Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2008"

New routes to sustainable photovoltaics: evaluation of Cu2ZnSnS4 as an alternative absorber material

Abstract: Thin film heterojunction solar cells based on chalcopyrites such as Cu(In,Ga)Se2 have achieved impressive efficiencies. However concern about the long term sustainability of photovoltaics based on scarce or expensive raw materials has prompted the search for alternative absorber materials. In this work, films of the p-type absorber Cu2ZnSnS4 (CZTS) were prepared by electroplating metallic precursors sequentially onto a molybdenum-coated glass substrate followed by an nealing in a sulfur atmosphere. The polycrystalline CZTS films were characterized by photoelectrochemical methods, which showed films were p-type with doping densities of the order of 1016 cm–3 and a band gap of 1.49 eV, close to the optimum value for terrestrial solar energy conversion. Preliminary results obtained for solar cells fabricated with this material are promising. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Negative compressibility, negative Poisson's ratio, and stability

Abstract: The role of negative compressibility is considered in detail. Experimental observations of negative bulk modulus in pre-stained foam are presented and interpreted. A constrained microscopic model which exhibits negative compressibility is proposed. Some other negative and counterintuitive properties of matter; like begative Poisson's ratio, negative thermal expansion, negative specific heat, and negative pressure are also discussed. A simple thermodynamic model with negative thermal expansion is presented.

On the properties of auxetic meta‐tetrachiral structures

Abstract: Auxetics are systems which get fatter when stretched and thinner when compressed i.e. they exhibit a negative Poisson's ratio. Here, we present an analysis of a novel class of structures (referred to as “metal-chiral”) which belong to the class of auxetics constructed using chiral building blocks. We show through analytical modelling that some of these systems can exhibit negative Poisson's ratios, the extent of which will depend, amongst other things, on the geometry of the system.

Complex oxide scintillators: Material defects and scintillation performance

Abstract: An overview of recognized structural defects, impurities and related trapping levels and their role in the scintillation mechanism is provided and discussed in single crystal materials belonging to tungstate, Ce- or Pr-doped aluminum perovskite, garnet and finally to Ce-doped silicate scintillators. New achievements and open problems in deeper understanding of electron and hole self-trapping phenomena and of the nature of defects in the crystal structure and their ability to localize migrating charge carriers are indicated. Fast optical ceramics and nanocomposite materials are pointed out as possible future advanced scintillators. Such novel technologies can in principle explore materials which are not available in the bulk single crystal form, but their figure-of-merit is dramatically dependent on the surface-interface defect states and related trapping and nonradiative recombination phenomena. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Epitaxial graphene : a new material

Abstract: Grapbene, a two-dimensional sheet of sp2-bonded carbon arranged in a honaycomb lattice, is not only the building block of fullerenes, carbon nano tubes (CNTs) and graphite, it also has interesting properties, which have caused a flood of also has interesting properties white have caused a flood of activities in the past few years The possibility to grow graph-itic films with thicknesses down to a single graphene layer epitaxially on-SiC{0001} surfaces is promising for for future applications The two-dimensional nature of epitaxial graphene films make them ideal objects for surface science techniques such as photoelectron spectroscopy, low-energy electron diffraction, and scanning probe microscopy The present article summarizes results from recent photoemission studies covering a variety of aspects such as the growth of epitaxial graphene and few layer graphene, the electronic and structural properties of the interface to the SiC substrate, and the elecinterface to the SiC substrate, and the electronic structure of the epitaxial graphene stacks

Doping asymmetry in wide-bandgap semiconductors: Origins and solutions

Abstract: Wide-bandgap (WBG) semiconductors are essential materials for making short-wavelength and transparent optoelectronic devices. However, a serious obstacle to realizing WBG semiconductor-based devices is their common doping asymmetry problem, i.e., a WBG semiconductor can be doped easily either p-type or n-type, but not both. This paper reviews the possible origins for doping asymmetry problems and updates our recent progress in searching for approaches to overcome the doping bottleneck in WBG semiconductors. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Plasmonic nanostructures in aperture-less scanning near-field optical microscopy (aSNOM)

Abstract: Apertureless scanning near-field optical microscopy offers superb spatial resolution, but interpreting the recoreded signal can still be a challenge. Especially images of eigenmodes in plasmonic nanostructures are very often obscured by concurrent scattering from the tip and/or coupling effects in hte tip sample system. We show here how the use of orthogonal polarizations in excitation and detection affords us with an elegant method to map near-fields of plasmonic eignenmodes and other optical phenomenta. We demonstrated with a variety of samples possible applications of this cross-polarization scheme, such as verification of functional nanooptical structures, systematic studies of localized and propagating plasmonic eignenmodes, and their susceptibility to disturbance from structural defects. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Effect of nitrogen doping on Raman spectra of multi‐walled carbon nanotubes

Abstract: Multi-walled carbon nanotubes (CNTs) and nitrogen-doped carbon (CNx) nanotubes have been synthesized by the aerosol-assisted chemical vapour deposition (CVD) method. The samples were prepared from toluene and acetonitrile taken in different proportion. The structure of CNTs and CNx nanotubes was compared using first- and second-order Raman spectra. The disorder-induced D band was found to up-shift with increase of the acetonitrile fraction. The I-D/I-G and I-G/I-G ratios showed the opposite dependence on the doping level of CNTs. Both kinds of the substitute nitrogen: graphitic-like and pyridinic-like contribute to the disorder in CNx nanotube layers. [GRAPHICS] The first- and second-order Raman spectra of CNTs produced from toluene and CNx nanotubes produced from acetonitrile. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Nonprecious metal catalysts for the molecular oxygen-reduction reaction

Abstract: This review covers the electrocatalytic activities, the corresponding influence factors and the synthesis methods of nonprecious-metal electrocatalysts towards oxygen-reduction reaction (ORR) for fuel-cell systems. In particular, the chalcogenides and the macrocycles containing Co or Fe metals have been focused upon. Due to the scarcity of Pt metal, the Co- and Fe-based chalcogenides are potentially interesting substitutes of Pt although they still show lower catalytic activities for ORR and lower electrochemical and chemical stability in the present fuel-cell electrolytes. Compared to Pt electrocatalysts, the Co- and Fe-based macrocycles, such as TMPP, TPP, Pc and TAA, are among the most promising substitutes for Pt because of their comparable catalytic activities towards ORR and the higher insensitivity to methanol (in direct methanol fuel cells – DMFC) although they still have some drawbacks, namely lower stability and shorter durability. Novel catalyst synthesis methods, modification techniques of catalysts as well as supporting substrates are expected to be developed in the future to satisfy the requirements of commercial fuel cells. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

On the auxetic properties of rotating rhombi and parallelograms: A preliminary investigation

Abstract: Auxetics exhibit the unusual property of expanding when uniaxially stretched (negative Poisson's ratio), a property that is usually linked to particular geometric features and deformation mechanisms One of the mechanisms which results in auxetic behaviour is the one involving rotating rigid units, for which, systems made from triangles, squares or rectangles have aleady been considered In this work we extend this study by considering systems which can be constructed from either connected rhombi or connected parallelograms We show that various types of such systems can exist and we discuss in detail the properties of one type of 'rotating rhombi' and one type of 'rotating parallelograms' We also show that the Poisson's ratio of these systems, which can be positive or negative, is anisotropic and dependent on the shape of the parallelograms/rhombi and the degree of openness of the system

Lifetime-killing defects in 4H-SiC epilayers and lifetime control by low-energy electron irradiation

Abstract: Carrier lifetimes in n-type 4H-SiC epilayers have been investigated by differential microwave photoconductance decay measurements. Through a correlation study between lifetime and various deep levels, the Z1/2 and/or EH6/7 centers have been identified as effective recombination centers. When the Z1/2 (and EH6/7) concentration is higher than 1013 cm–3, the inverse carrier lifetime is in proportion to the trap concentration, and the lifetime increases with increasing excitation intensity (density of irradiated photons). Alternartively, other recombination processes limit the lifetime when the Z1/2 concentration is less than 1013 cm–3. In this case, the carrier lifetime is decreased by increasing the excitation intensity. Surface recombination and recombination in the substrate have been suggested based on numerical analyses as the other recombination paths. By controlling the Z1/2 (and EH6/7) concentration by low-energy electron irradiation, lifetime control has been achieved. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Double layer supercapacitor properties of onion‐like carbon materials

Abstract: This paper describes the electrochemical performance of carbon onions as electrode materials in electrical double layer capacitors with water electrolytes. Onion-like carbon (OLC) species were produced by vacuum annealing of detonation nanodiamonds (NDs) powder at 1170-2170 K. The material capability of the charge accumulation in the electric double layer was estimated using the voltammetric technique. The correlations between specific capacitance, surface area and conductivity were found. Charge-discharge measurements revealed a dependence of the specific capacitance of OLC on the degree of diamond particle graphitization and defectness of particle surfaces. Promising capacitance values, ranging within 20-40 F g -1 and within 70-100 F g-1 were found for the OLC materials operating respectively in acid electrolytic solution (1 M H 2 S0 4 ) and alkaline electrolytic solution (6 M KOH).

Recombination of free and bound excitons in GaN

Abstract: We report on recent optical investigations of free and bound exciton properties in bulk GaN. In order to obtain reliable data it is important to use low defect density samples of low doping. We have used thick GaN layers (of the order of 1 mm) grown by halide vapour phase epitaxy (HVPE) with a residual doping down to <1016 cm–3 in this work. With such samples all polarisation geometries could also easily be exploited. The influence of the surface states on the photoluminescence (PL) experiments is analysed, it is concluded that surface recombination plays an important role for the free exciton (FE) recombination. The electronic structure of the FEs is discussed in detail, including the influence of spin-exchange and polariton effects, and compared with polarised PL spectra at 2 K. The detailed structure of excited states from the PL spectra is discussed, but further data are needed to fully explain all the peaks observed. The polarized FE spectra at room temperature allow a determination of the bandgap as 3.437 eV at 290 K, assuming an exciton binding energy of 25 meV. The PL transient of the A FE is very short (about 100 ps) for the no-phonon (NP) line interpreted as dominated by nonradiative surface recombination. The longitudinal-optical (LO) phonon replicas of the A FE exhibit a longer decay of about 1.4 ns at 2 K, suggested to represent the bulk lifetime of the FE. The corresponding decay time at 290 K is 9 ns in our samples, a value that might be affected by nonradiative recombination. The Si and O donor bound exciton (DBE) spectra with sharp NP lines at 3.4723 eV and 3.4714 eV respectively, are well resolved together with the so-called two-electron transitions (TETs) and several optical phonon replicas. The electronic structure of the DBE states including excited rotational states is discussed and compared with experiment. The well-resolved TET lines allow an accurate determination of the ground state binding energy of the Si donor as 30.4 meV and 33.2 meV for the O donor. The PL transients of the DBEs reveal a non-exponential decay for the NP lines. The DBE NP transient lineshape is assumed to be influenced by optical dispersion and scattering in the vicinity of exciton resonances, as well as by surface effects. The DBE decay time can most properly be deduced from the PL decay of the respective TETs and LO replicas, leading to values in the range of 1.1–1.8 ns. These values differ significantly from previous theoretical predictions, where values about two orders of magnitude shorter were obtained. A tentative discussion of the main observed features of acceptor bound excitons (ABEs), which are much less studied in GaN, is given. A decay time of about 0.9 ns for the shallowest 3.466 eV ABE is estimated, i.e. shorter than that for the shallow donor BEs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Micro‐ and nanomechanical structures for silicon carbide MEMS and NEMS

Abstract: Silicon carbide (SiC) is recognized as the leading semiconductor for high power and high temperature electronics owing to its outstanding electrical properties combined with mature processing technologies for monolithic structures. SiC has long been known for its outstanding mechanical and chemical properties making it equally attractive for mechanical structures in micro- and nanoelectromechanical systems (MEMS and NEMS). Recent advancements in bulk and surface micromachining have led to the development of SiC analogues to common Si-based devices (i.e., resonators, pressure sensors). This paper presents an overview of MEMS and NEMS technologies that incorporate SiC as a key component in their mechanical structure, providing both a historical perspective as well as recent developments. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Auxetic behaviour from rotating rhombi

Abstract: Auxeticity (i.e. negative Poisson's ratios) is associated with particular geometrical features and deformation mechanisms of a system. Among the potential systems that may exhibit such behaviour are rotating rigid units. Here we discuss how rigid rhombi of the same shape and size can be connected in two different ways to give rise to two distinct systems: the Type a and Type (3 rotating rhombi. We derive the mechanical properties. for the Type (β system and compare it to those of the Type a to highlight the differences between the two systems namely that Type a systems are highly anisotropic and can exhibit both negative and positive Poisson's ratios which depend on the shape of the rhombi and the angle between them whereas Type systems are isotropic with a constant in-plane Poisson's ratio of-1. Furthermore we show that by constructing the rhombi from five truss-type elements, where the one forming the diagonal has a different thermal expansion than the other four identical trusses, the Poisson's ratios for the Type a systems can also be made temperature dependent, in contrast with Type β systems which remain unaffected by temperature changes.

Development and prospects of nitride materials and devices with nonpolar surfaces

Abstract: The quest to use nonpolar surfaces of nitride materials and devices started a few years ago with the aim to avoid the strong internal electric fields in active regions of optoelec-tronic devices and to improve their efficiency. Starting with the growth optimizations, the progression via thorough understanding of new physical properties of the materials has led to significant improvement of device performance and to novel device concepts. In this review a historical survey of nonpolar nitride growth achievements is made. Along the way new challenges in material growth and characterization have been encountered and more sophisticated methods have been developed, which are briefly summarized. The main properties of the nitride materials grown in nonpolar directions are discussed with emphasis on the deviations from those of nitrides grown along the polar direction. Physical phenomena such as inherently present anisotropic in-plane strain and optical polarization anisotropy have been proposed for the realization of novel polarized light-emitting diodes, polarization-sensitive detectors and modulators. The present status of the nitride devices with nonpolar and semipolar surfaces is discussed, and an outlook of the future trends is presented. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fe in III–V and II–VI semiconductors

Abstract: Many theoretical and experimental studies deal with the realization of room-temperature ferromagnetism in dilute magnetic semiconductors (DMS). However, a detailed quantitative understanding of the electronic properties of transition metal doped semiconductors has often been neglected. This article points out which issues concerning electronic states and charge transfers need to be considered using Fe as an example. Methods to address these issues are outlined, and a wealth of data on the electronic properties of Fe doped III–V and II–VI compound semiconductors that have been obtained over a few decades is reviewed thoroughly. The review is complemented by new results on the effective-mass-like state consisting of a hole bound to Fe2+ forming a shallow acceptor state. The positions of established Fe3+/2+ and Fe2+/1+ charge transfer levels are summarized and predictions on the positions of further charge transfer levels are made based on the internal reference rule. The Fe3+/4+ level has not been identified unambiguously in any of the studied materials. Detailed term schemes of the observed charge states in tetrahedral and trigonal crystal field symmetry are presented including hyperfine structure, isotope effects and Jahn–Teller effect. Particularly, the radiative transitions Fe3+(4T1 6A1) and Fe2+(5E 5T2) are analyzed in great detail. An effective-mass-like state [Fe2+, h] consisting of a hole bound to Fe2+ is of great significance for a potential realization of spin-coupling in a DMS. New insights on this shallow acceptor state could be obtained by means of stress dependent and temperature dependent absorption experiments in the mK range. The binding energy and effective Bohr radius were determined for GaN, GaP, InP and GaAs and a weak exchange interaction between the hole and the Fe2+ center was detected. With regard to the Fe3+ ground state, 6A1, in GaP and InP, the hyperfine structure level Γ8 was found to be above the Γ8 level. All results are discussed with respect to a potential realization of a ferromagnetic spin-coupling in DMSs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of high performance 3C‐SiC vertical MOSFETs by reducing planar defects

Abstract: The planar defect density of 3C-SiC can be reduced by growing it on undulant-Si substrates. However, specific stacking faults (SFs) remain, that expose the Si-face on the (001) surface. These residual SFs increase the leakage current in devices made with 3C-SiC. They can be eliminated using an advanced SF-reduction method called switch-back epitaxy (SBE) that combines polarity conversion with homoepitaxial growth. Vertical metal–oxide–semiconductor field-effect-transistors (MOSFETs) are fabricated on 3C-SiC with SBE, varying in size from a single cell with an area of (30 × 30) μm2 to 12,000 hexagonal cells on a (3 × 3) mm2 chip. The MOSFET characteristics suggest that currents greater than 100 A are realistic for blocking voltages of 600–1,200 V by increasing the number of cells with reduced cell-pitch. The combination of blocking voltage capability with a demonstrable high current capacity shows that 3C-SiC is well-suited for use in vertical MOSFETs for high- and medium-power electronic applications. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A study of flexoelectric coupling associated internal electric field and stress in thin film ferroelectrics

Abstract: By discussions in the framework of tensor representations and phenomenological theory, general piezoelectricity equations that incorporate direct and inverse flexoelectric effects were analyzed and interlink between direct and inverse flexoelectricity discovered. Due to flexoelectric coupling, mechanical strain gradient is equivalent to internal electric field. In ferroelectrics above the Curie temperature, it was shown theoretically that strain gradient associated electric field leads to non-zero polarization without external electric field. It was also shown theoretically that polarization (or electric field) gradient is an extra source of internal stress. Magnitudes of flexoelectricity associated internal electric field and stress were analyzed for heteroepitaxial thin films of barium strontium titanate. Flexoelectric coupling is expected to intensify with reducing lateral dimensions and thickness and get large enough at nanoscale to modify ferroelectric phase transitions and functional response. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Charge transport and single-electron effects in nanoscale systems

Abstract: In this review, we consider single-electron effects in transport through nanoscale devices. These effects are ubiquitous in quantum dot physics, but in recent years their occurrence in molecular transport has triggered important new research efforts. In this case, the experimental results show a rich variety of features which make it possible to extract a wealth of information about the physics of these structures. We shall show that most of this information can be extracted in the case where the coupling of the active region to the leads is weak. However, between the strong and weak coupling regimes, that is, at intermediate coupling, we can observe the richest behaviour. We shall very briefly outline the physics of different transport processes through three-terminal devices and then focus on the single-electron effects. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamic percolation of carbon nanotube agglomerates in a polymer matrix: comparison of different model approaches

Abstract: A series of samples with different content of multi-walled carbon nanotubes (Q.5-7.5 wt%) in polycarbonate was investigated by electrical conductivity measurements. During isothermal annealing of the as-produced samples in the melt state an increase of the conductivity with time was observed. A similar conductivity recovery was found after a short shear deformation of the melt. The time-dependent transition from insulating to conductive state is explained by dynamic formation of a conductive network of interconnected carbon nanotube agglomerates. The conductive networks were found to be stable after cooling the samples below their glass transition temperature. Comparison of the room temperature conductivity plotted vs. nanotube content for as-produced and annealed samples shows a tremendous shift of the insulator-conductor transition from above 5 wt% to about 0.75 wt%, respectively. For the description of the insulator-conductor transition three different approaches were tested: classical percolation, general effective medium model (GEM), and Foumier equation. All three models fit the data satisfactory. Dynamic percolation was described by the GEM model coupled to a kinetic equation for CNT agglomeration.

Disorder and instability processes in amorphous conducting oxides

Abstract: The behaviour of ionic amorphous oxide semiconductors (transparent conducting oxides) such as In2O3, SnO2, and ZnO is contrasted with that of covalent amorphous semiconductors such as amorphous Si. These oxides have an s-like conduction band minima, which leads to high electron mobilities, smaller effects of disorder, and the ability to move the Fermi level well into the conduction band. This results in an absence of the bias stress instability due to the bond breaking mechanism in oxide thin film transistors, which limited the stability of a-Si:H transistors. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The hexachiral prismatic wingbox concept

Abstract: In this work the concept of using a chiral honeycomb as the internal structure for a passively morphing wing is explored. The use of a chiral honeycomb, which features an in-plane negative Poisson's ratio,potentially offers high deformability whilst maintaining structural integrity of the wing box. A finite element simulation was carried out, coupling a complete two-dimensional model of the wing and internal structure to a panel code flow solver. An iterative process was then used in order to predict the wing camber change with respect to air-flow. The concept was validated experimentally through the construction of a prototype wing for wind-tunnel testing. The measured response was non-linear with respect to aerodynamic loading, but the final deflected shape at the design case compared well with the finite element prediction. The concept was proven to work for small camber changes, with larger deflections predicted by tailoring the mechanical properties of the hexachiral core.

Structural, electronic and optical properties of spinel MgAl2O4 oxide

Abstract: The structure, band structure, total density of states, dielectric function, reflectivity, refractive index and loss function have been calculated for spinel MgAl2O4 oxide using density functional theory. The full-potential linearized augmented plane wave method was used with the generalized gradient approximation. Calculations of the optical spectra have been performed for the energy range 0–40 eV. It is shown that the material is transparent at visible wavelengths and the dispersion curve of the refractive index is fairly flat in the long-wavelength region and rises rapidly towards shorter wavelengths. The refractive index is 1.774 at 800 nm near the visible region. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier density and effective mass calculations in carbon nanotubes

Abstract: In this work, the intrinsic carrier concentration and doping effects have been investigated for different carbon nanotubes. Electronic structure and the density of states functions have been utilized to derive analytical equations for the carrier concentration under high and low doping concentrations. The temperature dependence has also been investigated. The results show how varying diameters and wrapping angles of carbon nanotubes change their concentration profile. The implications of these results allow us to use the same material (carbon nanotube) for a wide range of applications only by varying the chiral vector (n,m). The calculations performed in this work set an upper limit for a wide range of applications including carbon nanotube interconnects and carbon nanotube field effect transistors. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Elastic and electronic properties of hexagonal and cubic polymorphs of tungsten monocarbide WC and mononitride WN from first‐principles calculations

Abstract: We have performed accurate ab-initio total energy calculations using the full-potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange-correlation potential to investigate the elastic, electronic and cohesive properties of hexagonal and cubic polymorphs of WC and WN. The optimized lattice parameters, independent elastic constants (Cij), bulk moduli (B), shear moduli (G), as well as band structure, density of states, electron density distribution and cohesive energies are obtained and analyzed in comparison with the available theoretical and experimental data. In addition, for the first time, the numerical estimates are made for some elastic parameters of the polycrystalline WC and WN ceramics (in the framework of the Voigt–Reuss–Hill approximation), namely bulk and shear moduli, compressibility (β), Young's moduli (Y), Poisson's ratio (ν) and Lame's coefficients (μ, λ). (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Exciting prospects for solids: Exact-exchange based functionals meet quasiparticle energy calculations

Abstract: Focussing on spectroscopic aspects of semiconductors and insulators we will illustrate how quasiparticle energy calculations in the G0W0 approximation can be successfully combined with density-functional theory calculations in the exact- exchange optimised effective potential approach (OEPx) to achieve a first principles description of the electronic structure that overcomes the limitations of local- or gradient-corrected DFT functionals (LDA and GGA). (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theory of the elastic constants of graphite and graphene

Abstract: Born's long wave method is used to study the elastic properties of graphite and graphene. Starting from an empirical force constant model derived from full inplane phonon dispersions of graphite [Mohr et al., Phys. Rev. B 76, 035439 (2007)] we calculate the tension coefficients of graphene. Extending the model by interplanar interactions, we calculate the elastic constants of LE N3 graphite. The agreement of our theoretical values with inelastic x-ray scattering results on elastic constants of graphite [Bosak et al., Phys. Rev. B 75 153408 (2007)] 13 is very satisfactory.

Pressure‐induced phase transitions in silicon studied by neural network‐based metadynamics simulations

Abstract: We present a combination of the metadynamics method for the investigation of pressure-induced phase transitions in solids with a neural network representation of high-dimensional density-functional theory (DFT) potential-energy surfaces. In a recent illustration of the method for the complex high-pressure phase diagram of silicon [Behler et al., Phys. Rev. Lett. 100, 185501 (2008)] we have shown that the full sequence of phases can be reconstructed by a series of subsequent simulations. In the present paper we give a detailed account of the underlying methodology and discuss the scope and limitations of the approach, which promises to be a valuable tool for the investigation of a variety of inorganic materials. The method is several orders of magnitude faster than a direct coupling of metadynamics with electronic structure calculations, while the accuracy is essentially maintained, thus providing access to extended simulations of large systems. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Strip and gap plasmon polariton optical resonators

Abstract: A rectangular silver strip and two strips separated by a narrow gap are considered as optical resonators. A surface integral equation method is used to study theoretically their scattering cross section and local enhancement of the electric field relative to the incident wave. Peaks in the scattering spectra are shown to be related to the excitation of standing waves caused by forward and backward travelling slow plasmon polaritons (PPs) in the silver strip or in the gap between two strips. This is shown via analysis of the electric near-field inside and outside the strips, and by showing that an increase of the length of strips equal to half the slow PP wavelength re- sults in a scattering resonance at app. the same wavelength. We study the broadening of resonances with strip thickness or gap size, respectively, for the two types of resonators. For field enhancement purposes we consider taking advantage of the fact that the electric field component of slow PPs along the axis of PP propagation is dominating inside the metal strips. Due to this fact and the electromagnetics boundary conditions forcing the surface normal electric field component to jump across the silver-air interface a small (5 nm) gap introduced in one of the strips leads to a factor ∼25 enhancement of the field magnitude. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)