bluuebunny/crossref_metadata_2025 huggingface dataset:
Loss minimization through recombination center identification in solar cells
Authors: Lu Fu‐Hsing
Published: July 2007

Carrier recombination, one of the major loss mechanisms in solar cells, has been found to be quite sensitive to the charge character of the Shockley‐Read‐Hall recombination center. Varying either the capture cross sections of the carriers or the density of the recombination centers results in a substantial change of the open‐circuit voltage as well as the fill factor in a solar cell. Compared to a neutral center, the center which remains repulsive to the opposite carrier after capturing a first carrier is beneficial to the open‐circuit voltage, when maintaining a constant density of recombination centers, whereas the center which becomes attractive to the minority carrier is detrimental to the open‐circuit voltage.
Excitonic effects in the light‐harvesting Chl a/b–protein complex of higher plants
Authors: Voigt J. , Renger Th. , Schödel R. , Schrötter Th. , Pieper J. , Redlin H.
Published: July 2007

Exciton states and their coupling to long‐wavelength protein vibrations are investigated both, experimentally and theoretically, in Chl a/b pigment–protein complexes acting as light‐harvesting antennae in higher plants. It is shown that the 12 chlorophyll molecules located within the involved protein‐monomers form extended exciton states and act cooperatively in the light absorption process. Besides one‐exciton excitations also two‐exciton excitations are demonstrated at higher photon densities incident on the monomer clusters resulting in nonlinear transmission and emission effects. The distribution of excitons over the exciton energy levels follows a Boltzmann law as shown by emission and nonlinear transmission experiments. The exciton–vibration coupling is weak as indicated by a resonance‐like zero‐phonon emission line which shows a strong reabsorption. Conclusions are drawn about the structure–function relationship of this antenna type. Interesting similarities between the biological structure investigated and semiconductors or semiconductor structures are discussed.
Stochastic theory of cluster dynamics
Authors: Pal L.
Published: July 2007

An exactly soluble stochastic model of the clusterization process is developed and the time dependence of the probability of the cluster size is calculated. The condition of stationarity is derived and the correlation function of the stationary cluster size fluctuations is determined. The properties of the spectral density function and that of the distribution function of the cluster lifetime are analysed.
A non‐planar crack analysis using continuously distributed sinusoidal edge dislocations and linear elasticity
Authors: Anongba P. N. B.
Published: July 2007

Non‐planar cracks, propagating along crystallographic directions in the cleavage fracture of brittle materials and fluctuating about a mean plane, are represented in a simple model by a distribution of sinusoidal edge dislocations in a linear elastic medium; the dislocations are perpendicular to the crack propagation direction. The opening mode I loading is assumed. It is shown that, at the front of the sinusoidal crack and out of the mean fracture plane, the stresses are bounded and the derivative G of the energy decrease of the system with respect to crack area is zero. Only the positions of the crack front located on the mean fracture plane are associated with a nonzero value of G always lower than the well‐known crack opening force G0 = K(1−v2)/E per unit edge length of the planar crack (K1 is the stress intensity factor of mode I loading and E is Young's modulus). It is concluded that the sinusoidal crack cannot start propagating from positions on the crack front that are out of the mean fracture plane; instead, crack propagation could begin very near to the intersections of the crack front with the mean fracture plane if certain assumptions about the anisotropy of the surface energy are made.
Electrical and thermal studies of NaNO2
Authors: Ahmed S. A. , Ali M. H.
Published: July 2007

The present paper reports on experimental results on polycrystalline NaNO2 employing the dc resistivity, DTA, and unit cell parameters (measured by X‐ray diffractometry) in the temperature range from 298 to 480K. All the presented results confirm, in addition to the well‐known phase transitions at 436.5 and 438 K, the existence of a second‐order phase transition occurring at 448 K. We found clear evidences that the ferroelectric phase transition of NaNO2 is of second order close to the first‐order one, and the rotation of the NO radical mostly takes place around the c‐axis. The high‐temperature phase transformation occurring at 448 K is thought to be due to the completely disordered arrangement of the NO radicals in the normal paraelectric phase.
Eight‐band k·p Hamilton matrix for strained tetrahedral semiconductors: 4 × 4 block diagonalization for symmetric k‐directions
Authors: Enders P. , Woerner M.
Published: July 2007

In its mostly used form, the eight‐band k·p Hamilton matrix for diamond and zincblende structure semiconductors exhibits a twofold (Kramers) degenerated spectrum throughout the Brillouin zone (BZ), though its general block diagonalization (being valid for all k‐vectors) into two 4 × 4 blocks is not known yet. As a step in this direction, the 4 × 4 block diagonalization is achieved for k being parallel to the cubic symmetry axis. Using this, the calculation of electronic properties can be largely simplified, as demonstrated explicitly for the optical transition matrix elements. These results apply both to unstrained and to uni‐or biaxially strained materials.
The hot electron distribution function under impurity breakdown conditions
Authors: Kundrotas J. , Dargys A. , Cesna A.
Published: July 2007

The Boltzmann transport equation is solved by sparse‐matrix method under conditions that are typical for experiments of neutral impurity breakdown at low lattice temperatures. A strong cooling effect due to impact‐ionizing collisions is found on the hot carrier distribution function in moderately doped semiconductors. Implications of the cooling effect on the impact ionization coefficient are discussed. The results of simulation are presented for parameter values that are typical for n‐type GaAs doped with shallow donors and compensating acceptors, and compared with experimental data.
Magnetic measurements and crystal field investigations on Yb(BrO3)3 · 9 H2O
Authors: Neogy D. , Chakrabarti P. K. , Chattopadhyay K. N. , Chatterji A.
Published: July 2007

Ytterbium bromate (Yb(BrO3)3 · 9H2O) single crystals were grown, and the magnetic susceptibility perpendicular to the symmetry axis (χ⟂) and the magnetic anisotropy (Δχ = χ∥ − χ⟂) were measured in the temperature range 300 to 14 K and 300 to 90 K, respectively. The magnetic anisotropy at 300 K is considerable (≈ 23%). The smooth variation of χ⟂ down to 14 K does not suggest any structural phase transition or symmetry breaking. An analysis of the observed results undertaken for the first time invoking crystal field theory based on rare‐earth site symmetry D3h provides a satisfactory explanation to the experimental results.
Extended novel slab model analysis of overlaid bent step‐index single‐mode fiber half‐couplers. II. Coupled‐mode approach and design of overlays
Authors: Haubenreisser W. , Ecke W.
Published: July 2007

We present for the modal analysis of overlaid bent step‐index single‐mode fiber half‐couplers (OL‐BSSF) a coupled‐mode approach where the overlay (OL) and the BSSF structure are analyzed in isolation and the design criterion of suited BSSF and OL parameters is based on the phase‐matching condition of the coupled‐mode theory. The BSSF is mapped modally by an equivalent slab waveguide (ESW) structure. For the OL structure a general three‐layered overlay is considered which takes into account lossless, lossy, leaky, and plasmon active properties. Analytical expressions for the propagation characteristics of bound, bound surface plasmon polariton (SPP), leaky, and leaky SPP overlay modes are derived on which OL parameters at given BSSF/ESW parameters can be designed.
A non‐quadratic irrationality associated to an enneagonal quasiperiodic tiling of the plane
Authors: Franco B. J. O. , da Silva F. W. O. , Inácio E. C.
Published: July 2007

A enneagonal tiling of the plane is proposed. A self‐similar pattern is obtained by using eight basic shapes. This pattern presents rotational symmetry and no translational invariance.
X‐ray diffraction behaviour of W/Ti superlattices
Authors: Pan F. M. , Feng J. W. , Jin G. J. , Hu A. , Jiang S. S.
Published: July 2007

The structural properties of periodic and quasiperiodic W/Ti superlattices have been investigated by means of X‐ray diffraction. Intensity attenuation of even ordered peaks occurs in the spectra of W/Ti periodic superlattices with near‐equal‐layer thicknesses. This phenomenon is theoretically explained. Two series of quasiperiodic (Fibonacci) samples are prepared, one with thickness ratio dA/dB=τ, and the other dA/dB = 1/τ. X‐ray diffraction results of these quasiperiodic superlattices are discussed and compared. The possible application of Fibonacci films as optical elements in the soft X‐ray region is discussed. Furthermore some X‐ray diffraction spectra are simulated by computer calculation in order to understand the detailed structures. The calculated results are in good agreement with measured ones.
Analysis of the pressure dependence of the thermal expansivity of NaCl crystal
Authors: Thomas L. M. , Shanker J.
Published: July 2007

The pressure dependence of the thermal expansivity of NaCl crystal has been studied using various relations between α and the Anderson parameter δ The variation of α with pressure has been estimated and taken into account for determining the pressure dependence of α. A comparison of the results derived from various models has been presented with available experimental data. It is found that, the Anderson power law for the pressure dependence of δ is more consistent with the experimental data.
Tunnel electron current induction in n+‐I‐M structures
Authors: Volkov R. A. , Dyakov I. V.
Published: July 2007

A novel effect of tunnel electron current induction (TECI) is theoretically considered in n+‐I‐M structures with a penetrable potential barrier (I‐layer) resulting from heating the n+‐layer electrons by an external electric field and tunnelling them into the n+‐layer through the I‐insulator. A tunnel electron current which is thus induced leads to charging the structure and to a potential difference ‐ the tunnel electromotive force (TEMF)‐between the n+‐ and M‐layers. TEMF is a practically significant consequence of the TECI effect. An equation for TEMF has been deduced using numerical calculations. Approximate analytical solutions of this equation and their application limits are cited in electron temperature approximation as functions of the external field, lattice temperature, and structure parameters. Calculations of the TEMF build‐up time are presented as functions of the external field. All expressions are calculated for lattice temperatures of 300 and 77 K. Experimental observation possibilities of TECI are also briefly viewed.
Grain boundary scattering in DC transport in thin metallic films
Authors: Gerlach E.
Published: July 2007

The energy‐loss transport theory is applied to calculate the resistivity due to grain boundary scattering in a thin metallic film adjacent to two dielectric media, e.g. vacuum/Ag/Si. The common “bulk” result by Mayadas and Shatzkes for small film thicknesses is found to be reduced by about 20% due to image effects.
Effect of high hydrostatic pressure on the magnetoresistance quantum oscillations of the quasi‐two‐dimensional organic superconductor x‐(BEDT‐TTF)2i3
Authors: Helm P. , Kraak W. , Schweitzer D.
Published: July 2007

High‐field magnetotransport investigations of single crystals of the quasi‐two‐dimensional organic compound x‐(BEDT‐TTF)2I3 under hydrostatic pressure up to 9 × 108 Pa are presented. Pronounced Shubnikov‐de Haas oscillations are observed in magnetic fields up to 12 T over the whole pressure range. These quantum oscillations are affected remarkably by hydrostatic pressure. All of the four fundamental fields detected rise with increasing pressure corresponding to an enhancement of the extremal areas of the Fermi surface in x‐(BEDT‐TTF)2I3. The experimentally determined cyclotron effective masses of the charge carriers decrease with increasing pressure. The FFT amplitudes of the Shubnikov‐de Haas oscillations show a very complex pressure dependence reflecting a more complicated structure of the electron system than the relatively simple topology of the calculated Fermi surface involves. The pressure‐induced changes of the Fermi surface are discussed in the framework of an effective dimer model and compared with equivalent investigations on the isostructural compound x‐(ET)2Cu(NCS)2.
Finite cluster approximation study of spin‐1 ising model in transverse and crystal fields
Authors: Song Wei‐Ji , Yang Chuan‐Zhang
Published: July 2007

A spin‐1 Ising model with the presence of transverse and crystal fields (CTIM) is studied within the scheme of the finite cluster approximation (FCA). The phase diagrams and the other critical behaviour are numerically investigated for the honeycomb lattice. FCA yields results which are superior to those of the standard mean‐field approximation (MFA).
Effect of the topological transition on the optical absorption edge of sulphur‐based glasses
Authors: Tsiulyanu D. I. , Marian S. I.
Published: July 2007

Exponential absorption edge of chalcogenide glassy alloys with sulphur in the region of topological transition is studied. It is shown that the slope of the tail depends on mean coordination number and stoichiometry of the materials, showing a common behaviour with position of the edge.
Optical properties of Er3+ ions in zinc borosulphate glasses
Authors: Kumar V. V. Ravi Kanth , Jayasankar C. K. , Jagannathan R.
Published: July 2007

The optical properties of Er3+ ions in zinc borosulphate (ZnSO4 · 7 H2O + B2O3) and alkali zinc borosulphate (R2SO4 + ZnSO4 · 7H2O + B2O3, R2SO4 = Li2SO4 · H2O, Na2SO4, or K2SO4 and Gd2(SO4)3 · 8H2O) glasses are reported. The assigned energy level data of Er3+(4f11) in borosulphate glasses are analyzed in terms of a parametrized model Hamiltonian and in turn satisfactory eigenvalues and eigenvectors for intensity analyses are obtained. The Judd‐Ofelt theory has been applied to predict various spectroscopic parameters for the fluorescent levels of Er3+ : borosulphate glasses and these results are compared and discussed with similar results.
Temperature dependence of elastic constants and thermal expansion coefficient for NaCl crystals
Authors: Thomas L. M. , Shanker J.
Published: July 2007

The second‐order elastic constants C11, C12, and C44, isothermal bulk modulus BT, and volume thermal expansion coefficient α are calculated for NaCl crystals in the temperature range (294 to 766 K) using Tallon's model based on the concept of modified Anderson‐Grüneisen parameters. The results obtained in the present study are compared with the experimental values reported in the literature. There is good agreement between calculated and experimental values for V/V0, C11, C44, BT, and α. It is found that C11, C44, and BT decrease whereas C12 and α increase with increase in temperature.
Luminescent associates of indium ions with interstitials and vacancies in an x‐irradiated KCl: In crystal
Authors: Aceves R. , Flores M. Barboza , Nagirnyi V. , Salas R. Perez , Usarov A. , Zazubovich S.
Published: July 2007

The spectral, polarization, and kinetic characteristics of luminescence are studied for In+ and In2+ ions associated with interstitials and vacancies in an X‐irradiated KCl: In crystal. It is shown that the 2.08 eV emission is caused by the electronic transitions from the tetragonal minima of the triplet relaxed excited state of an In+ ion perturbed by an interstitial halide atom (H centre) and a cation vacancy v. The emission bands peaking in the region from 1.9 to 1.7 eV are ascribed to various types of associates of an In2+ ion with v or with v and one or two H centres. It is concluded that the anisotropic impurity colour centres studied are produced by X‐irradiation mainly on the basis of the In3+v dipoles existing in the nonirradiated KCl:In crystal due to the changeable valence of indium. The processes of the production, destruction, and mutual transformation of the impurity colour centres of various types are studied in the temperature range from 4.2 to 400 K.
Non‐markovian behavior of quantum confined electronic systems
Authors: Bertoncini R. , Mauser N.
Published: July 2007

The holomorphic memory function method is modified to include geometrical confinement. A ‘ballistic memory function’ is identified, showing that energy quantization due to lateral confinement is associated with a retardation in the system. Consequently, the ballistic conductance quantization of waveguides is shown to be caused by long‐lived correlations between values of the applied field at different points in the device. A renormalized force correlation function shows the mutual influence of geometrical confinement and scattering mechanisms: confinement‐induced memory effects modify the elastic scattering white‐relaxation spectrum and transform it into a scattering mechanism with enhanced retardation properties. Evidence of these phenomena is given through the numerical evaluation of the ac conductance and time‐dependent current for a rectangular crosssection wire of finite length.
Extended chain approximation in quasi‐one‐dimensional ising model
Authors: Muraoka Y. , Idogaki T.
Published: July 2007

The extended chain approximation (ECA) in the quasi‐one‐dimensional Ising model is proposed and is found to give the same results as the Bethe‐Peierls approximation for the ferromagnetic Ising model. It is discussed that the ECA can be applied to complicated systems such as the axial next‐nearest‐neighbour Ising (ANNNI) model, etc.
Visible photoluminescence in porous silicon prepared in different conditions ‐ temperature dependence and decay
Authors: Ciurea M. L. , Pentia E. , Manea A. , Belu‐Marian A. , Baltog I.
Published: July 2007

Measurements of the visible photoluminescence and the decay of the photoluminescence, performed at different temperatures on porous silicon films, prepared by the anodization process under different conditions on (100) p‐type silicon wafers are reported. A separation of the broad visible band of the photoluminescence in two subbands with maxima situated at 1.54 and 1.72eV at room temperature related to the preparation conditions is obtained. This separation is still present at low temperatures but is very weak. The PL decay process is well described by a bimolecular recombination. The decay time α−1 is in the order of microseconds at room temperature and nanoseconds at liquid nitrogen temperature.
Charge transfer in the bonding of gaas
Authors: Chandrasekaran K. S. , Mohanlal S. K. , Saravanan R.
Published: July 2007

Charge transfer in GaAs is analysed utilising our earlier precise and extensive X‐ray diffraction data. The h + k + l = 4n+ 2 structure factors are weak, “quasi‐forbidden” with the constituent atoms of nearly equal but opposite scattering. The measured structure factors are all systematically larger than their calculated values over the entire range of sin θ/λ up to about 10 nm−1. Moreover, they exhibit non‐equivalence of inversereflections with non‐zero indices which can arise only from bonding distortion or anharmonicity. A model calculation for the charge transfer inbonding from the III to the V atom, described by a single parameter x, valid over the entire reciprocal space, is proposed and found to yield a precise estimate for x, namely 0.52(0.12) for GaAs (in electron units). This result is in good agreement with the earlier models, and indicates an appreciable core contribution to the charge transfer.
Boundary least squares method for the solution of schrödinger's equation in quantum wires
Authors: Sirenko Yu. M. , Mitin V.
Published: July 2007

We present a method for the approximate semianalytical calculation of wave functions and eigenenergies in systems consisting of domains with knownbulk solutions of the corresponding Schrödinger equation (e.g. with piecewise constant potential). The trial wave function is written as a normalized linear combination of several bulk solutions for the given energy. The coefficients of the linear combination are found by minimization of the integral of square mismatch along the boundaries. Mathematically the problem is equivalent to the minimization of the Rayleigh quotient or solution of the generalized eigenproblem for the vector of coefficients. The value of the residual provides an estimation of the accuracy of the results and gives thepossibility to choose an optimal set of trial functions. We illustrate the use of the method by calculation of eigenfunctions of infinitely long triangular and quadrilateral wires.
Two‐LO‐phonon resonant Raman scattering in ii‐vi semiconductors
Authors: Garcí‐cristóbal A. , Cantarero A. , Trallero‐giner C. , Limmer W.
Published: July 2007

Recently, absolute values of second‐order Raman scattering efficiency have been measured around the E0 and E0 + Δ0 critical points of several II‐VI semiconductor compounds. The measurements were performed in the z(x, x)z̄ backscattering configuration on (001) (ZnSe and ZnTe) and (110) (CdTe) surfaces. They show strong incoming and outgoing resonances around the band gap and larger scattering efficiencies as compared to III‐V compounds. A theoretical model which includes excitons as intermediate states in the Raman process is shown to give a very good quantitative agreement between theory and experiment. Only a small discrepancy exists, while in III‐V compounds the discrepancies were close to one order of magnitude. A discussion of the differences in the scattering efficiencies between II‐VI and III‐V compounds is also reported.
Correlations between the cohesive energy and a crystallographic ratio in group‐II chalcogenides
Authors: Koh A. K.
Published: July 2007

Correlations between the cohesive energy per mole and a crystallographic ratio in group‐II chalcodes are presented. It is shown that the linear combination of the anion radius and the nearest‐hbour distance is a constant in these materials. Results obtained agree with works reported earlier. This work highlights the significance of the anion radius in determining the cohesive energy in group‐II chalcogenides.
Constraints, Fisher renormalization, and phase transitions in random‐field models
Authors: Busiello G.
Published: July 2007

We discuss the possibility that the critical behavior of the experimental random‐field Ising model may be interpreted as a constrained behavior. Emphasis is given to the problem of a first‐order phase transition which seems to be present in some experimental and theoretical results.
Phonon‐assisted two‐photon magnetoabsorption in an indirect band gap semiconductor quantum well
Authors: Kapatkar S. B. , Kubakaddi S. S. , Mulimani B. G.
Published: July 2007

We present a theory of phonon‐assisted two‐photon absorption in an indirect band gap semiconductor quantum well (QW) in a quantizing magnetic field for polarizations in the plane and perpendicular to the layer within a three‐band model. It is found that for both polarizations all transitions are possible between Landau levels due to phonon involvement unlike the case in direct band gap semiconductor QWs. Numerical calculations of the absorption coefficient are given for the SixGe1−x/Si QW.
Electron transport in GaAs at low lattice temperatures
Authors: Ghorai A. K. , Bhattacharya D. P.
Published: July 2007

Through numerical methods, investigations have been carried out to see how the finite energy of the deformation potential and piezoelectric acoustic phonons affects the ohmic and non‐ohmic mobility characteristics of n‐type GaAs taking the non‐parabolicity of the conduction band into account and under the conditions that, unlike the traditional approximations, the energy of the acoustic phonons can neither be neglected in comparison with the carrier energy nor the phonon population be represented by the equipartition law. The results show interesting features in that they are significantly different from those that follow from the traditional approximation of negligible phonon energy and also provide better agreement with the available experimental data. The inadequacies of the present theory and the scope for further refinements are discussed.
Ladder Ising spin configurations II. magnetic properties
Authors: Mejdani R. , Gashi A. , Ciftja O. , Lambros A.
Published: July 2007

In this paper we consider ladders with two, three, and four coupled Ising spin chains, characterized by interchain and intrachain couplings, to study in detail their magnetic behaviour for different ratios of the interaction constants and different values of magnetic field, by using a transfer matrix method and a complex algorithm, realizing the diagonalization and the differentiation. We have proposed a combined algorithm to realize also the generation of the elements of the transfer matrix for more complicated ladder configurations. The problem of Ising ladders is straightforward with a crossover between a 2D high‐temperature region and a 1D low‐temperature region when the 2D correlation length equals the width. To emphasize the fact that there is no finite magnetization in absence of the magnetic field, we study also analytically this case for the ladder with two coupled Ising chains. In presence of the magnetic field, for very weak magnetic fields (of the order 10−8, in reduced units) the magnetization goes from the value one to the value zero in a narrow interval of nonzero temperatures, where also the susceptibility exhibits a very high peak. This narrow interval, increasing the number of chains, is displaced towards higher temperatures. Also, in the case of interchain antiferromagnetic couplings, the formation of interchain spin pairs becomes clear.
The study of 57Fe Mössbauer spectra for La0.7Sr0.3FeO3−δ with different structure
Authors: Li Guangshe , Li Liping , Zhao Muyu
Published: July 2007

Samples of La0.7Sr0.3FeO3−δ were obtained by sintering the precursor prepared by the Polyethylene Glycol (PEG) method. The measurements of XRD and 57Fe Mössbauer spectra showed that the heat treatment of the precursor at different temperatures leads to phase transformations from the orthorhombic to the cubic and finally to the rhombohedral phase, to an increase of average grain size, and to changes of the electronic states of the Fe ions. It was thought that in the perovskite oxides the conditions of synthesis or treatment may have an important influence on structure and properties by adjusting the valence state or/and introducing oxygen vacancies.
Ground‐state zero‐field splitting of Fe3+ ion in ZnO and CdSe crystals
Authors: Kuang Xiao‐Yu
Published: July 2007

The ZnO and CdSe crystals have similar hexagonal wurtzite structures with a contraction along the c‐axis of the crystal, however, different electronic fine structures for ZnO:Fe3+ (D « 0) and CdSe: Fe3+ (D » 0) have been displayed in EPR experiments. This interesting phenomenon has not yet been interpreted at present. In this paper, a reasonable calculation is given by considering the covalent spin‐orbit coupling interaction. Our results show that the ground‐state splitting in ZnO:Fe3+ is due to the trigonal distortion, whereas the main physical mechanism of the splitting in CdSe:Fe3+ can be attributed to the divergent contributions of |t2〉 and |e〉 sub‐orbitals to the spinorbit coupling interaction.
Temperature dependence of the spontaneous polarization in Rochelle salt and thiourea
Authors: Piekara† A. H. , Wiȩckowski A. B.
Published: July 2007

The occurrence of ferroelectric properties characterized by two (and three) Curie temperatures is explained phenomenologically by a model of structural antiferroelectrics with two sublattices, involving hydrogen‐like bonds with asymmetric potential wells. On the assumption of a slight discrepancy between the differences in energy levels ΔU1 and ΔU2 of the two sublattices, a non‐zero spontaneous polarization is obtained for certain well‐defined ranges of temperature. The model permits an explanation of the temperature dependence of polarization in Rochelle salt and thiourea.
The peculiarities of colour centre production in doped alkali halides
Authors: Vale G.
Published: July 2007

The peculiarities of colour centre production and recombination in KBr, KI crystals doped with Tl, In are examined. The photostimulated dopant ion luminescence (PSL response) follows that of the F‐centre production in the subnanosecond time range. It is concluded that the radiation defects in a doped crystal are spatially distributed in the near vicinity of the dopants. To a limited extent the photo‐ and thermostimulated defect recombination also takes place near the dopant.
Diffusion in the intermetallic phase Fe3Si under hydrostatic pressure
Authors: Gude A. , Freitag K. , Sepiol B. , Vogl G. , Mehrer H.
Published: July 2007

Diffusion of iron in DO3 ordered Fe76Si24 has been studied under hydrostatic pressure at 934 K. The diffusion coefficients were determined using the radiotracer technique with the implanted radioisotope 59Fe and a precision grinding device for serial sectioning. The resulting activation volume of diffusion is ΔV = 1.48 × 10−29 m3 = 1.27Ω (Ω atomic volume). The large value of ΔV can be attributed to vacancy‐mediated Fe diffusion, in which divacancies or even larger vacancy clusters might be involved. This assumption is corroborated by positron annihilation experiments, which show a strong increase of the lifetime of trapped positrons in Fe75Si25 at higher temperatures.
A note on solute enhanced diffusion
Authors: Pelleg J. , Rabinovitch A.
Published: July 2007

A reformulation of the solute/solvent diffusion enhancement problem is discussed by introducing a new set of two empirical parameters. A comparison with experimental data, both for solvent and solute enhancements, shows good agreement. A constant u is introduced relating the preexponential factor and the activation energy in the Arrhenius equation for the diffusion coefficient. This constant is attributed to the solute/solvent enhancement‐reduction transition temperature.
Lattice distortion effects within the generalized transverse Ising model
Authors: Kokshenev V. B. , Pires A. S. T.
Published: July 2007

A qualitative analysis of lattice vibration effects including lattice distortions, spin‐phonon interaction, and phonon anharmonicity is given within the extended transverse Ising spin model (TIM). It is argued that the rigid‐lattice TIM is a credible model for real crystal in the sense that no new anisotropic‐type interactions emerge due to higher‐order lattice distortions. Instead of the known first‐order Kobayashi model a new effective second‐order nonrigid‐lattice TIM is proposed. Recent developments in the TIM spin dynamics are briefly discussed.
Investigation of electronic structure of the H.C.P. Ni75Nb25 MX phase
Authors: Pan J. S. , Huang X. Y. , Fan Y. D.
Published: July 2007

The total energy and electronic structure of the Ni75Nb25 MX phase with a h.c.p. structure have been calculated by means of discrete variational Xα (DV‐Xα) method. The total energy of this metastable crystalline phase was calculated as a function of lattice constant α. The predicted lattice constants agree well with the experimental result. The calculation gives an interpretation of the nature of bonds of this MX phase.
Disordered spin‐one Ising model with biquadratic exchange interaction within functional integration method
Authors: Rudavskii Yu. , Vatamaniuk O. , Savenko V.
Published: July 2007

A disordered spin‐one Ising model with biquadratic exchange interaction is investigated within a functional integration approach. The free energy is presented in the form of a functional expansion in a series over the Fourier‐transformed density fluctuations. In random phase approximation expressions for phase transition temperatures are obtained. The structural disorder is simulated by means of hard core system structure factors.
Phonon energy renormalization in KDP‐type ferroelectrics
Authors: Wesselinowa J. M. , Apostolov A. T. , Filipova A.
Published: July 2007

A Green's function technique is used to study the influence of spin‐phonon interactions in the Ising model with a transverse field including higher‐order anharmonic terms. The renormalized phonon energy has been evaluated for the first time beyond the RPA. At low temperatures both, the anharmonic spin‐phonon and phonon‐phonon interactions including cubic and quadratic terms play an important role, whereas near and above the Curie temperature only the anharmonic phonon‐phonon interaction contributes to the renormalization of the phonon energy.
Determination of the Pressure Dependence of Band‐Structure Parameters by Two‐Photon Spectroscopy
Authors: Reimann K. , Haselhoff M. , Rübenacke St. , Steube M.
Published: July 2007

Two‐photon spectroscopy has been used to determine the pressure dependence of band‐structure parameters such as band gap, exciton binding energy, biexciton binding energy, exchange energy, and Luttinger parameters. Compared to linear spectroscopy it yields not only a higher precision, but is often able to determine a larger number of parameters. Results are presented for ZnTe and CuCl nanocrystals in a LiCl matrix.
Electronic Structure and Pressure Dependence for Some Ternary Calcium Nitrides
Authors: Vansant P. R. , Van Camp P. E. , Van Doren V. E. , Martins J. L.
Published: July 2007

In this paper we present pseudopotential LDA calculations using a plane wave basis set. We investigate the structure of Ca3AsN and Ca3PN and consider its pressure dependence. Between the structures considered no phase transitions were found for positive pressures. Experimentally Ca3AsN and Ca3PN were found to have an orthorhombic structure which corresponds to a distorted cubic structure. Also theoretically this orthorhombic structure was found to be energetically the most favourable. Its corresponding lattice constants are in good agreement with the experimental data. Besides the lattice constants, the bulk moduli and its pressure derivatives we also calculate the electronic bandstructure.
Metal‐Insulator Transition in Degenerately Doped Si and Ge under High Uniaxial Pressure
Authors: Baidakov V. V. , Ermakov V. N. , Gorin A. E. , Kolomoets V. V. , Stuchinska N. V. , Shenderovskii V. A. , Tunstall D. P.
Published: July 2007

Strain‐induced metal‐insulator (MI) transitions have been observed in degenerately doped Si:Sb and Ge:Sb crystals at T = 4.2 K and for extremely high uniaxial pressures up to 5 GPa. It is shown that in both Si and Ge the transition from a metallic‐type to an activated conductivity is determined by strain‐induced changes of the conduction band parameters. The data obtained demonstrate that the effective mass donor approach and the scaling theory of localization describe well the MI transition phenomenona observed.
Si in GaN — On the Nature of the Background Donor
Authors: Wetzel C. , Chen A. L. , Suski T. , Ager J. W. , Walukiewicz W.
Published: July 2007

A characterization of the Si impurity in GaN is performed by Raman spectroscopy. Applying hydrostatic pressure up to 25 GPa we study the behavior of the LO phonon‐plasmon mode in a series of high mobility Si doped GaN films. In contrast to earlier results on unintentionally doped bulk GaN crystals no freeze out of the free carriers could be observed in Si doped samples. We find that Si is a shallow hydrogenic donor throughout the pressure range studied. This result positively excludes Si incorporation as a dominant source of free electrons in previously studied bulk GaN samples.
Skyrmion Excitations in a Two‐Dimensional Electron Gas under Hydrostatic Pressure
Authors: Maude D. K. , Potemski M. , Henini M. , Portal J. C. , Eaves L. , Hill G. , Pate M. A.
Published: July 2007

The spin excitation gap at Landau filling factor v = 1 has been determined from thermally activated transport measurements in the limit of small Landé g‐factor. The application of hydrostatic pressure allows to probe the region −0.11 < g < 0.055. The observed 50% reduction in the spin excitation gap at g = 0 is fully consistent with the recently proposed spin texture excitations (Skyrmions).
Pressure Dependence of the Electronic Subband Structure of Strained In0.2Ga0.8As/GaAs MQWs
Authors: Li G. H. , Goñi A. R. , Syassen K. , Hou H. Q. , Feng W. , Zhou J. M.
Published: July 2007

We have measured low‐temperature photoluminescence (PL) and optical absorption spectra of an In0.2Ga0.8As/GaAs multiple quantum well (MQW) structure at pressures up to 8 GPa. Below 4.9 GPa, PL shows only the emission of the n = 1 heavy‐hole (HH) exciton. Three new X‐related PL bands appear at higher pressures. They are assigned to spatially indirect (type‐II) and direct (type‐I) transitions from Xz states in GaAs and XXY valleys of InGaAs, respectively, to the HH subband of the wells. From the PL data we obtain a valence band offset of 80 meV for the strained In0.2Ga0.8As/GaAs MQW system. Absorption spectra show three features corresponding to direct exciton transitions in the quantum wells. In the pressure range of 4.5 to 5.5 GPa an additional pronounced feature is apparent in absorption, which is attributed to the pseudo‐direct transition between a HH subband and the folded Xz states of the wells. This gives the first clear evidence for an enhanced strength of indirect optical transitions due to the breakdown of translational invariance at the heterointerfaces in MQWs.
A General Approach to Measurement of Band Offsets of Near‐GaAs Alloys
Authors: Whitaker M. F. , Dunstan D. J. , Missous M. , Gonzalez L.
Published: July 2007

Band offsets can be found by high‐pressure spectroscopy if the X‐minima of a heterostructure are type II. However, many heterostructure systems do not have a type II X‐minimum and then this method does not work. We therefore propose a new approach to measuring band offsets. It is capable of obtaining the band offsets of any near‐GaAs alloy against GaAs by comparing the photoluminescence energies of alloy and GaAs quantum wells with AIGaAs barriers below and above crossover. The method is demonstrated for the case of InGaAs. There are other candidate systems which may usefully be used in the same manner as GaAs/AIGaAs. Before they can be used it is necessary to check whether the X‐minimum is type II or not. We present data confirming that the GaInP/GaAs system also has a type II X‐minimum and can therefore also be used.
Zn1−xMgxSe Samples and a ZnSe/Zn0.93Mg0.07Se Quantum Well under Pressure: Optical Absorption and Photoluminescence
Authors: Griebl E. , Haserer B. , Frey T. , Reisinger T. , Gebhardt W.
Published: July 2007

We present optical measurements of freestanding thin Zn1−xMgxSe films and of a ZnSe/Zn0.93Mg0.07Se single quantum well (SQW) under hydrostatic pressure applied by a diamond anvil cell. For the Zn1−xMgxSe compounds the pressure shift of the fundamental gap was determined from absorption measurements at room temperature for X ⩽ 0.37. A hydrostatic deformation potential a(x) = −4.85 + 0.67x eV is derived. The transition pressures Ptr to high pressure phases were determined by visual observation. The pressure dependent absorption and photoluminescence of the ZnSe SQW was investigated at 2 K. The energy shift of two SQW‐related transitions was recorded and is reproduced by a simple theoretical model. LO‐phonon replica were visible in photoluminescence at the sharp 11hh transition.
Nonmonotonic Behavior of the Threshold Current of Compressively Strained 1.3 μm InGaAsP Lasers with Hydrostatic Pressure
Authors: Patel D. , Menoni C. S. , Bernussi A. A. , Temkin H.
Published: July 2007

Hydrostatic pressure measurements of the threshold current of 1.3 μm compressively strained InGaAsP multiple quantum well lasers show an initial decrease up to 1.2 GPa followed by an increase at higher pressures. This behavior is explained by considering the changes in the differential gain and optical losses determined from measurements of the spontaneous emission and of the external differential efficiency as a function of pressure, respectively.
Bulk Amorphous GaSb Semiconductors Prepared by Thermobaric Treatment: Formation and Properties
Authors: Barkalov O. I. , Kolesnikov A. I. , Antonov V. E. , Ponyatovsky E. G. , Dahlborg U. , Dahlborg M. , Hannon A.
Published: July 2007

The multistage process of solid state amorphization and subsequent crystallization of the quenched “white tin” high pressure phase of Ga‐Sb alloys containing 20 to 80 at% Sb was studied at ambient pressure by differential scanning calorimetry. The heats of both amorphization and crystallization were exothermic and equal to (3.5 ± 0.5) and (8.3 ± 1.0) kJ/mol, respectively. Formation of the bulk amorphous alloys free of any crystalline inclusions was observed for compositions with 47.5 to 52.5 at% Sb. The structure of the amorphous GaSb produced by solid state amorphization was studied by neutron diffraction. It is found to be nearly identical to that of a sample prepared earlier by sputtering.
Pressure Induced Phase Transitions in Alkaline Earth Tungstates
Authors: Christofilos D. , Ves S. , Kourouklis G. A.
Published: July 2007

We present a Raman study of the pressure induced structural phase transitions in the tungstates CaWO4 and SrWO4. Raman spectra up to 24 GPa at room temperature reveal reversible pressure induced phase transitions at 10 and 11.5 GPa for CaW)4 and SrWO4, respectively. The lowest Bg mode, associated with the WO4‐WO4 vibration along the c‐axis, exhibits a softening in the scheelite structure, while in the high‐pressure phase its slope becomes positive. Above the phase transition the modes originating from the antisymmetric v3(Eg) and v3(Bg) stretching modes of the WO4 group exhibit a frequency jump to higher and lower frequencies. In the high pressure phase the modes originating from v3 show an unusual, strongly nonlinear softening in a small pressure region before becoming almost pressure independent.
Optical Properties of C60 Single Crystals at High Pressure
Authors: Meletov K. P. , Christofilos D. , Ves S. , Kourouklis G. A.
Published: July 2007

The Raman scattering, luminescence, and absorption spectra of C60 single crystals at T = 300 K and pressures up to 15 GPa were measured. The pressure dependence of the Raman‐active intramolecular phonons indicates that two phase transitions are induced at pressures of 0.4 and 2.4GPa. The magnitude of the pressure shifts of phonon modes, luminescence and absorption spectra, the values of the Grüneisen parameters, and the deformation potential have been determined.
Influence of Randomly Distributed Surface Steps on Photoemission Lineshape
Authors: Beckmann A. , Meinel K. , Heiler M. , Ammer Ch. , Neddermeyer H.
Published: July 2007

The model of randomly distributed steps is examined with respect to the possibility of non‐lifetime broadening in photoemission spectra from sp‐band surface states. The peak shape is discussed in terms of hole lifetime, terrace width, and angle of emission. The model calculations are based on the parameters of the Cu(III) sp‐surface state.
Electronic Energy Structure of the Ternary Carbide System TiAlC
Authors: Ilyasov V. V. , Ya. Nikiforov I. , Ilyasov Yu. V.
Published: July 2007

With the coherent potential method of the multiple scattering theory the electronic structure of the ternary carbide system Ti1−xAlxC (x = 0.17) is calculated. The cluster approximation with muffintin crystal potential is used. With the same method the electronic structure of the binary TiC system has been calculated for comparison. It was found that Ti1−xAlxC (x = 0.17) can be considered as a semiconductor with a narrow energy gap (2.18 eV). Aluminium doping results in an increase of the valence band width and a decrease of the cohesive energy.
The Influence of the Interaction between Phonons on the Properties of the Exciton in Polar Crystals
Authors: Xiao Wei , Xiao Jing‐Lin
Published: July 2007

Some properties of excitons in polar crystals are studied. The effective Hamiltonian of the exciton in polar crystals is derived by a perturbation method. If we consider the interaxtion between phonons of different wave vectors in the recoil process, the influence on the effective potential between electron and hole, the self‐trapping energy, and the self‐trapping condition are discussed.
Magnetic Alloys Containing Crystal Field Split Impurities: Magnetoresistance
Authors: Kumar A. , Ahluwalia P. K. , Sharma K. C.
Published: July 2007

The phenomena of magnetoresistance and thermal conductivity in the presence of a magnetic field are investigated using the Kondo interaction Hamiltonian within the framework of Abrikosov's pseudofermion representation in the presence of a crystalline electric field up to second order for the Ce‐based system CeCu2Si2. The results for both the properties for different values of magnetic field are presented and using the lowest four nondegenerate levels, these are found to be quite consistent with the experimental data.
Spin Wave Spectrum in Magnetic Superlattices with Anisotropic Fields
Authors: Anselmo D. H. A. L. , Albuquerque E. L.
Published: July 2007

We present a microscopic theory, based on the Heisenberg model, to investigate the spin wave spectrum in a magnetic superlattice whose constituents may present the uniaxial and nonuniaxial com‐ ponents of the single‐ion anisotropy. Our calculations are carried out for the exchange‐dominated regime, stressing the contribution of the bulk as well as the surface modes associated with the truncation of the superlattice. We derive general dispersion relations which encompass previous works in the field. Interesting new aspects of the spectrum are obtained due to the presence of the nonuniaxial component of the anisotropic field.
Ps formation in molecular media: low temperature hydrocarbons
Authors: Stepanov S. V. , Byakov V. M.
Published: July 2007

A simplified model of low temperature radiolysis of hydrocarbons sufficient for adequate interpretation of radiation chemical and positron annihilation data is suggested. The model describes non‐monotonic accumulation of trapped electrons and other radiolytic products with the absorbed dose in γ ‐radiolysis experiments, as well as variation of the shape of e+ annihilation lifetime spectra with time of the measurements. The developed model is compared with experiments in polyethylene and n‐tridecane. It is shown that intratrack radicals significantly affect Ps formation and time evolution of the e+ states. Some values of intratrack reaction rate constants are estimated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)