bluuebunny/crossref_metadata_2025 huggingface dataset:
Point Defects in CdTe Containing Simultaneously Lithium and a Group IV Impurity
Authors: Zielińska‐Purgał R. , Piwowarczyk J. , Nazarewicz W.
Published: July 2007

Infrared absorption associated with localized vibrational modes in CdTe containing both Li and a group IV impurity (Si, Ge, or Sn) is studied. On the basis of the obtained experimental results it is established that the following point defects play a dominant role: 1. isolated Licd, 2. Licd‐Li, nearest‐neighbour pairs, and 3. Licd Mcd next‐nearest‐neighbour pairs, where Mcd denotes a Si, Ge, or Sn impurity at a Cd site. It is also found that some localized vibrational modes of Licd‐Mcd pairs are connected with the coupled motion of Licd and Mcd impurities and that the model of a weakly coupled impurity pair, usually used to analyze experimental results, is not applicable in this case.
Order Parameters of Two Phase Transitions in Mixtures of Ellipsoidal and Spherical Molecules
Authors: Mejdani R. , Gashi A.
Published: July 2007

The model is the ES mixture of ellipsoidal (E) and spherical (S) molecules dispersed on a b.c.c. lattice with a ratio of concentrations different from 1:1. Minimising numerically the effective thermodynamic potential, obtained in a “modified” mean‐field approximation, the variation of the orientational and positional order parameters for different values of concentration, molecular parameter (of ellipsoidal molecules), and interaction constants are studied. The coupling effects arising from the interplay between orientational and positional degree of freedom are also checked. Determining the critical temperatures for two kinds of transitions the phase diagram (the critical temperature‐concentration diagram) is obtained, where the transition lines define four regions corresponding to the completely disordered mixture, the only positionally ordered mixture, the only orientationally ordered mixture, and the positionally and orientationally ordered mixture. Thus, in large lines, the model of the ES mixture reproduces qualitatively some features of the behaviour of plastic and liquid crystals.
Optical Properties and Structure of a‐Si1‐xGex:H Films
Authors: Sotiropoulos J. , Fuhs W.
Published: July 2007

A study of the optical spectra in alloys of a‐Si1‐xGex: H in the energy range 0.5 to 10 eV is presented. The absorption spectra and the reflectivity are measured for films with 0 ≦ x ≦ 1 and the imaginary part of the dielectric function, ϵ2(ω), is calculated by Kramers‐Kronig analysis. The spectra are discussed by considering the bonding structure of the films and the behavior is compared with that of other amorphous alloys (a‐Si1‐xCx : H, a‐Si1‐xNx: H). Whereas in the latter alloys the optical properties are determined by the SiSi bonds up to x ≈ 0.5 the contribution of the various bonds to ϵ2(ω) cannot be separated in a Si/Ge:H due to the fact that in this case the bond strengths of the SiSi, SiGe, and GeGe bonds are very similar.
Ferromagnetic Resonance Studies of Anisotropy in Co/Cu(111) Multilayers
Authors: Soliman A. , Schreiber F. , Meckenstock R. , Bödeker P. , Bröhl K. , Pelzl J.
Published: July 2007

Ferromagnetic resonance (FMR) measurements at X‐band frequencies are used to study in‐plane magnetic anisotropies of a number of molecular‐beam epitaxy (MBE) Co/Cu(111) multilayer films. X‐ray and reflection high‐energy electron diffraction (RHEED) measurements confirm the epitaxial growth of Co on Cu(111) with f.c.c.(111) structure. The FMR experimental data are fitted by using energy density expressions that include a combination of growth induced uniaxial (second‐) and sixth‐order anisotropies. Values for the effective demagnetizing field (4πMs)eff, the growth induced uniaxial field HKu' and the sixfold in‐plane anisotropy field HK4 parameters are obtained from a multi‐parameter χ2 function minimization routine.
Short‐Range Order in Binary Solid Solutions The Change of Electronic Density of States on Ordering
Authors: Petrenko P. V. , Grabovskii Yu. E. , Kulish N. P. , Melnikova N. A. , Repetskii S. P.
Published: July 2007

Concentration dependences of the electronic density of states near the Fermi level and the Cowley short‐range order parameters are presented for the Ag‐A1 solid solutions after varius thermomechanical treatments. The density of states is calculated from the data of the electronic heat capacity. The results obtained are analysed alongside with similar results for the Cu‐Al solid solutions. It is shown that the electronic density of states decreases with ordering in the alloys. This decrease attains 40% for some alloys and treatments. The concentration dependences of the electronic density of states are non‐monotonous. A minimum is observed in the range where the short‐range order is essential. The explanation of these results is given in terms of the inhomogeneous order model. According to this model regions with different types of SRO are formed in a‐solid solutions. In the ideal case the type of ordering in these regions corresponds to higher concentration phases, while the ordering in the α‐solid solution corresponds to a superstructure of LI2‐type. The order type of these regions is defined by the temperature and concentration of the alloy. The model explanations are confirmed by calculations based on formulas obtained with the experimentally measured short‐range order parameters.
Overdamped Magnons in a Doped Two‐Dimensional Antiferromagnet
Authors: Sherman A. , Schreiber M.
Published: July 2007

Hole and magnon spectral functions of the 2D t‐J model are calculated for a finite hole concentration x, using the self‐consistent Born approximation. The magnon spectrum is shown to feature a branch of overdamped magnons near the I and M points of the Brillouin zone, if a critical concentration xc of mobile holes is exceeded. For parameters which are adequate for La2CuO4 we estimate xc ≈ 1.5%. The appearance of such excitations means a sharp decrease of the instantaneous spin correlation length from a value determined by temperature fluctuations in the 2D system to a value which depends on x. We can relate xc to the hole concentration at which the 3D long‐range ordering is destroyed in La2‐xSrxCuO4. The overdamped branch coexists with the usual magnon branch. An overall softening of the latter branch with an increase of x is obtained.
Gain Processes in ZnTe Epilayers on GaAs
Authors: Majumder F. A. , Klingshirn C. , Westphäling R. , Kalt H. , Naumov A. , Stanzl H. , Gebhardt W.
Published: July 2007

Optical gain processes in ZnTe epitaxial layers grown by MOVPE on GaAs substrates are reported. Depending on the excitation conditions, optical gain related to exciton‐exciton scattering or due to recombination in an electron‐hole plasma is identificd by means of the variable‐stripe‐length method. The experimental gain spectra are compared to model calculations for the respective gain processes. Stimulated emission up to room temperature is observed under intense picosecond excitation.
Photoionization of Shallow Donor Impurities in Finite‐Barrier Quantum Wells
Authors: Ilaiwi K. F. , El‐Said M.
Published: July 2007

The dependence of the photoionization cross‐section on photon energy is calculated for shallow donors in finite‐barrier GaAs/Ga1−xAlxAs quantum wells as well as the binding energy as a function of well width. The effect of a magnetic field is also considered.
Magnetoresistance for the Sondheimer‐Wilson Model in a Metal and for a Mixed Nondegenerate Semiconductor
Authors: van Vliet Carolyne M. , Avello Miriam Y.
Published: July 2007

The overlapping two‐band model for the transverse magnetoresistance, originally proposed by Sondheimer and Wilson (1947), and the magnetoresistance in a mixed nondegenerate semiconductor with contributions from both the conduction and the valence bands is reconsidered. It is shown that the classical answers for both cases are readily obtained by using the tensor‐relaxation time formalism. introduced by Van Vliet and Marshak (1976). More detailed extensions are also given.
In‐Situ Characterization of II‐VI Molecular Beam Epitaxy
Authors: Gaines J. M. , Ponzoni C. A.
Published: July 2007

In‐situ monitoring of crystal growth can provide valuable information on growth processes and feedback to control growth. The use of reflection high‐energy electron diffraction (RHEED) and cathodoluminescence (CL) to examine molecular beam epitaxy (MBE) of II/VI wide‐band‐gap semiconductors is described.
Study of Characteristics of Photocurrent Spectra in β‐FeSi2 Thin Films
Authors: Shen W. Z. , Tang W. G. , Li Z. Y. , Shen S. C. , Wang L. W.
Published: July 2007

The absorption and photocurrent spectra of polycrystalline β‐FeSi2 thin films are studied in relation with the substrate temperature. The photocurrent spectra involving the fundamental interband, extrinsic defect transitions of β‐FeSi2, and the intrinsic transitions of Si substrates are observed. These transition energies are in good agreement with the results of absorption measurements. The advantages of photocurrent measurements for studying the energy band structure of β‐FeSi2 are revealed.
Photoemission Calculations Including Both Band Structure Effects and Photon Field Variation
Authors: Das P. , Kar N.
Published: July 2007

Photoemission cross‐sections are calculated, using band wave functions and a spatially varying photon field. The solid is considered to be composed of a stack of identical layers of atoms parallel to the surface with a potential of muffin‐tin form. The photoemission matrix element is calculated by appropriate expansion of the initial and final state wave function and the vector potential of the photon field in the muffin‐tin, interstitial, and vacuum regions. The case of normal photoemission from the (100) face of aluminium is considered as an application.
Polaritons in multiple quantum wells
Authors: Andreani L. C.
Published: July 2007

A description of polariton effects on excitons in multiple quantum wells (MQWs), superlattices (SLs), and semiconductor microcavities is given, which is based on a nonlocal semiclassical theory in a transfer‐matrix formulation. The radiative width in MQWs at k = 0 increases first with the number of wells N, then goes to zero as N → ∞, where stationary polariton states are recovered. A very similar behavior is found by treating the MQW with a local dielectric model. In the SL limit, the exciton‐polariton dispersion at long wavelengths reduces to that of a light wave in a uniaxial material, described by an effective dielectric tensor. Comparing with the bulk, the polariton splitting is increased in SLs, and reduced in microcavities. Nevertheless, polariton effects in microcavities are easily observable, since spatial coherence along the growth direction is not required.
Carrier cooling in intermixed GaAs/ AlGaAs quantum dots and wires using high excitation and transient spectroscopy
Authors: Adler F. , Burkard M. , Schweizer H. , Benner S. , Haug H. , Klein W. , Tränkle G. , Weimann G.
Published: July 2007

Buried low‐dimensional integrated dot and wire structures fabricated by masked implantation enhanced intermixing are compared directly. Using high excitation and transient spectroscopy hot carrier effects as well as phase‐space filling in the low‐dimensional structures are observed. The carrier cooling curves of the wires and dots show a bend at about 350 ps which may be an indication of two dominant relaxation mechanisms. The influences of the carrier capture in low‐dimensional systems and the relaxation on the carrier temperature are separated by measurements with different excitation wavelengths.
Ultrafast nonlinear optical response of Fano resonances in GaAs under high magnetic field
Authors: Siegner U. , Mycek M.‐A. , Glutsch S. , Chemla D. S.
Published: July 2007

The coherent dynamics of Fano resonances in GaAs are studied under magnetic field using femtosecond four‐wave mixing (FWM). In this system Coulomb coupling bel ween magnetoexciton and continuum states gives rise to Fano interference. By time‐resolving the FWM signal it is shown that dephasing of the coherent emission from a Fano resonance is dominated by the coupling to the continuum. Unlike Lorentzian resonances, the decay of the time‐integrated FWM signal is not related to dephasing. This surprising behavior is due to the interplay between Fano interference and Coulomb correlation in a semiconductor.
The role of coherence for carrier relaxation in photo‐excited semiconductors
Authors: Kuhn T. , Haas S. , Rossi F.
Published: July 2007

The role played by the dynamics of the interband polarization for the correct description of the generation of carriers by a short laser pulse is investigated. The calculations are based on the semiconductor Bloch equations which are solved by means of a generalized Monte Carlo simulation. It turns out that some of the approaches commonly used for a microscopic modelling of the dephasing process fail in describing correctly the effect of carrier‐carrier interaction in the low‐density limit. By including terms which have the structure of “in‐scattering” terms (vertex corrections) for the interband polarization, the experimentally observed features in the carrier dynamics are well reproduced in the whole density range.
Coherent interband effects in quantum kinetics
Authors: Thoai D. B. Tran , Bányai L. , Reitsamer E. , Haug H.
Published: July 2007

The quantum kinetics of a laser pulse exciting electrons and holes interacting with LO‐phonons is studied in [l] within the generalized Kadanoff‐Baym approximation, but the spectral Green functions are approximated by damped free propagators. After including the Hartree‐Fock coherent terms due to the bare Coulomb interaction, oscillations (“quantum beats”) of the incoherently summed polarization with the LO‐phonon frequency have been found in [2]. Here we include the Coulomb Hartree‐Fock terms as well as the optical pump field selfconsistently for both the spectral and particle functions. This procedure takes the phonon‐assisted transitions and excitonic effects fully into account. Our calculations in three‐dimensional and particularly in two‐dimensional semiconductors show an enhancement of the coherent “quantum beat” effects in the incoherently summed polarization. Further we show that these quantum beats are well resolved in the photon echo signal.
Quantum kinetics of the coupled carrier‐phonon system in photoexcited semiconductors
Authors: Schilp J. , Kuhn T. , Mahler G.
Published: July 2007

A quantum kinetic analysis of the coupled carrier–LO‐phonon dynamics in photoexcited semiconductors is presented. The emission of phonons leads to the build‐up of a substantial nonequilibrium phonon distribution which is responsible for a reduced energy relaxation and an increased dephasing with increasing carrier density. Deviations from the semiclassical case are particularly pronounced at small phonon wave vectors, which cannot be reached by classical scattering processes. It turns out that the damping as introduced usually in quantum kinetic calculations leads to a violation of the conservation of the total energy.
Dephasing of acceptor bound excitons in II–VI semiconductors
Authors: Lummer B. , Heitz R. , Kutzer V. , Wagner J.‐M. , Hoffmann A. , Broser I.
Published: July 2007

Degenerate four‐wave‐mixing experiments (DFWM) at the neutral acceptor bound exciton complex in CdS and ZnSe are reported. ps‐ as well as fs‐spectroscopy are carried out to investigate the dephasing processes at this complex. The results are discussed concerning scattering of free excitons, phonons, and interactions with impurity centers. Higher orders of diffraction in the DFWM process are modeled within the density matrix equations for noninteracting two‐level systems including propagation effects. From nonlinear quantum beat spectroscopy the binding energics of the acceptor bound biexciton in CdS and ZnSe are estimated.
A Green's function approach to the description of optical properties of disordered semiconductors
Authors: Stroucken T. , Anthony C. , Knorr A. , Thomas P. , Koch S. W.
Published: July 2007

A Green's function approach to the optical response of disordered semiconductors is presented. The theory is illustrated studying light propagation in semiconductor multiple quantum wells with well‐width fluctuations and interface roughness. The combined influence of disorder, Coulomb, and propagation effects leads to characteristic features of the transmitted pulse.
Relaxation kinetics and photoluminescence lineshape of excitons in II–VI semiconductor quantum wells
Authors: Runge E. , Schülzgen A. , Henneberger F. , Zimmermann R.
Published: July 2007

Exciton localization by local potential fluctuations is studied theoretically and compared with energy‐ and time‐resolved photoluminescence experiments under resonant excitation for MBE grown ZnCdSe/ZnSe and ZnSe/ZnMnSe samples. Within the exciton lifetime of about 100 ps a narrowing of the luminescence band and a shift of the maximum position to lower energies is seen. This relaxation is described by a kinetic equation for particles moving independently in a spatially correlated disordered potential. The resulting exciton distribution at low temperatures has strongly nonthermal character. Good quantitative agreement with experimental data is found. An approximately linear temperature dependence of the radiative lifetime is expected theoretically. The photoluminescence of a 2 nm ZnCdSe quantum well shows, indeed, an increase of the exciton lifetime from 80 ps (at 5 K) to 160 ps (at 40 K).
On thermometer operation of ultrasmall tunnel junctions
Authors: Müler H.‐O. , Hädicke A. , Krech W.
Published: July 2007

The temperature dependence of the I–V characteristics of many single‐electron tunneling devices enables thermometer operation of these systems. Two normal conducting kinds of them are investigated, a) a single junction in a high‐impedance environment and b) a double junction. The characterstics of botu devices show a cross‐over from Coulomb blockade at low temperatures to ohmic behavior at high temperatures. The related differential conductivity dip allow the determination of the temperature of the junctions. Both configurations a) and b) are expected to work at least within the range 0.5 ≦ βEc ≦ 10, where EC the coulomb energy of the system under investigation. An analytical solution for both low‐and high‐temperature cases of a) and b) as well numerical results and their fit are presented, including the effect of co‐tunneling in case of a double junction.
Theoretical model of the density of states for random dilute binary alloys
Authors: Brezini A. , Bouamrane R. , Hamdache F. , Depolier C.
Published: July 2007

A theoretical model of the density of states for random binary alloys is presented in the dilute limit based on a mean‐field approach. The model incorporates both diagonal and off‐diagonal disorder as well as short‐range order. Extensive results are reported for the density of states and various physical quantities in the limit of weak concentration and compared to other theories.
Periodic wave solution and its stability in antiferrornagnet Ni(C2H8N2)2NO2ClO,4
Authors: Liu Wu‐Ming , Wang Heng , Zhao Xiaopenc , Zhou Ben‐Lian
Published: July 2007

Introducing the coherent state ansatz and the time‐dependent variational principle, two partial differential equations of motion are obtained from the Hamiltonian. Employing the method of multiple scales, these equations are reduced into the envelope function equation and the amplitude function satisfying a nonlinear Schrödinger equation is found. The periodic wave solution and analysis of its stability in the antiferromagnet Ni(C2H8N2)2NO2ClO4 is derived.
Spatial correlation of laser‐generated electrons and holes in quantum wells
Authors: Belitsky V. I. , Cantarero A. , Pavlov S. T. , Cardona M. , Korovin L. I. , Lang I. G.
Published: July 2007

The spatial correlation of hot electrons and holes generated by light in a semiconductor quantum well (QW) is studied. For hot electron‐hole pairs in a polar material, this correlation is determined by the interaction with LO‐phonons. We analyze the distribution FN(r, K) of electrons and holes which are created in a given light absorption process, with respect to their relative separation r and total quasimomentum ħK, after the emission of a number N of LO‐phonons. The relationship between the spatial distribution of electrons and holes in these intermediate states and the cross‐section of multi‐phonon resonant Raman scattering (MPRRS) is established. Spatial correlation effects are strongly enhanced in low‐dimensional systems, a fact which is also reflected in the MPRRS efficiency.
Ginzburg‐Landau Equations and the Upper Critical Magnetic Field in Graphite Intercalation Compounds
Authors: Konsin P.
Published: July 2007

The Ginzburg‐Landau equations of a dirty two‐band superconductor, where superconductivity is driven by the interband electron‐phonon interaction are derived. On using these Ginzburg‐Landau equations, the obtained formulas for the critical field Hc2 describe well the temperature and angular dependences of Hc2 in graphite intercalation compounds (GICs). The calculated H∥ for C6K show a linear temperature dependence in a broad temperature range up to the values of the reduced temperature t = T /Tc ≈ 0.3 which is a general feature of GICs.
Low‐Temperature Anomalies in the CdI2Cd Crystalline System
Authors: Bolesta I. M. , Kityk I. V. , Filipecki J. , Zount H.
Published: July 2007

From the measurements of elastic coefficients, piezooptical constants, and dilatometrical lengthening the existence of an improper ferroelastic phase transition of first order in the CdI2Cd single crystalline system within the temperature range 40 to 60 K is found. Different values of the temperature hysteresis are revealed for the piezooptical and elastic constants. From the data obtained an essential role of electron‐phonon interaction in the appearance of the mentioned phase transitions is concluded. The obtained results clearly show that in the CdI2Cd single crystalline system at lower temperatures long‐range forces appear between the impurity subsystems, which due to the specific electron‐phonon interactions tend to the appearance of the phase transitions. The behaviour of the elastic constants and the lengthening indicate the possible ferroelastic nature of these transitions. Simultaneously the existence of the temperature hysteresis reflects the first‐order nature of the mentioned transitions.
Magnetopolaron in a Cylindrical Quantum Wire
Authors: Pokatilov E. P. , Klimin S. N. , Balaban S. N. , Fomin V. M.
Published: July 2007

The dependence of the electron energy spectrum on a magnetic field is obtained for the case of a cylindrical quantum wire with an infinite interface barrier potential and a parabolic potential within the wire. The results are represented for the dependence of bulk‐like and interface phonon contributions to polaron energy levels on a magnetic field. The magnetopolaron resonance is considered and the splitting of polaron levels is found.
Correlation between the Porous Silicon Morphology and the Photoluminescence Efficiency
Authors: Goudeau P. , Naudon A. , Vezin V. , Halimaoui A. , Bomchil G. , Lambert B.
Published: July 2007

Small‐angle X‐ray scattering is applied to investigate the microstructure and the morphology of three photoluminescent porous silicon samples of the same porosity (80%) but prepared using different electrochemical treatments: (A) as‐prepared, (B) as‐prepared 65% porosity, followed by an oxidation treatment and an oxide dissolution in HF, (C) as‐prepared 65% porosity followed by a HF dissolution. Although the PL spectra are almost similar in energy, there are differences in their intensities: the luminescence intensity of the as‐prepared sample A is a bit greater than the one observed for sample B and much more greater than the one corresponding to sample C. If one assumes the same surface passivation (Si‐H) for the three samples then these differences in luminescence intensity may be due to the different PS morphologies which are evidenced in SAXS curves at low q values.
Saturation and Voltage Quenching of the Porous Silicon Luminescence and Importance of the Auger Effect
Authors: Romestain R. , Vial J. C. , Mihalcescu I. , Bsiesy A.
Published: July 2007

Two important observations for porous silicon, the saturation and the voltage selective quenching of the photoluminescence, are presented. Their similarities are pointed out and discussed in two phenomenological models, the saturation of the absorption and an Auger effect. The consequences of carrier accumulation in quantum crystallites are emphasized in both cases.
The effects of LO‐phonons to Electrons in Quantum Wires
Authors: Onwuagba B. N. , Wei Bao‐Hua
Published: July 2007

The microscopic model is used to describe the interaction between bulk‐like phonon modes and electrons in one‐dimensional (1D) quantum wires. The total transition rate for phonon modes and electrons in 1D quantum wires is discussed in the framework of the generalized 1D Fröhlich Hamiltonian. Numerical results are presented and compared with previously related work.
Conduction Band Edge Charge Densities in InxGa1−xSb
Authors: Bouarissa N. , Aourag H.
Published: July 2007

The empirical pseudopotential method coupled with the virtual crystal approximation which incorporates compositional disorder as an effective potential is used to compute the electronic conduction band edge charge densities at the Γ and X points for the ternary alloy InxGa1–xSb. It is found that these charge densities are strongly dependent upon the stochiometric coeficient x. Such differences are crucial for a comprehensive understanding of interstitial impurities and the response of specific band states to perturbation in ternary alloy semiconductors.
Magnetic Field Induced Polarization of Luminescence in Paramagnetic and Spin‐Glass Phases of Cd1–xMnxTe
Authors: Kusraev Yu. G. , Koudinov A. V.
Published: July 2007

The magnetic field induced polarization of photoluminescence is studied at different temperatures in Cd1–xMnxTe samples with x ranging from 0.005 to 0.38. The mechanism of polaron alignment is shown to depend on manganese content and temperature. In paramagnetic samples polarization is governed by thermal disorder of magnetic polaron axes, while in the spin‐glass phase the magnetic anisotropy of the lattice spin system predominates. The change of the mechanisms becomes visible when plotting polarization versus temperature.
Bragg‐Williams Model of Thermal Defect Formation in Ordering Binary Alloys with D03 Structure: Applications to Fe3Al
Authors: Spišák D. , Mehrer H.
Published: July 2007

Thermal defect formation in ordering binary alloys with DO3 structure is studied in the framework of a Bragg‐Williams model including nearest and next‐nearest neighbour bonds and applied to the case of Fe3AI. Values for vacancy concentrations on the various sublattices are deduced including the configurational entropy of the system but neglecting defect formation entropies. Vacancy concentrations are found to be fairly high especially on the Fe sublattices. Antisite atom concentrations are calculated and found to be several orders of magnitude higher than vacancy concentrations. Information about short‐range ordering is also achieved. Some consequences for diffusion in DO3 ordering alloys are discussed.
Compton Profile of Ruthenium
Authors: Kothari R. K. , Joshi K. B. , Sharma M. D. , Ahuja B. L. , Sharma B. K.
Published: July 2007

The first measurement of the Compton profile of polycrystalline ruthenium is reported and compared with recent APW calculations of Papanicolaou et al. with and without correlation effects. A comparison with our calculation based on the renormalised‐free‐atom (RFA) model is also made. It is seen that the incorporation of electron correlation effect improves slightly the agreement between APW theory and experiment but the best agreement is found for 4d7.65S0.4 configuration within the RFA model.
Electrical Study of 123 HTSC (77 to 400 K) with Varying Fraction of Y. Explanation of the Normal State Behaviour
Authors: Roy D. , Ghosh B. , Neogy C. , Deb S. K. , Nag A.
Published: July 2007

By varying the fraction n of yttrium in copper oxide based ceramic superconductors (nRE)BaCuO (RE = Y/Dy) it is found that below a certain n the dc percolation path through superconducting grains is lost and the material behaves insulator‐like with variable range hopping mode of electrical transport (In ϱ ∞ T−1/4. Experiments also show that the variation of resistivity ϱ with temperature T in the normal state for samples (n ≧ 0.25) exhibiting a superconducting transition is far from linear as the temperature is raised. By an extension of Mott's original ideas to polaron hopping with decreased localisation it is found that In ϱ should vary as T in satisfactory agreement with experiment. Thus, the so‐called “anomalous temperature variation of resistivity” in the metal‐like normal statc as well as in the insulator‐like state with increased disorder is explained within a single framework of hopping of localised carriers.
2D Scaling Properties of the Angular Dependence of the Critical Current Density in (YBa2Cu3O7)3/(PrBa2Cu3O7)8 Superlattice
Authors: Li K. , Qi Zh.
Published: July 2007

The critical current density Jc of the [(YBa2Cu307)3/(PrBa2Cu307)8 (YBCO)3/(PrBCO)8] superlattice as a function of the angle θ between the direction of the magnetic field and the c‐axis is measured in several magnetic fields at T = 55.8 K. Experimental results show that the angular dependence of Jc can be well scaled by a 2D model, i.e., only the perpendicular field component is relevant. The pinning mechanism in this film is discussed.
Theory of the Thermopower and Nernst Effect in Random Two‐Component Solid Systems
Authors: Fishchuk I. I.
Published: July 2007

The theory of the thermopower and the Nernst effect in inhomogeneous solid systems which consist of random high‐ and low‐conductivity regions is developed using the effective‐medium theory. An approximation is discussed in the case that the effective conductivity depends merely on the pair of conductivities in the components as this would happen in the absence of thermoelectricity. It is shown that this approximation can be used in the presence of thermoelectricity in almost all cases of random two‐component systems. It is found that the Nernst coefficient has the minimum value in the vicinity of the percolation threshold.
Carrier Relaxation in a‐Si:H/a‐Sic:H Multilayers Studied by Picosecond Transient Reflectometry
Authors: Galeckas A. , Petrauskas M. , Wang F. , Schwarz R.
Published: July 2007

The dynamics of photocarriers under high excitation in a series of a‐Si:H/a‐SiC:H multilayer structures with different bilayer thicknesses is studied with picosecond resolution using pump‐and‐probe reflectivity measurements. By varying both the photon energy and the intensity of the pump beam, the processes of carrier thermalization from extended into localized states on a picosecond time scale and free carrier trapping assisted processes on a nanosecond time scale are identified. The obtained relaxation time dependence on bilayer thickness is explained in terms of the competition of carrier scattering and enhanced recombination processes at the interfaces. The parallel and perpendicular transport peculiarities of photogenerated carriers are discussed.
The Off‐Center Effect on the Diffusion Coefficient of Cu+ and Li+ in the KCl Lattice
Authors: Despa F.
Published: July 2007

It is well‐known that the diffusion coefficients of the Cu+ cation in the NaCl and KCl lattices exceed by three or four orders of magnitude the corresponding self‐diffusion coefficients in the intrinsic temperature regions. This fast diffusion of the Cu+ has been explained in many papers as an interstitial diffusion although the optical spectra do not confirm the existence of interstitial Cu+. In this paper a new mechanism for fast diffusion is proposed. The model assumes that the equilibrium positions of the cationic impurities are noncentral and that the diffusion proceeds by hopping across the potential barrier along the nonlinear paths with the highest probability. The main result shows that the off‐center position considerably enhances the diffusion. Theoretical diffusion coefficients are obtained by modelling the potential barrier. Changes of the configuration entropy and the vibration spectra due to the presence of the noncentral impurity are included in the model. The numerical results of the diffusion coefficient computed in this way are in agreement with the experimental data and it is shown that if the impurity is placed close to the central site the due diffusion coefficient is close to that for the cationic self‐diffusion. The Li+ cation case is dealt with as the case of the Cu+ cation.
A Simple Analysis of the Einstein Relation in Ultrathin Films of Nonparabolic Semiconductors in the Presence of an Arbitrarily Oriented Magnetic Field
Authors: Ghatak K. P. , Bhattacharya D. , Basu D. , Nag B.
Published: July 2007

An attempt is made to study the Einstein relation of the diffusivity—mobility ratio of carriers in ultrathin films of nonparabolic semiconductors in the presence of an arbitrarily oriented magnetic field. This is done on the basis of a new electron dispersion law. It is found, taking Hg1−xCdxTe and In1−xGaxAsyP1−y lattice‐matched to InP as examples, that the diffusivity‐mobility ratio increases with increasing surface electron concentration and decreasing alloy composition and film thickness in various oscillatory manners. The magnetic field enhances the numerical value of the diffusivity‐mobility ratio and an experimental method is suggested to determine it in degenerate materials with arbitrary dispersion laws. In addition, the well‐known corresponding results for relatively wide‐gap materials in the absence of a magnetic field are obtained as special cases of our generalized analysis.
Theoretical Investigations of the Sideband Effect in Mössbauer Spectra of FeBO3
Authors: Maier Ch. , Gonser U.
Published: July 2007

A theoretical investigation of the sideband effect in Mössbauer spectra of FeBO3 induced by radio frequency (rf) magnetic fields is presented. Because the interpretation of the sideband effect of FeBO3 with existing theoretical models reveals no satisfactory results modifications are developed to improve the description. Especially nonlinear acoustic properties of the sample under investigation are taken into account. It can be shown that new resonances occur leading to additional sidebands at fractional integers of the applied rf frequency which have also been experimentally observed. Taking into account this feature an improved empirical description of the sideband effect of FeBO3 is developed. This implies that nonlinear acoustic properties must be considered to obtain an appropriate description of the sideband generation in FeBO3.
A Model B.C.C. Iron Including Angular Interactions. The Self‐Interstitial
Authors: Simonelli G. , Pasianot R. , Savino E. J.
Published: July 2007

Self‐interstitial configurations in Fe are simulated using an embedded‐atom‐method (EAM) interatomic potential and an embedded‐defect (ED) one recently developed by the authors. The latter potential includes angular‐dependent many‐body local terms. Formation energy and lattice distortion for the 〈110〉 and 〈111〉 oriented dumbbells are calculated. The relative contributions of the pair and many‐body interaction terms to these properties are discussed. Comparisons with other many‐body potentials from the literature are made.
Intersubband Transitions in Thin Bismuth Films
Authors: Peschke Ch.
Published: July 2007

The reflectivity and transmittivity of thin monocrystalline bismuth films on barium fluoride are measured in the spectral range of 80 to 4000 cm−1. The measurements are performed at room temperature and at 5 K. At 5 K, thickness dependent absorption maxima occur if the film thickness is less than 50 nm. These maxima are interpreted as transitions between subbands of valence and conduction band at the L‐points of the Brillouin zone. The spectral position of the maxima is explained assuming an elevated gap energy due to lattice mismatch between films and substrates. Conclusions on the boundary condition of the electron wave function are supported by an analysis of the plasma frequencies of the films.
On the Influence of Weak Spatial Disorder on the Stress Fields of Finite Dislocation Walls
Authors: Raabe D.
Published: July 2007

Numerical simulations of the three‐dimensional elastic stress fields associated with finite ordered and weakly disordered dislocation walls are carried out. The walls consist of discrete dislocations which are regarded as piecewise linear defects embedded within an otherwise isotropic elastic medium. The dislocation line vectors and Burgers vectors involved constitute a simple cubic lattice. The dislocations are subdivided into small segments. Two basic types of dislocation walls are studied, namely, a finite small angle tilt boundary and a finite dislocation cell wall consisting of 45° edge dislocation dipoles. The tilt boundary consists of 100 straight parallel dislocations. The dipole wall contains 100 alternately antiparallel edge dislocations. For both wall types the influence of weak spatial disorder on their stress fields is investigated. For this purpose non‐straight dislocations with a fractal dimension of 1.05 are used instead of straight dislocations. The study substantiates that weak spatial disorder considerably affects the stress field in the vicinity of the dislocation walls. However, the course of the long‐range stress is not predominantly determined by spatial disorder of the dislocation lines but by the finite geometry of the walls. The maximum shearing stress which is associated with the finite tilt boundary reveals a strong (but not exponential) decay in the direct vicinity of the wall. A local minimum occurs within a distance which exceeds the dislocation spacing in the wall by less than one order of magnitude. A local stress maximum is generated within a distance which exceeds the dislocation spacing by about two orders of magnitude. Finally, a long‐range stress field is predicted which corresponds to the solution obtained for a corresponding super‐dislocation with bsuper = 100bsingle (bsuper the Burgers vector of the super‐dislocation, bsingle the Burgers vector of the dislocations in the wall). The stress field of the dipole wall is not negligible but reveals short‐ and intermediate‐range stress components which decay with 1/distance.
A Bound State of Positrons with Sulphate Ions
Authors: Serna J. , Troev T.
Published: July 2007

Positron lifetime spectra are obtained in four different single crystals of sulphates: sodium, strontium, barium, and lead sulphate. The spectra can be resolved into four components, although only the intermediate ones are significant since the other components, with a very low intensity percentage, are associated to positronium formed at the interface of the source with the specimen. The intermediate short‐lived component is associated with the annihilation of the outer electrons of the oxygen ions. The annihilation of positrons in cation vacancies or in interstitial impurities may also contribute to this component. Annihilation in other defects, like clusters of vacancies, is associated with the intermediate long‐lived component, whose main contribution seems to be a bound state of positrons with electrons belonging to the covalent bonds between sulphur and oxygen.
Calculated Electronic Structure and Hyperfine Interactions on γ′Fe4Z (Z  N, C)
Authors: Kong Yong , Zhou Rongjie , Li Fashen
Published: July 2007

The spin‐polarized linear muffin‐tin orbital (LMTO) method is used to calculate the electronic energy bands of γ′‐Fe4Z (Z = N, C) and to study the hyperfine interactions in Fe4Z. In comparison with the effect of N atoms, the C atom enhances the itineracy of electrons and then in turn prevents the exchange splitting. The C atom brings about a larger charge transfer among the atoms in the unit cell. The results show that the interaction between the Z and the nearest Fe atoms (FeII) is stronger than that between the Z and the next‐nearest Fe atoms (FeII). According to the calculated Fermi contact hyperfine magnetic field (HFC), the proportionality constant between the core contribution of HFC and the local magnetic moment of the atom is evaluated as ≈︁  13 T/μB. The calculated values of HFC and the Mössbauer isomer shift (IS) indicate that the FeIFeII and FeZ interactions strongly depend on the Z atom. The results of Hhf and IS are qualitatively in agreement with the experimental results.
Study of Balance Equations for Hot‐Electron Transport in an Arbitrary Energy Band
Authors: Wu Hang‐Sheng , Wu M. W.
Published: July 2007

In view of the recent disputes of Huang and Wu and Lei about Lei's balance equation theory for an arbitrary energy band, the balance equations for hot‐eléctron transport in an arbitrary energy band are reformulated. In the new formulation the adoption of the effective Hamiltonian method is avoided and all calculations are carried out in laboratory coordinates. In the mean time the crystal momentum k is carefully distinguished from the physical momentum. By introducing a correct equilibrium density matrix of the drifting electron gas in place of that used by Lei, the balance equations together with the distribution function can be derived directly with no need to distinguish the degree of freedom of the center of mass from the relative degree of freedom of the electrons, which is in our opinion neither possible nor necessary at least for a nonparabolic energy band. Some problems associated with the foundation of the balance equation theory are also examined. The result shows that the comment to Lei's theory given by Huang and Wu is correct and to the point. Nevertheless, contrary to their assertion that this theory is inapplicable to the nonparabolic band transport, Lei's balance equations, after a few, however, important corrections, are still applicable to transport for nonparabolic energy bands.
The Perspectives of Hydrogenated Amorphous Germanium as an Electronic Material
Authors: Chambouleyron I. , Graeff C. F. , Zanatta A. R. , Fajardo F. , Mulato M. , Campomanes R. , Comedi D. , Marques F. C.
Published: July 2007

This paper refers to the perspectives of the use of hydrogenated amorphous germanium and its alloys (deposited by the rf sputtering method). It is shown that considerable progress has been made in the optimization of aGe: H films. The deposition conditions leading to a good quality material and the corresponding properties are discussed. The problem of material stability, as well as recent progress in the understanding of the n‐ and p‐type doping mechanisms in the aGe: H network are presented. The main issues and perspectives of hydrogenated GeSi, GeN, and GeSn alloys are analyzed. Finally, results in the aGe: H device area are presented.
Mössbauer effect in atomic ordered and disordered Fe0.75−xMnxAl0.25 alloys
Authors: Babu T. V. S. M. Mohan , Bansal C.
Published: July 2007

A Mössbauer effect study of the effect of chemical order on hyperfine magnetic field distributions in the quasi‐binary alloy system Fe0.75−xMnxAl0.25 (based on substitution of Fe by Mn in Fe3Al) shows that a preferential substitution of Mn at one of the sites leads to a B2 order in the ternary system. The kinetics of the ordering process in Fe0.45Mn0.3Al0.25 shows distinctly different relaxation behaviours at the two possible inequivalent sites in the ordered structure.
Spin‐lattice relaxation in diluted magnetic semiconductors
Authors: Scalbert D.
Published: July 2007

The contribution of pairs and triads of nearest‐neighbor spins to the spin‐lattice relaxation of magnetization is evaluated in Mn‐based diluted magnetic semiconductors. Modulation of the exchange interaction by lattice vibrations allows transitions in the cluster which change the total magnetization when the Dzyaloshinski‐Moriya interaction is taken into account. Spin‐lattice relaxation rates are calculated for pairs and open triads due to a two‐phonon Orbach process. In Cd1−xMnxTe it is found that the main contribution to relaxation of magnetization comes from triads. For temperatures ranging from 10 to 35 K and for x between 0.002 and 0.02 relaxation of triads can account very well for the observed relaxation rates, if one assumes rapid spin diffusion. Fitting of the experimental data yields an approximate value of a = J/(dJ/dr) ∼ 0.35 A, where J is the nearest‐neighbors exchange constant and r the Mn‐anion bond length, in good agreement with the expected value for a superexchange interaction.
Decoupling approach for the Hubbard model
Authors: Gehlhoff L.
Published: July 2007

The U = ∞ one‐band Hubbard model with large spin degeneracy N formulated with X operators is considered. The single‐particle Green's function is determined by applying the equation‐of‐motion method and a decoupling approach which is exact in the large‐N limit.
Single crystal EPR studies of water bridged one‐dimensional paramagnet diacetato‐copper (II)‐bis‐(p‐toluidine) trihydrate
Authors: Datta S. , Pal A. K.
Published: July 2007

X‐band EPR measurements of g‐factor, lineshape, and linewidth in three mutually orthogonal planes of diacetato‐copper(II)‐bis‐(p‐toluidine) trihydrate at 77 and 300 K are reported. The lineshape is found to be Lorentzian in all directions including the chain and the magic angle. Excellent fitting of the linewidth anisotropy is obtained on the basis of the anisotropic exchange (AE) theories of McGregor and Soos and Ritter et al. The z‐direction of the AE tensor is found to be coincident with the chain direction. Values of isotropic and anisotropic exchange parameters derived from the above fitting are found to be in good agreement with those obtained from magnetic susceptibility measurements and the well‐known Moriya relation, respectively.
Magnetization curves of disordered cellular domain patterns in thin magnetic films
Authors: Zablotskii V. A. , Mamalui Yu. A. , Soika E. N.
Published: July 2007

Observations of the coarsening of two‐dimensional cellular domain patterns with increasing film temperature at a constant bias field in magnetic garnet films are reported. It is shown that during this coarsening the value of the relative film magnetization increases just as it occurs on a magnetizing film by an external magnetic field at a fixed temperature. The dependences of the film magnetization on temperature as well as the bias field strength are calculated.
Recent advances of the photovoltaic activities in Japan
Authors: Hamakawa Y.
Published: July 2007

A review is given on recent progress in solar photovoltaic technology in Japan and its contributions as a clean energy resource for solving the worldwide environmental issue. First, a new concept to reform the energy strategy for future civilization is postulated taking account of the environmental issue. Organizations and objectives of the New Sunshine Project are introduced. Second, some unique advantages of solar photovoltaics are described and compared with conventional electric power generation technologies. Third, key issues to develop photovoltaic technology are revealed, and recent R & D efforts to solve them are demostrated with live technologies in progress. Finally, possible new roles to contribute to the global environment issues by the photovoltaic technology are proposed and discussed.
Heterojunctions in photovoltaic applications
Authors: Elstner L. , Conrad E. , Eschrich H. , Füssel W. , Flietner H.
Published: July 2007

Numerical simulations on silicon solar cells with heteroemitter layers made from various wide‐gap semiconductors are described and the dependence of the I–V curves on the band offsets is explained. The main advantage utilizing heteroemitters on silicon is the chance for an improved response in the blue region of the solar spectrum which may achieve an increase of the total photocurrent up to 10%. Experimental results on test solar cells with μc‐Si, ZnO, and SIPOS heteroemitters are presented and compared with these predictions.