Showing papers in "Physical Review B in 2005"

Linear response approach to the calculation of the effective interaction parameters in the LDA + U method

Abstract: In this work we reexamine the $\mathrm{LDA}+\mathrm{U}$ method of Anisimov and co-workers in the framework of a plane-wave pseudopotential approach. A simplified rotational-invariant formulation is adopted. The calculation of the Hubbard $U$ entering the expression of the functional is discussed and a linear response approach is proposed that is internally consistent with the chosen definition for the occupation matrix of the relevant localized orbitals. In this way we obtain a scheme whose functionality should not depend strongly on the particular implementation of the model in ab initio calculations. We demonstrate the accuracy of the method, computing structural and electronic properties of a few systems including transition and rare-earth correlated metals, transition metal monoxides, and iron silicate.

Real-space grid implementation of the projector augmented wave method

Abstract: A grid-based real-space implementation of the projector augmented wave (PAW) method of Bl\"ochl [Phys. Rev. B 50, 17953 (1994)] for density functional theory (DFT) calculations is presented. The use of uniform three-dimensional (3D) real-space grids for representing wave functions, densities, and potentials allows for flexible boundary conditions, efficient multigrid algorithms for solving Poisson and Kohn-Sham equations, and efficient parallelization using simple real-space domain-decomposition. We use the PAW method to perform all-electron calculations in the frozen core approximation, with smooth valence wave functions that can be represented on relatively coarse grids. We demonstrate the accuracy of the method by calculating the atomization energies of 20 small molecules, and the bulk modulus and lattice constants of bulk aluminum. We show that the approach in terms of computational efficiency is comparable to standard plane-wave methods, but the memory requirements are higher.

String-net condensation: A physical mechanism for topological phases

Abstract: We show that quantum systems of extended objects naturally give rise to a large class of exotic phases---namely topological phases. These phases occur when extended objects, called ``string-nets,'' become highly fluctuating and condense. We construct a large class of exactly soluble 2D spin Hamiltonians whose ground states are string-net condensed. Each ground state corresponds to a different parity invariant topological phase. The models reveal the mathematical framework underlying topological phases: tensor category theory. One of the Hamiltonians---a spin-$1∕2$ system on the honeycomb lattice---is a simple theoretical realization of a universal fault tolerant quantum computer. The higher dimensional case also yields an interesting result: we find that 3D string-net condensation naturally gives rise to both emergent gauge bosons and emergent fermions. Thus, string-net condensation provides a mechanism for unifying gauge bosons and fermions in 3 and higher dimensions.

Weak ferromagnetism and magnetoelectric coupling in bismuth ferrite

Abstract: We analyze the coupling between the ferroelectric and magnetic order parameters in the magnetoelectric multiferroic $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ using density functional theory within the local spin density approximation (LSDA) and the $\mathrm{LSDA}+\mathrm{U}$ method. We show that weak ferromagnetism of the Dzyaloshinskii-Moriya type occurs in this material, and we analyze the coupling between the resulting magnetization and the structural distortions. We explore the possibility of electric-field-induced magnetization reversal and show that, although it is unlikely to be realized in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$, it is not in general impossible. Finally, we outline the conditions that must be fulfilled to achieve switching of the magnetization using an electric field.

First-principles study of spontaneous polarization in multiferroic Bi Fe O 3

Abstract: The ground-state structural and electronic properties of ferroelectric $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ are calculated using density functional theory within the local spin-density approximation (LSDA) and the $\mathrm{LSDA}+U$ method. The crystal structure is computed to be rhombohedral with space group $R3c$, and the electronic structure is found to be insulating and antiferromagnetic, both in excellent agreement with available experiments. A large ferroelectric polarization of $90--100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{C}∕{\mathrm{cm}}^{2}$ is predicted, consistent with the large atomic displacements in the ferroelectric phase and with recent experimental reports, but differing by an order of magnitude from early experiments. One possible explanation is that the latter may have suffered from large leakage currents. However, both past and contemporary measurements are shown to be consistent with the modern theory of polarization, suggesting that the range of reported polarizations may instead correspond to distinct switching paths in structural space. Modern measurements on well-characterized bulk samples are required to confirm this interpretation.

Raman spectroscopy of hydrogenated amorphous carbons

Abstract: We present a comprehensive multiwavelength Raman investigation of a variety of hydrogenated amorphous carbons $(a\text{\ensuremath{-}}\mathrm{C}:\mathrm{H})$, ranging from polymeric $a\text{\ensuremath{-}}\mathrm{C}:\mathrm{H}$ to diamond-like $a\text{\ensuremath{-}}\mathrm{C}:\mathrm{H}$ and ta-C:H, which allows us to derive values for their bonding, density, band gap, hydrogen content, and mechanical properties. The Raman spectra of $a\text{\ensuremath{-}}\mathrm{C}:\mathrm{Hs}$ show two different trends. In one case, the $G$ peak width increases with $G$ peak dispersion. In the second case, the opposite trend is found. In the first case, the Raman parameters vary with optical, structural, and mechanical properties in the same way as in hydrogen-free carbon films. In the second case, typical of polymeric $a\text{\ensuremath{-}}\mathrm{C}:\mathrm{H}$, the $G$ peak width correlates with the density, while the $G$ peak dispersion varies with the optical gap and hydrogen content. This allows a unified picture of bonding and disorder of all carbon films. UV Raman is particularly useful for $a\text{\ensuremath{-}}\mathrm{C}:\mathrm{Hs}$, as it gives clear measurements in the $D$ and $G$ peaks spectral region even for highly hydrogenated samples, for which the visible Raman spectra are overshadowed by photoluminescence. On the other hand, the slope of the photoluminescence background in visible Raman spectra can be used to estimate the H content. UV Raman measurements also allow the detection of $\mathrm{C}\mathrm{H}$ stretching vibrations.

First-principles determination of the structural, vibrational and thermodynamic properties of diamond, graphite, and derivatives

Abstract: The structural, dynamical, and thermodynamic properties of diamond, graphite and layered derivatives (graphene, rhombohedral graphite) are computed using a combination of density-functional theory total-energy calculations and density-functional perturbation theory lattice dynamics in the generalized gradient approximation. Overall, very good agreement is found for the structural properties and phonon dispersions, with the exception of the c/a ratio in graphite and the associated elastic constants and phonon dispersions. Both the C-33 elastic constant and the F to A phonon dispersions are brought to close agreement with available data once the experimental c/a is chosen for the calculations. The vibrational free energy and the thermal expansion, the temperature dependence of the elastic moduli and the specific heat are calculated using the quasiharmonic approximation. Graphite shows a distinctive in-plane negative thermal-expansion coefficient that reaches its lowest value around room temperature, in very good agreement with experiments. Thermal contraction in graphene is found to be three times as large; in both cases, bending acoustic modes are shown to be responsible for the contraction, in a direct manifestation of the membrane effect predicted by Lifshitz over 50 years ago. Stacking directly affects the bending modes, explaining the large numerical difference between the thermal-contraction coefficients in graphite and graphene, notwithstanding their common physical origin.

Device model for the operation of polymer/fullerene bulk heterojunction solar cells

Abstract: We have developed a numerical device model that consistently describes the current-voltage characteristics of polymer:fullerene bulk heterojunction solar cells. Bimolecular recombination and a temperature- and field-dependent generation mechanism of free charges are incorporated. It is demonstrated that in poly[2-methoxy-5-(3('),7(')-dimethyloctyloxy)-p-phenylene vinylene]- (OC1C10-PPV-) and [6,6]-phenyl C-61-butyric acid methyl ester- (PCBM-) (1:4 wt. %) based solar cells space-charge effects only play a minor role, leading to a relatively constant electric field in the device. Furthermore, at short-circuit conditions only 7% of all free carriers are lost due to bimolecular recombination. The model predicts that an increased hole mobility together with a reduction of the acceptor strength of 0.5 eV will lead to a maximum attainable efficiency of 5.5% in the PPV/PCBM-based solar cells.

Improved analytical fit of gold dispersion: Application to the modeling of extinction spectra with a finite-difference time-domain method

Abstract: We propose an accurate description for the dispersion of gold in the range of 1.24--2.48 eV. We implement this improved model in an FDTD algorithm and evaluate its efficiency by comparison with an analytical method. Extinction spectra of gold nanoparticle arrays are then calculated.

Martensitic transitions and the nature of ferromagnetism in the austenitic and martensitic states of Ni-Mn-Sn alloys

Abstract: Structural and magnetic transformations in the Heusler-based system ${\mathrm{Ni}}_{0.50}{\mathrm{Mn}}_{0.50\ensuremath{-}x}{\mathrm{Sn}}_{x}$ are studied by x-ray diffraction, optical microscopy, differential scanning calorimetry, and magnetization. The structural transformations are of austenitic-martensitic character. The austenite state has an $L{2}_{1}$ structure, whereas the structures of the martensite can be $10M$, $14M$, or $L{1}_{0}$ depending on the Sn composition. For samples that undergo martensitic transformations below and around room temperature, it is observed that the magnetic exchange in both parent and product phases is ferromagnetic, but the ferromagnetic exchange, characteristic of each phase, is found to be of different strength. This gives rise to different Curie temperatures for the austenitic and martensitic states.

Atomistic calculations of elastic properties of metallic fcc crystal surfaces

Abstract: Elastic properties of crystal surfaces are useful in understanding mechanical properties of nanostructures. This paper presents a fully nonlinear treatment of surface stress and surface elastic constants. A method for the determination of surface elastic properties from atomistic simulations is developed. This method is illustrated with examples of several crystal faces of some fcc metals modeled with embedded atom potentials. The key finding in this study is the importance of accounting for the additional relaxations of atoms at the crystal surface due to strain. Although these relaxations do not affect the values of surface stress (as had been determined in previousworks), they have a profound effect on the surface elastic constants.Failure to account for these relaxations can lead to values of elastic constants that are incorrect not only in magnitude but also in sign. A possible method for the experimental determination of the surface elastic constants is outlined.

Systematic treatment of displacements, strains, and electric fields in density-functional perturbation theory

Abstract: The methods of density-functional perturbation theory may be used to calculate various physical response properties of insulating crystals including elastic, dielectric, Born charge, and piezoelectric tensors. These and other important tensors may be defined as second derivatives of an appropriately defined energy functional with respect to atomic-displacement, electric-field, or strain perturbations, or as mixed derivatives with respect to two of these perturbations. The resulting tensor quantities tend to be coupled in complex ways in polar crystals, giving rise to a variety of variant definitions. For example, it is generally necessary to distinguish between elastic tensors defined under different electrostatic boundary conditions, and between dielectric tensors defined under different elastic boundary conditions. Here, we describe an approach for computing all of these various response tensors in a unified and systematic fashion. Applications are presented for two materials, hexagonal ZnO and rhombohedral $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$, at zero temperature.

Effect of nanotube alignment on percolation conductivity in carbon nanotube/polymer composites

Abstract: Percolation conductivity of a stick network depends on alignment as well as concentration. We show that both dependences exhibit critical (power-law) behavior, and study the alignment threshold in detail. The highest conductivity occurs for slightly aligned, rather than isotropic, sticks. Experiments on single wall carbon nanotube composites are supported by Monte Carlo simulations. These results should be broadly applicable to percolating networks of anisotropic conductors.

Observation of zigzag and armchair edges of graphite using scanning tunneling microscopy and spectroscopy

Abstract: The presence of structure-dependent edge states of graphite is revealed by both ambient and ultrahigh-vacuum (UHV) scanning tunneling microscopy and scanning tunneling spectroscopy observations. On a hydrogenated zigzag (armchair) edge, bright spots are (are not) observed together with a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ superlattice near the Fermi level (${V}_{S}\ensuremath{\sim}\ensuremath{-}30\phantom{\rule{0.3em}{0ex}}\mathrm{mV}$ for a peak of the local density of states) under UHV, demonstrating that a zigzag edge is responsible for the edge states, although there is no appreciable difference between as-prepared zigzag and armchair edges in air. Even in the hydrogenated armchair edge, however, bright spots are observed at defect points, at which partial zigzag edges are created in the armchair edge.

Optical properties of coupled metallic nanorods for field-enhanced spectroscopy

Abstract: The optical properties of coupled metallic nanorods are studied to investigate the use of coupled plasmonic structures in field-enhanced spectroscopies. Light scattering by coupled nanorods is calculated with the boundary element method, including retardation. The modes of coupled nanorod systems are calculated by the boundary charge method and discussed in terms of their symmetry. Similar scattering behavior for isolated nanorods and pairs of nanorods can mask the very different local responses that produce near-field enhancement. The response of isolated rods redshifts with increasing rod length because intrarod restoring forces are reduced. The near- and far-field responses increase monotonically with increasing rod length increasing polarization along the rod. For coupled nanorods, coupling localizes charge at the gap between the rod ends and splits degenerate modes. The localized charge depolarizes the intrarod response and provides an additional redshift. Moreover, the near-field enhancement in the gap between the nanorods is dramatically increased by coupling-induced charge localization at the gap. For short nanorods, the near-field response in coupled systems is determined by the geometry of the rod ends that define the gap. For longer nanorods, the response in coupled systems is determined by the rod length. Changing the dimensions and geometry of the nanorods to modify the interrod coupling has a major effect on the local-field enhancement. The effects of the environment and the actual metallic material do not have as big an influence on the field enhancement.

Magnetoelectric phase diagrams of orthorhombic R MnO 3 ( R = Gd , Tb, and Dy)

Abstract: Magnetoelectric phase diagrams have been investigated for rare-earth manganites with orthorhombically distorted perovskite structure, $R{\mathrm{MnO}}_{3}$ ($R=\mathrm{Gd}$, Tb, and Dy). A variety of magnetic and electric phases emerge with varying $R$-site ion, temperature, and magnetic field in these systems. The magnetoelectric phase diagram varies sensitively with the direction of a magnetic field relative to the crystallographic axes. Although the ground state of ${\mathrm{GdMnO}}_{3}$ with the largest ionic radius of $R({r}_{R})$ is not ferroelectric in zero magnetic fields $(H=0)$, a ferroelectric phase with electric polarization $(P)$ along the $a$ axis appears by applying $H(g\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ along the $b$ axis. Both ${\mathrm{TbMnO}}_{3}$ and ${\mathrm{DyMnO}}_{3}$ show a ferroelectric order with $P$ along the $c$ axis even at $H=0$ below a lock-in transition temperature where nonzero wave vectors for magnetic and lattice modulations become nearly constant. These systems also exhibit a flop of the ferroelectric polarization ($P\ensuremath{\Vert}c$ to $P\ensuremath{\Vert}a$) when $H$ is applied along the $a$ or $b$ axis. By contrast, the application of $H$ above $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ along the $c$ axis completely suppresses the ferroelectricity in ${\mathrm{TbMnO}}_{3}$. Possible origins of the observed evolution of magnetoelectric phases are discussed in consideration of magnetism and lattice distortion in the perovskite rare-earth manganites.

Quantum vacuum properties of the intersubband cavity polariton field

Abstract: We present a quantum description of a planar microcavity photon mode strongly coupled to a semiconductor intersubband transition in presence of a two-dimensional electron gas. We show that, in this kind of system, the vacuum Rabi frequency ${\ensuremath{\Omega}}_{R}$ can be a significant fraction of the intersubband transition frequency ${\ensuremath{\omega}}_{12}$. This regime of ultrastrong light-matter coupling is enhanced for long-wavelength transitions, because for a given doping density, effective mass and number of quantum wells, the ratio ${\ensuremath{\Omega}}_{R}∕{\ensuremath{\omega}}_{12}$ increases as the square root of the intersubband emission wavelength. We characterize the quantum properties of the ground state (a two-mode squeezed vacuum), which can be tuned in situ by changing the value of ${\ensuremath{\Omega}}_{R}$, e.g., through an electrostatic gate. We finally point out how the tunability of the polariton quantum vacuum can be exploited to generate correlated photon pairs out of the vacuum via quantum electrodynamics phenomena reminiscent of the dynamical Casimir effect.

Decoherence in a superconducting quantum bit circuit

Abstract: Decoherence in quantum bit circuits is presently a major limitation to their use for quantum computing purposes. We present experiments, inspired from NMR, that characterize decoherence in a particular superconducting quantum bit circuit, the quantronium. We introduce a general framework for the analysis of decoherence, based on the spectral densities of the noise sources coupled to the qubit. Analysis of our measurements within this framework indicates a simple model for the noise sources acting on the qubit. We discuss various methods to fight decoherence.

Anion vacancies as a source of persistent photoconductivity in II-VI and chalcopyrite semiconductors

Abstract: Using first-principles electronic structure calculations we identify the anion vacancies in II-VI and chalcopyrite $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{III}\text{\ensuremath{-}}{\mathrm{VI}}_{2}$ semiconductors as a class of intrinsic defects that can exhibit metastable behavior. Specifically, we predict persistent electron photoconductivity ($n$-type PPC) caused by the oxygen vacancy ${V}_{\mathrm{O}}$ in $n$-ZnO, originating from a metastable shallow donor state of ${V}_{\mathrm{O}}$. In contrast, we predict persistent hole photoconductivity ($p$-type PPC) caused by the Se vacancy ${V}_{\mathrm{Se}}$ in $p\text{\ensuremath{-}}{\mathrm{CuInSe}}_{2}$ and $p\text{\ensuremath{-}}{\mathrm{CuGaSe}}_{2}$. We find that ${V}_{\mathrm{Se}}$ in the chalcopyrite materials is amphoteric having two ``negative-$U$''-like transitions, i.e., a double-donor transition $\ensuremath{\epsilon}(2+∕0)$ close to the valence band and a double-acceptor transition $\ensuremath{\epsilon}(0∕2\ensuremath{-})$ closer to the conduction band. We introduce a classification scheme that distinguishes two types of defects: type $\ensuremath{\alpha}$, which have a defect-localized-state (DLS) in the band gap, and type $\ensuremath{\beta}$, which have a resonant DLS within the host bands (e.g., the conduction band for donors). In the latter case, the introduced carriers (e.g., electrons) relax to the band edge where they can occupy a perturbed-host state. Type $\ensuremath{\alpha}$ is nonconducting, whereas type $\ensuremath{\beta}$ is conducting. We identify the neutral anion vacancy as type $\ensuremath{\alpha}$ and the doubly positively charged vacancy as type $\ensuremath{\beta}$. We suggest that illumination changes the charge state of the anion vacancy and leads to a crossover between $\ensuremath{\alpha}$- and $\ensuremath{\beta}$-type behavior, resulting in metastability and PPC. In ${\mathrm{CuInSe}}_{2}$, the metastable behavior of ${V}_{\mathrm{Se}}$ is carried over to the $({V}_{\mathrm{Se}}\text{\ensuremath{-}}{V}_{\mathrm{Cu}})$ complex, which we identify as the physical origin of PPC observed experimentally. We explain previous puzzling experimental results in ZnO and ${\mathrm{CuInSe}}_{2}$ in the light of this model.

Geometric, electronic, and magnetic structure of Co2FeSi: Curie temperature and magnetic moment measurements and calculations

Abstract: In this work a simple concept was used for a systematic search for materials with high spin polarization It is based on two semiempirical models First, the Slater-Pauling rule was used for estimation of the magnetic moment This model is well supported by electronic structure calculations The second model was found particularly for ${\mathrm{Co}}_{2}$ based Heusler compounds when comparing their magnetic properties It turned out that these compounds exhibit seemingly a linear dependence of the Curie temperature as function of the magnetic moment Stimulated by these models, ${\mathrm{Co}}_{2}\mathrm{FeSi}$ was revisited The compound was investigated in detail concerning its geometrical and magnetic structure by means of x-ray diffraction, x-ray absorption, and M\"ossbauer spectroscopies as well as high and low temperature magnetometry The measurements revealed that it is, currently, the material with the highest magnetic moment $(6{\ensuremath{\mu}}_{B})$ and Curie temperature (1100 K) in the classes of Heusler compounds as well as half-metallic ferromagnets The experimental findings are supported by detailed electronic structure calculations

Theoretical and computational studies of carbon nanotube composites and suspensions : Electrical and thermal conductivity

Abstract: Monte Carlo simulations have been performed, aimed at finding a critical fractional volume (CFV) associated with the onset of percolation for randomly oriented nanotubes (or, indeed, any conductive particles with large aspect ratios) that are randomly dispersed in a low thermo- or electroconductive medium. The nanotubes were treated as capped interpenetrating conductive cylinders (``sticks'') with high (up to 2000) aspect ratio $a$. It has been found that for these aspect ratios the CFV is inversely proportional to $a$ resulting in surprisingly low filler volume loadings, of the order of 0.01%, required to achieve percolation in such systems. By studying fluctuations of the CFV and the density of the percolation clusters, various critical indices of the percolation theory have been calculated including the critical index of conductivity, $t$. For three-dimensional systems it has been found that $t$ decreases substantially with an increase in the aspect ratio. The calculated thermal and electrical conductivity of the nanotube suspensions and composites as functions of the nanotube loading is in good agreement with recent experimental data.

Quantum cutting by cooperative energy transfer in Yb x Y 1 − x P O 4 : Tb 3 +

Abstract: We present experimental evidence for cooperative energy transfer from ${\mathrm{Tb}}^{3+}$ to two ${\mathrm{Yb}}^{3+}$ ions and a determination of the energy-transfer rate. Energy transfer from ${\mathrm{Tb}}^{3+}$ to ${\mathrm{Yb}}^{3+}$ was investigated by luminescence measurements on $({\mathrm{Yb}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x})\mathrm{P}{\mathrm{O}}_{4}$ doped with 1% ${\mathrm{Tb}}^{3+}$. Time-resolved luminescence experiments were analyzed using Monte Carlo simulations based on theories for phonon-assisted, cooperative, and accretive energy transfer. The luminescence decay curves of the $^{5}D_{4}$ emission from ${\mathrm{Tb}}^{3+}$ show an excellent agreement with simulations based on cooperative energy transfer via dipole-dipole interaction, while a phonon-assisted or an accretive energy-transfer mechanism cannot explain the experimental results. The energy-transfer rate to two nearest-neighbor ${\mathrm{Yb}}^{3+}$ ions is $0.26\phantom{\rule{0.3em}{0ex}}{\mathrm{ms}}^{\ensuremath{-}1}$. This corresponds to an upper limit of the energy-transfer efficiency of 88% in $\mathrm{Yb}\mathrm{P}{\mathrm{O}}_{4}$. Application of cooperative energy transfer has prospects for increasing the energy efficiency of crystalline Si solar cells by photon doubling of the high energy part of the solar spectrum.

Functional designed to include surface effects in self-consistent density functional theory

Abstract: We design a density-functional-theory (DFT) exchange-correlation functional that enables an accurate treatment of systems with electronic surfaces. Surface-specific approximations for both exchange and correlation energies are developed. A subsystem functional approach is then used: an interpolation index combines the surface functional with a functional for interior regions. When the local density approximation is used in the interior, the result is a straightforward functional for use in self-consistent DFT. The functional is validated for two metals (Al, Pt) and one semiconductor (Si) by calculations of (i) established bulk properties (lattice constants and bulk moduli) and (ii) a property where surface effects exist (the vacancy formation energy). Good and coherent results indicate that this functional may serve well as a universal first choice for solid-state systems and that yet improved functionals can be constructed by this approach.

Flexural wave propagation in single-walled carbon nanotubes

Abstract: The paper presents the study on the flexural wave propagation in a single-walled carbon nanotube through the use of the continuum mechanics and the molecular dynamics simulation based on the Terroff-Brenner potential. The study focuses on the wave dispersion caused not only by the rotary inertia and the shear deformation in the model of a traditional Timoshenko beam, but also by the nonlocal elasticity characterizing the microstructure of carbon nanotube in a wide frequency range up to THz. For this purpose, the paper starts with the dynamic equation of a generalized Timoshenko beam made of the nonlocal elastic material, and then gives the dispersion relations of the flexural wave in the nonlocal elastic Timoshenko beam, the traditional Timoshenko beam and the Euler beam, respectively. Afterwards, it presents the molecular dynamics simulations for the flexural wave propagation in an armchair (5,5) and an armchair (10,10) single-walled carbon nanotubes for a wide range of wave numbers. The simulation results show that the Euler beam holds for describing the dispersion of flexural waves in the two single-walled carbon nanotubes only when the wave number is small. The Timoshenko beam provides a better prediction for the dispersion of flexural waves in the two single-walled carbon nanotubes when the wave number becomes a little bit large. Only the nonlocal elastic Timoshenko beam is able to predict the decrease of phase velocity when the wave number is so large that the microstructure of carbon nanotubes has a significant influence on the flexural wave dispersion.

Analytical potential for atomistic simulations of silicon, carbon, and silicon carbide

Abstract: We present an analytical bond-order potential for silicon, carbon, and silicon carbide that has been optimized by a systematic fitting scheme. The functional form is adopted from a preceding work [Phys. Rev. B 65, 195124 (2002)] and is built on three independently fitted potentials for $\mathrm{Si}\mathrm{Si}$, $\mathrm{C}\mathrm{C}$, and $\mathrm{Si}\mathrm{C}$ interaction. For elemental silicon and carbon, the potential perfectly reproduces elastic properties and agrees very well with first-principles results for high-pressure phases. The formation enthalpies of point defects are reasonably reproduced. In the case of silicon stuctural features of the melt agree nicely with data taken from literature. For silicon carbide the dimer as well as the solid phases B1, B2, and B3 were considered. Again, elastic properties are very well reproduced including internal relaxations under shear. Comparison with first-principles data on point defect formation enthalpies shows fair agreement. The successful validation of the potentials for configurations ranging from the molecular to the bulk regime indicates the transferability of the potential model and makes it a good choice for atomistic simulations that sample a large configuration space.

Continuous-time quantum Monte Carlo method for fermions

Abstract: We present a numerically exact continuous-time quantum Monte Carlo algorithm for fermions with a general interaction nonlocal in space-time The new determinantal grand-canonical scheme is based on a stochastic series expansion for the partition function in the interaction representation The method is particularly applicable for multiband, time-dependent correlations since it does not invoke the Hubbard-Stratonovich transformation The test calculations for exactly solvable models, as well results for the Green function and for the time-dependent susceptibility of the multiband supersymmetric model with a spin-flip interaction are discussed

Exciton binding energies in carbon nanotubes from two-photon photoluminescence

Abstract: Excitonic effects in the linear and nonlinear optical properties of single-walled carbon nanotubes are manifested by photoluminescence excitation experiments and ab initio calculations. One- and two-photon spectra showed a series of exciton states; their energy splitting is the fingerprint of excitonic interactions in carbon nanotubes. By ab initio calculations we determine the energies, wave functions, and symmetries of the excitonic states. Combining experiment and theory we find binding energies of $0.3\char21{}0.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for nanotubes with diameters between 6.8 and $9.0\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$.

Magnetic ordering at the edges of graphitic fragments: Magnetic tail interactions between the edge-localized states

Abstract: To understand the formation mechanism of magnetic moments at the edges of graphitic fragments, we carry out first-principles density-functional calculations for the electronic and magnetic structures of graphitic fragments with various spin and geometric configurations. We find that interedge and interlayer interactions between the localized moments can be explained in terms of interactions between the magnetic tails of the edge-localized states. In addition, the dihydrogenated edge states as well as Fe ad-atoms at the edge are studied in regard to the magnetic order and proximity effects.

Planar metal plasmon waveguides: frequency-dependent dispersion, propagation, localization, and loss beyond the free electron model

Abstract: A numerical analysis of surface plasmon dispersion, propagation, and localization on smooth lossy films is presented. Particular attention is given to determining wavelength-dependent behavior of thin Ag slab waveguides embedded in a symmetric SiO2 environment. Rather than considering Ag as a damped free electron gas, the metal is defined by the experimentally determined optical constants of Johnson and Christy and Palik. As in free electron gas models, analytic dispersion results indicate a splitting of plasmon modes—corresponding to symmetric and antisymmetric field distributions—as film thickness is decreased below 50 nm. However, unlike free electron gas models, the surface plasmon wave vector remains finite at resonance with the antisymmetric-field plasmon converging to a pure photon mode for very thin films. In addition, allowed excitation modes are found to exist between the bound and radiative branches of the dispersion curve. The propagation characteristics of all modes are determined, and for thin films (depending upon electric field symmetry), propagation distances range from microns to centimeters in the near infrared. Propagation distances are correlated with both the field decay (skin depth) and energy density distribution in the metal and surrounding dielectric. While the energy density of most long-range surface plasmons exhibits a broad spatial extent with limited confinement in the waveguide, it is found that high-field confinement does not necessarily limit propagation. In fact, enhanced propagation is observed for silver films at ultraviolet wavelengths despite strong field localization in the metal. The surface plasmon characteristics described in this paper provide a numerical springboard for engineering nanoscale metal plasmon waveguides, and the results may provide a new avenue for integrated optoelectronic applications.

Wavelet analysis of extended x-ray absorption fine structure data

Abstract: Fourier transform (FT) is a fundamental step for the data reduction and interpretation of extended x-ray absorption fine structure (EXAFS) spectra. The FT separates backscattering atoms by their radial distance from the absorbing atom (so-called shells). We suggest to routinely complement the FT by a wavelet transform (WT), which provides not only radial distance resolution, but resolves in $k$ space. This information eases the discrimination of atoms by their elemental nature, especially if these atoms are at the same distance. We present an in-depth analysis of the Morlet wavelet, which has specific advantages for EXAFS analysis, including the possibility to estimate Morlet parameter values optimized either for elemental or for spatial resolution. Using an experimental spectrum of $\mathrm{Zn}\text{\ensuremath{-}}\mathrm{Al}$ layered double hydroxide, we demonstrate the discrimination of $\mathrm{Al}$ and $\mathrm{Zn}$ at a similar crystallographic position, in spite of destructive interference substantially reducing signal information. Finally, the extension to multiple scattering paths leads to a deeper understanding of the resolution properties of the WT.