the spin‐3/2 transverse Ising model on a cubic lattice (N = 6) is studied within the effective field theory. The total longitudinal and transverse moments are obtained. A number of interesting phenomena is found in these quantities, due to the fluctuation of exchange interactions and the applied transverse field.
MBE growth of cubic GaN on GaAs substrates
Authors: Yang H. , Brandt O. , Ploog K.
Published: July 2007

We review our recent results on plasma‐assisted molecular beam epitaxy of cubic GaN films on GaAs(001). The crucial role of the nucleation stage for obtaining epitaxial growth is highlighted by reflection high‐energy electron diffraction and transmission electron microscopy. For the subsequent growth, it is found that the surface stoichiometry determines the phase composition of the epilayers as determined by X‐ray reciprocal space mapping, Raman and cathodoluminescence spectroscopy. For an in situ control of the surface stoichiometry, we develop a method based on the transitions of the surface reconstructions of cubic (001)GaN. Films grown under conditions optimized by this method are single‐phase cubic and exhibit intense excitonic luminescence up to room temperature.
XRD investigations of WSiN and LaB6 layers on GaAs
Authors: Nebauer E.
Published: July 2007

The microstructures of WSiN and LaB6 thin layers (amorphous or crystalline) on GaAs substrates are studied in dependence on the deposition and annealing treatments by means of the TFXRD parallel‐beam technique. From the I(2θ) diffractograms the recrystallization behaviour of the sputtered a‐WSiN and a‐LaB6 layers can be especially investigated. These results are of relevance for the choice of suitable materials for diffusion barriers in contact systems.
Optical interband properties of some semiconducting silicides
Authors: Lange H. , Henrion W. , Fenske F. , Zettler Th. , Schumann J. , Teichert S.
Published: July 2007

Optical transmission and reflection as well as ellipsometric studies are performed for the semiconducting phases CrSi2, MnSix, and Ir3Si5 in the energy region from 0.2 to 5.0 eV. Onsets of strong interband transitions are found around 0.5 eV for the former two compounds and 1.2 eV for the latter one. Optical data analysis for the former two compounds suggests them to be indirect semiconductors whereas for the latter to be a direct one. Main interband features for CrSi2 and MnSi1.73 appear between 1 and 2 eV and for Ir3Si5 at 3 eV. An explanation of these findings is given in terms of existing band structure and density of states considerations.
Optical anisotropy in superlattices as a consequence of dielectric mismatch
Authors: Jahne E.
Published: July 2007

The “bulk‐slab” continuum model for the dielectric function of a superlattice is reconsidered. It is demonstrated how the optical anisotropy of the effective medium depends on the dielectric mismatch between the layers. As examples the dielectric mismatch and layering‐induced anisotropy are studied for both phonon and electronic interband excitations in GaAs/AlAs and GaSb/InAs superlattices. It turns out that the dielectric mismatch and hence the anisotropy for phonon excitations is large in comparison to that for electronic interband transitions. Angular dependencies of longwavelength optical phonons for long period superlattices are derived from the angular dependence of the dielectric function of the extraordinary ray in the effective medium.
Extended novel slab model analysis of overlaid bent step‐index single‐mode fiber half‐couplers. I. Basic formalism and normal‐mode approach
Authors: Haubenreisser W. , Ecke W.
Published: July 2007

An extended novel slab model (ENSM) in strong guidance approximation (SGA) is presented for the modal analysis of overlaid bent step‐index single‐mode fiber half‐couplers (OL‐BSSF) where now proper (bound) and leaky modes are included as well. Within the frame of ENSM and SGA a basic formalism is elaborated to evaluate the propagation characteristics as well as the dissipative and/or leakage propagation losses of OL‐BSSF structures in a great measure analytically. The powerfulness of the formalism is demonstrated on a six‐layered OL‐BSSF/ESW basic structure with a general three‐layered overlay (OL) where the BSSF structure is mapped modally by an equivalent slab waveguide (ESW) structure. Such a multilayered slab basic structure can describe the modal and optical features of all hitherto implemented configurations of all‐fiber in‐line components as special cases straightforwardly based on OL‐BSSF structures.
Covalent mechanism of cooperative optical transitions in lanthanide exchange‐coupled pairs. Intensity calculations for double transitions in a M2L11 f1–f1 dimer
Authors: Mironov V. S. , Kaminskii A. A.
Published: July 2007

The covalent mechanism of cooperative optical transitions in exchange‐coupled pairs of lanthanide ions (which is closely related to the magnetic superexchange mechanism) is analyzed in detail for a simple M2L11 f1–f1 model dimer (two ML6 octahedra sharing one bridging ligand L). New effective electron transfer pathways in the M(A)–L–M(B) exchange system are considered and intensities of cooperative transitions from the (00) ground state to (ij) doubly‐excited states of the f1–f1 dimer are calculated including polarization effects.
Efficient 2 μm stimulated emission in the 4F9/2 → 4I11/2 channel from monoclinic BaYb2F8: Er3+ crystals
Authors: Kaminskii A. A. , Butashin A. V. , Mironov V. S. , Bagaev S. N. , Eichler H.‐J.
Published: July 2007

New spectroscopic and laser data on monoclinic Ba(Yb1−xErx)2F8 crystals are obtained. Efficient 2 μm stimulated emission of Er3+ ions in the 4F9/2 → 4I11/2 channel and 2 and 3 μm cascade lasing of the sequential 4F9/2 → 4I11/2 → 4I13/2 transitions were excited at room temperature under Xe flashlamp pumping. Intensity characteristics of manifold‐to‐manifold (JJ′ transitions) such as Einstein coefficients AJJ, line strengths sJJ′, branching ratios βJJ′, and radiative lifetimes τrad for the initial laser states 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, and 2H(2)9/2 were analyzed in detail. All observed laser transitions between individual Stark levels were identified. Unlike well‐known 2 μm lasers with Ho3+, Tm3+, Co2+, and Ni2+ ions in many oxide and fluoride compounds, BaYb2F8: Er3+ crystals operate on a pure four‐level generating scheme.
Field domains in semiconductor superlattices
Authors: Schwarz G. , Schöll E.
Published: July 2007

Field domain formation in the regime of vertical high‐field transport in semiconductor superlattices is studied theoretically. Depending upon the mean doping density ND, either stable stationary domains (for high ND), or self‐sustained oscillations of the domains (for intermediate ND) are found. Our simulations show that this behaviour is strongly affected by growth‐related imperfections like small fluctuations of the doping density, and that weak disorder on microscopic scales can be quantitatively detected by the global macroscopic current‐voltage characteristics. The role of the different realizations of the microscopic disorder and different boundary conditions are discussed. Equal areas rules for domain formation are derived under simplifying approximations.
Transport properties of high‐Tc mercury cuprates
Authors: Onbasli Ü. , Wang Y. T. , Naziripour A. , Tello R. , Kiehl W. , Hermann A. M.
Published: July 2007

Measurements of dc and ac susceptibility, resistivity, Hall effect, and thermoelectric power (TEP) on pure phase Hg‐1223 and mixed phase Hg‐1212–1223 cuprates are reported. The mixed phase compounds show the highest critical temperature (magnetic susceptibility onset of 138 K). Both Hall effect and TEP data support an underdoped state with hole‐like conduction. Oxygen annealing reduces the Hall coefficient (increasing the concentration of holes) and lowers the TEP (increasing the Fermi energy) features which can be explained by a simple Fermi‐gas picture. The Hall coefficient which increases linearly with temperature and the negative temperature coefficient of the TEP, however, cannot be explained by single‐band Fermi‐gas or ‐liquid theory, unless one introduces energy dependent carrier scattering. Estimates of the highest critical temperature based on the universality observed in the thermopower data for the cuprates suggest that 138 K is near the maximum one might expect for the Hg cuprates under optimal doping at atmospheric pressure.