bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrasonic Attenuation in Semiconductor Quantum Heterostructures under a High Magnetic Field
Authors: Rodrigues C. , Fonseca A. L. A. , Nunes O. A. C.
Published: July 2007

A theory of ultrasonic attenuation in a quasi‐two‐dimensional electron gas in a quantum well semiconductor heterostructure in the presence of a quantizing magnetic field is presented. The effect of the high magnetic field on the ultrasonic attenuation coefficient is studied in the extreme quantum limit, when all the electrons are accomodated in the lowest Landau level and electro‐subband. Numerical examples are shown and discussed.
Nonradiative Capture of Electrons in Quantum Wells and Wires by Shallow Donor Impurities
Authors: Sirenko Y. M. , Mitin V.
Published: July 2007

The capture of electrons in a quantum well or wire by shallow donor impurities due to the interaction with an acoustic phonon is considered. Wave functions of several lowest bound states are found by the variational method. The rates of transitions between the free and bound states as well as between different bound states are calculated. In the case of well or wire width L less than or of the order of the effective Bohr radius and greater than the phonon wavelength, the transition rates are proportional to ϵ−5 (nϵ + 1/2 ± 1/2) L−6, where ϵ is the transferred energy, and nϵ a phonon occupation number.
Oxygen Content, Crystal Structure, and Superconductivity in YSr2Cu2.75Mo0.25O7+δ
Authors: Cao Guanghan , Qian Yitai , Wei Qingqiao , Chen Zuyao , Lu Bin , Mao Zhiqiang , Shi Lei , Zhang Yuheng
Published: July 2007

Polycrystalline samples of YSr2Cu2.75Mo0.25O7+δ with different oxygen contents are investigated by powder X‐ray diffraction and selected area electron diffraction. The temperature dependence of resistivity and the oxygen content are determined. The air‐sintered sample is tetragonal and semiconductive, while the O2‐sintered sample is orthorhombic and superconducting at 26 K. It is suggested that the structural variation originates from the (Cu(1), Mo)O1+δ layers and the superconductivity is induced by the excess oxygen rather than charge redistribution between Cu(1) and Cu(2).
Tranverse Scattering Resonances in Quantum Wires
Authors: Wilkinson M. , Austin E. J.
Published: July 2007

A theory for the effect of scattering by impurities in a quantum wire or electron waveguide is described. Scattering is strongly influenced by resonances which occur when a new mode of propagation appears as the Fermi energy is increased. These resonances can either increase or decrease the probability of backscattering by many orders of magnitude.
High Field Positive Magnetoresistance in the Variable Range Hopping Regime in n‐Type CuInSe2
Authors: Essaleh L. , Wasim S. M. , Galibert J. , Leotin J.
Published: July 2007

Electrical conduction by the variable range hopping mechanism of Mott type is observed in n‐type CuInSe2 over a wide temperature range up to about 50 K. A detailed study of the field dependence up to 15 T of the localization length ξ and T0 is made at different temperatures. The increase of ξ with B at lower values is consistent with the magnetic field induced delocalization effect whereas at higher values it decreases due to the shrinkage of the wave functions of the impurity states. The variation of experimental T0 with B shows a similar trend as predicted by the theory but its magnitude is much smaller. This is attributed to the increase in the dielectric constant due to correlation effects not taken into account in the theoretical expression for the density of states.
The Role of an Applied Transverse Field in a Diluted Mixed Spin Ising System
Authors: Kaneyoshi T.
Published: July 2007

The phase diagrams and magnetization curves of a diluted mixed spin‐1/2 and spin‐1 Ising system on honeycomb lattice in a transverse field Ω are investigated by the use of the effective‐field theory with correlations. The influence of Ω on the magnetic properties is discussed and some interesting phenomena are found in them, such as the transverse field induced magnetic ordering in the system with a concentration at or very near the critical concentration where the system with Ω = 0 becomes paramagnetic.
Phase Diagram and Magnetization Moments of the Spin‐5/2 Diluted Ising Ferromagnet in a Transverse Field
Authors: Saber M. , Tucker J. W.
Published: July 2007

The phase diagram and the magnetization moments of the spin‐5/2 diluted Ising ferromagnet in a transverse field is studied by an effective field method within a single‐site cluster theory. The effective field equations are derived using a probability distribution obtained with the aid of generalized van der Waerden identities that account for the single‐site spin kinematic relations. Correlations between the site disorder and the local configurational‐dependent thermal averages of the spin variable are included. The dependence of the transition temperature for longitudinal ordering is studied both as a function of the transverse field strength and the site dilution. Selected results for the thermal dependence of the longitudinal and transverse components of the magnetization and their higher moments are also presented.
The Coherent Potential Approximation in the Description of Spin Wave Resonance
Authors: Urbaniak‐Kucharczyk A.
Published: July 2007

The coherent potential approximation method is applied to spin wave resonance in thin ferromagnetic films. The approach takes into account a natural disorder connected with different states of spin components. The resonance spectra with a finite line‐width are calculated for the b.c.c. (100) film and S = 1/2. The results obtained show that the half‐widths of the surface as well as the volume peaks depend on the surface anisotropy parameter.
Excitonic Transitions and Band Offset Calculations in a Short Period ZnS‐ZnSe Superlattice
Authors: Abounadi A. , Rajira A. , Averous M. , Calas J.
Published: July 2007

An investigation of the photoreflectance and photoluminescence spectra of a 45 period ZnS‐ZnSe strained layer superlattice having well and barrier widths of 3.3 and 3.7 nm, respectively, is reported. Heavy hole (e1hh1) and light hole (e1lh1) excitons are well resolved in the 2 K photoreflectance spectrum. The energies of these transitions are calculated within the envelope function framework, generalized to strained layers. The valence and conduction band offsets are the principal parameters in this calculation. Thus, two different approaches are set in competition to determine these quantities. The first is based on theoretical models while the second uses a “semi‐experimental” method. The latter method gives a free‐standing type‐I superlattice with optimal agreement between photoreflectance measurements and theory. Also the importance of a precise knowledge of the exciton binding energy (enhanced because of the reduced dimensionality) for an accurate band offset determination is shown. This binding energy is estimated for the heavy hole exciton from a variational calculation.
Cd+(5s1) Centres in Rb2ZnCl4 Single Crystals
Authors: Nistor S. V. , Bouwen A. , Schoemaker D.
Published: July 2007

Electron paramagnetic resonance and optical absorption spectra attributed to 5s1 trapped electron Cd+(I) and Cd+(II) centres are observed over a broad temperature range in Rb2ZnCl4 single crystals doped with cadmium, after X‐ray irradiation at 80 K and room temperature, respectively. The low temperature Cd+(I) centre, which seems to be a substitutional Cd+ ion in a Zn2+ site, transforms into a Cd+(II) centre at 190 K by trapping an anion vacancy, which has become mobile at this temperature, in its nearest‐neighbour positions. The suggested structural models of the Cd+ centres are supported by the analysis of the spectral data considering covalency effects.
Equations of State and Pressure Dependence of Bulk Modulus for NaCl Crystals
Authors: Thomas L. M. , Shanker J.
Published: July 2007

Various equations of state reported in the literature are used to investigate the variation of the bulk modulus for NaCl crystals under the effect of pressure up to 300 × 108 Pa. The results are compared with the corresponding values obtained in the present study using interionic potentials as well as with the available experimental data. Values of bulk modulus as a function of pressure are also calculated by taking into account the variation of the Anderson parameter with pressure. The results thus obtained are found to present best agreement with those estimated from the universal equation of state.
X‐Ray Study of the System BP1−xBPIx
Authors: Santos M. Lopes Dos , Kiat J. M. , Almeida A. , Chaves M. R. , Klöpperpieper A. , Albers J.
Published: July 2007

Accurate X‐ray diffraction measurements are carried out as function of temperature in antiferroelectric betaine phosphate (BP), ferroelectric betaine phosphite (BPI), and mixed compounds of BP1−xBPIx. In agreement with previous studies, the phases which derive from a high‐temperature phase with P21/c symmetry are clearly detected. However, in contrast with previous assignments, for 0.90 < x ≦ 1.0 a transition occurs to the P21 ferroelectric phase. For 0 ≦ x < 0.20, the two successive transitions are to a P21, phase and subsequently to a Pc phase with a simultaneous doubling of the unit cell. For 0.9 ≦ x < 1.0 the results confirm previous assignments of a single transition to a P21 phase. In the intermediate region 0.20 < x < 0.90 a clear evidence of an isomorphous transition to a glass phase is obtained, where the relative dominance of the ferroelectric or antiferroelectric interaction depends on the concentration values. Furthermore an orientational glass phase near x = 0.80 is identified. The existence of a strong anisotropy in all the studied elements of the family and of a weak coupling between the ordering of the dipoles and the lattice is evidenced. The role of the absent oxygen in the behaviour of the lattice parameters as well as its subsequent influence on the competitive interactions between dipoles are discussed.
Energy Transfer from Gd3+ to Er3+ in Y0.34Gd0.65Er0.01F3
Authors: Collins J. M. , Dibartolo B.
Published: July 2007

The energy transfer from the 6P7/2 level of Gd3+ to the 2P3/2 level of Er3+ is studied in the phosphor Y0.34Gd0.65Er0.01F3. The lifetime of the 6P7/2 level of Gd3+ decreases substantially in the presence of Er, indicating that the transfer is nonradiative. The transfer is studied in the temperature range from 20 to 300 K, and is found to be more efficient at low temperatures. A model is proposed to explain this behavior, in which we consider the distribution of the ions in the Stark levels involved in the transfer, and the presence of one‐ and two‐phonon processes.
Dynamics of Carrier Hopping in Exponential Band Tails of Quasi‐One‐Dimensional Systems in Electric Field
Authors: Singh M. , Tarutani Y. , Takagi K.
Published: July 2007

The dynamics of carrier hopping in band tails of a quasi‐one‐dimensional (QOD) system is studied in the presence of temperature and electic field. A method is used in which it is assumed that localized states are distributed randomly in both space and energy coordinates and the hopping of carriers occurs in both coordinates. The exponential form of the density of states for band tails is considered. The hopping rate and the time dependent demarcation energy are calculated in the activated and nonactivated relaxation regimes. The hopping rate varies with the electric field as (1  β2)−1/γ and exp [(1  β2)] for the AR and the NAR regions, respectively. Here β and γ are directly proportional to the electric field and the temperature, respectively. Numerical calculations are performed for the hopping rate and the demarcation energy as a function of β and γ for both regions.
Oscillation Phenomena on the Magnetoresistance of the Organic Superconductor K‐(BEDT‐TTF)2I3
Authors: Helm P. , Kraak W. , Schweitzer D. , Keller H. J.
Published: July 2007

High‐field magnetotransport properties of single crystal samples of the quasi‐two‐dimensional organic compound K‐(BEDT‐TTF)2I3 are investigated at low temperatures in magnetic fields up to 12 T. Pronounced quantum oscillations of the magnetoresistance (Shubnikov‐de Haas effect) are observed in magnetic fields above 8 T. In the magnetic field orientation perpendicular to the high conducting b−c plane four different SdH oscillations with fundamental fields 570, 2710, 3280, and 3850 T are detected. The corresponding extremal areas of the cross sections of the Fermi surface are found to be in good agreement with theoretical band structure calculations. For the first time “giant” angular oscillations of the magnetoresistivity are observed in K‐(BEDT‐TTF)2I3. These oscillations are periodic in tan θ in agreement with the quasi‐classical explanation by Yamaji. From the experimental data the electronic parameter π/akF ≈ 0.58 can be estimated.
Longitudinal and Transverse Random‐Field Ising Model with Arbitrary Spin (S)
Authors: Wang Yong‐Qiang , Li Zhen‐Ya
Published: July 2007

The Ising model with transverse and longitudinal random fields, which have trimodal and bimodal distribution, respectively, for arbitrary spin (S), is investigated by combining the pair approximation with the discretized path integral representation (DPIR). The phase diagrams for S = 1, 1.5, and 2 are obtained, where some fascinating phenomena are observed.
Early Stages of Pore Formation on Si During Etching in HF Solutions
Authors: Zur Mühlen E. , Niehus H.
Published: July 2007

The fabrication of porous silicon (PS) with controlled pore size distributions is of importance for reproducible manufacturing as well as it may help to elucidate the origin of light emission from this material. The pore formation on p‐doped Si(111) surfaces is studied. For this purpose hydrogen passivated surfaces are prepared by wet chemical treatment. After introduction into UHV system a 1 × 1 LEED pattern is readily obtained without any further treatment. These surfaces are etched at the open circuit potential (OCP) in HF solutions with pH‐values ranging from 1 to 10. Atomic force microscopy is used to characterize the dependence of pore formation on time and acidity. It is found that narrow pore size distributions can be obtained when HF solutions with high pH‐values are used.
Radiative and Nonradiative Recombination in Porous Silicon. What Can We Learn from Spin Resonance?
Authors: Stutzmann M. , Brandt M. S.
Published: July 2007

Recent results of spin‐resonance investigations of porous silicon are presented. Special attention is given to nonradiative and radiative recombination processes as detected by spin‐dependent recombination (spin‐dependent photoconductivity, SDPC, and optically detected magnetic resonance, ODMR). Pb‐like defects are identified as the main recombination centers which quench both photoconductivity and visible photoluminescence in porous Si. Strongly localized triplet excitons are found as the initial states of visible luminescence.
Efficient Visible‐Light Emission from Si/CaF2 (111) Heterostructures Grown by Molecular Beam Epitaxy
Authors: Vervoort L. , Bassani F. , Mihalcescu I. , Vial J. C. , d'Avitaya F. Arnaud
Published: July 2007

The elaboration by molecular beam epitaxy of a new luminescing Si‐based material is presented, i.e. small grain polycrystalline Si/CaF2 multi‐quantum wells. The static luminescence properties are studied as a function of Si layer thickness, and the time dependent luminescence features as a function of wavelength and temperature. There is a rather striking similarity with the photoluminescence of porous Si, both in spectral position as in decay dynamics. The disappearance of the luminescence when the Si layers become too thick, and the blue‐shift of the spectra when the Si layer thickness decreases, favour the quantum confinement hypothesis for the origin of the light emission. The important role of oxygen as passivating agent after ageing in air, is also pointed out.
New Hydrogen‐Localized Vibrational Modes in Proton‐Implanted GaAs Activated by γ‐Ray Irradiation
Authors: Wang L. P. , Zhu W. X. , Qin G. G.
Published: July 2007

Two new hydrogen‐localized vibrational modes (LVMs) in proton‐implanted GaAs activated by γ‐ray irradiation are observed at 1726 and 1688 cm−1 (295 K), and at 1771 and 1702 cm−1 (4.2 K) in infrared absorption spectra, respectively. The integrated absorption intensities of both bands increase with increasing γ‐ray radiation dose. A tentative structural model for the new hydrogen LVMs is suggested.
Phonon‐Assisted Two‐Photon Absorption in the Reststrahl Region of Excitons in Alkali Halides
Authors: Fröhlich D. , Rübenacke St. , Schlutt Ch. , Stolz H.
Published: July 2007

The two‐photon spectrum of RbI is investigated in the energy region of the lowest IS exciton. A significant increase of the two‐photon signal is observed when crossing the transverse energy and scanning through the excitonic reststrahl region. This phenomenon is explained by a LO‐phononassisted absorption process based on the Fröhlich exciton‐phonon coupling. Evaluating the two‐photon excitation within a two‐band model allows a quantitative analysis, which shows good agreement with the experimental data. Besides a scaling factor, the only fit parameter is the exciton Bohr radius that is found in reasonable agreement with the Wannier exciton model.
On the Problem of the Existence of Davydov Solitons
Authors: Todorović D. , Ristovski Lj. , Tošić B. S.
Published: July 2007

Davydov's soliton model, which introduces autolocalised excitations in a soft molecular chain, is applied also for explaining the energy transport process in the α‐helix protein being considered as a system consisting of three coupled molecular chains (spines). However, there is an inconsistency in the consideration of the single chain case compared to the case of coupled chains (α‐helix). Namely, in the Hamiltonian of the single molecular chain some relevant exciton‐photon terms are omitted which are, on the other hand, included in the Hamiltonian of the coupled chains. Our numerical analysis shows that the omitted term can have a very important influence on the soliton properties in Davydov's model.
Quasicluster Electronic Structure Calculations of Point Defects in Crystals
Authors: Titov S. A.
Published: July 2007

An earlier proposed quasicluster embedding scheme is applied to the calculations of the ground state potential surfaces, energies, and probabilities of electronic transitions in neutral and charged point defects in KCI, SiO2, and diamond crystals. The model is closely related to the periodic supercell approach (providing similar electronic structure characteristics) and the molecular cluster one. Two modified versions of the model are proposed, which improve results of calculations of ground state properties (especially for small quasiclusters) and transition energies. Ground state potential surfaces calculated for defects in ionic and semiionic crystals are better than those obtained using supercell and molecular cluster models.
Magnetic Susceptibility of the Anderson Lattice with Finite f‐Band Width
Authors: Panwar S. , Singh I.
Published: July 2007

An extension of the periodic Anderson model by considering the finite f‐band width is considered. A variational method is used to study the temperature dependence of the valence nf and of the magnetic susceptibility χs for various values of f‐band width. As the f‐band width is reduced, the low temperature peak in the susceptibility χs gets sharpened.
Two‐Photon Excitation to the High‐Lying Triplet State of F Color Centers in LiF Crystals
Authors: Ter‐Mikirtychev V. V. , Tsuboi T.
Published: July 2007

The excitation spectrum for the anti‐Stokes luminescence band with a peak at 538 nm, which arises from the electronic transition of the singlet excited state 1E to the ground state 1A2 in F+3 centers of LiF crystals, is obtained for the first time using the first harmonic radiation of a tunable Al2O3:Ti3+ laser. From the quadratic dependence of the luminescence intcnsity on the average pumping power, the luminescence is understood to be caused by two‐photon excitation from the ground state 1A1 to the triplet state 3A2. The peak of the two‐photon excitation band was determined to be located at 24331 cm−1 from the 1A1 ground state.
Temperature Dependence of Core and Valence Thermal Vibrations in Germanium
Authors: Jeyakanthan J. , Saravanan R. , Mohanlal S. K.
Published: July 2007

An X‐ray analysis is made for the core and valence thermal vibrations in germanium, using powder X‐ray intensity data collected at various temperatures (298, 573, and 733 K). The overall thermal vibration is split into the components of core and valence thermal vibrations, because the valence electrons oscillate with different phase and amplitude than the core electrons due to bond polarizability. The data collected at the above temperatures are analyzed using locally developed computer programs. In the first phase, the overall Debye‐Waller factors are estimated. Then in the second phase, the core and valence contributions of the thermal vibrations are refined. The error in the fitting procedure is found to be less than 2%, and in the third phase an attempt is made to refine the individual anharmonic thermal parameters (βcore and βval), which result in unrealistic thermal parameter values. Hence only the overall anharmonic thermal parameters are evaluated and the results are reported.
Numerical Study of One‐Dimensional Strongly Correlated Systems
Authors: Kumari K. , Singh I.
Published: July 2007

The path‐integral formulation along with the quantum Monte Carlo method is used to study the physical properties of the one‐dimensional Hubbard model for various band fillings and values of interaction strength U. Results for singlet and triplet pairing correlations, local square magnetic moment, total energy, probability of double occupancy, and nearest‐neighbour charge‐density correlations are presented. These results are explored at β (inverse temperature) = 2.0, 1.0, and 0.5 for the linear chain N = 16. These results show that the effect of correlation on the physical quantities is small for low band‐filling and it becomes most evident for the half‐filled band at finite temperatures.
The Renormalization of the Energetical Spectrum and the Peculiarities of High Density Paraexciton Luminescence Band
Authors: Bobrysheva A. I. , Moskalenko S. A. , Shmiglyuk M. I. , Russu S. S.
Published: July 2007

The renormalization of the energetical spectrum of paraexcitons in the presence of a Bose‐Einstein condensate (BEC) of orthoexcitons or at least of their saturation is studied. Due to the conversion of a pair of orthoexcitons into a pair of paraexcitons an absolute instability in the energy spectrum of paraexcitons appears. The appeared paraexcitons can be downscattered by optical phonons T5 (88 cm−1) into a state above the bottom of the paraexciton band. This can be the reason of the observed peculiarities in the paraexciton luminescence band. On this basis a new explanation of the appearance of an extra peak in the luminescence spectrum of the uniaxially stressed Cu2O crystal is suggested. It has the same origin as the bump of the luminescence band in unstrained crystals. The spectral positions of the extra peak and of the bump differ due to the diminution of the ortho‐para splitting in uniaxially stressed crystals. The difference of their widths is due to different densities of the condensed orthoexcitons in stressed and unstrained crystals.
Surface Barrier at the Edge of a Uniaxial Anisotropic Type‐II Superconductor
Authors: Saif A. G. , El‐Sabagh M. A.
Published: July 2007

The structure of the surface barrier, using the uniaxial anisotropic London equation, for a high‐temperature type‐II supcrconducting quarter‐space, is studied. The role of a transport current on the annihilation of the surface barrier is investigated. It is found that this barrier depends on the external field strength, the value of the transport current flowing through the sample, and the orientation of the external field. It is shown that the results obtained will reduce to some known results in some limiting case.
Superconductivity and Structural Transition in Mo‐Stabilised R1–xCaxSr2Cu3Oy Compounds (R = Pr, Nd, and Y)
Authors: Zhao Y. , Zhang Y. L. , Choi C. H. , Chen B. , Kennedy H. , Sorrell C. C.
Published: July 2007

Sr‐based 123 compounds R1–xCaxSr2Cu2.7Mo0.3Oy with R = Pr, Nd, and Y are successfully stabilised by partial substitution of Mo for Cu. A tetragonal‐orthorhomic‐tetragonal transition is observed for R = Y and Nd, which is induced by different oxygen content in the samples. Tc of the samples is sensitive to the oxygen content other than the structure. For Pr‐contained samples the resistivity decreases systematically with increasing the Ca dopant and oxygen content. The results show that the hybridisation of the conduction electrons with the 4f band of Pr in Sr‐based 123 compounds is weaker than that in Ba‐based 123 compounds.
Thermoelectric Power of Solid Solutions (PbTe)1–x(Bi2Te3)x with 0 ≦ x ≦ 0.02
Authors: Christakudi T. A. , Christakudis G. Ch. , Borissova L. D.
Published: July 2007

The thermoelectric power (Seebeck coefficient) S which is one of the parameters determining the figure of merit Z of thermoelectric materials is measured for polycrystalline (PbTe)1 ‐x(Bi2Te3)x solid solutions as a function of temperature T (in the interval from 80 to 350 K) and composition x (in the range 0 ≦ x ≦ 0.02). The experimental results are analyzed on the basis of the two‐band nonparabolic Kane model and the transport coefficient theory of AIVBVVI semiconductors. Some information on the phonon‐drag term in this coefficient is specially extracted from the data.
The Temperature Dependences of the Magnetization and the Quadrupolar Moment for the Spin‐1 Ising Model
Authors: Tóth L.
Published: July 2007

The temperature dependence of the magnetization and the quadrupolar moment is studied for the spin‐1 Ising model on a honeycomb lattice in one‐spin cluster approximation. Nearest‐neighbour ferromagnetic exchange interactions J with concentration p and exchange interactions αJ (|α|≦1) with concentration (1 − p ) are considered.
Phase Transition of a Ferromagnetic Ising Superlattice in a Transverse Field
Authors: Wu Yinzhong , Zhong Jialin
Published: July 2007

Using the effective‐field theory with corrections, the critical behavior of a magnetic superlattice consisting of two different ferromagnets is examined in a spin‐1 transverse Ising model. The dependence of the critical temperature on the thickness of the constituents in a unit cell is investigated for different cases. A critical value of the interface exchange coupling (Jab)c above which the interface magnetism appears is found, and the effect of the transvcrsc field on (Jab)c is also investigated. All the results are understandable by phase diagrams.
On the Theory of Diffusion in Amorphous Metals
Authors: Davydov L. N. , Spolnik Z. A.
Published: July 2007

Diffusion in solids with random activation energy of individual jumps is analyzed. The experimentally observed relation between the activation energies of diffusion for metal glasses and for the corresponding crystals is explained under reasonable assumptions. The activation energy is shown to be not the most probable buth rather close to the minimum allowed (by the percolation mechanism) activation energy of individual jumps. For an interstitial solute the activation energy parametrically depends on the mean solute concentration. The expression for the diffusion coefficient is obtained. The short‐range diffusion is also considered.
Relativistic Positronium Planar Channeling. Ground State Properties
Authors: Boev O. V.
Published: July 2007

The ground state of a relativistic positronium (Ps) atom moving in the planar channeling regime in a Si crystal is investigated within the continuum potential approximation. The strong dependence of the internal structure of the Ps atom and its spatial distribution inside the crystal is found to be a function of the total energy of Ps. The squeezing of the atom and destroying of the spherical symmetry of the Ps ground state in a crystal with increasing Ps total energy is demonstrated. A brief discussion of applications and extensions is given.
Lattice Dynamics of Superionic Cerium Dioxide
Authors: Sinha M. M. , Ashdhir P. , Gupta H. C. , Tripathi B. B.
Published: July 2007

A modified rigid ion model is used to investigate the lattice dynamics of the superionic cerium dioxide compound with an appropriate effective charge in the electrostatic interaction and third‐neighbour interaction for the short‐range forces. The present model is more physical than others in predicting the phonon frequencies in superionic compounds.
On the Dielectric Constants of AIBIIIC Chalcopyrite Semiconductor Compounds
Authors: Márquez R. , Rincón C.
Published: July 2007

Values of the static, ϵ0, and high‐frequency, ϵ∞, dielectric constants of I‐III‐VI2 chalcopyrite compounds are estimated by using an empirical model proposed by Nag for cubic semiconductors, which relates these constants to the average atomic number of its constituent atoms Zav. It is found, as in the cubic semiconductors, that the inverse of both these constants is linearly related to Zav. It is also suggested that ϵ0 and ϵ∞ thus obtained should be more reliable than those calculated from the energy gap Eg, due to the wide range of values of this parameter reported in some I‐III‐VI2 chalcopyrite compounds.
Thermal Expansion and Compression of Alkaline Earth Oxides and Cesium Halides at High Temperature and High Pressure
Authors: Upadhyay S. P. , Kumar M.
Published: July 2007

Temperature and pressure dependence of thermal expansion and compression of alkaline earth oxides and cesium halides with NaCl‐ and CsCl‐type structures is investigated. The theory of interionic potentials based on the quantum mechanical form of the overlap repulsive energy is used to investigate the interatomic separations as a function of temperature and pressure from room temperature up to the melting temperature and from zero pressure up to the structural transition pressure. The values of van der Waals dipole‐dipole and dipole‐quadrupole energies are also included in the model. The results are used to predict the temperature and pressure dependence of the coefficient of volume thermal expansion using a formula derived on the basis of the thermodynamic analysis. The results are compared with available experimental data and discussed in the light of recent research.
Correlations between the Szigeti Charge and the Differences in Phase Volumes in the Alkali Halides
Authors: Koh A. K.
Published: July 2007

Correlations between the Szigeti charge and the difference in phase volume in the alkali halides are presented. It is shown that the lattice bonding is enhanced if an alkali halide shows a bigger fractional change in volume at melting. The fractional change in volume for RbF can be obtained by extrapolation.
Polarons in a Cylindrical Quantum Wire with Finite‐Barrier Well
Authors: Pokatilov E. P. , Klimin S. N. , Balaban S. N. , Fomin V. M.
Published: July 2007

The eigenfunctions and eigenenergies of an electron in a finite‐barrier cylindrical quantum wire are obtained. The case of different electron band masses in the cylinder and the barrier material is discussed. The polaron binding energy and the polaron effective mass due to the interaction of an electron with two cylindrical‐interface optical modes as well as with cylindrical bulk‐in and cylindrical bulk‐out confined LO‐phonon modes are calculated, and their dependence on the radius of the cylinder is studied in detail. The comparison with the results for a 2D polaron is performed.
Study of the Electronic Structure and the Role of Gallium 3d Electrons in Gallium Nitride
Authors: Jhi S.‐H. , Ihm J.
Published: July 2007

Ab‐initio pseudopotential total energy calculations are performed for gallium nitride in both wurtzite and zincblende structures. The 3d electrons of gallium are treated as valence states and the behavior of the 3d electrons and their influence on higher‐lying valence states are investigated. It is found that the explicit treatment of the gallium 3d electrons as interacting valence states rather than inert core states systematically improves various structural and electronic properties.
Impurity Binding Energy of a Cylindrical Quantum Wire in a Magnetic Field
Authors: Xiao Zhigang , Zhu Jiqian , He Fenglai
Published: July 2007

By using the strong‐perturbation theory, suitable trial wave functions are chosen and the ground‐state energy and the binding energy of the ground state of a hydrogenic donor in a quantum wire are calculated in the presence of a uniform magnetic field applied parallel to the wire axis. The quantum wire is assumed to be a cylinder of GaAs having an infinite confining potential. The ground‐state energy and the binding energy increase as the strength of the magnetic field increases. Our results agree with those of previous calculations. The calculating method is novel and has general significance.
Persistent Currents in Finite‐Width Mesoscopic Rings. The Role of the Interchannel Coupling and of the Electron‐Electron Interaction
Authors: Wendler L. , Fomin V. M.
Published: July 2007

The persistent currents of few interacting electrons with spin in mesoscopic rings are calculated in the ballistic regime. The energy bands of the finite‐width multi‐channel ring are calculated with the transfer‐matrix method including the interchannel coupling induced by the impurity scattering, and the electron—electron interaction. It is shown that in the case of a narrow‐width ring the electron—electron interaction and the confining potential build a rotating Wigner crystal of the few electrons, where the relative angular motions are harmonic oscillations if the impurity potential is weak. As long as this picture remains valid, the persistent current is not influenced by the electron—electron interaction. It is shown that the width of the mesoscopic ring is the critical parameter for the influence of the electron—electron interaction on the persistent current. With increasing width of the mesoscopic ring, the electron—electron interaction decreases the amplitude of the persistent current at low temperatures and increases it at higher temperatures. Further, for a finite width of the mesoscopic ring the interchannel coupling becomes important, which results in the occurrence of higher harmonics in the persistent current.
Structural and Magnetic Studies of Amorphous and Icosahedral Al65Cr10Fe10Ge15 Synthesized through Mechanical Alloying
Authors: Bahadur D. , Singh K. , Srivastava C. M. , Ivanov E.
Published: July 2007

The mechanical alloying is used to synthesize the amorphous powder of the nominal composition Al65Cr10Fe10Ge15. On the basis of DTA studies, the compound is heat treated at 530 K. The X‐ray diffraction of the heat treated phase can be indexed to icosahedral indices. The magnetic susceptibility measurement particularly exhibits some interesting features. A qualitative explanation based on the competition between on‐site Coulomb repulsion, which favours local moment formation and the s‐d interaction that leads to itinerancy, is attempted which is found to be well correlated with the structural disorder in the system.
Could the Oxygen Vacancy in Oxides of the Perovskite Family Be a Dipole Center?
Authors: Prosandeyev S. A. , Osipenko I. A.
Published: July 2007

The lattice polarization in oxides of the perovskite family is demonstrated to give rise to symmetry breaking of one‐electron orbitals at F centers. This can provide an explanation of recent experiments on paraelectric KTaO3 where, by the second harmonic generation method, approximately 1018 cm−3 dipole centers were found, the concentration of which depends on reduction and oxidation processes.
Temperature Dependent Elastic and Lattice Dynamical Behaviour of Solids. An Effective Approach
Authors: Saxena S. K. , Agrawal R. M. , Rathore R. P. S.
Published: July 2007

An effective approach in terms of core and ionic radii is developed to study the form factor, energy wave number characteristics, phonon frequencies, and elastic constants of some scarcely studied transition metals, i.e. b.c.c. zirconium and b.c.c. titanium. The radial and tangential force constants extending up to 15th neighbours are computed for these metals. A reasonably good agreement between the computed and experimental findings on phonon dispersion and elastic constants is obtained.
The Ionicity Character Seen by the Charge Density in Copper Halides
Authors: Zaoui A. , Ferhat M. , Certier M. , Soltani M. , Khelifa B.
Published: July 2007

The electronic charge densities of copper halides CuX (X = Cl, Br, I) are calculated using the nonlocal empirical pseudopotential method in order to obtain their ionicities. The resulting ionicity values are compared with those of the Phillips scale. A favorable agreement is found.
Influence of Temperature on Magnetophonon Resonances in Four‐Component Solid Solution of ZnxCdyHg1−x−yTe
Authors: Sheregii E. M. , Polit J. , Cebulski J. , Andrukhiv A. M.
Published: July 2007

The influence of temperature on oscillations of the transverse magnetoresistance of ZnxCdyHg1−x−yTe (x = 0.08, y = 0.11) in the range 77 to 200 K and under magnetic fields of up to 2.5 T is studied. It is shown that the structure of the observed oscillations cannot be interpreted by participation of one kind of phonons only in magnetophonon resonance. The interpretation of experimental peaks with the help of magnetophonon resonances at different phonon frequencies describing the different sublattices of the solid solution (HgTe‐like, CdTe‐like, ZnTe‐like) is given.
Magnetic Excitation in Peierls‐Hubbard Model
Authors: Chen Bin , Yu Chaofan , Fang Hui , Jiao Zhengkuan , Zhang Qirui
Published: July 2007

Based on the Peierls‐Hubbard Hamiltonian, the influence of electron‐phonon interaction on the spin excitation of itinerant systems has been analyzed. It is shown that the interaction not only changes the dynamic response properties, but also decreases the gap between two spin subbands, which makes the electron‐hole pairs easy to be excited singly. It should be emphasized that the spin excitation spectrum has an energy gap due to the presence of the electron‐phonon interaction mentioned above.
Optical Properties of Interband Transitions in the Oxygen‐Plasma‐Anodised Film on Gallium Arsenide 〈100〉
Authors: Xu Xiao‐Liang , Zhu Li‐Xin , Chen T. P. , Fung S.
Published: July 2007

A first study of optical emission and excitation properties from interband transitions in an oxygen‐plasma‐anodised (OPA) film on GaAs 〈100〉 is reported. Six optical emission (EM) structures ranging from 2.83 to 4.60 eV are observed using cathodoluminescence (CL) and photoluminescence (PL) methods. The PL excitation spectra (EX) obtained have also demonstrated that the nine absorption transitions are related to the EM bands.
Nanostructures and Semiconductor Electronics
Authors: Lüth H.
Published: July 2007

In a short introduction limitations of present semiconductor microelectronics are discussed. At the beginning of the next century both technological and physical limits will severely affect further miniaturisation of integrated circuits (IC) into the deep submicron, i.e. below 100nm feature size range. Several novel device concepts being based on the use of quantum effects in nanostructures are discussed which might open new pathways to faster, lower consumption, and more compact semiconductor ICs: Resonant tunneling structures might play an important role as multifunctional devices which allow the integration of more complex logical functions in one and the same device under more relaxed lithography conditions. A further approach in quantum electronics is focussed on electron interference in high mobility two‐dimensional electron gases (2DEG) at semiconductor heterointerfaces. Novel processor concepts might be possible by use of quantum dot arrangements and single electron tunneling. The underlying physics is explained and several experimental approaches for the realisation of quantum dots and wires are presented.
Scattering of Small Molecules at Surfaces
Authors: Heiland W. , Schlathölter T. , Vicanek M.
Published: July 2007

The interaction of energetic molecules with surfaces is of interest for nuclear fusion experiments, for the erosion of space vehicles, and for the understanding of basic charge exchange phenomena between metal surfaces and molecules. For the latter reason experiments with hyperthermal molecules are also of interest for catalytic processes and for the functioning of solid state sensors. In many cases the interaction of molecules with surfaces is governed by charge exchange processes. An obvious case is the catalytic oxidation of CO at surfaces where a negative carbon dioxide species is identified as an intermediate of the reaction. Negative carbon dioxide species are also found in beam experiments. Their yield and the correlated dissociation probabilities show the expected dependencies on e.g. the coverage of the metal with alkali metal layers. In the case of hydrogen molecules the influence of alkali layers is quite different. In chemisorption experiments alkalis cause a decrease of the dissociative chemisorption of hydrogen. In our beam experiments an increase of the molecular survival with alkali coverage is found accordingly. The interpretation of the findings gives more detailed insight into the dissociation mechanisms of small molecules.
Building Nanostructures by Controlled Manipulation of Single Atoms and Molecules with the Scanning Tunneling Microscope
Authors: Meyer G. , Neu B. , Rieder K.‐H.
Published: July 2007

Our present experiences in manipulating single atoms and molecules on solid surfaces by using the tip of a scanning tunneling microscope (STM) are reviewed. The construction and performance characteristics of our temperature variable STM is shortly described, which allows investigations in the temperature range between 15 and 300 K. As example for horizontal manipulation, in which the adatoms or admolecules are pulled along the surface by the STM tip, and the formation of man‐made surface structures, the formation of the letters ‘F’ and ‘U’, which form the logo of our university is presented. The adspecies used in these experiments are CO molecules and the substrate a Cu(211) surface. An example of vertical manipulation, in which an adparticle is picked up by the tip, carried to another place and released back to the surface, concerns Xe on the same substrate. The latter technique allows also to vary the chemical composition of the tip apex in a well‐defined manner. Consequences in imaging the surface with tips of different compositions are presented and discussed. Finally, STM observations of ordered high density phases of CO on the Cu(211) substrate are discussed.
Ultrathin Films of F.C.C. (001) Co and Cu: Electronic Structure, Photoemission, and Magnetic Dichroism
Authors: Henk J. , Scheunemann T. , Halilov S. V. , Feder R.
Published: July 2007

A relativistic Green function theory of the layer‐KKR type has been applied to calculate the electronic structure and its manifestation in valence‐band photoemission of ferromagnetic layer systems of the form Co/Cu/Co(001) and Cu/Co/Cu(001). The spin‐ and layer‐resolved density of states in Co films of one to ten monolayers on Cu(001) is almost mirror symmetric with respect to the central Co plane. For Cu films on Co(001), minority spin “quantum well states” have constant weights in the Cu monolayers and a stronger weight in the first Co monolayer. For all systems, spin‐resolved photoemission spectra mostly reflect the spin character of the initial states with modifications by spin‐orbit coupling. As a consequence, the spin‐averaged photocurrent by p‐polarized light from films with in‐plane magnetization shows a pronounced dependence on the magnetization direction, i.e. magnetic linear dichroism. For perpendicular magnetization, circularly polarized light produces sizeable magnetic circular dichroism in photoemission.
Characterization of Metal Alloy Systems by Scanning Tunneling Microscopy and Low‐Energy Ion Backscattering
Authors: Niehus H.
Published: July 2007

Low‐energy ion backscattering and scanning tunneling microscopy (STM) can be used in combination to study the surface crystallography of single crystals, in particular also for alloy and ordered intermetallic compound systems. The elemental composition as well as the location of the atoms is determined by low‐energy noble gas impact collision ion scattering spectroscopy with neutral detection (NICISS). Microscopic information on an atomic scale is obtained with the help of STM. The influence of adsorption‐induced surface modifications is investigated at Cu3Au and compared with Cu surfaces. From the NICISS investigation the AuCu termination is inferred for clean Cu3Au(100) and (110). After careful preparation of the clean Cu3Au(110) surface the prominent (2 × 1) LEED superstructure changes into a pronounced (4 × 1) LEED pattern. The (4 × 1) structure can be explaining by row pairing of CuCu and AuAu rows in the topmost layer. Upon oxygen exposure of the Cu3Au compounds, Cu tends to segregate to the surface. In case of Cu3Au(110) a (2 × 1)O superstructure appears which is in part similar to the known added row structure obtained for Cu(110)(2 × 1)O. Comparison with the Cu(110)O surface shows, however, that the known periodically ordered structure of CuO stripes at undersaturation with oxygen does not show up at Cu3Au(110)‐O.
Characterization of Supported Catalyst Systems with Surface Spectroscopies
Authors: Taglauer E. , Knözinger H.
Published: July 2007

Supported catalysts of transition metals and their oxides are widely used for a variety of reactions. They typically consist of a high surface area oxide as support material (e.g. Al2O3, TiO2, SiO2) and finely dispersed metal or metal oxide particles (e.g. Rh, MoO3, V2O5) as active components (or their precursor) and promoters. The catalytic properties are determined by these components, their structural arrangement, and their interactions. Studies of real and model catalyst systems are reported for which low‐energy ion scattering (LEIS) is used in combination with other spectroscopies, such as RBS, AES, TDS, XPS, and Raman spectroscopy. The applicability of these techniques to real systems and the relevance of model catalysts is discussed. The results shown demonstrate the usefulness of these spectroscopies for the characterization of supported catalysts, in particular for the analysis of the composition and the arrangement of the topmost atomic layers. Among the examples given are special aspects of catalyst impregnation, the investigation of solid‐solid wetting which may be relevant to catalyst preparation and strong metal‐support interactions (SMSI effect) in supported Rh metal catalysts.

Übergangsmetalle und ihre Oxide werden in Trägerkatalysatoren für eine große Zahl von Reaktionen verwendet. Sie bestehen typisch aus einem Oxid mit großer spezifischer Oberfläche als Trägermaterial (z. B. Al2O3, TiO2, SiO2) und fein verteiltem Metall oder Metalloxid als aktiver Komponente (z. B. Rh, MoO3, V2O5) oder deren Vorläufer und als Promotoren. Die Eigenschaften des Katalysators (Aktivität, Selektivität) werden von diesen Komponenten. ihrer strukturellen Anordnung und ihrer chemischen Wechselwirkung bestimmt. Es werden Untersuchungen an Real‐ und Modellkatalysatoren dargestellt, für die die Niederenergie‐Ionenstreuung in Kombination mit anderen Spektroskopien wie RBS, AES, TDS, XPS und Ramanspektroskopie verwendet werden. Die Ergebnisse demonstrieren die Nützlichkeit dieser Methoden für die Charakterisierung von Trägerkatalysatoren, insbesondere für die Analyse der Zusammensetzung und geometrischen Anordnung der obersten Atomlagen. Die angeführten Beispiele betreffen spezielle Aspekte der Imprägnierung, die Untersuchung von Festkörper‐Fest‐körper‐Benetzung als einer Möglichkeit der Präparation von Katalysatoren und die sogenannte starke Metall‐Träger‐Wechselwirkung am Beispiel von getragenen Rh‐Modellkatalysatoren.