bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear Dynamics of Optical Switching Fronts in CdS
Authors: Schmolke R. , Schöll E. , Nägele M. , Gutowski J.
Published: July 2007

A two‐level model for impurity‐related optical switching in lightly doped CdS crystals is developed which explains dynamical switching between a low and a high transmissivity state by a bleaching front traversing the crystal with a velocity of several 108 cm/s and offers detailed insight into the internal nonlinear spatio–temporal dynamics of the process. The predicted front is detected experimentally by a 90° pump‐and‐probe configuration. The front velocity in the experiment agrees well with the theoretical prediction.
Excitonic radiative dynamics in multiple quantum wells
Authors: Citrin D. S.
Published: July 2007

The fundamental intrinsic radiative process associated with excitons in semiconductor multiple quantum wells is discussed within the context of exciton polaritons. Multiple quantum wells in which each well is randomly displaced from its ideal periodic position are discussed in order to investigate the effect of disorder on excitonic radiative widths. The coherent transpori of excitons in multiple quantum wells is discussed.
Polaritons in PbI‐based self‐organized superlattices
Authors: Gippius N. A. , Mulyarov E. A. , Tikhodeev S. G.
Published: July 2007

The polariton reflection spectra in PbI‐based compounds – self‐organized semiconductor/insulator superlattices – for different polarizations of incoming light and within a nonlocal model for the exciton response are calculated.
On the existence of quasi‐1D quantum exciton crystals
Authors: Ivanov A. L. , Haug H.
Published: July 2007

The development of a ground state of the squeezed quasi‐1D system of Wannier‐Mott excitons (x) into a quantum exciton crystal (EC) is examined. This quantum EC is due to repulsive interactions between singlet x (or biexcitons). Spatial squeezing of the x‐system can be realized by the surface and interface potentials of a quasi‐1D quantum wire. Stability and melting of the quantum EC are studied both within the Lindemann and Mott criteria. The phase diagram of the squeezed quasi‐1D x‐system is proposed. According to the estimates within the Heitler‐London approximation, the quantum EC can exist for moderate x densities (1 < rs ≦ 5.5). The expected short condensation times (ps) will allow an EC formation in, e.g., GaAs/Ga1−xAlxAs quantum‐well wires with relatively short x lifetimes. At low temperatures the quantum EC favors collective, coherent emission in the form of superradiance instead of incoherent luminescence.
Physics of gain and stimulated emission in ZnSe‐based quantum well lasers
Authors: Ding J. , Kozlov V. , Kelkar P. , Salokatve A. , Nurmikko A. V.
Published: July 2007

Spectroscopic work is reviewed which focuses on the microscopic mechanism of gain in ZnSe‐based quantum well (QW) lasers, under optical and electrical injection, respectively. In particular, the lasing process in optically pumped ZnCdSe/ZnSe QW lasers at cryogenic temperatures is shown to be entirely an excitonic process. This conclusion is supported by results from resonant pumping experiments and femtosecond pump–probe measurements. A phenomenological model is presented, which can explain the experimental observation rather well. More strikingly for this strong quasi‐two‐dimensional (2D) exciton system, recent studies on room temperature ZnCdSe/ZnSSe/ZnMgSSe diode lasers show that pairwise electron–hole Coulomb correlation remains relevant in this case as well.
Transient response of a short‐cavity semiconductor laser
Authors: Pompe G. , Rappen T. , Wehner M. , Knop F. , Wegener M.
Published: July 2007

The transient response of short‐cavity semiconductor lasers excited by 250 fs optical pulses is investigated. The emission is time‐ and spectrally‐resolved by an up‐conversion technique. The experimental evidence is compared with simulations employing a simple resonator model, which allow for a comparison of the energetic position, width, and intensity of the laser emission as a function of time. The time scale of the lasing process is governed by heating due to stimulated emission. Caused by the changing carrier distribution functions, inducing changes of the refractive index, the resonance frequency of the cavity experiences a rapid red shift in time, which cannot be followed adiabatically by stimulated emission. This transient mismatch qualitatively explains the observed temporal evolution of the linewidth.
Carrier dynamics and lasing in epitaxial ZnTe layers on GaAs
Authors: Majumder F. A. , Kalt H. , Klingshirn C. , Stanzl H. , Gebhardt W.
Published: July 2007

By means of ps‐time resolved luminescence measurements the carrier dynamics in ZnTe/GaAs epitaxial layers of different thicknesses are investigated. A linear decay process corresponding to exciton recombination at low and a quadratic process corresponding to the recombination in an electron–hole plasma (EHP) at high excitation, respectively, are observed. The decay processes are dominated by the carrier diffusion into the substrate. The lifetime, binding energy, and temperature of the plasma are extracted from the time‐resolved luminescence spectra. The carrier density found from the luminescence fit is used to describe the band‐gap renormalization effect in ZnTe/GaAs epilayers at high excitation. Laser emission from all samples is observed at low and even up to room temperatures.
Lateral quantization effects in the Luminescence of InGaAs/InP quantum wires
Authors: Forchel A. , Kieseling F. , Braun W. , Ils P. , Wang K. H.
Published: July 2007

Cw and time‐resolved luminescence spectra of InGaAs/InP quantum wires with lateral extensions down to 10 nm are investigated. The luminescence of the wires shows shift to higher energy as well as features due to transitions of up to four lateral subbands when the wire widht is reduced to less than 50nm. Time‐resolved studies with subpicosecond time resolution indicate that the typical relaxation times for resonantly excited carriers in the wires are below 10 ps. If the laser is polarized perpendicular to the wires the direct absorption in the wires is suppressed. This is used to study the population of the wires by carrier capture from the unpatterned InP sections of the wires due to lateral quantization.
Time and frequency resolved luminescence from excitonic nanostructures
Authors: Chernyak V. , Wang N. , Mukamel S.
Published: July 2007

Time and frequency resolved luminescence spectroscopy of molecular and semiconductor nanostructures provides a direct probe for exciton transport and relaxation. A microscopic Green function theory which incorporates effects of inelastic scattering induced by exciton–phonon interactions, and elastic scattering due to static disorder is developed.
Interferometric cross‐correlation measurements of interband polarization decay in GaAs
Authors: Böhne G. , Freundt S. , Arlt S. , Pfister D. , Nácke C. , Nagerl J. S. , Ulbrich R. G.
Published: July 2007

The method of interferometric cross‐correlation is used to obtain precise information about the phase and amplitude of the laser pulse induced interband polarization in semiconductors. With this technique it is possible to study the linear as well as nonlinear response of a sample. The light transmitted through a thin GaAs crystal after excitation with femtosecond pulses near the band gap is measured. The phase shift between the driving laser pulses and the induced polarization is measured in the time domain. The influence of additional pump pulses on the decay of the polarization is studied. The instantaneous frequency of the polarization depends on the laser center frequency, on the pump pulse delay time, and on the relative polarization of the pump pulse with respect to the test pulse.
Non‐markovian polarization decay in semiconductor quantum wells studied by three‐beam four‐wave mixing
Authors: Löser F. , Klatt D. , Pantke K.‐H. , Leo K. , Bakker H. , Shah J. , Köhler K.
Published: July 2007

The polarization and population dynamics of resonantly generated excitons in GaAs/AlGaAs quantum well structures are investigated using transient three‐pulse four‐wave mixing. The decay dynamics show strongly nonexponential behavior for both variation of the delay between the first two pulses reflecting the polarization dynamics and variation of the delay between the second two pulses, which reflects the population dynamics. A comparison with a theoretical model shows that the dynamics of both polarization and population are mainly determined by frequency fluctuations and not by scattering processes or by recombination. The characteristic time of the fluctuations TC has to be energy‐dependent to fully describe the results.
Four‐wave mixing in modulation‐doped GaAs quantum wells under strong magnetic fields
Authors: Bar‐Joseph I. , Finkelstein G. , Bar‐Ad S. , Shtrikman H. , Levinson Y.
Published: July 2007

Time‐resolved four‐wave mixing is used to study electron scattering in modulation‐doped GaAs quantum wells in high magnetic fields up to 8T. A strong increase in dephasing times in strong magnetic fields is observed and attributed to a decrease in the scattering rates relative to zero field. An early time signal in high magnetic field is found, which is due to a polarization interaction process. A strong enhancement of the signal at the absorption edge is measured which is attributed to the Fermi edge singularity.
Phase relaxation of excitons in Zn1−xCdxSe/ZnSe multiple quantum well structures
Authors: Weckendrup D. , Saschek M. , Neukirch U. , Gutowski J. , Ferreira S. O. , Sitter H.
Published: July 2007

The coherent dynamics of excitons in Zn1−xCdx,Se/ZnSe multiple quantum wells measured by means of spectrally integrated as well as spectrally resolved transient degenerate four‐wave mixing are reported. When simultaneously exciting the heavy‐hole and light‐hole excitons by spectrally broad femtosecond pulses a quantum beating between heavy‐ and light‐hole excitons is observed. In spectrally resolved measurements an additional contribution from the 2s heavy‐hole exciton to the four‐wave‐mixing signal occurs which dephases very rapidly. By use of narrow‐band ps pulses the energy dependence of the dephasing rate in localized heavy‐hole exciton states is investigated. The dephasing times deduced from the experimental data increase for decreasing energy up to 4 ps in deeply localized states.
Renormalization effects in dense neutral magnetoplasma photoexcited in CdTe/CdMnTe quantum wells
Authors: Tyashlov M. , Lebedev M. V. , Kulakovskii V. D. , Forchel A. , Yakovlev D. R. , Waag A. , Landwehr G.
Published: July 2007

An optical study of the dense neutral electron‐hole magnetoplasma is carried out in an undoped CdTe/Cd0.88Mn0.12Te quantum well in magnetic fields up to 14 T at 2 K. The renormalization of the Landau‐level transition energies is found to be qualitatively similar to that in magnetoplasmas photoexcited in the InGaAs/GaAs quantum wells, but to differ quantitatively. The difference appears due to the enhanced Coulomb interaction and electron‐phonon coupling in CdTe quantum wells.
Negative differential gap renormalization in type‐II superlattices
Authors: Langbein W. , Hallstein S. , Kalt H. , Schönherr H. P. , Nötzel R. , Ploog K.
Published: July 2007

A quantitative determination of the band gap renormalization in type‐II GaAs/AlAs superlattices under high excitation is reported. The time‐resolved photoluminescence after intense picosecondexcitation is analyzed using a detailed luminescence lineshape fit. The renormalization of the type‐I transition is found to be comparable to that in type‐I superlat tices, whereas the type‐II transition reveals a negative differential renormalization. This is attributed by a quantitative calculation to the space‐charge effects caused by the carrier separation.
Atomic hydrogen at anion sites in LiFH− and NaH− crystals
Authors: Egranov A. V. , Chernyago B. P.
Published: July 2007

Anion hydrogen atom centres are studied by optical and ESR echniques in LiFH− and NaFH− crystals. The ESR spectra of these centres are observed after X‐ray irradiation of the crystals at 77K. The optical absorption spectra of H centres consists of two bands at 2.4 and 3.5 eV for LiFH− and at 2.95 and 3.8eV for NaFH−,which are due to the electron charge transfer from a flurine ion to the l s state of the hydrogen atom.
Charge ordering of asymmetrical clusters in mixed‐valence molecular crystals: Mössbauer spectra
Authors: Klokishner S. I. , Gheikhman B. D.
Published: July 2007

A crystal consisting of asymmetrical d‐d mixed‐valence (MV) clusters is considered. It is supposed that the cluster distortion results from the difference of crystal fields acting on centers a and b as well as from other factors providing the difference of single‐electron wave functions of centers a and b. The model accepted takes into account Heisenberg‐type and double exchange along with cluster asymmetry and intercluster dipole‐dipole interaction. Conditions of charge‐ordered phase stabilization are elucidated. Ferro‐and antiferroelectric states of crystals comprising asymmetrical MV dimers are investigated. The phase transitions in such crystals take place gradually. Computer simulation of Mössbauer spectra is carried out. The experiments of Mössbauer spectra of a series of biferrocenium derivatives performed by the Hendrickson group are qualitatively interpreted.
Effect of interface energy anisotropy on thermal stability and transformation of lamellar Structures: II. Transformation of lamellae
Authors: Mazurski M. I. , Salishchev G. A.
Published: July 2007

Thermodynamic regularities of the transformation of a lamellar shape into a spherical one is analyzed on the basis of the comparison of interface energies of spheres and disks. It is revealed that the motive force of spheroidization in two‐phase systems with a high level of anisotropy occurs during high‐temperature deformation due to developing recrystallization processes. In systems with a low level of interface energy anisotropy the motive force considerably increases due to recrystallization. The causes of a specific behaviour of systems consisting of phases with different types of interatomic bonds are noted.
Resonant tunneling and confining phenomena in symmetrical rectangular triple‐barrier structures with C‐type double‐wells
Authors: Yamamoto H. , Tsuji K. , Taniguchi K.
Published: July 2007

Resonant tunneling is studied theoretically in symmetrical rectangular triple‐barrier structures composed of triple barriers and C‐type double‐wells which are coupled through a central thin barrier. Analytical expressions for the transmission coefficient and the resonance conditions are derived for the first time. Wave functions of an electron at the resonance level are examined and the confining phenomena are studied.
A modified Hückel calculation on the structures and ionization potentials of Ag clusters
Authors: Zhao Jijun , Chen Xiaoshuang , Wang Guanghou
Published: July 2007

The geometrical structures and ionization potentials of silver clusters are studied by a modified Hückel theory including the variable Coulomb integral resulting from different coordinate environments. The stable atomic configurations of the Agn clusters are obtained by comparing the energy of different structural isomers. The clusters with magic number atoms are found to adopt the near spherical host‐like structure with higher symmetry, corresponding to the complete electronic shell. Based on those favorable structures the size dependent ionization potentials of Agn are calculated up to 68 atoms by using this new developed modified Hückel theory. The theoretical values are in quite agreement with the experimental results as well as ab initio configuration interaction (CI) calculations. A transition from “electronic order” to “atomic order” around n = 40 is proposed.
The elastic energy loss related to the lattice instabilities above room temperature in Bi1.7Pb0.3Sr2Ca2Cu30y
Authors: Chen S. O. , Shen H. M. , Huang Y. N. , Xu J. , Wang X. M. , Wang Y. N.
Published: July 2007

The tensile test, the torsional pendulum test, and X‐ray analysis are used to investigate the lattice instabilities of Bi1.7Pb0.3Sr2Ca2Cu3Oy ceramics from room temperature to 250 °C. The results show that there are three energy loss peaks near 35, 95, and 155 °C, which display of strain amplitude dependence and frequency independence. In the vicinity of these peak temperatures step‐like changes of lattice parameters associated with lattice instabilities are observed.
Comparison of Surface‐Enhanced Raman and Hyper‐Raman Spectra of the Triphenylmethane Dyes Crystal Violet and Malachite Green
Authors: Schneider S. , Brehm G. , Freunscht P.
Published: July 2007

Vibrational spectra of two dyes, malachite green and crystal violet are presented. Raman and hyper‐Raman spectra are recorded as surface‐enhanced (hyper‐) Raman spectra on silver colloids. These data are compared to the respective IR spectra with respect to the point group of the molecules (C2 and C3). Theoretical calculations (PM3 semiempirical) and polarized Raman spectra are used to give an assignment of some vibrational bands. The advantage of SERS (surface‐enhanced Raman scattering) and SEHRS (surface‐enhanced hyper‐Raman scattering) is discussed for the analysis of vibrational spectra.
Vibronic Two‐Photon Intensities in the 1La and 1Lb Transitions of Anthracene
Authors: Hohlneicher G. , Wolf J.
Published: July 2007

Vibronic intensities for a two‐photon transition from the electronic ground state of anthracene to the first two excited singlet states (1La and 1Lb) are calculated within first‐order Herzberg‐Teller theory. Dushinsky effects are fully included. The results are compared to recent experimental observations.
Level Correlations and Dephasing‐Induced Resonances in Molecular Aggregates
Authors: Chernyak V. , Mukamel S.
Published: July 2007

Dephasing‐induced resonances in the nonlinear optical response of molecular aggregates are expressed in terms of level correlations. The third‐order optical response is related to the three‐dimensional spectral density function χ(ε′, ε,ε″). Expressions for χ(ε′, ε,ε″) derived using the Frenkel exciton model show the existence of collective (dephasing‐induced) resonances.
On the Phase Transition Sequence in Deuterated Betaine Calcium Chloride Dihydrate under Hydrostatic Pressure
Authors: Chaves M. R. , Almeida A. , Kiat J. M. , Tolédano J. C. , Schneck J. , Glass R. , Schwarz W. , Ribeiro J. L. , Klöpperpieper A. , Albers J.
Published: July 2007

An elastic neutron scattering study of deuterated betaine calcium chloride dihydrate under hydrostatic pressure is presented. It is shown that a previous phenomenological model based on a one‐dimensional order parameter P, and involving a term of the form P2(δP/δz)2 remains adequate to account satisfactorily for the effects of the hydrostatic pressure. From the study of the temperature dependence of the integrated intensity of the satellites it is inferred that the critical exponent β related to the primary order parameter is close to 0.5 for 2.9 × 108, 3.4 × 108, and 4 × 108 Pa.
Energy Relaxation of Hot‐Electrons in a Quantum Well. Confined Optical and Acoustic Phonon Effects
Authors: León‐Avila F. , Rodríguez‐Coppola H. , Comas F.
Published: July 2007

Energy‐loss rates of hot electrons are calculated for a semiconductor quantum well (QW) of the GaAs/AlGaAs prototype. Couplings with acoustic phonons (AP) and with confined polar‐optical phonons (OP) are considered. Hot electrons are assumed in internal equilibrium at a temperature Te (the temperature model). Comparison with previous calculations of other authors shows that OP confinement with the proper dispersion relation is of utmost importance for the correct interpretation of energy‐loss rates in the temperature range of 25 to 100 K. In the lower temperature regime the coupling with AP cannot be neglected. Thus reasonable agreement with experimental data is obtained even within the relatively simple model for the calculation when both mechanisms are taken into account. Other effects (hot phonons, etc.) are not considered and also no fitting parameters of any kind are introduced.
Effect of Forming Pressure on the Critical Temperature in YBa2Cu3O7−δ Superconductors
Authors: Al‐Shibani K. , Al‐Aql A. , Al‐Muezzin A. , Humeida H. , Nakhmedov E. P. , Firat T. , Öztürk O. , Saçli Ö. A.
Published: July 2007

The effect of pressure on the critical temperature Tc is studied for YBa2Cu3O7–δ compounds prepared under various forming pressures. A systematic decrease in lattice parameters is observed with forming pressure. Resistivity measurements show an increase of the onset temperature Tc0 with forming pressure. A value of dTc0/dP ≈ (0.070 ± 0.010) × 10−8 K/Pa is determined. A new mechanism based on the variation of the interlayer tunneling integral t⟂ is used to analyse the experiment results.
Influence of Surface Electric Field on Exciton Photoluminescence of CdS. Self‐Reversal of Spectral Lines
Authors: Rakovich Yu. P. , Yablonskii G. P. , Gladyshchuk A. A. , Smal A. S.
Published: July 2007

The structure of luminescence spectra of free excitons in CdS single crystals at T = 77 K is investigated. The dip formation is found in the top part of free A and B exciton resonant peaks of photoluminescence (PL) spectra after heating of the CdS crystals followed by fast cooling down to 77 K. Similar effects are observed when an external electric field is applied. It is shown that the structure of exciton PL spectra is due to self‐absorption of resonance radiation in the near‐surface layer with low exciton concentration. Our calculations demonstrate that the formation of an exciton concentration gradient may be caused by the influence of a surface electric field on the binding of electron‐hole pairs to excitons. It is assumed that the increase of surface electric fields after heat treatment is caused by the thermostimulated adsorption of oxygen, which results in the formation of electron trap levels.
Electronic Structure of Vanadyl Phthalocyanine. Investigations of Thin Film Samples by Electron Energy‐Loss Spectroscopy
Authors: Dahms U. , Mrwa A.
Published: July 2007

The energy‐loss function Im (−ε−1) of a thin polycrystalline vanadyl phthalocyanine film is determined in the range from 1 to 37 eV from energy‐loss experiments with 50 keV electrons. The electronic structure of the film investigated is analyzed by computing the complex dielectric function ε(E), optical constants (n(E) + ik(E), R(E), α(E)), and neff(E) using the Kramers‐Kronig analysis of the energy‐loss function. The spectrum of valence electron excitations is primarily interpreted in terms of π−π* and σ−σ* transitions of individual molecules and in terms of the collective excitation of the valence electron ensembles (σ + π plasmon).
Al3+ Ion Site Symmetry in (NH4)2[AlF5 · H2O] Single Crystals
Authors: Darabont Al. , Borodi Gh. , Nistor S. V. , Velter‐Stefănescu M.
Published: July 2007

The production of pure and Cr3+ doped (NH4)2[AlF5 · H2O] single crystals and the crystal structure of this compound are described. The infrared spectra recorded experimentally at room temperature show – using the results of group theory – that the [AlF5 · H2O]2− complex ion has at most the symmetry of the C4v group. EPR measurements are carried out for a more exact determination of the site symmetry of Al3+ ions using the Cr3+ ion on Al3+ site as “probe ion”. The determined values of spin‐Hamiltonian parameters suggest that the symmetry of the [AlF5 · H2O]2− complex ion has at most the symmetry of the C2v group.
Theory of the AC Hall Effect and Magnetoresistance in Polycrystalline Systems
Authors: Fishchuk I. I.
Published: July 2007

The theory of the ac Hall effect and magnetoresistance for systems which consist of randomly oriented anisotropic crystallites is developed. The effective‐medium theory is used. The large frequency region, where ω < τ−1 (τ is the electron mean free time) is investigated. It is shown that the real parts of the dynamic conductivity, the Hall conductivity, the Hall mobility, the Hall coefficient, the magnetoconductivity, and the magnetoresistance have low‐ and high‐frequency plateaus. The high‐frequency plateau begins in the vicinity of the frequency ω0 ≈ τ, where τM is the Maxwell relaxation time (τM » τ).
Pair‐Breaking and Magnetic Ordering in the Pseudo‐Quaternary Intermetallic Series (Y, Gd)Ni2B2C
Authors: El Massalami M. , Budko S. L. , Giordanengo B. , Fontes M. B. , Mondragon J. C. , Baggio‐Saitovitch E. M. , Sulpice A.
Published: July 2007

Alloying influences on the superconducting and magnetic properties of the (GdxY1−x)Ni2B2C series are studied. For x < 0.25, Tc is degraded linearly with x as expected from the theory of Abrikosov‐Gorkov. Magnetization and magnetoresistivity measurements for x = 0.22 show that the magnetic saturation is attained only for fields as high as 80 kOe implying that even for such a low Gd concentration, antiferromagnetic couplings are quite strong. On the other hand, the magnetic dilution in the Heisenberg antiferromagnet GdNi2B2C causes a linear depression of the magnetic ordering temperatures as expected from molecular field theory. The abrupt quench of superconductivity for x > 0.25 and the coexistence of superconductivity with long‐range antiferromagnetic order for 0.1 < x < 0.25 are discussed.
Estimating Critical‐Point Parameters of Modulated Reflectance Spectra
Authors: Hosea T. J. C.
Published: July 2007

Two new methods, based on the 3‐point method and a Kramers‐Kronig transformation, were recently published for estimating the energy, broadening, phase, and amplitude of critical points in modulated reflectance spectra which are describable by Aspnes' third derivative functional form. The use of the techniques is demonstrated by applying them to example experimental spectra, which are photo‐reflectance measurements of stress‐induced splitting of the direct band gap E0 transition in bulk GaAs. The results are compared and contrasted with those obtained from a least‐squares fit of a theoretical model to the experimental spectra.
Ultrafast Carrier Dynamics in Porous Silicon
Authors: Fauchet P. M.
Published: July 2007

The carrier dynamics in porous silicon is investigated by two time‐resolved techniques. The blue photoluminescence of oxidized porous silicon is measured with 100 ps time resolution as a function of the oxidation method, emission wavelength, excitation intensity, and measurement temperature. The blue luminescence has a distinct origin from the well‐studied rcd luminescence and is attributed to defects in the oxide. Femtosecond photoinduced absorption measurements are performed on thin red‐emitting porous silicon films. The wavelength and intensity dependence of the recovery are interpreted in terms of trapping and of Auger recombination at high excitation intensity. It is shown that red‐emitting porous silicon is not a direct‐gap semiconductor.
Light Emission from Silicon Nanostructures Produced by Conventional Lithographic and Reactive Ion Etching Techniques
Authors: Nassiopoulos A. G. , Grigoropoulos S. , Papadimitriou D. , Gocolides E.
Published: July 2007

Sub‐ten nanometer diameter silicon pillars and silicon walls of the same thickness are fabricated by using conventional optical lithography based on deep‐UV exposure and reactive ion etching using fluorine only containing gases. The produced structures are studied for their luminescence properties. Visible photoluminescence with a peak in the range 580 to 650 nm is observed under Ar laser irradiation.
Internal Friction and Sound Wave Velocities in Pure C60 Fullerite
Authors: Kobelev N. P. , Soifer Ya. M. , Bashkin I. O. , Gurov A. F. , Moravskii A. P. , Rybchenko O. G.
Published: July 2007

The temperature dependence of the attenuation and sound wave velocity of compacted pure C60 fullerite is measured in the temperature interval from 100 to 300 K at a frequency of about 5 MHz. Two internal friction peaks are observed at 250 and 215 K. The conclusion is made that the internal friction peak at 250 K is connected with the structural phase transition and the peak at 215 K is due to the relaxation by reorientation of C60 molecules in the stress field of ultrasonic waves. The magnitudes of the elastic moduli of C60 fullerite are determined by sound velocity measurements. The results are compared with theoretical estimations of the elastic moduli and experimental data of other authors.
EPR of CrO Radicals in High Temperature Phases of (NH4)3H(SeO4)2 Crystal
Authors: Aucustyniak M. A. , Hoffmann S. K.
Published: July 2007

EPR X‐band studies of CrO radicals substituting SeO4, groups in (NH4)3H(SeO4)2 crystals are performed in the temperature range 280 to 400 K, where transitions to two superionic phases with protonic conductivity appear at 304 and 330 K. In the room temperature ferroelastic phase four types of CrO radicals with different g‐factors and direction cosines are identified. Three of them (with 40% of the total spectrum intensity) are identified as Halperin‐Varma‐type defects. These defects vanish in superionic phases. The electronic structure of the CrO3‐4 radical is different from that, predicted by the geometry of the host SeO, groups, and is described by the d ground state, characteristic of an elongated tetrahedron of distorted D2d symmetry. Only part of the CrO3‐4 radicals show the dynamics expected for SeO4 groups and follow their behaviour at the phase transitions. The transition to the superionic phase II at 304 K is observed as a rapid EPR line narrowing due to an increase in the crystal lattice dynamics and an increase in the SeO4 reorientation rate. In the phase transition to the superionic phase I (330 K) a shift and broadening of the lines are observed. It allows to suggest that the crystal structure of phase I is dynamically disordered with two equally populated orientations of the SeO4 group.
The Dependence of the Debye Temperature on the Elastic Constants
Authors: Siethoff H. , Ahlborn K.
Published: July 2007

Sophisticated methods have been put forward in the literature to calculate Debye temperatures from the elastic constants cik. As a simple alternative method for cubic crystals, a relation is often used that correlates Debye temperature and bulk modulus (c11 + 2c12)/3 by a square‐root law. It is shown that, with exception of the alkali metals, such a law is only poorly fulfilled for other cubic elements and compounds. If one takes, however, elastic moduli such as [c44(c11 − c12) (c11 + c12 + 2c44)]1/3 or {c44[(c11 − c12)c44/2]1/2 (c11 − c12 + c44)/3}1/3 instead of the bulk modulus the square‐root law is established for various cubic material classes. This procedure eventually allows to easily evaluate hitherto unknown Debye temperatures from the elastic constants with high precision.
DSC, Dilatometric, Dielectric, and 1H NMR Studies of Phase Transitions and Molecular Motions in [N(C2H5)4]3M2Cl9 (M = Sb, Bi) Crystals
Authors: Zaleski J. , Medycki W. , Piślewski N. , Jakubas R.
Published: July 2007

Results in the dependence of stoichiometry of obtained tetraethylammonium (TEA) chloroantiomonatc and chlorobismuthate salts on the molar ratio of reactants used in the synthesis are presented. Seven tetraethylammonium salts are obtained: (TEA)6M8Cl30, TEAMCl4, (TEA)3M2Cl9 (M = Sb, Bi) and (TEA)2SbCl5. Preliminary X‐ray diffraction studies on (TEA)3M2Cl9 (M = Sb, Bi) show that they are isomorphous, crystallizing at room temperature in monoclinic symmetry. The dilatometric, dielectric, and DSC studies show that (TEA)3Bi2Cl9 undergoes two phase transitions at Tc2 = 144K and at Tc1 = 322K while (TEA)3Sb2Cl9 undergoes three transitions at Tc3, = 185. Tc2 = 219, and Tc1 = 321 K. Both crystals are ferroelastic below the ferro‐paraelastic transition at Tc1. The room temperature domain structure belongs to the Aizu species 6/mmmF2/m. The spin‐lattice relaxation times and the values of the second moments of the 1 NMR line for the (TEA)3Sb2Cl9 crystal are measured in the temperature range of 50 to 350K. Three minima of T1 are observed. Two low‐temperature minima are attributed to CH3 group reorientations, while the high‐temperature one to reorientations of the whole cations. The activation energies for the processes are determined.
X‐Ray Diffraction Study of Thin Porous Silicon Layers
Authors: Lomov A. A. , Bellet D. , Dolino G.
Published: July 2007

High resolution X‐ray diffraction measurements are performed on porous silicon samples (p+ type, 40% porosity) of various thicknesses (from 0.03 to 20 μm). In the thicker samples narrow Bragg peaks are observed for as‐formed samples, with profiles close to those of dynamical diffraction theory. For thin samples (in the 0.1 μm range) the Bragg peak of the porous layer is not resolved, but appears as a shoulder on the substrate peak. Thickness fringes are observed, from which simulations give information on these thin PS layers, showing that the porous layer surfaces are well defined.
Consistent Analysis of the Tl and Pb Center ESR Parameters
Authors: Nistor S. V. , Schoemaker D.
Published: July 2007

The electron spin resonance parameters of the Tl and Pb dimer centers produced by electron trapping at pairs of neighboring Tl+ or Pb2+ impurity ions in alkali chlorides are reanalyzed. The analysis which uses a consistent set of molecular parameters is based on the molecular modcl previously employed to describe the ESR properties of the homonuclear Vk self‐trapped hole center, which has a complcrnentary electronic configuration. In a rough approximation the analysis is extended to the self‐trapped electron (STEL) center in PbCl2 crystals, which is a bent Pb molecular ion in a low symmetry crystal field.
Computer Simulations of I‐Center Annealing in KCl and KBr Crystals. Theoretical Interpretation of Thermostimulated Experiments
Authors: Eglitis R. I. , Popov A. I. , Kotomin E. A.
Published: July 2007

Results of computer simulations of the kinetics of correlated annealing of pairs of close α–I and F–I centers in KCI and KBr crystals, enhanced by I‐center diffusion and Coulomb or elastic attractions, respectively, are presented. Special attention is paid to the conditions under which multi‐stage annealing stages arise as it has been observed experimentally more than once. Our general conclusions are: (i) a weak elastic interaction affects the recombination kinetics and the survival probability even for relatively well‐separated F–I pairs, the more so is true for the case of Coulomb attraction between charged α–I pairs; (ii) the multi‐step (kink) structure arises only for close (typically, up to fourth‐nearest neighbour) defects. To explain the strongly separated stages observed experimentally for α–I (17 to 21 K, 30 to 39 K in KCI and 18 to 22 K and 27 to 30 K in KBr) a very particular initial distribution of defects is required consisting of two groups of close and well‐separated defects. It gives strong support to the results of recent computer simulations that an interstitial ion created due to electron trapping by an H center can be transformed into a crowdion which could be displaced athermally by a long distance from a vacancy (five to seven interionic distances).
Nonequilibrium Statistical Theory of Channeling. Energy–Momentum Balance Equation and Transverse Quasitemperature of a Channeled Particle Subsystem
Authors: Kashlev Yu. A.
Published: July 2007

Planar channeled particles (protons, helium ions, etc.) are considered as an individual thermodynamic subsystem. A statistical operator for the nonequilibrium system under conditions of multiple electron scattering and an energy‐momentum balance equation are obtained on the basis of the nonequilibrium statistical thermodynamics. The formalism presented is applied to analyze the transverse quasitemperature of the channeled particle subsystem in steady state, its quasiequilibrium energy distribution, and the diffusion coefficient of the particles.
Electronic and Cohesive Properties of NbC, NbN, and NbO
Authors: Onwuagba B. N. , Sundararajan V.
Published: July 2007

The electronic and cohesive properties of the refractory metal compounds NbC, NbN, and NbO are calculated using the tight‐binding linear muffin‐tin orbital (TB‐LMTO) method. The electronic structure agrees very well with the previous calculations obtained by other methods. The hybridisation increases as we go from the C to O compounds. The cohesive energies and the bulk moduli are increased compared to the pure Nb metal indicating the role of C, N, and O in the enhancement of binding.
Segregation Kinetic Simulation and Segregation Energy Determination on a CuAg(111) Alloy
Authors: Jouaiti A. , Mosser A. , Romeo M.
Published: July 2007

Using a method based on the kinetic tight binding Ising model (KTBIM), the segregation energy on the (111) face of a CuAg 0.45 at% alloy, is derived as a function of surface coverage, from segregation kinetics measured at 763 K. Nearly the same segregation energy value as from equilibrium experiments is obtained. Besides, it is shown that a surface size effect term, proportional to the square of the surface concentration, has to be introduced in the segregation energy, in order to take into account the surface strain energy which occurs in the first layer when the silver atoms segregate at the surface.
Density of States in Ordered and Disordered Pd2TiAl and Pd2TiSn Heusler Alloys
Authors: Jezierski A.
Published: July 2007

The band structure of the ordered and disordered Pd2TiAl and Pd2TiSn Heusler alloys is studied by using the LMTO and LMTO‐CPA methods. The self‐consistent spin‐polarized LMTO calculations show that these systems are not magnetic and the shape of the density of states is in good agreement with the ultraviolet photoemission spectra for the Pd2TiAl alloy. Our theoretical calculations indicate the existence of a small sub‐peak near E = −9 eV in the UPS spectra for Pd2TiSn.
Random Impurity Potential and Hopping Conduction in ZnSe
Authors: Kasiyan V. A. , Nedeoglo D. D. , Timchenko I. N.
Published: July 2007

Hall effect, electrical conductivity, carrier mobility, and magnetoresistance of Cu‐doped n‐ZnSe bulk crystals are studied in the temperature range 2 to 300 K. Compensating Cu impurity amplifies the random impurity potential, modulates the band edges, and leads to electron localization in the potential wells of the conduction band bottom in slightly doped, highly compensated crystals (K = 0.82 to 0.97). High‐temperature hopping conductivity through the localized states of such type observed in the range 100 to 200 K and accompanied by negative magnetoresistance is first investigated in n‐ZnSe.
Electron Transport in Vapor Deposited Molecular Glasses. The Role of Group Dipole Moments
Authors: Borsenberger P. M. , Gruenbaum W. T. , Magin E. H.
Published: July 2007

Electron mobilities are measured in vapor deposited layers of N1N′‐bis(1,2‐dimethylpropyl)‐1, 4, 5, 8‐naphthalenetetracarboxylic diimide (NTDI). NTDI is a weakly polar compound with a molecular dipole moment of 0.3 D. The molecular moment is the vector sum of two group moments, each approximately 3.3 D. The results are described by a formalism based on disorder, due to Bässler and coworkers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameter of the formalism is the energy width of the hopping site manifold. For NTDI, the width is 0.093 eV. The width can be described by a model based on dipolar disorder. The model assumes that the width is comprised of a dipolar component and a van der Waals component. A comparison of the experimental results with predictions of the model leads to the conclusion that the dipolar component to the width is determined by group dipole moments of two carbonyl groups adjacent to the imide acceptor functionalities. The positional disorder parameter is 2.3 and attributed to packing constraints. The prefactor mobility is 1.0 × 10‐2 cm2/Vs.
Structural and Optical Properties of the System (Ca, Sr, Eu)5(PO4)3CI
Authors: Nötzold D. , Wulff H. , Herzog G.
Published: July 2007

Polycrystalline materials of the composition Sr5−x−yCaxEuy(PO4)3Cl with 0 ≦ x ≦ 4.99 and 0 ≦ y ≦ 0.1 crystallize in the hexagonal crystal system. The space group is P63/m. The alkaline earth ions occupy two different crystallographical sites. In addition to X‐ray diffraction measurements, the optical properties are studied at room temperature. Moreover the emission spectra are measured at liquid nitrogen temperature. Refinement of the structure parameters is carried out using the Rietveld method. A varying calcium portion influences the lattice constants a0 and c0 and therefore the luminescence of the samples. The luminescence arises from transitions between 4f65d and 4f7 configurations of the Eu2+ ion.
Self‐Diffusion of Ni in Single and Polycrystals of Ni3Al. A Study of SIMS and Radiotracer Analysis
Authors: Frank St. , Södervall U. , Herzig Ch.
Published: July 2007

Diffusion of nickel in the intermetallic compound Ni3Al is measured in the temperature range from 1004 to 1422 K by applying two different techniques for the determination of the concentration profiles. The diffusion of the radioactive 63Ni tracer in Ni3Al polycrystals (75.2 at% Ni) is analyzed in the temperature region from 1223 to 1422 K by the conventional serial sectioning technique using a precision parallel grinding device and an improved technique of detecting the low‐energy β‐decays of 63Ni with high efficiency by liquid scintillation counting. In the low‐temperature region (1004 to 1259 K) the concentration profiles are determined by secondary ion mass spectrometry (SIMS) using the highly enriched stable 64Ni tracer and Ni3Al single crystals (75.9 at% Ni). The SIMS technique is considered as particularly suitable for profile detection, owing to its potential of measuring also low diffusion coefficients with high accuracy. The temperature dependence of the diffusion coefficient DNiv follows a perfect linear Arrhenius relation in the investigated temperature range for Ni3Al. The frequency factor D0 = (3.59) × 10−4 m2s−1 and the activation enthalpy Q = (303.0 ± 5.3) kj mol−1 correspond to a vacancy mechanism. An enhanced diffusivity at lower temperatures, which was previously reported in the literature and ascribed to the presence of constitutional vacancies, is not observed. It is concluded that Ni self‐diffusion in Ni3Al only occurs via thermal vacancies. This result is in agreement with recent positron annihilation studies by Badura and Schaefer where, independent of the composition, no constitutional vacancies are detected in the Ni3Al compound.
The Dynamics of Dislocation (Kink) in the Frenkel‐Kontorova Model in the Case of a Nonzero Environment Temperature
Authors: Landau A. I.
Published: July 2007

Computer simulation is used to study the dependence of the velocity of a dislocation (kink) on the applied motive force and temperature in the Frenkel‐Kontorova model (discrete sine‐Gordon system). The applied forces used exceed the Peierls‐Nabarro barrier. Heating of the atomic chain by the environment is simulated by introducing random thermal forces and frictional forces acting on individual atoms of the system into the Frenkel‐Kontorova equations. It is found that though in this model the velocity of a dislocation depends on temperature, this dependence is very weak. In the range of dislocation velocities studied, smaller velocities monotonically rise with growing temperature, while larger velocities depend on temperature nonmonotonically, first decreasing and then increasing with rising temperature.
Lattice Dynamics of Dichlorodifluoromethane
Authors: Zeng W. Y. , Anderson A.
Published: July 2007

A simple force constant model is applied to the vibrations of crystalline CCl2F2 at low temperatures, and good agreement with recent spectroscopic results is obtained. The calculations suggest that two weak Raman peaks observed in the lattice region should be re‐assigned as fundamentals. Principal intermolecular interactions are identified, and it is shown that the intramolecular force constants change only slightly from their gas phase values.
Si/SiGe Quantum‐Well Electron Mobility. Main Scattering Mechanisms
Authors: Tutor J. , Comas F.
Published: July 2007

Mobility calculations for a symmetrically strained Si/SiGe quantum well are reported considering conduction along the Si channel. The Boltzmann transport theory is applied taking into account two scattering mechanisms:electron scattering by acoustic phonons (via deformation‐potential coupling) and electron scattering by ionized impurities (both remote and background). It is compared with experimental results reported by Schäffler et al., where the QW was grown avoiding threading dislocations in the Si channel, and an excellent agreement is achieved. As in our previous works only one adjustable parameter is introduced: the deformation‐potential energy.
About a Conjunction between Electrical and Optical Phenomena in p‐Type Heavily Doped Silicon at Room Temperature
Authors: Van Cong H. , Debiais G.
Published: July 2007

The conjunction between electrical and optical phenomena in p‐type heavily doped silicon at room temperature is investigated, based on our accurate empirical models for the band‐gap narrowing, apparent band‐gap narrowing, and unperturbed Fermi energy shift. The numerical results of optical band gap, reduced interacting Fermi energy, and reduced interacting density‐of‐valence band states effective mass are presented and compared with other theoretical results and experiments. It is concluded that, taking into account the unperturbed Fermi energy shift effect, the values of band‐gap narrowing, obtained from electrical and optical phenomena, are found to be the same, removing the disagreements or discrepancies in theoretical and experimental values of band‐gap narrowing existing so far in the literature.
Concentrations of Atomic Defects in B2FexAl1−x. An Ab‐Initio Study
Authors: Mayer J. , Elsässer C. , Fähnle M.
Published: July 2007

The concentrations of various atomic defects are calculated within the framework of a grand canonical formalism for the ordered compound FexAl1−x with B2 structure as functions of temperature and composition. The input parameters for the grand canonical formalism are determined by the ab‐initio mixed‐basis pseudopotential theory in local‐density approximation. It is shown that FexAl1−x is neither a compound with pure antisite disorder nor a compound with pure triple‐defect disorder but that it exhibits a hybrid behaviour in which vacancies on the Fe sublattice and antisite atoms on both sublattices appear. The concentration of vacancies on the Al sublattice is negligibly small. The implications of this result for the modelling of diffusion mechanisms in FexAl1−x are discussed.
Hole Transport in a Vapor Deposited Phenylenediamine Molecular Glass
Authors: Borsenberger P. M. , Shi J.
Published: July 2007

Hole mobilities have been measured in vapor deposited amorphous layers of N,N′‐bis(1‐naphthalenyl)‐N,N′‐diphenyl‐4,4′‐phenyldiamine (NPPDA) over a wide range of fields and temperature. The results are described within the framework of a formalism based on energetic disorder of the hopping sites. The variance of hopping site energies is 0.098 eV. The energy variance can be described by a model based on dipolar disorder. The model is based on the assumption that the total variance is comprised of a dipolar component and a van der Waals component. For NPPDA, the dipolar component is 0.080 eV and the van der Waals component is 0.067 eV.
Influence of Electrostatic Field on Excess Conductivity of Thin YBaCuO Films
Authors: Sorkin B. , Konsin P.
Published: July 2007

The influence of a strong electrostatic field perpendicular to the electric current on the excess conductivity of YBa2Cu3O7−y thin films near Tco (R = 0) and also in those with weak links is investigated by modelling with an Arrhenius‐type law. One film with weak links is treated by Aslamazov‐Larkin formulas, too. It is shown that the excess or paraconductivity depends on the polarity and strength of the electric field and this dependence may be explained as a change of the dimension of fluctuation conductivity with the increase of the hole concentration.