Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2011"
TL;DR: In this article, the authors summarize the underlying electrostatics in condensed matter and derive a novel defect scheme for point defects, which overcomes limitations of previous schemes with respect to applicability, systematic improvement, and formal justification.
Abstract: Most theoretical calculations for point defects employ the supercell approach. The supercell consists of a few dozen or 100 atoms of the bulk material with a single defect, and is subject to periodic boundary conditions. However, the large density and periodic arrangement of the defects introduce artifacts. They need to be corrected for to extrapolate to the isolated-defect limit. This is particularly important for electrostatic interactions between charged defects, which decay only very slowly (asymptotically like L ―1 ) with increasing supercell lattice constant L. In this paper, we summarize the underlying electrostatics in condensed matter. A novel defect scheme is derived from this analysis. It overcomes limitations of previous schemes with respect to applicability, systematic improvement, and formal justification. Good performance is demonstrated for vacancies in diamond and GaAs.
363 citations
363 citations
TL;DR: The Heyd-Scuseria-Ernzerhof (HSE) screened hybrid density functional as mentioned in this paper was proposed to solve the band gap problem in solid state physics and materials science.
Abstract: Density functional theory (DFT) is the most widely used technique in the realm of first-principles electronic structure methods. Principally, this is because DFT in the Kohn–Sham (KS) formalism offers the appealing combination of relatively high accuracy and relatively low computational cost. Despite their great successes, traditional semilocal functionals fail to describe some important problems in solid state physics and materials science, the most conspicuous example being the notorious band gap problem. More sophisticated functionals providing greater accuracy without sacrificing computational efficiency are therefore needed. The Heyd–Scuseria–Ernzerhof (HSE) screened hybrid density functional [J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003); J. Heyd and G. E. Scuseria, J. Chem. Phys. 121, 1187 (2004)] successfully addresses some of the chief problems which plague semilocal functionals by including only the important parts of exact nonlocal Hartree–Fock-type exchange. This work discusses some of the concepts underlying HSE and provides illustrative examples highlighting the successes of HSE in numerous solid state applications.
264 citations
TL;DR: In this article, hybrid density functional calculations applied to defect charge transition levels are explored in the attempt to overcome the band-gap problem of semilocal density functionals, and the validity of the empirical procedure which consists in tuning the fraction of nonlocal exchange to a value which gives a theoretical band gap reproducing the experimental one is explored.
Abstract: Hybrid density functional calculations applied to defect charge transition levels are explored in the attempt to overcome the band-gap problem of semilocal density functionals. Charge transition levels of a large set of point defects calculated with semilocal and hybrid density functionals are found to correspond closely when aligned with respect to the average electrostatic potential. This strongly suggests that the defect levels defined in this way are already accurately described at these levels of theory. In particular, this then also applies to the energy separation between different defect levels, which is directly accessible experimentally. At variance, within the same alignment scheme, the band edges obtained with hybrid functionals are found to undergo significant shifts with respect to those obtained with semilocal functionals. While these shifts systematically give larger band gaps, the agreement with experiment is not always satisfactory when a fixed fraction of exact exchange is admixed. This describes a current limitation of hybrid functional schemes. In the attempt of identifying a viable theoretical description within the class of one-parameter hybrid functionals based on bare exchange, we explore the validity of the empirical procedure which consists in tuning the fraction of nonlocal exchange to a value which gives a theoretical band gap reproducing the experimental one. Comparisons with experiment for band offsets and specific defect levels record very encouraging results. Despite its inherent limitations, such an empirical scheme based on hybrid functionals represents a definite improvement with respect to semilocal functionals.
236 citations
TL;DR: The FIREBALL method as discussed by the authors uses separable pseudopotentials and goes beyond the minimal sp3 basis set of the SN method, allowing for double numerical (DN) basis sets with the addition of polarization orbitals and d-orbitals to the basis set.
Abstract: One of the outstanding advancements in electronic-structure density-functional methods is the Sankey–Niklewski (SN) approach [Sankey and Niklewski, Phys. Rev. B 40, 3979 (1989)]; a method for computing total energies and forces, within an ab initio tight-binding formalism. Over the past two decades, several improvements to the method have been proposed and utilized to calculate materials ranging from biomolecules to semiconductors. In particular, the improved method (called FIREBALL) uses separable pseudopotentials and goes beyond the minimal sp3 basis set of the SN method, allowing for double numerical (DN) basis sets with the addition of polarization orbitals and d-orbitals to the basis set. Herein, we report a review of the method, some improved theoretical developments, and some recent application to a variety of systems.
201 citations
TL;DR: In this paper, the effects of dilution at the surface, the surface exchange interaction, and the shell coupling on the magnetization profiles are investigated, and they find a number of characteristic phenomena for them.
Abstract: Magnetic properties (phase diagram and magnetization) of a cylindrical Ising nanowire or nanotube are investigated by the use of the effective-field theory with correlations. Particular emphasis is given to the effects of the surface and its dilution on them. Much attention is paid to the thermal variation of the magnetization when the spins at the surface are coupled antiferromagnetically to the ferromagnetic core spins by the negative shell coupling. The effects of dilution at the surface, the surface exchange interaction, and the shell coupling on the magnetization profiles are investigated. We find a number of characteristic phenomena for them.
139 citations
137 citations
TL;DR: In this paper, the authors systematically review several charge-transport approaches and illustrate their relationship in order to display their capabilities regarding the goal of efficient charge transport in organic semiconductors, which is the key for their application in organic Electronics.
Abstract: Efficient charge transport in organic semiconductors is the key for their application in Organic Electronics. Therefore the most important design ansatz is directed to improve carrier mobilities by means of available tools such as chemical and/or structural modifications of the organic materials. Thereby theoretical input can provide guidelines towards possible realizations of high-mobility or, more general, highly functional materials. In particular the structure–property relationship is at the heart of theoretical studies which are extremely helpful if they achieve predictive power. In this paper we systematically review several charge-transport approaches and illustrate their relationship in order to display their capabilities regarding this goal. Special focus is put on the transport mechanism, the mobility anisotropy and temperature dependence of charge-carrier transport. As the central concept the inclusion of the strong coupling of the carriers with the vibrating lattice and, hence, the dressing of carriers to polarons is described. The availability of material parameters is tremendously important for transport description. We highlight their computation by ab initio theory in order to arrive at a satisfactory level beyond phenomenological approaches.
135 citations
TL;DR: In this article, the authors compare defect calculations based on density functional theory within the local density approximation (LDA), the orbital-dependent LDA, and using hybrid functionals.
Abstract: We describe and compare defect calculations based on density functional theory within the local density approximation (LDA), the orbital-dependent LDA + U, and using hybrid functionals. Limitations of the LDA in describing defect formation energies and transition levels are discussed, followed by corrections based on the LDA + U, and the use of the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE). The band-gap error in LDA leads to large uncertainties not only in defect transition levels but also in formation energies.
LDA + U provides a partial correction to the band gap and, when combined with LDA, provides an accurate method for predicting transition levels. Formation energies obtained from the LDA + U/LDA approach depend on the ability of LDA + U to correctly describe the position of the band edges on an absolute energy scale. Although computationally demanding, HSE is demonstrated to be a reliable method for predicting structure and electronic properties of semiconductors, including transition levels and formation energies of defects.
107 citations
TL;DR: In this paper, the key aspects of Raman spectroscopy of graphite and graphene are reviewed, focusing on the double resonant Raman modes such as the D, D* (also known as G' or 2D), and D' bands.
Abstract: We review the key aspects of Raman spectroscopy of graphite and graphene, focusing on the double resonant Raman modes such as the D, D* (also known as G' or 2D), and D' bands. We discuss the practical significance of Raman spectroscopy for the study of single- and multi-layer graphene.
98 citations
TL;DR: In this paper, the electronic band structure of antimony selenide using density functional theory (DFT) within the generalized gradient approximation (GGA) with GW corrections was investigated. But the results were limited to the Sb 2 Se 3 lattice.
Abstract: Antimony selenide (Sb 2 Se 3 ) has been proposed as an alternative material for a wide range of applications; however, the electronic structure of the Sb2Se 3 lattice is not clearly known yet. As a consequence, there are abundant contradictory interpretations of experimental results leading to incoherent determinations of its energy band gap and the type of optical transitions. Moreover, Sb 2 Se 3 is recently being synthesized in different types of nanostructures; therefore, detailed knowledge of the bulk electronic structure is necessary to evaluate deviations due to confinement or surface effects. In this paper, we study the electronic band structure of antimony selenide using density functional theory (DFT) within the generalized gradient approximation (GGA) with GW corrections. Our calculations show that Sb 2 Se 3 has an indirect energy band gap of 1.21 eV; however, a direct transition only 0.01 eV higher than the band gap (1.22 eV) is also possible. The calculated density of states agrees well with the experiments reporting photoemission spectra.
TL;DR: In this article, the authors discuss theoretical approaches for describing the phonon thermal transport in graphene and identify open questions and problems, and show that due to a fundamental ambiguity in the definition of the intrinsic thermal conductivity of two-dimensional (2-D) systems the calculations that use an arbitrary low-bound cut-off for phonon frequency in the thermal conductivities integral lead to erroneous results.
Abstract: We discuss theoretical approaches for description of the phonon thermal transport in graphene and identify open questions and problems. Specifically, we show that due to a fundamental ambiguity in the definition of the intrinsic thermal conductivity of two-dimensional (2-D) systems the calculations that use an arbitrary low-bound cut-off for the phonon frequency in the thermal conductivity integral lead to erroneous results. The problem of the relative contributions of the longitudinal acoustic (LA), transverse acoustic (TA), and out-of-plane phonon polarization branches to thermal transport in graphene is also discussed. Theoretical thermal conductivity data for graphene, obtained by different approaches, are compared with those for carbon nanotubes.
TL;DR: In this paper, the second-order dipole-dipole rate constants governing the main nonlinear quenching channel in CsI, csI:Tl, NaI, and NaI-Tl were investigated.
Abstract: Stopping of an energetic electron produces a track of high excitation density, especially near its end, and consequent high radial concentration gradient. The effect of high excitation density in promoting nonlinear quenching is generally understood to be a root cause of nonproportionality in scintillators. However, quantitative data on the kinetic rates of nonlinear quenching processes in scintillators are scarce. We report experimental measurements of second-order dipole–dipole rate constants governing the main nonlinear quenching channel in CsI, CsI:Tl, NaI, and NaI:Tl. We also show that the second of the extreme conditions in a track, i.e., radial concentration gradient, gives rise to fast (≤picoseconds) diffusion phenomena which act both as a competitor in reducing excitation density during the relevant time of nonlinear quenching, and as a determiner of branching between independent and paired carriers, where the branching ratio changes with dE/dx along the primary electron track. To investigate the interplay of these phenomena in determining nonproportionality of light yield, we use experimentally measured rate constants and mobilities in CsI and NaI to carry out quantitative modeling of diffusion, drift, and nonlinear quenching evaluated spatially and temporally within an electron track which is assumed cylindrical Gaussian in this version of the model.
TL;DR: In this paper, a review of recent developments in many-body perturbation theory (MBPT) calculations that have enabled the study of interfaces and defects is presented, starting from the theoretical basis of MBPT, leading to the GW and GWΓ approximations.
Abstract: We review some recent developments in many-body perturbation theory (MBPT) calculations that have enabled the study of interfaces and defects. Starting from the theoretical basis of MBPT, Hedin’s equations are presented, leading to the GW and GWΓ approximations. We introduce the perturbative approach, that is the one most commonly used for obtaining quasiparticle (QP) energies. The practical strategy presented for dealing with the frequency dependence of the self-energy operator is based on either plasmon-pole models (PPM) or the contour deformation technique, with the latter being more accurate. We also discuss the extrapolar method for reducing the number of unoccupied states which need to be included explicitly in the calculations. The use of the PAW method in the framework of MBPT is also described. Finally, results which have been obtained using MBPT for band offsets at interfaces and for defects are presented, with emphasis on the main difficulties and caveats.
TL;DR: In this article, the role of grain boundaries, free surfaces and interfaces in ferromagnetic behavior of GaN is discussed, in particular their presence permits to increase the Mn solubility in GaN without precipitation of secondary Ferromagnetic phases.
Abstract: The pure ZnO thin films were deposited by the wet chemistry ('liquid ceramics') method from the butanoate precursors on the single-crystalline (102) sapphire substrates. The films annealed in air (550 °C, 24 h) after butanoate pyrolysis have pronounced texture, and they reveal the ferromagnetic behaviour. Argon annealed films (650 °C, 30min) exhibit randomly oriented grains, where the ferromagnetism of these non-textured films is almost equal to that of bare substrate. In both cases the films consist of dense equiaxial nanograins with size ∼20 nm. We observed that grain boundaries (GBs) and related vacancies are the intrinsic origin for RT ferromagnetism in polycrystals [Straumal et al., Phys. Rev. B 79, 205206 (2009)]. Present results demonstrate that not only the specific area of GBs in nanograined ZnO alone determines the ferromagnetic behaviour of ZnO. The GB character distribution (i.e. GB misorientation and orientation) is different in the textured and non-textured films. Most probably, the GBs with different character possess also different magnetic properties. The role of GBs, free surfaces and interfaces in ferromagnetic behaviour of GaN is discussed. In particular, their presence permits to increase the Mn solubility in GaN without precipitation of secondary ferromagnetic phases.
Abstract: A common fingerprint of the electrically active point defects in semiconductors is the transition between their localized defect states upon excitation, which may result in characteristic absorption or photoluminescence spectrum. While density functional calculations have been very successful in exploring the ground-state properties like formation energies or hyperfine tensors the density functional theory (DFT), in principle, is not capable of providing reliable excitation spectrum. Time-dependent (TD)-DFT, however, addresses this issue which makes possible to study the properties of point defects associated with their excited states. In this paper, we apply the TD-DFT on two characteristic examples: the well-known nitrogen-vacancy defect in diamond and the less known divacancy in silicon carbide. The former defect is a leading candidate in solid state quantum bit applications where detailed knowledge about the excitation spectrum is extremely important. The excitation property of divacancy will be also studied and its relevance in different applications will be discussed.
TL;DR: M. Vazquez is particularly honoured to contribute with this study in homage of Prof. Manfred Fahnle with whom he shared quite interesting discussions on phase transition in amorphous alloys already 30 years ago.
Abstract: M. Vazquez is particularly honoured to contribute with this study in homage of Prof. Manfred Fahnle with whom he shared quite interesting discussions on phase transition in amorphous alloys already 30 years ago. The authors are very grateful to Dr O. Chubykalo-Fesenko and Dr R. Perez at the ICMM/CSIC, Madrid, to Dr V. Prida from University of Oviedo, and to Dr D. Altbir and Dr J. Escrig from the University of Santiago de Chile for their essential support and supply of results in the preparation of this feature article. The study has been supported by the Spanish Ministry of Science and Innovation under project MAT2010-20798-C05. L. G. Vivas acknowledges the International Iberian Nanotechnology Laboratory (INL) for the doctoral grant.
TL;DR: In this paper, a review of semi-polar GaN surfaces, of interest for light emitting devices, from both theoretical and experimental perspectives, is presented, and the current status of such templates as grown by hydride vapour phase epitaxy is presented.
Abstract: This paper reviews semi-polar GaN surfaces, of interest for light emitting devices, from both theoretical and experimental perspectives. Theoretical results on polarization charges at InGaN/GaN heterointerfaces and In incorporation into InGaN films are presented for polar (0001), semi-polar (1122) and non-polar (1100) surfaces. Specific features of semi-polar InGaN/GaN structures are emphasized which can be beneficial for improving optical and transport properties of quantum-well-based light emitting devices. The analysis favours semi-polar surfaces such as the (1122) surface as growth plane for long-wavelength light emitters. Therefore, the experimental sections emphasize progress towards long-wavelength LEDs and lasers by growth of InGaN/AlGaN/GaN(1122) heterostructures on large-area GaN(1122)/m-sapphire templates. The current status of such templates as grown by hydride vapour phase epitaxy is presented. The implementation of an epitaxial lateral overgrowth method on such templates to improve device performances is demonstrated.
TL;DR: The organic magnetoresistance (OMAR) as mentioned in this paper has been investigated in the context of spin transport and spin interactions in organic semiconductors, and three main models are proposed: the bipolaron model, the electron-hole pair model and the exciton-charge interaction model.
Abstract: Organic semiconductors are interesting materials for spintronics applications because of their long spin lifetimes. In addition, organic spintronics offers the possibility to add magnetic functionality to existing organic electronics. Two main topics of organic spintronics are discussed.
First, in organic spin valves, spin transport occurs through an organic spacer layer. As a main source of loss of spin polarization the interactions of the spins with random hyperfine fields originating from surrounding hydrogen nuclei has been identified. Recent progress in the development of organic spin valves and related approaches are discussed, addressing conductivity mismatch and the question whether the observed magnetoresistance is from spin injection or direct tunneling.
Second, an intrinsic magnetoresistance is observed in many organic semiconductors. This, so-called, organic magnetoresistance (OMAR) is interesting because it shows large effects at room temperature, making it interesting for applications, and poses fundamental questions about spin transport and spin interactions in these organic materials.
An overview of the main characteristics of OMAR is given and the three main models are discussed. These models are the bipolaron model, the electron–hole pair model and the exciton–charge interaction model. Finally, a comparison is made between the proposed models and experimental results, where it is concluded that the exact origin of OMAR is still open for debate.
TL;DR: In this article, a decomposition of the XY plane with the elastic constant ratio is introduced to facilitate the interpretation of cubic materials under hydrostatic pressure,, and isotropic tension, which are two qualitatively different situations.
Abstract: The global maximum and global minimum Poisson's ratio (PR) surfaces, regions of different auxetic behaviour and the domains of the different extreme directions of cubic materials are shown and discussed in terms of the elastic moduli ratios, , where K is the bulk modulus and are shear moduli. A straightforward way is given to classify any cubic material as being auxetic, nonauxetic and partially auxetic, as well as calculating its extreme PR values. A decomposition of the XY plane with the elastic constant ratio is introduced to facilitate the interpretation of cubic materials under hydrostatic pressure, , and isotropic tension, , which are two qualitatively different situations. Using this representation it is demonstrated that cubic materials with can be auxetic only under ‘negative’ pressure (). It is shown that the influence of pressure on the auxetic behaviour is different for the positive and negative cases. In particular, a cubic material with may be auxetic at negative as well as positive pressure. The work demonstrates the crucial role of P in obtaining desired auxetic behaviour. Microscopic mechanisms which tune the cubic system to targeted regions in the XY plane are investigated with a fcc crystal of particles interacting via the pairwise spherically symmetric potential, . It is found that all fcc static models in which the range of interaction is only between nearest neighbour particles are placed on a universal curve in the XY plane. It is shown that auxetic behaviours can be achieved with simple static analytic forms. The influence of the next neighbour interactions on the universal curve is also considered. In order to assess the role of thermal fluctuations, molecular dynamic simulations for two different (the Lennard–Jones (LJ) and the tethered particle potentials) were performed. At low temperatures the studied systems are well represented by the universal curve and on increasing temperature a systematic departure from it is observed.
TL;DR: In this article, the elastic properties of all known superconducting MAX phases (Nb 2 SC, Nb 2 SnC, nb 2 AsC, as well as the machinability indexes were analyzed using first-principles density functional calculations.
Abstract: Using first-principles density functional calculations, a systematic study of the elastic properties for all known superconducting MAX phases (Nb 2 SC, Nb 2 SnC, Nb 2 AsC, Nb 2 InC, Mo 2 GaC, and Ti 2 InC) was performed. As a result, the optimized lattice parameters, independent elastic constants, indicators of elastic anisotropy, and brittle/ductile behavior as well as the so-called machinability indexes were calculated. We also derived bulk and shear moduli, Young's moduli, and Poisson's ratios for ideal polycrystalline MAX aggregates. The results obtained are discussed in comparison with available theoretical and experimental data and elastic parameters for other layered superconductors.
TL;DR: A detailed analytical model for novel hexagonal honeycomb structures which are composed of alternate layers of re-entrant and non reentrant features is presented in this paper, where deformation from hinging, flexure and/or stretching of the cell walls can lead to zero Poisson's ratios, a property which is highly desirable in niche applications.
Abstract: A detailed analytical model for novel hexagonal honeycomb structures which are composed of alternate layers of re-entrant and non re-entrant features is presented. It is shown that deformation from hinging, flexure and/or stretching of the cell walls can lead to zero Poisson's ratios, a property which is highly desirable in niche applications.
TL;DR: In this article, the minimum energy geometrical structure of the regioregular head-to-tail poly(3-hexylthiophene) (rr-HT-P3HT) polymer has been addressed by means of density functional theory (DFT) calculations which include long-range (van der Waals) interactions.
Abstract: The minimum-energy geometrical structure of the regioregular head-to-tail poly(3-hexylthiophene) (rr-HT-P3HT) polymer has been addressed by means of density functional theory (DFT) calculations which include long-range (van der Waals) interactions. The problem of the P3HT structure has been debated in the literature in the last decades mainly for what concerns the arrangement of the alkyl side chains of the polymer and the type and content of the crystalline primitive cell. The main result of our calculations is that the energetically favored structure of the crystalline polymer at T = 0 K corresponds to polythiophene chains with slightly (∼16°) non co-planar rings and a fishbone arrangement of tilted alkyl side chains with complex internal structure. The alkyl side chains are negligibly interdigitated with those of the adjacent polymer layers; moreover the five terminal carbon atoms of each alkyl side chain are co-planar in all-trans staggered conformation. The optimized geometrical structure obtained for the rr-HT-P3HT polymer is in agreement with measured X-ray spectra of high molecular weight P3HT crystalline samples, and confirms that two non-equivalent polymer chains, mutually shifted along the backbone axis, are contained in an orthorhombic primitive cell.
TL;DR: In this paper, carbon fiber laminates were produced with matched through-the-thickness modulus, but with negative (auxetic), positive and near zero through the thickness Poisson's ratios.
Abstract: Carbon fibre laminates were produced with matched through-the-thickness modulus, but with negative (auxetic), positive and near zero through-the-thickness Poisson's ratios. These were indented with noses of diameter 2, 12.7 and 20 mm. Enhancements in indentation resistance were seen for the auxetic laminates with smaller, more localized damage areas for the two larger diameter indentors where delamination was the dominant failure mechanism. Little difference was seen in mechanical properties for the 2 mm diameter indentor where fibre breakage was the dominant failure mechanism, though smaller damage areas are still obtained in this case due to delamination growth suppression.
TL;DR: It is demonstrated that full Kohn–Sham density functional theory calculations can be performed in a time only marginally greater than tight binding implementations and a route is opened to achieve full and demonstrable convergence with respect to basis size.
Abstract: We present a review of methodological and implementation details of the AIMPRO Kohn-Sham density functional code. It is demonstrated that full Kohn-Sham density functional theory calculations can be performed in a time only marginally greater than tight binding implementations and a route is opened to achieve full and demonstrable convergence with respect to basis size. Topics covered will include both the kernel and functionality of the current code, a discussion of recent developments as well as future research directions and perspectives. Also, a broad discussion regarding the application of these methods is made that, it is hoped, will serve as a useful guide to application specialists.
TL;DR: In this paper, the integration of the 3D structures into a conventional laser design is presented, and the results of integration of 3D GaN pyramids into conventional laser designs are presented.
Abstract: Selective-area epitaxy is used to form three-dimensional (3D) GaN structures providing semipolar crystal facets. On full 2-in. sapphire wafers we demonstrate the realization of excellent semipolar material quality by introducing inverse GaN pyramids. When depositing InGaN quantum wells on such a surface, the specific geometry influences thickness and composition of the films and can be nicely modeled by gas phase diffusion processes. Various investigation methods are used to confirm the drastically reduced piezoelectric polarization on the semipolar planes. Complete electrically driven light-emitting diode test structures emitting in the blue and blue/ green spectral regions show reasonable output powers in the milliwatt regime. Finally, first results of the integration of the 3D structures into a conventional laser design are presented.
TL;DR: In this article, the number of RDs with crystallographically equivalent interfaces (CEIF) was determined for all combinations of the two-dimensional (2D) point symmetries of substrate and epilayer, including cases of aligned and misaligned symmetry directions.
Abstract: The formation of rotation domains (RDs) in heteroepitaxy depends fundamentally on the relation of the crystal symmetries of substrate and epilayer. We assume well-defined crystallographic axes of substrate and epilayer along the growth direction and determine from group theory the number of RDs with crystallographically equivalent interfaces (CEIF) for all combinations of the two-dimensional (2D) point symmetries of substrate and epilayer, including cases of aligned and misaligned symmetry directions. Additional domains can arise from multi-domain substrates. Accidentally or nearly fulfilled additional symmetries of the substrate allow further domains with crystallographically inequivalent interfaces (CIIF). Schematic RDs for various rotation symmetries of substrate and epitayer.
TL;DR: In this paper, the authors review recent advances in the theory of defects in solids from the perspective of first-principle calculations, focusing on methods that improve the description of band gaps, leading to results that can be directly compared to experiments.
Abstract: Defects and impurities are often decisive in determining the physical properties of most materials. The process of defect identification and characterization is typically difficult and indirect, usually requiring an ingenious combination of different experimental techniques. First-principles calculations have emerged as a powerful microscopic tool that complements experiments or sometimes even serves as the sole source of atomistic information due to experimental limitations. Still, first-principles calculations based on density functional theory in the local density or generalized gradient approximations suffer from serious limitations when describing defects in solids. Recent advances in electronic structure methods, rapid increases in computing power, and the development of efficient algorithms indicate a promising future for computational defect physics. We review recent advances in the theory of defects in solids from the perspective of first-principles calculations. We focus in particular on methods that improve the description of band gaps, leading to results that can be directly compared to experiments on a quantitative level. We discuss the use of LDA+U in wide-band-gap materials, screened hybrid functionals, the quasiparticle GW method, and the use of modified pseudopotentials. Advantages and limitations of these methods are illustrated with examples.
TL;DR: In this article, the authors compare two types of parameterized perturbations, i.e., the addition of onsite potentials and the mixing of non-local Fock exchange in hybrid-functionals.
Abstract: Lattice defects in semiconductors and wide-gap materials which create deep levels in an open-shell electronic configuration can give rise to so-called defect bound small polarons. This type of defects present a challenge for electronic structure methods because the localization of the defect state and the associated energy levels depend sensitively on the ability of the total-energy functional to satisfy the physical condition that the energy E(N) must be a piecewise linear function of the fractional electron number N. For practical applications the requirement of a linear E(N) is re-cast as a generalized Koopmans condition. Since most functionals do nnt fulfill this condition accurately, we use parameterized perturbations that cancel the non-linearity of E(N) and recover the correct Koopmans behavior. Starting from standard density functionals, we compare two types of parameterized perturbations, i.e., the addition of on-site potentials and the mixing of non-local Fock exchange in hybrid-functionals. Surveying a range of acceptor-type defects in II―VI and III―V semiconductors, we present a classification scheme that describes the relation between hole localization and the lattice relaxation of the polaronic state.
TL;DR: In this paper, the authors used X-ray microtomography to study the uniaxial tensile response of low-density polyurethane foam and constructed microstructurally faithful finite element (FE) models from the initial tomographs.
Abstract: X-ray microtomography has been used to study in situ the uniaxial tensile response of low-density polyurethane foam. Two variants have been examined, one before and one after treatment to generate auxetic behaviour. For both variants, microstructurally faithful finite element (FE) models have been constructed from the initial tomographs. For each variant a series of tomographs have been collected during progressive straining. Poisson's ratios of 0.30 (conventional, non-auxetic) and -0.22 (auxetic) have been measured for the two variants by digital image correlation (DIC) between successive images. By comparison, the FE models exhibited Poisson's ratio's of 0.5 and -0.3, respectively. Key micromechanical mechanisms responsible for the auxetic effect have been observed during straining, such as the straightening of bent ribs and rotation of nodes (joints), compared to changes in the angles between essentially straight struts for the non-auxetic variant. The microstructurally faithful FE models confirm the mechanisms observed in the experiments and enable characteristic rib and node behaviour to be followed in greater detail. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim