bluuebunny/crossref_metadata_2025_split huggingface dataset:
Dielectric function and plasmon dispersion relation of a quasi‐two‐dimensional electron gas
Authors: Khanh Nguyen Quoc 
Published: July 2007

AbstractIn this paper we generalize the method suggested by Lee and Spector to calculate the dielectric function of a quasi‐two‐dimensional electron gas without strict restriction on the wave vector q, the conducting layer thickness d, and the dielectric constants of the conducting and surrounding media ϵ, ϵ1, ϵ2. In the case of a thin layer, the plasmon dispersion equation obtained by means of the dielectric function is solved exactly to get the dispersion relation of the collective excitations. We compare our exact solution to the approximated result of Aharonian et al. and find that the latter is valid only for very small values of qd.
Quantized photoemission from rectangular quantum wires of narrow‐gap semiconductors
Authors: Das N. R. , Ghosh K. K. , Ghoshal D. 
Published: July 2007

AbstractIn the present work a theoretical study has been made on the photoemission from rectangular quasi‐one‐dimensional structures of narrow‐gap semiconductors. The effect of reduced dimensionality on the photoemission is seen to be quite prominent. The computations are performed for a temperature of 4.2 K taking strongly degenerate n‐InSb as an example. It is observed from the study that the photoemission exhibits a stair‐case function of incident photon energy and an oscillatory function of the transverse dimensions.
Critical temperatures of the diluted spin‐1/2 Ising system with Gaussian type exchange integrals
Authors: Zheng S. M. 
Published: July 2007

AbstractThe site‐ and bond‐disordered spin‐1/2 Ising ferromagnetic system is studied in which the exchange integral between nearest‐neighbour sites is expressed by the Gaussian probability distribution function. The general expression of the magnetization is derived. The critical temperatures of the system are calculated as functions of concentration and disorder parameter, using the improved correlated effective field theory. For comparison, the corresponding results for the Handrich‐Kaneyoshi type probability distribution function of the exchange integral are also given. It is shown that the Gaussian probability function of the exchange integrals gives more reasonable results.
ESR study of high‐pressure phases of C60 fullerene
Authors: Kozlov M. E. , Zakhidov A. A. , Yakushi K. , Tokumoto M. 
Published: July 2007

AbstractFace‐centered cubic and rhombohedral phases of pressure‐temperature treated C60 were investigated by ESR spectroscopy in the 5 to 550 K temperature range. It was found that the ESR spectra of the former differ only slightly from those of the starting C60 powder. The intensity of the ESR signal from this phase reduces upon heating the specimens above 480 K. In contrast, the rhombohedral phase shows an intense narrow ESR line which obeys the Curie law and does not change much upon heating. The origin of paramagnetic centers in the high‐pressure phases of C60 and their relationship to the structure of the substances are discussed.
Magnetic phase transition of ultrathin Fe layers in Fe/Au multilayers
Authors: Yamazaki H. 
Published: July 2007

AbstractThe magnetic phase transition of Fe layers is investigated in Fe(2 Å)/Au(d) multilayers with d = 5 to 200 Å by means of magnetization measurements. For d < 10Å, the Curie temperature Tc is above 300 K. With increasing d, an overall decrease of Tc is observed. Reference samples including a single Fe(2 Å) layer show Tc ≈ 80 K, which is lower than Tc ≈ 150 K for 200 Å ≧ d > 100 Å. The interlayer coupling between Fe layers mediated by Au layers remains even for long distances exceeding 100 Å. It is expected that even a weak interlayer coupling suppresses the spin fluctuation in two‐dimensional magnets and shifts Tc to a higher temperature.
Spin‐one Ising model with biquadratic exchange interaction within functional integration method
Authors: Vatamaniuk O. , Rudavskii Yu. 
Published: July 2007

AbstractA spin‐one Ising model with biquadratic exchange interaction is investigated within a functional integration approach. The phase diagram is analyzed by means of Landau's free energy expansion. The dependence of the features of the system on the ratio of bilinear to biquadratic exchange interaction constants is discussed. In the random phase approximation, expressions for phase transition temperatures and the tricritical point for the ferromagnetic transition are obtained.
Evaluation of the effective second harmonic generation coefficient of monomethyl urea single crystals
Authors: Romani S. , Razzetti C. , Zha M. , Paorici C. 
Published: July 2007

AbstractThe effective second harmonic generation coefficient of monomethyl urea has been determined relatively to urea by phase matched interaction. Both Type I and Type II phase matching have been achieved by propagating the pump beam at small angles to natural faces of solution grown crystals. The conversion efficiency for the doubling of Nd:YAG fundamental is similar in urea and methylurea.
Quantitative analysis of dislocations in relation with the martensitic transformation in Cu‐Zn‐Al alloys
Authors: Rodriguez P. L. , Condo A. M. , Lovey F. C. 
Published: July 2007

AbstractA quantitative evaluation of the dislocation energies in relation with the β → 18R martensitic transformation in Cu‐Zn‐Al alloys has been performed. Dislocations produced by macroscopic deformation of the parent or martensitic phase have been considered. The crystallographic changes of the dislocations upon transformation have been established and the anisotropic theory of dislocations has been used to calculate their energy when embedded in β or martensite. Good agreement was found between the calculated separation of the partials and the corresponding line orientations for the minimum energy and the experimental results from transmission electron microscopy observations.
Temperature and mass dependence of anharmonic defect dynamics
Authors: Salamatov E. I. 
Published: July 2007

AbstractWithin the framework of an approach with allowance for fluctuations of the mean energy of a particle interacting with a thermostat, the partial density of vibrational states (PDVS) of a crystal lattice defect located in a two‐well potential has been investigated. The results obtained are in good agreement with those of a numerical study of the corresponding Langevin equation. It has been shown that the mass dependence of the anharmonic defect PDVS is quite complicated and is determined by temperature.
Intense laser field effect on confined hydrogenic impurities in quantum semiconductors
Authors: Fanyao Q. , Fonseca A. L. A. , Nunes O. A. C. 
Published: July 2007

AbstractThe influence of an intense high‐frequency laser field on the binding energy of a shallow neutral donor impurity in a quantum semiconductor structure such as GaAs/AlGaAs is reported. By making use of a nonperturbative theory and the variational approach it was found that for an on‐centre hydrogenic impurity in the given quantum well, the binding energy for different well widths increases, reaches a maximum, and then decreases quite rapidly with increasing laser field amplitude of the quiver motion of the electron.
Variational approach for phonon renormalization effects in photoexcited quantum wires and quantum wells
Authors: Güven K. , Tanatar B. 
Published: July 2007

AbstractWe investigate the effects of screening on polaronic corrections to the effective band edge in photoexcited quasi‐one‐dimensional GaAs quantum wires and two‐dimensional quantum wells. We develop a variational method to calculate the polaron energy of a two‐component plasma (electrons and holes) coupled to LO‐phonons. Screening effects are incorporated within a dynamical scheme. We find that the screening effects and finite well width considerably reduce the polaron energy as the plasma density increases. Many‐body corrections beyond the random‐phase approximation are also considered.
Plasmapolaron selfenergy and effective mass in uniaxial polar crystals
Authors: Capone G. , Cataudella V. , Ninno D. , Iadonisi G. 
Published: July 2007

AbstractWe study the polaron effects in a uniaxial polar semiconductor with electrons in the conduction band. We describe the system as an external charge interacting with three boson fields: two optical longitudinal phonons and a plasmon. The combined effects of these three fields lead to a quasiparticle which we call plasmapolaron. The selfenergy and the anisotropic effective mass of the plasmapolaron as a function of the anisotropy parameters and the electron density are calculated in the intermediate regime for the electron‐phonon and electron‐plasmon coupling. We find that, when the crystal has high polarizability along the c‐axis and a large polaron radius in the a‐b plane, the screening effects due to the plasmons on the electron‐phonon interaction are small, so that the latter is present even at relatively high electron density. In these conditions the induced charges are localized mainly on the a‐b plane. We also calculate the selfenergy of the plasmapolaron in the strong coupling limit and compare it with that calculated in the intermediate regime. We find that, increasing the electron density, the selfenergy in the intermediate regime becomes always lower than that in the strong limit. A possible connection of our results with the high‐Tc superconductors is discussed.
The ground state energy of a bound polaron in the presence of a magnetic field
Authors: Zorkani I. , Belhissi R. , Kartheuser E. 
Published: July 2007

AbstractA theoretical calculation for the ground state energy of a bound magneto‐polaron as a function of the magnetic field is presented. The theory is based on a variational approach using the trial function , representing the envelope wave function of the ground state already proposed by Devreese et al. in absence of a magnetic field. This function is adequate for all electron‐phonon coupling α and γ0 values, where èM0 is the ratio between the LO phonon energy and the Coulomb one. For the electronic part of the trial wave function we have adopted a new linear combination function which covers all magnetic field domains. Analytical results are obtained in the weak coupling limit.
Far‐Infrared Stimulated Emission in p‐Ge under High Uniaxial Pressure
Authors: Altukhov I. V. , Chirkova E. G. , Kagan M. S. , Korolev K. A. , Sinis V. P. , Yassievich I. N. 
Published: July 2007

AbstractStimulated emission of far‐infrared radiation from uniaxially compressed p‐Ge in strong electric fields is shown to be due to population inversion of strain‐split acceptor levels. A peak corresponding to the optical transitions between split‐off and ground acceptor states is found in the spectrum of stimulated emission. A strong frequency tuning by stress due to the pressure dependence of the energy splitting of these states is obtained. The inversion takes place as the split‐off acceptor state is in the valence band continuum.
Time‐Resolved Photoluminescence Study of Ordered Ga0.5In0.5P under High Pressure
Authors: Kobayashi T. , Minaki M. , Takashima K. , Uchida K. , Nakahara J. 
Published: July 2007

AbstractWe have measured time‐resolved photoluminescence (TR‐PL) spectra in ordered and disordered Ga0.5In0.5P alloys at 12 K and at pressures up to 3 GPa to investigate the characteristics of the main broad PL spectrum at ≈︁1.91 eV in nominally ordered alloys. At normal pressure, unlike the disordered alloy, the PL‐decay profiles for the ordered alloys change remarkably for small changes in detection energy. This indicates the presence of adjacent overlapping emissions of different nature. Together with the prolonged nonexponential decay curves, a gradual red‐shift of the low‐energy peak with delay time is observed for the first time, indicating spatially indirect recombinations. With increasing pressure, the constituent emissions shift toward higher energies at rates slightly smaller than that for the disordered alloy. At 3 GPa, the PL decay appears to be insensitive to the detection energy, and a faster decay is observed.
Effects of Conduction Band Structure and Dimensionality of the Electron Gas on Transport Properties of InSe under Pressure
Authors: Errandonea D. , Segura A. , Royo J. F. Sánchez , Muñoz V. , Ulrich C. , Grima P. , Chevy A. 
Published: July 2007

AbstractWe report Hall effect and resistivity measurements in InSe under pressure. The electron concentration strongly decreases under pressure in samples exhibiting 3D transport behaviour. This is explained by the existence of an excited minimum in the conduction band moving to lower energies under pressure. The related impurity level traps electrons as it reaches the band gap and approaches the Fermi level. In samples exhibiting 2D behaviour the electron concentration remains constant. This behaviour, together with the pressure dependence of the Hall mobility, is consistent with a previous model which considers high mobility 3D electrons and low mobility 2D electrons to contribute to charge transport along the layers.
Coexistence of DX and A1 States in Highly Doped GaAs:Ge, Te and GaAs:Si, Te
Authors: Wasik D. , Baj M. , Przybytek J. , Słupiński T. , Kudyk K. 
Published: July 2007

AbstractWe studied highly localized states of germanium and silicon impurities in GaAs. Hall effect and resistivity measurements as a function of hydrostatic pressure up to 1.5 GPa and temperature ranging from 4.2 up to 300 K were performed on highly doped LEC GaAs:Ge, Te and LEC GaAs:Si, Te. For Ge impurities our data confirm the two‐electron character of GeDX states and evidence that the GeA1 state does not reveal any metastable features down to liquid helium temperature. In the case of Si impurities we analyzed Hall concentration and Hall mobility dependencies on pressure and illumination in order to investigate the coexistence of DX and A1 states.
Photoluminescence Study of Rare‐Earth Doped Semiconductors under Pressure
Authors: Takarabe K. 
Published: July 2007

AbstractHigh pressure photoluminescence (PL) studies are reported for InP:Yb and GaAs:Er, O. In InP:Yb, the thermally quenched intra‐4f luminescence is recovered by applying pressure near room temperature. Simultaneously the band‐edge related emission decreases. This result is interpreted qualitatively by a thermal equilibrium between the photocreated electron‐hole pair of the host InP and the excited state of the 4f shell. In MOCVD grown GaAs:Er, O, the sharp luminescence from the intra‐4f transition of Er is observed at 1.538 μm at atmospheric pressure. This is the main emission associated with the ErGa‐2O center which has C2v symmetry. For increasing pressure the PL intensity of the Er intra‐4f emission increases and also some new PL lines appear at low temperatures. These pressure‐induced new lines are assigned to other Er centers with different atomic configurations.
Pressure and Time‐Resolved Photoluminescence Studies of Mg‐Doped and Undoped GaN
Authors: Teisseyre H. , Kozankiewicz B. , Leszczynski M. , Grzegory I. , Suski T. , Bockowski M. , Porowski S. , Pakula K. , Mensz P. M. , Bhat I. B. 
Published: July 2007

AbstractPressure and time‐resolved photoluminescence (PL) experiments were performed on undoped and Mgdoped GaN homoepitaxial layers. The pressure coefficients of the following PL lines were determined: (i) exciton bound to a neutral donor (3.472 eV), (ii) exciton bound to a neutral acceptor (3.463 eV), (iii) line centered at about 3.27 eV and its two LO (92 meV) phonon replicas. In the time‐resolved measurements, we observed that the latter PL line corresponds to a slow process (delay time is in the millisecond range). This observation strongly supports the conclusion that donor‐acceptor pairs are responsible for this radiative transition. Identification of the radiative centers and a new explanation of the radiative mechanisms (including optically active E12 phonons) are proposed.
An Investigation of the Formation Mechanisms of Electric Field Domains in GaAs/AlAs Superlattices
Authors: Sun Baoquan , Jiang Desheng , Liu Zhenxing , Zhang Yaohui , Liu Wei 
Published: July 2007

AbstractWe have studied the vertical transport and formation mechanisms of electric field domains in doped weakly‐coupled GaAs/AlAs superlattices. Under hydrostatic pressure two kinds of sequential resonant tunneling are observed within the pressure range from 0 to 4.5 kbar. A transition from Γ‐Γ to Γ‐X sequential resonant tunneling occurs at Pt ≈︁ 1.6 kbar. For P < Pt, the high electric field domain is formed by the Γ‐Γ process, while for P > Pt it is preferentially formed by the Γ‐X process.
Structural and Optical Properties of HgO at High Pressure
Authors: Zhous T. , Schwarz U. , Hanfland M. , Liu Z. X. , Syassen K. , Cardona M. 
Published: July 2007

AbstractWe have investigated high‐pressure properties of HgO by optical spectroscopies and powder X‐ray diffraction. At pressures up to 14 GPa we observe a pronounced softening of two oxygen‐related Raman modes of orthorhombic HgO which is interpreted as a signature of increasing coupling between ‐OHgO‐ chains. The lattice parameters reveal a strongly anisotropic compressibility in this pressure range. At a pressure of 14 GPa, discontinuous changes of Raman active modes indicate a structural phase transition, in agreement with changes in the X‐ray diffraction diagrams. The high‐pressure phase is assigned as tetragonal with the mercury partial structure being close to face centered cubic. An increase in near‐infrared oscillator strength, which starts at 28 GPa as seen in optical reflectivity, is attributed to an insulator‐metal transition resulting from a monotonic increase of valence and conduction band overlap.
Stability of Diamond at Megabar Pressures
Authors: Scandolo S. , Chiarotti G. L. , Tosatti E. 
Published: July 2007

AbstractThe stability of diamond at megabar pressures (1 Mbar = 100 GPa) is studied by means of ab‐initio simulations. Under hydrostatical pressure, diamond is found to be locally stable (i.e. a local enthalpy minimum) up to a pressure as high as 3000 GPa, where it is predicted to collapse into a sixfold coordinated metallic phase never considered so far. On the other hand, if a considerable uniaxial component is added to the stress tensor, as in diamond‐anvil‐cell experiments, a completely different scenario applies whereby much lower pressures are required to bring diamond to mechanical instability. Qualitative considerations on the maximum pressure achievable in diamond anvil cells are presented.
High Pressure Stability in III–V and II‐VI Binary Compounds and the Cmcm Phase A Theoretical Study
Authors: Mujica A. , Needs R. J. , Muñoz A. 
Published: July 2007

AbstractWe present the results of a theoretical study based on the density functional theory of the structural stability of several III‐V and II‐VI binary compounds (GaAs, Gap, InAs, InP, ZnSe, and ZnTe, among others). The recent developments in X‐ray diffraction techniques have led to the identification of several new phases and to a major change in the accepted trends for these families of compounds. A remarkable feature of the emerging new picture is the existence of an orthorhombic phase which is stable in many of the binary compounds studied so far, a distortion of the rocksalt structure which has been named Cmcm after its space group. Our theoretical study focuses on this novel and rather general phase. We have investigated the instability of the rocksalt cell with respect to the distortion leading to Cmcm. The results of our ab initio study are in line with recent experimental studies of these compounds.
Structural Stability of the Group‐VIb Elements under High Pressure
Authors: Nishikawa A. , Niizeki K. , Shindo K. , Ohno K. 
Published: July 2007

AbstractWe have performed band structure calculations of a high‐pressure phase of the group‐VIb elements within the local density‐functional formalism and the norm‐conserving pseudopotential method to investigate the structural stability. This paper mainly deals with the β‐Po type rhombohedral phase of sulphur, and we discuss the effect of the rhombohedral distortion on the electronic structure.
Nonequilibrium Phase Transformations in Diamond and Zincblende Semiconductors under High Pressure
Authors: Lyapin A. G. , Brazhkin V. V. , Popova S. V. , Sapelkin A. V. 
Published: July 2007

AbstractThe problem of nonequilibrium phase transformations is reviewed for compression of diamond, zincblende, and amorphous tetrahedral semiconductors and for decompression of their high‐pressure phases. The importance of lattice or network dynamical properties for the nature of nonequilibrium transformations is shown. Using low‐temperature quenching of high‐pressure phases, we obtained new Crystalline phases for Ge and Ge‐Gash solid solutions, which have X‐ray patterns very similar to corresponding data for rhombohedral R8 silicon. We established that a number of anomalous features, like elastic softness, pressure‐induced geometrical distortion, and strong bulk modulus softening (δB/δP > 0), distinguish the pressure behavior of the amorphous tetrahedral network in a‐GaSb from its crystalline counterpart.
Spectroscopic Study of KTiOAsO4 Single Crystals Doped with In, Sc, Fe
Authors: Mashkovtsev R. I. , Isaenko L. I. 
Published: July 2007

AbstractElectron paramagnetic resonance and optical absorption in the visible and infrared regions have been used to study flux‐grown KTiOAsO4 crystals doped with In, Sc, and Fe. An unstable broad absorption band and paramagnetic hole centres in KTA: In or Sc after γ‐irradiation are observed. The analysis of EPR data leads to a model wherein the holes are trapped on bridging oxygen ions between two titanium ions. The optical absorption band at 2.7 eV is ascribed to the light‐induced transfer of holes from one oxygen ion to another. Infrared absorption bands at 3584 and 3594 cm−1 are observed in all crystals and possible positions of the OH group in the structure are discussed.
On the Theory of Huang X‐Ray Scattering Caused by Point Defects in Hexagonal Crystals
Authors: Michelitsch Th. , Wunderlin A. 
Published: July 2007

AbstractThe theory of Huang scattering of point defects in hexagonal crystals is treated. The concept of “elementary defects” which is based on group theoretical methods is introduced. From this point of view, point defects in hexagonal crystals can be understood as unique superpositions of three types of incoherent elementary scattering defects. These elementary defects reflect characteristical defect symmetries. This concept yields a simple analytical expression for the Huang intensity in the case of arbitrary scattering vectors. The derived intensity function may be a useful tool for the investiga‐ tion of defect structures in h.c.p. metals by Huang scattering measurements.
Half‐Space Jellium and Its Natural Occupation Numbers: Generalized Neutrality Sum Rule and Correlation Entropy
Authors: Ziesche P. , Gersdorf P. 
Published: July 2007

AbstractA conceptional analysis of half‐space jellium in terms of its oneparticle density matrix (diagonalized by natural orbitals and natural occupation numbers as eigenfunctions and eigenvalues) is presented. This allows to apply the concept of correlation entropy as a measure of correlation strength. Exact expressions for the electron density, the kinetic energy density, and the correlation entropy density are given. A generalized Sugiyama‐Langreth neutrality sum rule for the phases which describe the bulk asymptotics of the natural orbitals is derived. Kinetic surface energy and correlation surface entropy depend on these phases and on the bulk momentum distribution of the uniform electron gas.