The question of temperature-dependent intrinsic dipoles in chalcogenide glasses

The claim that the kinetics of photoinduced anisotropy in chalcogenide glasses prove the presence of a temperature-dependent concentration of electric dipoles is re-examined. It is concluded that neither a temperature dependence of electric dipoles nor even the presence of such dipoles is necessary to explain the experimental observations.     

Three-dimensional direct optical writing and Raman characterization of resulting structural changes in As2S3 glasses

We report on optical writing of typically 300–1000?nm diameter dots within the volume of a bulk As₂S₃ chalcogenide glass using a focused 1–3?mW beam of a cw He–Ne laser. The Raman spectra taken from the dots in the course of the writing indicate that the photorefractivity is manifested by an increased polarizability of bonds within intrinsic As₄S₄ (realgar) groups as well as of S–S bonds within the matrix of the As₂S₃ glass. The photorefractivity is also accompanied by softening of the As–S–As vibrational frequencies, characteristic of AsS₃ pyramids, which are the main structural units of the As₂S₃ glass.     

Photoinduced and thermally induced bleaching of amorphous Ge─S films

Abstract

The photoinduced changes of the optical properties of Ge₂₇S₇₃ thin films have been examined using visible and infrared spectroscopy. It was found that illumination of the virgin thin film in vacuum is accompanied by a blue shift of the optical gap and by an increase of the Ge─S bond density. If the film is illuminated in air, the blue shift of the gap is observed too, but the density of Ge─S bonds decreases and new extrinsic absorption bands around 820 and 695 cm^?1 appear in the spectrum, indicating most probably the presence of Ge─O bonds. It is supposed that in the former case the photoinduced changes are most probably intrinsic (decreases of the randomness of the film structure), while in the second case the photoinduced bleaching is accompanied by photoinduced oxidation of the film.     

Improved representation of metallic bonding in atomistic simulations

Previous use of the Sutton-Chen glue potential to model deformation processes in aluminium showed marked deviation from the results of an ab initio simulation prior to yield. The origin of this problem is identified, and a revised scheme is suggested and tested. It is based on the fact that the atomic volume is a key quantity in the metallic bonding of sp bonded elements, and our idea should therefore have wide applicability to interatomic bonding models. The scheme uses a new approximate but rapidly computable formula for the local atomic volume in an inhomogeneous system.     

Electrical properties and local structure of n-type conducting amorphous indium sulphide

An n-type amorphous chalcogenide, In₄₉S₅₁, having a band gap of 1.9eV, has been found. The conductivity in as-prepared films was ～10^?4?S?cm^?1, which increased to 1?×?10^?1?S?cm^?1 on post-annealing at 125°C in vacuum, accompanied by a reduction in the sulphur content of the films. TEM observations showed the amorphous nature of the films before and after annealing. Both Seebeck and Hall coefficients are negative, indicating that the major carriers are electrons. The Hall mobility can be as large as 26?cm²?V^?1?s^?1 at 300?K. No significant changes to the optical absorption were observed upon annealing. Analysis of the X-ray radial distribution function reveals that the sulphur atoms have four-fold coordination, making the structure more rigid than conventional amorphous chalcogenides in which the chalcogen is alloyed to elements of group IV or V of the periodic table. We tentatively associate the electron carrier generation with the formation of sulphur vacancies.     

Ab initio pseudopotential studies on an Al Σ = 9 grain boundary: Effects of Na and Ca impurities

The electronic properties of an Al = 9 tilt grain boundary with segregated Na and Ca impurity atoms have been calculated by a first-principles pseudopotential method. The results show that the boundary expands and the charge density decreases significantly over the whole boundary by the segregation of Na and Ca, and there are no stronger bonds than those associated with metallic bonding in the boundary even with the impurities. It is concluded that the embrittlement promotion mechanism by Na or Ca segregation in the Al boundaries is one kind of 'decohesion model'.     

First-principles pseudopotential study of an aluminium grain boundary containing sulphur atoms

The electronic structure of an aluminium grain boundary with segregated sulphur impurity atoms has been calculated by a first-principles pseudo-potential method. It is found that a sulphur atom bonds to only one of the neighbouring aluminium atoms. This bond is a mixed-character metallic-covalent bond which is stronger than the metallic Al-Al bonds. Electrons that participate in forming this bond are 3p electrons of sulphur but not its 3s electrons. Other Al--S bonds in the boundary contain no covalent character. From the nature of Al--S bonds in the boundary it cannot be decided whether the embrittlement promotion mechanism by sulphur segregation should be classified as a 'bond mobility model' or a 'decohesion model'.     

Book Reviews

Book reviewed in this article: Courage for Dialogue : Ecumenical Issues in Inter -Religious Relationships , by S. J. Samartha. Voices of Unity : Essays in Honour of Willem Vissert Hooft on the Occasion of his 80th Birthday , ed. Ans J van der Bent.     

Sliding on carbon nanotube arrays

Adhesion, friction and wear of Single-Walled Nanotube (SWNT) and multi-walled nanotube arrays have been investigated. SWNT arrays exhibit lower adhesion and friction values on account of the lower van der Waals’ forces resulting from lower packing density and higher flexibility. Wear tests show that both arrays do not wear. Tip wear and friction force in the SWNT array are lower due to lower adhesion and higher flexibility of the SWNTs, which lead to less resistance to the motion of the tip.     

Boron impurities on a vicinal Si(100) surface: A study of structural properties and binding energies

The equilibrium locations of boron on the steps of the Si(100) surface have been studied using the Hartree-Fock method at the semiempirical level. The step is represented by a cluster of silicon atoms whose dangling bonds are saturated by hydrogen atoms. The boron impurity is moved in the step in substitutional and interstitial positions and the optimal configuration of the step containing the impurity is evaluated from a steepest-descent energy minimization. The study focuses on the dependence of the properties of the boron-step interaction on the shape of the step. These effects are analysed in the light of the features of boron absorbed on the flat surfaces and in the bulk, which have been widely studied.     

Structural and thermal investigations of a high-strength Cu-Zr-Ti-Co bulk metallic glass

Cu 55 Zr 30 Ti 10 Co 5 bulk metallic glass exhibits a high compressive fracture strength of 2.31 GPa and Young's modulus of 130 GPa, values that are higher than those of other Cu- and Zr-based metallic glasses. The addition of Co to the ternary Cu-Zr-Ti alloy stabilizes the supercooled liquid. On heating, the Cu 55 Zr 30 Ti 10 Co 5 metallic glass devitrified and formed an intermediate intermetallic compound prior to reaching equilibrium by diffusion-controlled growth at constant nucleation rate. In the fully annealed state the structure consists of the equilibrium Cu 10 Zr 7 phase with the slightly reduced lattice parameters a = 0.933 nm, b = 0.928 nm and c = 1.254 nm and a small fraction of an unidentified phase.     

Structural effects on diffusion in metallic glasses: The pressure dependence of Ni diffusion in as-quenched and relaxed Co42Zr58

In order to clarify the role of thermal defects in diffusion in metallic glasses, we have measured the temperature and pressure dependences of diffusion of Ni, a probe for Co, in relaxed and non-relaxed amorphous Co₄₂Zr₅₈ alloys by means of the tracer technique in combination with secondary-ion mass spectroscopy. For the relaxed state, the activation energy and the pre-factor are Q _{= (}1:65=0:08₎ eV and D _{0 =} 2:4+3:9 10-6m2 s-1 respectively. The pressure - 1:9 dependence yields an activation volume V _{act = (}0:66 0:15₎Omega, with Omega being the average atomic volume of the alloy. This value is similar to activation volumes in crystalline materials and is indicative of diffusion via thermally generated defects. Unlike monovacancies in crystals, these defects appear to be spread out, judging from recent isotope-effect measurements. Comparison with literature data shows that the activation volume and hence the diffusion mechanism in metallic glasses clearly depend on the structure and composition even for the very same component. The activation volume in the as-quenched state was found to be _{( )} ^{0:93 0:25 Omega.}     

Potential fluctuations in phase change memory materials

Long-range potential fluctuations have been quantified in amorphous and crystallised thin films of a family of Ge–Sb–Te (GST) chalcogenide glasses. Among the compositions studied, the width of the potential fluctuations is the smallest for amorphous Ge₂Sb₂Te₅. This is also the most robust material in terms of the number of write–erase cycles for GST films when used as phase change memory materials. A plausible explanation for this observation is given and a criterion for selecting suitable compositions in optical memory devices is proposed.     

Simulation of adatom clustering on a stepped surface

Study of the interaction and evolution behaviour of adatoms is fundamental to many surface phenomena. In the present paper, a continuum mechanics method, based on the force dipole concept, is presented to study the evolution of adatoms adsorbed on a smooth or stepped surface. The effects of elastic and van der Waals interactions are examined. It is found that the elastic interaction among adatoms usually serves as the main driving force for the clustering behaviour of adatoms and that the presence of surface defects, such as steps, may significantly influence the evolutionary arrangement of interacting adatoms.     

Absence of photodegradation in amorphous chalcogenide films with a narrow optical bandgap

The time-dependent change in photocurrent during illumination has been studied in amorphous arsenic tritelluride (a-As Te ), which is an amorphous 2 3 chalcogenide material with a narrow optical bandgap. No photodegradation in the photocurrent is observed in this material, although photodegradation occurs in most wide-bandgap amorphous chalcogenides.     

Thermal expansion coefficients of Mo-Si compounds by first-principles calculations

Taking Mo-Si compounds as model systems, we show that the coefficients of thermal expansion (CTEs) of complex structures can be calculated precisely from first principles by incorporating the Debye model for acoustic response. Specifically, we obtain a nearly isotropic CTE in MoSi₂ but a highly anisotropic CTE in Mo₅Si₃. The CTE anisotropy in Mo₅Si₃ is due to an elastically more rigid basal plane and a higher anharmonicity along the c axis. As the structure of 5-3 compounds is modified from D8_m to D8_l by boron substitutions (Mo₅SiB₂), we predict a significant decrease in the CTE anisotropy, which is confirmed by experiments.     

Thermal expansion of icosahedral Al-Pd-Mn and decagonal Al-Cu-Co quasicrystals

Thermal expansion measurements have been performed by X-ray diffractometry on icosahedral Al-Pd-Mn (i-(Al-Pd-Mn)) in the temperature range between 10 and 700K and on decagonal Al-Cu-Co (d-(Al-Cu-Co)) in the range between 100 and 750K. The linear thermal expansion coefficient alpha(T) of i-(Al-Pd-Mn) is about half of that for the pure aluminium phase at room temperature and does not show a negative thermal expansion at low temperatures. The degree of anisotropy in the thermal expansion of d-(Al-Cu-Co) is small; the ratio of the average alpha between the tenfold periodic direction and a quasiperiodic direction perpendicular to it is 0:93 +/- 0:05. The Gruneisen parameter gamma(T) has been evaluated from the measured alpha(T). gamma for d-(Al-CuCo) is almost isotropic, similar to alpha. The gamma values for the two phases lie in the range between 1.5 and 1.8, which are comparable with those for conventional metallic crystals. They are almost constant over the temperature range studied.     

Introductory commentary

Abstract

Harriet Pappenheim. Einführender Kommentar.

Seit zwanzig Jahren führte die Postgraduate Psychoanalytic Society jedes Jahr eine Konferenz durch, zu der die Psychoanalytiker der Umgebung eingeladen wurden. Gut bekannte Gelehrte und Kliniker aus den Vereinigten Staaten wurden zum Vortragen ihrer Arbeiten eingeladen. Die Schwerpunkte waren auf neue, im Entstehen begriffene Aspekte der Psychoanalyse gerichtet, die nach unserer Meinung weiter studiert und beleuchtet werden sollten.

Harriet Pappenheim. Un Comentario introductorio

En cada uno de los últimos veinte años, la Postgraduate Psychoanalytic Society ha organizado la Conferencia Científica Anual en la ciudad de Nueva York a la cual la comunidad psicoanalítica del área ha sido invitada. Clínicos y colegiales prestigiados de todas partes de Estados Unidos han sido invitados a presentar trabajos y en todas las conferencias el foco ha sido las nuevas áreas que van emergiendo en psicoanalisis, que desde nuestro punto de vista, requirieron mayor elucidación y estudio.     

A thermodynamic model for nanocrystallization in bulk metallic glasses

The structural complexity of glass-forming alloys, which generally contain more than three components, can lead by partial crystallization during annealing to a dispersion of nanocrystals in an amorphous matrix, giving the material a very high mechanical strength. In the present study, the evolution of the driving force for crystallization is expressed as a function of the composition and the chemical potentials of the components. Application to Zr₆₀Al₁₀Cu₃₀ and Zr₆₀Al₁₀Cu₂₀Pd₁₀ bulk metallic glasses shows that the first crystallization step leads to a metastable equilibrium between nanocrystals of an intermetallic and a percolating amorphous phase. The effects of the number of components and of chemical bonding on the fraction crystallized is analysed and discussed.     

Estructura narrativa en niños y adolescentes: relatos del maltrato entre iguales en la escuela

Resumen

A partir de las posibilidades que permiten un enfoque narrativo para indagar el sentido que tienen las experiencias cotidianas, se utilizó una historia en viñetas (Almeida, del Barrio, Marques, Gutiérrez y van der Meulen, 2001; del Barrio, Almeida, van der Meulen, Barrios y Gutiérrez, 2003) para obtener las diferentes versiones o textos narrativos de niños y adolescentes (de 9 a 15 años) de la historia acerca de la victimización entre escolares. Por un lado, se estudiaron varios aspectos formales de los relatos: tipo de narración (tema o argumento), el uso de verbos mentales y físicos y el nivel de probabilidad en el progreso del relato. Por otro lado, se analizó el contenido de los textos en cuanto al final de la historia y la naturaleza de la relación entre los protagonistas. Los distintos relatos obtenidos muestran el paso desde unas representaciones infantiles más centradas en las conductas y la resolución ingenua de los conflictos, a un pensamiento más abstracto y probabilista en los participantes adolescentes, especialmente a los 15 años.