Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2005"
TL;DR: In this article, an alternative mode of deformation involving "rotating rigid units" is discussed, which also results in negative Poisson's ratios, and this mechanism may be construc- ted in two dimensions using "rigid polygons" connected together through hinges at their vertices.
Abstract: Auxetic materials exhibit the unexpected feature of becoming fatter when stretched and narrower when compressed, in other words, they exhibit a negative Poisson's ratio. This counter-intuitive behaviour imparts many beneficial effects on the material's macroscopic properties that make auxetics superior to conventional materials in many commercial applications. Recent research suggests that auxetic be-haviour generally results from a cooperative effect between the material's internal structure (geometry setup) and the deformation mechanism it undergoes when submitted to a stress. Auxetic behaviour is also known to be scale-independent, and thus, the same geometry/deformation mechanism may operate at the macro-, micro- and nano- (molecular) level. A considerable amount of research has been focused on the ‘re-entrant honeycomb structure’ which exhibits auxetic behaviour if deformed through hinging at the joints or flexure of the ribs, and it was proposed that this ‘re-entrant’ geometry plays an impor- tant role in generating auxetic behaviour in various forms of materials ranging from nanostructured polymers to foams. This paper discusses an alternative mode of deformation involving ‘rotating rigid units’ which also results in negative Poisson's ratios. In its most ideal form, this mechanism may be construc- ted in two dimensions using ‘rigid polygons’ connected together through hinges at their vertices. On application of uniaxial loads, these ‘rigid polygons’ rotate with respect to each other to form a more open structure hence giving rise to a negative Poisson's ratio. This paper also discusses the role that ‘rotating rigid units’ are thought to have in various classes of materials to give rise to negative Poisson's ratios. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
317 citations
TL;DR: Nikl et al. as mentioned in this paper showed the structure of a Lu3Al5O12:Ce single crystal, a possible high performance scintillator material, whose scintillation performance is degraded by the occurrence of antisite defects.
Abstract: The cover picture of the current issue refers to the Rapid Research Letter by Martin Nikl et al. [1]. It depicts the structure of a Lu3Al5O12:Ce single crystal, a possible high performance scintillator material. Its scintillation performance is, however, degraded by the occurrence of antisite defects. Such defects arise in the material structure sketched in the figure due to the occurrence of Lu3+ ions at the Al3+ octahedral sites as seen in the middle left octahedron. This defect constitutes a shallow electron trap, which delays energy delivery to the Ce3+ emission centers, and thus slows down the scintillation response of the material. Thermoluminescence measurements appear as sensitive diagnostic tool to evidence these defects in the single crystals grown.
Martin Nikl is senior scientist and head of the Laboratory of Luminescence and Scintillation Materials at the Institute of Physics of the Czech Academy of Sciences. His main research activities include the characterization of the luminescence and scintillation properties of wide band-gap materials and the influence of material defects on them.
197 citations
TL;DR: Auxetic composite materials can be produced either from conventional components via specially designed configurations or from auxetic components as mentioned in this paper, and the possibility of property enhancements in both low velocity impact and fibre pull out due to the negative Poisson's ratio.
Abstract: Auxetic composite materials can be produced either from conventional components via specially designed configurations or from auxetic components. This paper reviews manufacturing methods for both these scenarios. It then looks at the possibility of property enhancements in both low velocity impact and fibre pull out due to the negative Poisson's ratio. Tests revealed that auxetic carbon fibre composites made from commercially available prepreg show evidence of increased resistance to low velocity impact and static indentation with a smaller area of damage. Also, using auxetic fibres in composite materials is shown to produce a higher resistance to fibre pullout.
172 citations
TL;DR: In this paper, the lattice-matched AlInN/GaN was used to replace the Al(Ga)N/GAN material system for III-nitride Bragg reflectors, despite the poor material quality reported until very recently.
Abstract: We propose to use lattice-matched AlInN/GaN to replace the Al(Ga)N/GaN material system for III-nitride Bragg reflectors, despite the poor material quality of AlInN reported until very recently. We report an improvement of AlInN material that allowed for successful fabrication of a microcavity light emitting diode, a distributed Bragg reflector with 99.4% reflectivity and microcavities with a quality factor over 800. These results establish state-of-the-art values for III-nitrides, and announce the future importance of AlInN in GaN-based optoelectronics. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
155 citations
TL;DR: In this article, the authors describe the manufacturing and tensile testing of auxetic (negative Poisson's ratio) thermoplastic polyurethane foams, both under constant strain rate and sinusoidal excitation.
Abstract: The paper describes the manufacturing and tensile testing of auxetic (negative Poisson's ratio) thermoplastic polyurethane foams, both under constant strain rate and sinusoidal excitation. The foams are produced from conventional flexible polyurethane basis following a manufacturing route developed in previous works. The Poisson's ratio behaviour over tensile strain has been analyzed using an Image Data processing technique based on Edge Detection from digital images recorded during quasi-static tensile test. The samples have been subjected to tensile and compressive tests at quasi-static and constant strain-rate values (up to 12 s -1 ). Analogous tests have been performed over iso-volumetric foams samples, i.e., foams subjected to the same volumetric compression of the auxetic ones, exhibiting a near zero Poisson's ratio behaviour. The auxetic and non-auxetic foams have been also tested under sinusoidal cycling load up to 10 Hz, with maximum pre-strain applied of 12%. The hysteresis of the cycling loading curve has been measured to determine the damping hysteretic loss factor for the various foams. The measurements indicate that auxetic foams have increased damping loss factor of 20% compared to the conventional foams. The energy dissipation is particularly relevant in the tensile segment of the curve, with effects given by the pre-strain level imposed on the samples.
151 citations
TL;DR: This issue of pss as mentioned in this paper contains a collection of review articles on the rather controversially discussed topic of Electron-Phonon Interaction in High-Temperature Superconductors, guest-edited by Miodrag Kulic, Johann Wolfgang Goethe-Universitat Frankfurt/Main, Germany, with a Preface written by V. L. Ginzburg and E. G. Maksimov.
Abstract: This issue of pss (b) – basic solid state physics contains a collection of Review Articles on the rather controversially discussed topic of Electron–Phonon Interaction in High-Temperature Superconductors, guest-edited by Miodrag Kulic, Johann Wolfgang Goethe-Universitat Frankfurt/Main, Germany, with a Preface written by V. L. Ginzburg and E. G. Maksimov [1].
The cover picture, taken from the review [2] by T. Cuk et al., shows plots of the electron–phonon coupling vertex, g2(k, k′), where k, k′ are the initial and final electron momentum for electrons scattered by the bond-buckling phonon B1g (the out-of-phase vibration of the in-plane oxygen) in a tight-binding model of the copper–oxygen plane. The momentum dependence of this vertex, along with the d-wave superconducting gap and the van Hove singularity at the anti-node, accounts for the momentum dependence of the collective mode coupling seen in angle-resolved photoemission data on Bi2212.
The present issue also sees the start of our rapid research letters, the fastest peer-reviewed publication medium in solid state physics. For more information see www.pss-rapid.com and the Editorial by the Editor-in-Chief Martin Stutzmann on page 7 [3].
142 citations
TL;DR: Enyashin et al. as mentioned in this paper presented a view of two types of lepidocrocite-based cylindrical nanotubes (TiO2, light grey - titanium, red -oxygen).
Abstract: The front cover of this issue depicts a view of two types of lepidocrocite-based cylindrical nanotubes (TiO2, light grey – titanium, red – oxygen). These structures represent examples of a manifold of stable tubular forms of the titania layer modifications anatase and lepidocrocite. These structures were obtained using density-functional based calculations and characterize the essen-tial structural features of synthesized titania nanotubes.
Andrey Enyashin is a PhD student at the Institute of Solid State Chemistry at the Ural Branch of the Russian Academy of Sciences in Ekaterinburg. He performed this study during a visit at the Institute of Physical Chemistry and Electrochemistry (IPEC) of the TU Dresden together with Gotthard Seifert. Professor Seifert is head of the theory group at the IPEC. His research interests are in the areas of quantum chemistry, cluster physics and computational materials research.
122 citations
TL;DR: In this article, the flat-wise compression behavior of an innovative cellular structure configuration was investigated through classical analytical formulas for the linear buckling of thin plates and shells and a bifurcation analysis performed on a Finite Element model.
Abstract: This paper investigates the flat-wise compression behavior of an innovative cellular structure configuration. The considered layout has a hexagonal chiral geometry featuring cylinders, or nodes, joined by ligaments, or ribs. The resulting assembly is characterized by a number of interesting properties that can be exploited for the design of alternative honeycombs or cellular topologies to be used in sandwich construction. The flat-wise strength of the chiral geometry is investigated through classical analytical formulas for the linear buckling of thin plates and shells and a bifurcation analysis performed on a Finite Element model. The analytical expressions predict the global buckling behavior and the resulting critical loads, and can be directly compared with the results obtained from the Finite Element analysis. In addition, the Finite Element model predicts local buckling modes, which should be considered to evaluate the possible development of localized plasticity. A sensitivity study is performed to evaluate the influence of the geometry of the chiral structure on its buckling strength. The study shows that the considered topology can offer great design flexibility, whereby several parameters can be selected and modified to improve the flat-wise performance. The comparison with traditional, hexagonal centro-symmetric structural configurations concludes the paper and demonstrates the enhanced performance and the potentials of chiral noncentrosymmetric designs.
119 citations
TL;DR: In this article, the structural and electronic properties of perovskite-type compounds BaZrO3 and SRSO3 ceramics were analyzed using the full-potential linearized augmented plane wave (FP-LAPW) method.
Abstract: Ab initio calculations have been performed on the structural and electronic properties of perovskite-type compounds BaZrO3 and SrZrO3 ceramics. The Kohn–Sham equations were solved by applying the full-potential linearized augmented-plane-wave (FP-LAPW) method. In this approach, the generalized gradient approximation was used for the exchange-correlation potential. The ground state properties such as lattice parameter, elastic constants, bulk modulus and its pressure derivative were calculated and the results are compared with previous calculations and experimental data when available. The SrZrO3 perovskite should exhibit higher hardness and stiffness than BaZrO3. Furthermore, the electronic structure calculations show that these materials are weakly ionic and exhibit indirect and wide band gaps, which are typical of insulators. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
116 citations
TL;DR: In this article, a new mechanism of persistent luminescence was proposed to determine the energy levels of divalent lanthanide ions relative to the conduction band and valence band of host compounds.
Abstract: A recently developed method to determine the energy levels of divalent lanthanide ions relative to the conduction band and valence band of host compounds is applied to Sr2MgSi2O7 doped with Eu2+ and Dy3+. The level locations are not consistent with the widely adopted and generally believed mechanism of persistent luminescence in Eu2+ and Dy3+ doped aluminates and silicates. A new mechanism of persistent luminescence is proposed. In this mechanism Eu3+ and Dy2+ are the stable trapping centers and not Eu+ and Dy4+ as in the existing mechanism. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
110 citations
TL;DR: In this article, a summary of inelastic neutron scattering studies searching for signatures of a strong electron-phonon coupling in high temperature superconductors is given, focusing on anomalous dispersion, the unusually large linewidths and the anomalous temperature dependence observed for plane polarized copper-oxygen bond-stretching vibrations.
Abstract: This article gives a summary of inelastic neutron scattering studies searching for signatures of a strong electron–phonon coupling in high temperature superconductors. Special emphasis is laid on the anomalous dispersion, the unusually large linewidths and the anomalous temperature dependence observed for plane polarized copper-oxygen bond-stretching vibrations. It will be discussed in how far these results can be understood within conventional density functional theory or require recourse to theories for strongly correlated electrons, in particular the concept of charge stripe order. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, the connection between gradient theory and nonlocal theory is discussed for elasticity as well as for micropolar elasticity, and Nonsingular solutions for the elastic fields of screw and edge dislocations are given.
Abstract: In this paper we consider and compare special classes of static theories of gradient elasticity, nonlocal elasticity, gradient micropolar elasticity and nonlocal micropolar elasticity with only one gradient coefficient. Equilibrium equations are presented but higher-order boundary conditions are not of concern here, since they are not required for the problems considered. The connection between gradient theory and nonlocal theory is discussed for elasticity as well as for micropolar elasticity. Nonsingular solutions for the elastic fields of screw and edge dislocations are given. Both the elastic deformation (distortion, strain, bend-twist) and the force and couple stress tensors do not possess any singularity unlike ‘classical’ theories. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, the wave propagation characteristics of periodic two-dimensional auxetic lattice structures were investigated and a sensitivity analysis was performed to investigate the influence of band-gap and directional behaviors with respect to changes in the internal angle.
Abstract: The paper investigates the wave propagation characteristics of periodic two dimensional, auxetic lattice structures. Periodic structures in general feature unique wave propagation characteristics, whereby waves are allowed to propagate only within specific frequency bands. Two dimensional periodic structures complement this feature with a low frequency directional behavior. The combination of these unique characteristics makes two dimensional periodic structures ideal candidates for the design of pass-band directional mechanical filters. Focus is here placed on honeycomb lattice configurations. A sensitivity analysis is first presented to investigate the influence of band-gap and directional behaviors with respect to changes in the internal angle. The presented results demonstrate how re-entrant topologies feature enhanced wave attenuation capabilities with respect to hexagonal lay-outs. An optimization problem is then formulated by considering the internal angle as a design variable, and the width of the attenuation frequency ranges and angular range of propagation at low frequencies as objective functions. The identified optimal configurations feature combined properties which demonstrate the effectiveness of the analysis procedure.
TL;DR: In this article, a description of the non-equilibrium properties of a microcavity polariton fluid, injected by a nearly-resonant continuous wave pump laser, is presented.
Abstract: We present a description of the non-equilibrium properties of a microcavity polariton fluid, injected by a nearly-resonant continuous wave pump laser. In the first part, we point out the interplay between the peculiar dispersion of the Bogoliubov-like polariton excitations and the onset of polariton parametric instabilities. We show how collective excitation spectra having no counterpart in equilibrium systems can be observed by tuning the excitation angle and frequency. In the second part, we explain the impact of these collective excitations on the in-plane propagation of the polariton fluid. We show that the resonant Rayleigh scattering induced by artificial or natural defects is a very sensitive tool to reveal novel fascinating effects such as polariton superfluidity or to show new aspects of the Cherenkov effect. We present a comprehensive set of predicted far-field and near-field images for the resonant Rayleigh scattering emission. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: RefReflection, emission and luminescence spectra of Czochralski grown molybdate crystals have been investigated over a 8-295 K temperature range using VUV synchrotron radiation as mentioned in this paper.
Abstract: Reflection, emission and luminescence spectra of Czochralski grown molybdate crystals, i.e. MgMoO4, CaMoO4 and CdMoO4 have been investigated over a 8–295 K temperature range using VUV synchrotron radiation. Preliminary interpretation of the spectroscopic properties has been carried out on the basis of present knowledge of the electronic structure and emission properties of these materials. The results of this study support the conclusion that molybdate crystals have good prospect for application in the search for rare events as cryogenic phonon-scintillation detectors. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this paper, the phase diagrams of a ferroelectric small particle described by the transverse Ising model are investigated by the use of two theoretical frameworks, namely the standard mean field theory and the effective field theory corresponding to the Zernike approximation.
Abstract: The phase diagrams of a ferroelectric small particle described by the transverse Ising model (TIM) are investigated by the use of two theoretical frameworks, namely the standard mean-field theory and the effective-field theory corresponding to the Zernike approximation. The particle is represented by a two-dimensional array of pseudo-spins with two types of exchange interactions (J in bulk and JS on the surface) and two types of transverse fields (Ω in the bulk and ΩS on the surface). We study the phase diagrams of this model, changing the size S of the particle and the ratios of JS/J and ΩS/Ω. We find that the critical behaviors are a little different from those found for the phase diagrams of ferroelectric thin films described by the TIM. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this paper, a polymer with para-quaterphenyl rods laterally attached to the polymer main chain has been synthesized and X-ray scattering experiments were performed on the polymers both in the quiescent (unstretched) and the stretched states.
Abstract: Polymers containing para-quaterphenyl rods laterally attached to the polymer main chain have been synthesized. The molecular design choices which led to use of the quaterphenyl rod to achieve auxetic response will be discussed. Preparative chemistry of the monomers will be described along with the details of the polymerization reaction. Both polyethers and polyesters were prepared. The length of the flexible alkyl main chains which link the rods plays an important role in determining whether the resulting polymers will exhibit liquid crystallinity. Polymers with longer alkyl links exhibit nematic liquid crystallinity and the rods appear to have their long axes oriented roughly parallel and along the main chain direction. X-ray scattering experiments were performed on the polymers both in the quiescent (unstretched) and the stretched states. Results from these experiments suggest that, under tensile strain, site connectivity driven rod-reorientation occurs in these materials giving rise to an increase in the interchain distance for these polymers. This increase in the interchain separation is consistent with our concept of a molecular level auxetic mechanism for this type of designed polymer.
TL;DR: The purpose of this article is to summarize the available knowledge in the area of auxetics and to point out interesting directions for further research and development work.
Abstract: Materials with negative Poisson's ratio are characterized as auxetic materials. They arise quite rarely in nature, so that our engineering intuition can not help us understand their mechanical behaviour or, further, use them effectively for innovative products and processes. From a literature review and from analytical calculations confirmed with the use of numerical homogenization, it seems that some nonconvex-shaped (re-entrant) microstructure is the most understandable origin of the auxetic behaviour. The picture is quite clear in elastostatics, for which a quite large number of potential applications have been discussed in the engineering literature. Analogous problems in elastodynamics and in the area of viscoelasticity have not been discussed thoroughly in the open literature. The purpose of this article is to summarize the available knowledge in the area of auxetics and to point out interesting directions for further research and development work.
TL;DR: In this article, the structural properties of ternary chalcopyrite absorbers for solar-cell device applications studied by Raman Spectroscopy are reviewed according to the results of previous and present Raman and IR spectroscopic studies.
Abstract: This paper reviews the structural properties of ternary chalcopyrite absorbers for solar-cell device applications studied by Raman Spectroscopy. Raman mode assignment in CuGaSe 2 is reviewed according to the results of previous and present Raman and IR spectroscopic studies. The compositional dependence of Raman modes, in particular, the composition dependent frequency shift of the A 1 -mode, is suggested for the analysis of the chalcopyrite film structure. Order and disorder effects are discussed with reference to the excitation of phonon modes assigned to Cu-Au ordered phases and to defect related phases, respectively. Phase separation effects associated with the formation of secondary microcrystalline Cu x Se- or Cu x S-phases and the observation of Raman scattering from these phases are also discussed.
TL;DR: In this paper, the semiconducting properties of single-crystal TiO2 and their changes during prolonged oxidation at elevated temperatures and under controlled oxygen activity were monitored using measurements of electrical conductivity and thermo-electric power.
Abstract: The semiconducting properties of single-crystal TiO2 and their changes during prolonged oxidation at elevated temperatures and under controlled oxygen activity were monitored using measurements of electrical conductivity and thermo-electric power. Two kinetic regimes were revealed: Regime I – rapid oxidation, associated with the transport of oxygen vacancies, and Regime II – prolonged oxidation, which corresponds to the transport of titanium vacancies. The present data represent the first documented evidence for the formation and transport of titanium vacancies in TiO2. This finding allows the processing of p-type TiO2 without the incorporation of aliovalent foreign ions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this paper, the electronic properties of high-index free-standing silicon nanowires were calculated and it was shown that the magnitude of the gap depends strongly on the wire growth direction, which is due to the different effective confinement length and effective masses for the ΓX-derived silicon states.
Abstract: We calculated the electronic properties of high-index free-standing silicon nanowires. [11] nanowires are indirect semiconductors for diameters down to 0.8 nm; [110] wires have a direct band gap at the Γ-point, but the density of states is very small at the conduction band edge. Confinement arguments show that only [001] nanowires are expected to develop a direct gap with a large density of electronic states at the band edges for diameters in the nm range. The magnitude of the gap depends strongly on the wire growth direction, which is due to the different effective confinement length and effective masses for the ΓX-derived silicon states. Correcting for the extension of the wave functions we find our calculated energies to agree with recent scanning tunneling experiments. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, the authors measured the spin polarization of bulk Mn5Ge3 epilayers in the diffusive and ballistic regimes using density functional theory (DFT), which exceeds theoretical estimates of PDFT = 35 ± 5% and 10 ± 5%, respectively.
Abstract: Germanium-based alloys hold great promise for future spintronics applications, due to their potential for integration with conventional Si-based electronics. High-quality single phase Mn5Ge3(0001) films, grown by solid-phase epitaxy on Ge(111) and GaAs(111), exhibit strong ferromagnetism up to the Curie temperature TC ∼ 296 K. Point Contact Andreev Reflection (PCAR) measurements on Mn5Ge3 epilayers reveal a spin-polarization P = 42 ± 5% for both substrates. We also calculate the spin polarization of bulk Mn5Ge3 in the diffusive and ballistic regimes using density-functional theory (DFT). The measured spin polarization exceeds the theoretical estimates of PDFT = 35 ± 5% and 10 ± 5% for the diffusive and ballistic limits, respectively. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, an extension of the "on-site-on-time" approach to atoms, dimers, etc. and to dislocations is outlined, where well-established theories are adapted to identify atomic processes and extract quantitative information on diffusion parameters important for growth and stability of nanostructures, such as energies and prefactors.
Abstract: Morphological changes on surfaces have been followed in real time by scanning tunnelling microscopy on the time scale of seconds to days. This “on-site on-time” mode has been employed for the observation of decay, motion, coalescence, and shape fluctuations of islands on the low-indexed faces of Ag, Cu, Pt, and Au. For island changes under well-defined conditions, well-established theories are adapted to identify atomic processes and extract quantitative information on diffusion parameters important for growth and stability of nanostructures, such as energies and prefactors. Extension of the “on-site on-time” approach to atoms, dimers, etc. and to dislocations is outlined. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, a complex set of optical phenomena linked to the spin dynamics of exciton polaritons in semiconductor microcavities are addressed, including pump energy, pumping intensity and polarization.
Abstract: In this chapter we address a complex set of optical phenomena linked to the spin dynamics of exciton polaritons in semiconductor microcavities. When optically created, polaritons inherit the spin and dipole moment from the exciting light. Their state can be fully characterized by a so-called pseudospin accounting for both spin and dipole moment orientation. However, from the very beginning of their life in a microcavity, polaritons start changing their pseudospin state under effect of effective magnetic fields of different nature and due to scattering with acoustic phonons, defects, and other polaritons. This makes pseudospin dynamics of exciton polaritons rich and complex. It manifests itself in non-trivial changes in polarization of light emitted by the cavity versus time, pumping energy, pumping intensity and polarization. During the first years of theoretical research on exciton-polariton relaxation the polarization has been simply neglected. Later it has been understood that the energy and momentum-relaxation of exciton polaritons are spin-dependent. It is typically the case in the regime of stimulated scattering when the spin polarizations of initial and final polariton state have a huge effect on the scattering rate between these states. It appeared that critical conditions for polariton Bose-condensation are also polarization-dependent. In particular, the stimulation threshold (i.e. the pumping power needed to have a population exceeding 1 at the ground state of the lower polariton branch) has been experimentally shown to be lower under linear than under circular pumping. These experimental observations have stimulated the theoretical research toward understanding of mutually dependent polarization- and energy-relaxation mechanisms in microcavities. The authors of this chapter have been working on theoretical description of different specific effects of polariton spin-dynamics in microcavities for years. Here we attempted to put together all fragments and to formulate a general approach to the problem that would allow then to consider a variety of particular cases. We start from reminding the main spin-relaxation mechanisms known for free carriers and excitons. We then overview the most essential experimental results in this field before to present our original formalism which allowed us to interpret the key experimental findings. We are going to discuss only the strong coupling regime leaving aside all polarization effects in VCSELs.
TL;DR: In this paper, the authors simulate auxetic behavior by carrying out dynamic analyses of mesoscopic model structures, including four-node "shell" elements and eight-node 'brick' elements.
Abstract: We sought to simulate auxetic behavior by carrying out dynamic analyses of mesoscopic model structures. We began by generating nearly periodic cellular structures. Four-node 'Shell' elements and eight-node 'Brick' elements are the basic building blocks for each cell. The shells and bricks obey standard elastic-plastic continuum mechanics. The dynamical response of the structures was next determined for a three-stage loading process: (1) homogeneous compression; (2) viscous relaxation; (3) uniaxial compression. The simulations were carried out with both serial and parallel computer codes--DYNA3D and ParaDyn--which describe the deformation of the shells and bricks with a robust contact algorithm. We summarize the results found here.
TL;DR: In this paper, the electronic structure and disorder effects in beryllium chalcogenides alloys were investigated using the full potential-linearized augmented plane wave method (FP-LAPW) within density-functional theory.
Abstract: First-principles calculations have been used to investigate the electronic structure and disorder effects in beryllium chalcogenides alloys (BeSxSe1–x, BeSxTe1–x and BeSexTe1–x) using the full potential-linearized augmented-plane wave method (FP-LAPW) within density-functional theory. We used the local-density approximation within the generalized gradient correction as well as the Engel–Vosko GGA formalism to calculate the electronic structure at equilibrium volume. The ground-state properties are determined for the bulk materials (BeS, BeSe, and BeTe) as well as for the average concentration (x = 0.5) of the alloys. Using the approach of Zunger and coworkers, the microscopic origins of compositional disorder have been detailed and explained. The disorder parameter (gap bowing) is found to be mainly caused by the chemical charge-transfer effect, while the volume deformation and the structural relaxation contribute to the gap bowing parameter at smaller magnitude. This should be expected since there is a weak lattice mismatch between the binary compounds and a considerable electronegativity difference between Be and X (X = S, Se, Te) atoms. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this paper, Baughman et al. presented the stereographic projections of Poisson's ratio for a set of natural crystals with cubic, hexagonal and monoclinic symmetry.
Abstract: Auxetic behavior (negative values of Poisson's ratio) in some cubic metals for a stretch along a cube-face diagonal was investigated in detail by R. H. Baughman et al., F. Milstein, K. Huang, M. Jain, M. P. Verma. Directions with extreme values of Poisson's ratio in crystals for a stretch along a cube-face diagonal and corresponding lateral strain along the orthogonal cube-face diagonal and cube-axis were considered by the authors. In this paper the stereographic projections of Poisson's ratio for a set of natural crystals with cubic, hexagonal and monoclinic symmetry were computed. From these stereographic projections the Poisson's ratio for any possible directions of stretch and lateral strain in the crystal were calculated and orientations of stretch and lateral strain with extreme values of Poisson's ratio were obtained. Crystals with auxetic behavior were revealed. Orientations of stretch directions and lateral strain directions with negative values of Poisson's ratio were determined. By analysis of the above results cuts with optimal negative values of Poisson's ratio were revealed in zinc, molybdenum sulfide, carbon, graphite, polypropylene, monoclinic natural minerals labradorite and augite, a complex silicate and in a set of cubic alloys. The exotic behavior of change of cross-section of a cylindrical rod during stretching along the rod axis was revealed in the naturally-occurring mineral SiO 2 cristobalite - an example of a molecular auxetic. The elastic constants of crystals from the manual of Landolt-Bornstein were used in the calculations.
TL;DR: The Web of Science Record was created on 2007-08-31, modified on 2017-05-12 by as discussed by the authors, with a record set on 2007/08/31 and updated on 2017/05/12.
Abstract: Reference LOEQ-ARTICLE-2005-006View record in Web of Science Record created on 2007-08-31, modified on 2017-05-12
TL;DR: In this article, Monte Carlo simulations with variable shape of the periodic box were performed for a few two-dimensional, hard-body models showing elastically isotropic solid phases.
Abstract: Monte Carlo computer simulations with variable shape of the periodic box were performed for a few two-dimensional, hard-body models showing elastically isotropic solid phases. Two examples of homomolecular (i.e. consisting of identical molecules) systems of anisotropic molecules are discussed which form unusual solid phases, showing negative Poisson's ratio and know as auxetic ones, at densities and pressures higher than the elastically isotropic solid phases. Sharp extremes of the Poisson's ratio were observed for both systems at freezing of the orientational degrees of freedom of the molecules: a minimum was observed for the system for which the auxetic phase with frozen rotation is isotropic and a maximum was found for the other system. This indicates that isotropic auxetic phases can have auxetic precursors with much more negative Poisson's ratios than those characterizing the auxetic phases themselves. An example of a hetero-molecular (i.e. mixture consisting of different molecules), elastically isotropic system is also discussed - the polydisperse hard disc system. The simulations revealed a maximum of the Poisson's ratio in the close packing limit at any nonzero polydispersity. Although no manifestation of any auxetic behaviour has been found in this system, the obtained results indicate other unusual effect - a jump (discontinuity) of the Poisson's ratio at close packing when the polydispersity tends to zero. The results obtained in this work demonstrate that the Poisson's ratio can play the role of a sensitive indicator of (at least) some structural changes in solids.
TL;DR: In this article, the authors used analytical and molecular mechanics to study the structure and deformation mechanisms acting at the molecular level in the auxetic polymorph of crystalline silica (a-cristobalite) and showed that the deformation mechanism for transverse uniaxial loading of α-crisobalite is by concurrent dilation and cooperative rotation about the local a and c-axes of the SiO 4 tetrahedra.
Abstract: Analytical and Molecular Mechanics methods have been used to study the structure and deformation mechanisms acting at the molecular level in the auxetic polymorph of crystalline silica (a-cristobalite). The Molecular Mechanics simulations indicate a stress-induced phase transition from α-cristobalite to 'ordered' β-cristobalite occurs for uniaxial loading along the x 3 direction. This is in reasonable agreement with the previous prediction from an analytical model assuming deformation is by concurrent dilation and cooperative rotation (about axes in the x 1 -x 2 plane, passing through the midpoints of opposing edges - the a-axes) of the SiO 4 tetrahedral molecular sub-units, previously shown to predict the Poisson's ratio for loading in the x 3 direction. The analytical models have been extended to include cooperative rotation of each tetrahedron about its axis (the c-axis) mostly closely aligned with the principal unit-cell x 3 -axis. The new models enable significantly improved prediction of the Poisson's ratios of α-cristobalite when loaded in one of the transverse (x 1 or x 2 ) directions. Parametric fitting of the analytical models indicate that the deformation mechanism for transverse uniaxial loading of α-cristobalite is by concurrent dilation and cooperative rotation about the local a and c-axes of the SiO 4 tetrahedra.