bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magnetization and Critical Temperature of the Structurally Disordered s‐d Model
Authors: Ponedilok G. V. , Savenko V. P. 
Published: July 2007

AbstractA disordered s‐d model is considered and applied for the description of amorphous ferromagnetic compounds. The partition function is obtained in the form of a functional integral over Bose fields. The thermodynamical potential is calculated in RPA. The magnetization and critical temperature of the “ferromagnetic‐paramagnetic” phase transition of amorphous compounds with liquid‐type disorder are studied.
Low Temperature EPR Spectra of the (Pr, RE)BaCuO Ceramics in the Orthorhombic Phase
Authors: Guskos N. , Likodimos V. , Patapis S. K. , Kuriata J. , Wabia M. , Sadlowski L. , Koufoudakis A. , Mitros C. , Gamari‐Seale H. , Psycharis V. 
Published: July 2007

AbstractThe EPR spectra of the members of the series Pr0.5RE0.5Ba2Cu3O6+x (RE = Pr, La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Tm, Yb, and Lu) in the orthorhombic phase are investigated at low temperature. The EPR spectra arise from two different paramagnetic centers, one from divalent copper ions in orthorhombic local crystal field symmetry and another from exchange coupled pairs of copper ions whose origin is discussed. The EPR spectra essentially depend on the time exposition. The EPR spectra of some divalent and trivalent rare earth ions are recorded. The crystal field analysis is done for the trivalent ytterbium ions. The semiconducting behaviour of the dc conductivity can be attributed to thermally activated small polaron hopping motion.
Electron Paramagnetic Resonance and Superposition Model Analysis of Zero‐Field Splitting for Gd3+ in Eu2Co3(NO3)12·24H2O
Authors: Jain V. K. 
Published: July 2007

AbstractElectron paramagnetic resonance of Gd3+ in Eu2Co3(NO3)12 24H2O single crystals is studied at ≈ 9.45 GHz and room temperature. Gd3+ substitutes for the Eu3+ site. In addition to the allowed fine structure lines some weak low field lines, identified as forbidden transitions (ΔM = ±2, ±3) are observed. The superposition model is applied to the zero‐field splitting parameter b for Gd3 + in Eu2Co3(NO3)12·24H2O, Ce2Mg3(NO3)12·24H2O, and La2Mg3(NO3)12·24H2O.
Mossbauer and Magnetic Investigations of Iron‐Rich FeNiZr Amorphous Alloys
Authors: Zatroch M. , Kováč J. , Rohr L. , Aubertin F. , Gonser U. 
Published: July 2007

AbstractMössbauer, high‐field susceptibility, and thermomagnetic measurements indicate a correlation between reentrant spin glass (RSG)‐like behaviour and the presence of a broad hyperfine magnetic field distribution of Fe92−xNixZr8 glasses. The pure iron area, in which the competition between antiferromagnetic and ferromagnetic exchange takes place, is probably responsible for both. There is a transition from collinear to noncollinear magnetic structure at low temperatures for x = 0, 2, 5 due to this competition. The substitution of Ni for Fe or the hydrogenation suppresses the RSG‐like behaviour.
Solitary Excitations in the Antiferromagnet KCuF3
Authors: Liu Wu‐Ming , Zhou Ben‐Lian , Guo Li‐Ming 
Published: July 2007

AbstractEmploying the Holstein‐Primakoff transformation, first the spin‐Hamiltonian is transformed into a boson Hamiltonian. Then, using a coherent state ansatz, a time‐dependent variational principle, and the continuum approximation, two nonlinear coupled equations are obtained. Finally, these equations of motion, which describe the dynamics of the system, are reduced to one nonlinear Schrodinger equation with localized envelope soliton solutions. Introducing the inverse scattering transformation, single‐, two‐, and N‐soliton solutions are obtained and the solitary excitations in the antiferromagnet KCuF3 are investigated.
Spectral‐Angular Distribution and Polarization of Surface Parametric Optical Radiation
Authors: Andriyanchik A. A. , Baryshevskii V. G. , Kaminskii A. N. 
Published: July 2007

AbstractSurface parametric (quasi‐Cherenkov) optical radiation of a relativistic charged particle, moving close to the surface of a spatially periodic medium, is considered. The spectral‐angular distribution and polarization characteristics of the radiation are found. The possibility of experimental observation of the radiation described is discussed.
Ab Initio Calculation of Infrared and Raman Spectra for Hydrogenated Amorphous Silicon
Authors: Zou Xian‐Wu , Zhang Wen‐Bing , Zhuang Hong‐Zhang , Jin Zhun‐Zhi 
Published: July 2007

AbstractThe ab initio method with the STO‐3G basis set is used to calculate the infrared and Raman spectra of hydrogenated amorphous silicon. To remove the systematic deviation of the calculated frequencies due to the finite number of the basis functions and the small size of the cluster, a scaling method is used. The results are in good agreement with the experiments. The calculated results support that there are SiH, SiH2, SiH3, and (SiH2)n configurations in hydrogenated amorphous silicon. The doublet lines are associated with the SiH3 configuration.
Site‐Selective Upconversion Luminescence of Ho3+ ‐Doped CaF2 Crystals
Authors: Zhang Xiao , Jouart J.‐P. , Bouffard M. , Mary G. 
Published: July 2007

AbstractWith red dye laser excitation, site‐selective upconversion properties of three Ho3+ centres (C4v, C3v, and dimer) in CaF2, : 0.1 at% Ho3+ crystals are studied. The energy level diagrams for the three centres are deduced from the spectral data. The excited state absorption from the 5I7 level plays a very important role in the upconversion coming from C4v, and C3v centres, whereas the ground state absorption of 5I8, dominates for that from the dimer centre. The upconversion of C4v and C3v centres is explained by a sequential two‐photon excitation mechanism. For the dimer centre, the predominant upconversion mechanism is energy transfer.
Double Screening Problem in Dechanneling by Point Defects
Authors: Siddiqui A. M. , Harikumar V. , Pathak A. P. 
Published: July 2007

AbstractA simple double screening model is used to calculate the interaction potential between an energetic channeled μ+ and an impurity (O) in an otherwise perfect single crystal (Ta). The twofold screening of the nuclear charge of the impurity is first due to the atomic electrons and second due to conduction electrons of the host crystal (Ta). Using this potential the dechanneling cross‐section of muons due to the oxygen impurity in the lattice of Ta along various planes is evaluated. Three charge states of the oxygen impurity are considered and the corresponding dechanneling cross‐sections are predicted.
Optical and Structural Properties of ZnTeZnS Strained‐Layer Superlattices
Authors: Guan Z. P. , Lu Y. M. , Chen L. C. , Yang B. J. , Fan X. W. 
Published: July 2007

AbstractZnTeZnS strained‐layer superlattices are grown by atmospheric pressure metalorganic chemical vapor deposition on transparent BaF2(111) and GaAs(100) substrates. With X‐ray diffraction measurements, multi‐order satellites are observed, which confirms the formation of a periodic structure of the superlattices. Photoluminescence and optical absorption measurements are carried out at 77 K and room temperature. The photoluminescence spectra under different excitations, time‐resolved and time‐delayed spectra are employed to study the band band recombination at 77 K. The band structure and strain effects are studied using the uniaxial stress and the linear combination of atomic orbital theory for such high mismatch ZnTeZnS strained‐layer superlattices. It is noticed that the energy disparity of ZnTeZnS superlattices is large with ZnTe or ZnS buffer layers. So the band‐band emission can be observed over a wide wavelength region for ZnTeZnS superlattices.
Dynamics of Double‐Barrier Resonant Tunnelling Structures
Authors: Pandey L. N. , Muratov L. S. , Stockman M. I. , George Th. F. 
Published: July 2007

AbstractA time‐dependent Schrödinger equation is solved numerically for a double‐barrier, quantum‐well resonant tunnelling structure (RTS), such as modelling a semiconductor heterostructure, by the fast Fourier transform method. Results are shown for a resonance tunnelling structure with barriers doped by a negative δ‐function potential (δ‐potential) which broadens the widths of the resonances and in turn decrease the electron dwell times. The strength of the δ‐function can be such that it may form bound states in the barrier regions, but the states bound to the δ‐potential are very shallow. For this study, the δ‐potentials are replaced by equivalent barriers of different heights and widths. It is found that for a certain strength of the δ‐potential or parametric value of a barrier with an equivalent effect in the RTS, there are three resonance states very close together. The squares of the wave functions trapped in the well region for the states oscillate in time for a broad wave packet in k‐space, whereas the wave function trapped in the whole structure decays exponentially. The oscillating part resembles quantum beats.
Spin‐Glass Behavior in Semiconducting Single Crystalline ZnMn2As2
Authors: Laue I. , Vanacken J. , Nateprov A. N. , Herlach F. , von Ortenberg M. , Arushanov E. K. 
Published: July 2007

AbstractThe dc magnetization of single crystalline ZnMn2As2 is studied using an rf‐SQUID. It is found that the crystals are anisotropic, show a ferromagnetic transition around T = 310 K, a helical magnetic phase below T = 110 K, and there is evidence of a strong spin‐glass behavior below T = 33 K. The spin‐glass phase is studied from zero‐field cooled (ZFC) and rémanent magnetization experiments. An irreversibility line (de Almeida‐Thouless line) is clearly observed for both principal orientations of the crystal and a comparison is made between the magnetic moment relaxation measured using the ZFC and the rémanent methods.
Cherenkov Effect in the Context of Scanning Tunneling Microscopy
Authors: Smolyaninov I. I. , Keller O. 
Published: July 2007

AbstractThe intensity of the light emission from the STM tunneling junction between two metals is shown to depend exponentially on the tunneling distance. Assuming the exponential decay constant to be equal to the mean wave vector of the surface plasmons excited by the tunneling electrons, the plasmon phase velocity is estimated to be two times smaller than the Fermi velocity of the electrons. A new theoretical description of the light emission phenomenon based on a Cherenkov‐like resonant interaction between the tunneling electrons and the surface plasmons is developed.
Time‐Resolved Luminescence of Exciton‐Polaritons in Solid Xenon
Authors: Varding D. , Reimand I. , Zimmerer G. 
Published: July 2007

AbstractThe decay of excitons in solid Xe is investigated with time and spectrally resolved luminescence as a function of temperature and photon energy of excitation. The luminescence of free excitons exceeds the self‐trapped exciton luminescence by a factor of 50 (peak‐to‐peak). The FE‐decay curves yield a non‐exponential behaviour. They can be reproduced with Monte‐Carlo simulations of the exciton‐polariton transport to the sample/vacuum interface, taking into account phonon scattering and trapping. A new upper limit for the intrinsic self‐trapping rate in the lower 107 s−1 range and a surprisingly large experimental value for the scattering length ( ≈ 100 nm at T = 5 K) are deduced. It is indispensable to discuss the decay of excitons in solid Xe in terms of the exciton‐polariton model.
Isobaric Specific Heat and Thermal Expansion of Solids in the Debye Approximation
Authors: Askerov B. M. , Cankurtaran M. 
Published: July 2007

AbstractA theoretical study in the Debye approximation is made of the isobaric specific heat (CP) and thermal expansion of solids with a Bravais lattice. Analytical expressions are obtained for the variations of CP and the linear thermal expansion coefficient (xp) with temperature. The temperature dependence of CP is determined by two characteristic parameters of the solid: the Debye temperature θand the Gruneisen parameter yc. It is shown that the difference CPCv between the calculated isobaric and isochoric specific heat capacities is very small at low temperatures (T≪θ) and increases markedly at higher temperatures. In the range T≫θ, CP is almost independent of temperature and is.determined solely by γG. This provides a technique to estimate the Gruneisen parameter γG from the measurements of isobaric specific heat at high temperatures. The results obtained for αP, in the low‐ and high‐temperature limits, are in agreement with those in literature.
Electrical Piezoeffect in Semiconducting BiSb Alloys due to Anisotropy of Electron Mobility
Authors: Tolutis R. 
Published: July 2007

AbstractThe strong electrical piezoeffect caused by the deformation induced anisotropy of electron mobility in Bi1−xSbx semiconducting samples is described. Attention is drawn to the piezoeffect voltage, high in undoped samples, its nonsymmetric change under tensile and compressive deformation, and its sign inversion at small magnetic fields. All these effects are explained in terms of electron intervalley repopulation and scattering. Possible applications are discussed.
Two‐Photon Absorption in an Indirect‐Gap Semiconductor Quantum Well System. II. Excitonic Transitions
Authors: Hassan A. R. 
Published: July 2007

AbstractA theory of phonon‐assisted two‐photon exciton transitions in indirect‐band‐gap semiconductor layered quantum wells (QW) and quantum well wires (QWW) is developed. The expressions for the two‐photon absorption coefficients in one‐dimensional (1D), α,2D, α, and (bulk), α, respectively, are calculated. The spectral dependence of these expressions, the vicinity of the band edge, are found to obey the law (2ℏω ± ℏΩ – EG + E)β, where ℏω (ℏΩ) is the photon (phonon) energy, EG the effective indirect gap, and Ek the exciton binding energy. The values of β vary from – ½ up to ½ depending on the dimension of the system and the type of the coupling matrix element involved in each transition process. Before the edge, the final exciton states are of s‐ or p‐symmetry according to the photon polarizations with respect to the confinement directions in the QWs and to the selection rules allowed by the momentum matrix elements. A numerical estimation for the case Si0.5Ge0.5 shows that α for both photon polarizations (parallel and perpendicular to the confinement direction of the QW's) is enhanced over the values of α and α (allowed and forbidden transitions). Furthermore α is also enhanced over the α values of bulk materials. This behaviour of the αex's is interpreted as due to (i) the additional confinement of the carriers which occur going from 3D → 2D → 1D systems, (ii) the photon polarization configurations with respect to the confinement directions, and (iii) the coupling matrix elements.
Point Defects in CdTe Containing Simultaneously Lithium and a Group IV Impurity
Authors: Zielińska‐Purgał R. , Piwowarczyk J. , Nazarewicz W. 
Published: July 2007

AbstractInfrared 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

AbstractThe 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

AbstractA 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

AbstractFerromagnetic 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

AbstractConcentration 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

AbstractHole 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

AbstractOptical 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

AbstractThe 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

AbstractThe 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

AbstractIn‐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

AbstractThe 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.