# Showing papers in "Physical Review B in 1998"

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TL;DR: In this article, the authors improved the description of both electron energy loss spectra and parameters characterizing the structural stability of the material compared with local spin density functional theory by taking better account of electron correlations in the $3d$ shell of metal ions in nickel oxide.

Abstract: We demonstrate how by taking better account of electron correlations in the $3d$ shell of metal ions in nickel oxide it is possible to improve the description of both electron energy loss spectra and parameters characterizing the structural stability of the material compared with local spin density functional theory.

7,997 citations

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TL;DR: In this paper, a selfconsistent real-space multiple-scattering (RSMS) approach for calculations of x-ray-absorption near-edge structure (XANES) is presented and implemented in an ab initio code applicable to arbitrary aperiodic or periodic systems.

Abstract: A self-consistent real-space multiple-scattering (RSMS) approach for calculations of x-ray-absorption near-edge structure (XANES) is presented and implemented in an ab initio code applicable to arbitrary aperiodic or periodic systems This approach yields a quantitative interpretation of XANES based on simultaneous, self-consistent-field (SCF) calculations of local electronic structure and x-ray absorption spectra, which include full multiple scattering from atoms within a small cluster and the contributions of high-order MS from scatterers outside that cluster In addition, the code includes a SCF estimate of the Fermi energy and an account of orbital occupancy and charge transfer We also present a qualitative, scattering-theoretic interpretation of XANES Sample applications are presented for cubic BN, ${\mathrm{UF}}_{6},$ Pu hydrates, and distorted ${\mathrm{PbTiO}}_{3}$ Limitations and various extensions are also discussed

3,696 citations

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TL;DR: In this paper, an extension of the tight-binding (TB) approach to improve total energies, forces, and transferability is presented. The method is based on a second-order expansion of the Kohn-Sham total energy in density-functional theory (DFT) with respect to charge density fluctuations.

Abstract: We outline details about an extension of the tight-binding (TB) approach to improve total energies, forces, and transferability. The method is based on a second-order expansion of the Kohn-Sham total energy in density-functional theory (DFT) with respect to charge density fluctuations. The zeroth order approach is equivalent to a common standard non-self-consistent (TB) scheme, while at second order a transparent, parameter-free, and readily calculable expression for generalized Hamiltonian matrix elements may be derived. These are modified by a self-consistent redistribution of Mulliken charges (SCC). Besides the usual ``band structure'' and short-range repulsive terms the final approximate Kohn-Sham energy additionally includes a Coulomb interaction between charge fluctuations. At large distances this accounts for long-range electrostatic forces between two point charges and approximately includes self-interaction contributions of a given atom if the charges are located at one and the same atom. We apply the new SCC scheme to problems where deficiencies within the non-SCC standard TB approach become obvious. We thus considerably improve transferability.

3,070 citations

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TL;DR: The relativistic dual-space Gaussian pseudopotential was introduced in this paper for the whole Periodic Table and a complete table of pseudopoetic parameters for all the elements from H to Rn.

Abstract: We generalize the concept of separable dual-space Gaussian pseudopotentials to the relativistic case. This allows us to construct this type of pseudopotential for the whole Periodic Table, and we present a complete table of pseudopotential parameters for all the elements from H to Rn. The relativistic version of this pseudopotential retains all the advantages of its nonrelativistic version. It is separable by construction, it is optimal for integration on a real-space grid, it is highly accurate, and, due to its analytic form, it can be specified by a very small number of parameters. The accuracy of the pseudopotential is illustrated by an extensive series of molecular calculations.

2,661 citations

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TL;DR: In this paper, an atomistic imaging of dislocation nucleation during displacement controlled indentation on a passivated surface is presented, where defects are located and imaged by local deviations from centrosymmetry.

Abstract: We model indentation of a metal surface by combining an atomistic metal with a hard-sphere indenter. This work provides atomistic imaging of dislocation nucleation during displacement controlled indentation on a passivated surface. Dislocations and defects are located and imaged by local deviations from centrosymmetry. For a Au(111) surface, nucleation of partial dislocation loops occurs below the surface inside the indenter contact area. We compare and contrast these observations with empirical criteria for dislocation nucleation and corresponding continuum elasticity solutions.

1,684 citations

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TL;DR: In this paper, the stiffness of single-walled carbon nanotubes is estimated by observing their freestanding room-temperature vibrations in a transmission electron microscope, assuming that the vibration modes are driven stochastically and are those of a clamped cantilever.

Abstract: We estimate the stiffness of single-walled carbon nanotubes by observing their freestanding room-temperature vibrations in a transmission electron microscope. The nanotube dimensions and vibration amplitude are measured from electron micrographs, and it is assumed that the vibration modes are driven stochastically and are those of a clamped cantilever. Micrographs of 27 nanotubes in the diameter range 1.0--1.5 nm were measured to yield an average Young's modulus of $〈Y〉=1.25 \mathrm{TPa}.$ This value is consistent with previous measurements for multiwalled nanotubes, and is higher than the currently accepted value of the in-plane modulus of graphite.

1,571 citations

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TL;DR: In this article, it was shown that the minima are the collection of loci for Wood's anomaly, which occurs when a diffracted beam becomes tangent to the film, and that the maxima were the result of a resonant excitation of surface plasmons (SP's).

Abstract: Optically thick metal films perforated with a periodic array of subwavelength holes show exceptional transmission properties. The zero-order transmission spectra exhibit well-defined maxima and minima of which the positions are determined by the geometry of the hole array. We show that the minima are the collection of loci for Wood's anomaly, which occurs when a diffracted beam becomes tangent to the film, and that the maxima are the result of a resonant excitation of surface plasmons (SP's). SP's from both surfaces of the metal film are apparent in the dispersion diagram, independent of which side of the film is illuminated, indicating an anomalously strong coupling between the two sides. This leads to wavelength-selective transmission with efficiencies that are about 1000 times higher than that expected for subwavelength holes.

1,508 citations

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TL;DR: In this article, the authors studied the defect physics in a chalcopyrite semiconductor and showed that it takes much less energy to form a Cu vacancy in the semiconductor than to form cation vacancies in II-VI compounds and that defect formation energies vary considerably both with the Fermi energy and with the chemical potential of the atomic species.

Abstract: We studied the defect physics in ${\mathrm{CuInSe}}_{2},$ a prototype chalcopyrite semiconductor. We showed that (i) it takes much less energy to form a Cu vacancy in ${\mathrm{CuInSe}}_{2}$ than to form cation vacancies in II-VI compounds (ii) defect formation energies vary considerably both with the Fermi energy and with the chemical potential of the atomic species, and (iii) the defect pairs such as $({2\mathrm{V}}_{\mathrm{Cu}}^{\mathrm{\ensuremath{-}}}{+\mathrm{I}\mathrm{n}}_{\mathrm{Cu}}^{2+})$ and $({2\mathrm{C}\mathrm{u}}_{\mathrm{In}}^{2\mathrm{\ensuremath{-}}}{+\mathrm{I}\mathrm{n}}_{\mathrm{Cu}}^{2+})$ have particularly low formation energies (under certain conditions, even exothermic). Using (i)--(iii), we (a) explain the existence of unusual ordered compounds ${\mathrm{CuIn}}_{5}{\mathrm{Se}}_{8},$ ${\mathrm{CuIn}}_{3}{\mathrm{Se}}_{5},$ ${\mathrm{Cu}}_{2}{\mathrm{In}}_{4}{\mathrm{Se}}_{7},$ and ${\mathrm{Cu}}_{3}{\mathrm{In}}_{5}{\mathrm{Se}}_{9}$ as a repeat of a single unit of $({2\mathrm{V}}_{\mathrm{Cu}}^{\mathrm{\ensuremath{-}}}{+\mathrm{I}\mathrm{n}}_{\mathrm{Cu}}^{2+})$ pairs for each $n=4,$ 5, 7, and 9 units, respectively, of ${\mathrm{CuInSe}}_{2};$ (b) attribute the very efficient $p$-type self-doping ability of ${\mathrm{CuInSe}}_{2}$ to the exceptionally low formation energy of the shallow defect Cu vacancies; (c) explained in terms of an electronic passivation of the ${\mathrm{In}}_{\mathrm{Cu}}^{2+}$ by ${2\mathrm{V}}_{\mathrm{Cu}}^{\mathrm{\ensuremath{-}}}$ the electrically benign character of the large defect population in ${\mathrm{CuInSe}}_{2}.$ Our calculation leads to a set of new assignment of the observed defect transition energy levels in the band gap. The calculated level positions agree rather well with available experimental data.

1,124 citations

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TL;DR: In this paper, a model of the thermal conductivity and phonon transport in the direction perpendicular to the film plane of superlattices is established based on solving the phonon Boltzmann transport equation (BTE).

Abstract: Significant reductions in both the in-plane and cross-plane thermal conductivities of superlattices, in comparison to the values calculated from the Fourier heat conduction theory using bulk material properties, have been observed experimentally in recent years. Understanding the mechanisms controlling the thermal conductivities of superlattice structures is of considerable current interest for microelectronic and thermoelectric applications. In this work, models of the thermal conductivity and phonon transport in the direction perpendicular to the film plane of superlattices are established based on solving the phonon Boltzmann transport equation (BTE). Different phonon interface scattering mechanisms are considered, including elastic vs inelastic, and diffuse vs specular scattering of phonons. Numerical solution of the BTE yields the effective temperature distribution, thermal conductivity, and thermal boundary resistance (TBR) of the superlattices. The modeling results show that the effective thermal conductivity of superlattices in the perpendicular direction is generally controlled by phonon transport within each layer and the TBR between different layers. The TBR is no longer an intrinsic property of the interface, but depends on the layer thickness as well as the phonon mean free path. In the thin layer limit, phonon transport within each layer is ballistic, and the TBR dominates the effective thermal conductivity of superlattices. Approximate analytical solutions of the BTE are obtained for this thin-film limit. The modeling results based on partially specular and partially diffuse interface scattering processes are in reasonable agreement with recent experimental data on GaAs/AlAs and Si/Ge superlattices. From the modeling, it is concluded that the cross-plane thermal conductivity of these superlattices is controlled by diffuse and inelastic scattering processes at interfaces. Results of this work suggest that it is possible to make superlattice structures with thermal conductivity totally different from those of their constituting materials.

979 citations

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TL;DR: In this article, a super-prism phenomenon was demonstrated at optical wavelength in photonic crystals with three-dimensional-periodic structure fabricated on Si substrate, where the incident-angle dependence including negative refraction and multiple beam branching was interpreted from highly anisotropic dispersion surfaces derived by photonic band calculation.

Abstract: Extraordinary angle-sensitive light propagation, which we call a superprism phenomenon, was demonstrated at optical wavelength in photonic crystals with three-dimensional-periodic structure fabricated on Si substrate. The propagation beam was swung from $\ensuremath{-}90\ifmmode^\circ\else\textdegree\fi{}$ to $+90\ifmmode^\circ\else\textdegree\fi{}$ with a slight change in the incident angle within $\ifmmode\pm\else\textpm\fi{}12\ifmmode^\circ\else\textdegree\fi{}.$ This effect together with wavelength sensitivity is at least two orders of magnitude stronger than that of the conventional prism. The incident-angle dependence including negative refraction and multiple beam branching was interpreted from highly anisotropic dispersion surfaces derived by photonic band calculation. These phenomena will be available to fabricate microscale light circuits on Si with LSI-compatible lithography techniques.

957 citations

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TL;DR: In this paper, the authors measured the magnetic properties of a diluted magnetic semiconductor based on III-V semiconductors and determined the $p\ensuremath{-}d$ exchange between holes and Mn $3d$ spins.

Abstract: Magnetotransport properties of $p$-type ferromagnetic (Ga,Mn)As, a diluted magnetic semiconductor based on III-V semiconductors, are measured and the $p\ensuremath{-}d$ exchange between holes and Mn $3d$ spins is determined. The ferromagnetic transition temperatures calculated based on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction using the exchange reproduce remarkably well the observed ferromagnetic transition temperatures, demonstrating that ferromagnetism of (Ga,Mn)As has its origin in the RKKY interaction mediated by holes.

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TL;DR: A study of four Gd samples of different purities using ac susceptibility, magnetization, heat capacity, and direct measurements of the magnetocaloric effect in quasistatic and pulse magnetic fields revealed that all techniques yield the same value of the zero-field Curie temperature of 294(1) K as mentioned in this paper.

Abstract: A study of four Gd samples of different purities using ac susceptibility, magnetization, heat capacity, and direct measurements of the magnetocaloric effect in quasistatic and pulse magnetic fields revealed that all techniques yield the same value of the zero-field Curie temperature of 294(1) K. The Curie temperature determined from inflection points of the experimental magnetic susceptibility and heat capacity is in excellent agreement with those obtained from the magnetocaloric effect and Arrot plots. Above 2 T the temperature of this transition increases almost linearly with the magnetic field at a rate of $\ensuremath{\sim}6\mathrm{K}/\mathrm{T}$ in fields up to 7.5 T. The spin reorientation transition, which occurs at 227(2) K in the absence of a magnetic field, has been confirmed by susceptibility, magnetization, and heat-capacity measurements. Magnetic fields higher than 2--2.5 T apparently quench the spin reorientation transition and Gd retains its simple ferromagnetic structure from the ${T}_{C}(H)$ down to $\ensuremath{\sim}4\mathrm{K}.$ The nature of anomaly at $T\ensuremath{\cong}132\mathrm{K},$ which is apparent from ac susceptibility measurements along the $c$ axis, is discussed. The presence of large amounts of interstitial impurities lowers the second-order $\mathrm{paramagnetic}\ensuremath{\leftrightarrow}\mathrm{ferromagnetic}$ transition temperature, and can cause some erroneous results in the magnetocaloric effect determined in pulsed magnetic fields. The magnetocaloric effect was studied utilizing the same samples by three experimental techniques: direct measurements of the adiabatic temperature rise, magnetization, and heat capacity. All three techniques, with one exception, yield the same results within the limits of experimental error.

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TL;DR: In this paper, the field effect mobility in an organic thin-film transistor was studied theoretically. And the authors applied the theory to describe the experiments by Brown et al. on solution-processed amorphous organic transistors, made from polythienylene vinylene and from a small molecule (pentacene).

Abstract: The field-effect mobility in an organic thin-film transistor is studied theoretically. From a percolation model of hopping between localized states and a transistor model an analytic expression for the field-effect mobility is obtained. The theory is applied to describe the experiments by Brown et al. [Synth. Met. 88, 37 (1997)] on solution-processed amorphous organic transistors, made from a polymer (polythienylene vinylene) and from a small molecule (pentacene). Good agreement is obtained, with respect to both the gate voltage and the temperature dependence of the mobility.

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TL;DR: In this article, a comparative molecular dynamics simulation study of collision cascades in two elemental semiconductors and five fcc metals is performed to elucidate how different material characteristics affect primary defect production during ion irradiation.

Abstract: A comparative molecular dynamics simulation study of collision cascades in two elemental semiconductors and five fcc metals is performed to elucidate how different material characteristics affect primary defect production during ion irradiation. By using simulations of full 400 eV-10 keV collision cascades and contrasting the results on different materials with each other, we probe the effect of the mass, melting temperature, material strength, and crystal structure on the modification of the material due to the cascade. The results show that the crystal structure has a strong effect on many aspects of damage production, while other material characteristics are of lesser overall importance. In all materials studied, isolated point defects produced by the cascade are predominantly interstitials. In semiconductors, amorphous clusters are produced in the cascade core, whereas in metals most of the crystal regenerates, leaving only small vacancy-rich clusters. Large interstitial clusters found in a few events in the heavy metals were observed to form by the isolation of a high-density liquid zone during the recrystallization phase of a cascade.

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TL;DR: In this paper, the results of room-and low-temperature measurements of second-order Raman scattering for perfect GaN and AlN crystals as well as the Raman-scattering data for strongly disordered samples are presented.

Abstract: We present the results of room- and low-temperature measurements of second-order Raman scattering for perfect GaN and AlN crystals as well as the Raman-scattering data for strongly disordered samples. A complete group-theory analysis of phonon symmetry throughout the Brillouin zone and symmetry behavior of phonon branches, including the analysis of critical points, has been performed. The combined treatment of these results and the lattice dynamical calculations based on the phenomenological interatomic potential model allowed us to obtain the reliable data on the phonon dispersion curves and phonon density-of-states functions in bulk GaN and AlN. @S0163-1829~98!06840-4#

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TL;DR: In this article, a low-energy spin-fluctuation peak position shifts from ($\frac{1}{2}$ to ($ \frac{ 1}{2$\ifmmode\pm\else\textpm\fi{ \ensuremath{\delta}=x$, and the peak momentum width of the spin fluctuations at low energies is small throughout the superconducting concentration region.

Abstract: Systematic low-energy neutron-scattering studies have been performed on float-zone-grown single crystals of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ with $x$ extending from zero doping, $x=0$, to the overdoped, weakly superconducting regime, $x=025$ For $x$ beyond a critical doping value of ${x}_{c}\ensuremath{\approx}005$ the low-energy spin-fluctuation peak position shifts from ($\frac{1}{2}$, $\frac{1}{2}$) to ($\frac{1}{2}$\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}, $\frac{1}{2}$), and ($\frac{1}{2}$, $\frac{1}{2}$\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}); ${x}_{c}$ also represents the onset concentration for superconductivity For $006l~xl~012$ the incommensurability $\ensuremath{\delta}$ follows approximately the quantitative relation $\ensuremath{\delta}=x$ However, beyond $x\ensuremath{\approx}012$ the incommensurability tends to saturate around $\ensuremath{\delta}\ensuremath{\approx}1/8$ The superconducting-transition temperature ${T}_{c}(x)$ for stoichiometric samples at a given doping scales linearly with $\ensuremath{\delta}$ up to the optimal doping value of $x$ The peak momentum width of the spin fluctuations at low energies is small throughout the superconducting concentration region except in the strongly overdoped region An anomalously small width is observed for $x=\frac{1}{8}$ The incommensurate spatial modulation is found to be robust with respect to pair-breaking effects that lower ${T}_{c},$ such as deoxygenation of the sample or replacement of Cu by Zn

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TL;DR: In this paper, angular-resolved inverse photoemission spectroscopy (KRIPES) was used to monitor the progressive formation of crystalline graphite on silicon carbide surfaces.

Abstract: When annealed at elevated temperatures under vacuum, silicon carbide surfaces show a tendency towards graphitization. Using the sensitivity of empty conduction-band states dispersion towards the structural quality of the overlayer, we have used angular-resolved inverse photoemission spectroscopy (KRIPES) to monitor the progressive formation of crystalline graphite on $6H\ensuremath{-}\mathrm{SiC}(0001)$ surfaces. The KRIPES spectra obtained after annealing at 1400 \ifmmode^\circ\else\textdegree\fi{}C are characteristic of azimuthally oriented, graphite multilayers of very good single-crystalline quality. For lower annealing temperatures, the ordered interface already presents most of the fingerprints of graphite as soon as 1080 \ifmmode^\circ\else\textdegree\fi{}C. The observation of unshifted ${\ensuremath{\pi}}^{*}$ states, which reveals a very weak interaction with the substrate, is consistent with the growth of a van der Waals heteroepitaxial graphite lattice on top of silicon carbide, with a coincidence lattice of $(6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ symmetry. The growth of the first graphene sheet proceeds on top of adatoms characteristic of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction. These adatoms reduce the chemical reactivity of the substrate. A strong feature located at 6.5 eV above the Fermi level is attributed to states derived from Si vacancies in the C-rich subsurface layers of the SiC substrate. This strongly perturbed substrate can be viewed as a diamondlike phase which acts as a precursor to graphite formation by collapse of several layers. In this framework, previously published soft x-ray photoemission spectra find a natural explanation.

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IBM

^{1}TL;DR: In this article, the strength and effect of surface van der Waals forces on the shape of multiwalled and single-walled carbon nanotubes was investigated using atomic-force microscopy, continuum mechanics, and molecular-mechanics simulations.

Abstract: The strength and effect of surface van der Waals forces on the shape of multiwalled and single-walled carbon nanotubes is investigated using atomic-force microscopy, continuum mechanics, and molecular-mechanics simulations. Our calculations show that depending on the tube diameter and number of shells, the van der Waals interaction between nanotubes and a substrate results in high binding energies, which has also been determined experimentally. Nanotubes on a substrate may consequently experience radial and axial deformations, which significantly modify the idealized geometry of free nanotubes. These findings have implications for electronic transport and the tribological properties of adsorbed nanotubes.

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TL;DR: In this paper, an empirical tight-binding method for tetrahedrally coordinated cubic materials is presented and applied to group-IV and III-V semiconductors, and the method extends existing calculations by the inclusion of all five $d$ orbitals per atom in the basis set.

Abstract: An empirical tight-binding method for tetrahedrally coordinated cubic materials is presented and applied to group-IV and III-V semiconductors. The present ${\mathrm{spds}}^{*}$ method extends existing calculations by the inclusion of all five $d$ orbitals per atom in the basis set. On-site energies and two-center integrals between nearest neighbors in the Hamiltonian are fitted to measured energies, pseudopotential results, and the free-electron band structure. We demonstrate excellent agreement with pseudopotential calculations up to about 6 eV above the valence-band maximum even without inclusion of interactions with more distant atoms and three-center integrals. The symmetry character of the Bloch functions at the $X$ point is considerably improved by the inclusion of $d$ orbitals. Density of states, reduced masses, and deformation potentials are correctly reproduced.

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TL;DR: In this paper, the authors present a formula that relates the pumped current to the parametric derivatives of the scattering matrix of the system, and compute the statistical distribution of pumped current in the case of a chaotic quantum dot.

Abstract: A dc current can be pumped through a quantum dot by periodically varying two independent parameters ${X}_{1}$ and ${X}_{2},$ like a gate voltage or magnetic field. We present a formula that relates the pumped current to the parametric derivatives of the scattering matrix ${S(X}_{1}{,X}_{2})$ of the system. As an application we compute the statistical distribution of the pumped current in the case of a chaotic quantum dot.

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TL;DR: In this article, the lattice thermal conductivity of a quantum well limited by umklapp, impurity, and boundary scattering was investigated theoretically by taking into account dispersion of confined acoustic-phonon modes.

Abstract: Lattice thermal conductivity of a quantum well limited by umklapp, impurity, and boundary scattering was investigated theoretically by taking into account dispersion of confined acoustic-phonon modes. We show that strong modification of phonon group velocities due to spatial confinement leads to a significant increase in the phonon relaxation rates. From the numerical calculations, we predict a decrease by an order of magnitude of the lattice thermal conductivity in a 100-\AA{}-wide free-standing quantum well. Our theoretical results are consistent with recent experimental investigations of the lateral thermal conductivity of nitride/silicon/oxide membranes conducted in our group.

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TL;DR: In this paper, the authors demonstrate a dramatic reduction of the oscillator strength in quantum wells due to piezoelectric fields, showing a strong increase of the luminescence decay time of the dominating transition with increasing well width in parallel to a redshift of the emission peaks.

Abstract: We demonstrate a dramatic reduction of the oscillator strength in ${\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ quantum wells due to piezoelectric fields. Our study using time-resolved photoluminescence spectroscopy reveals a strong increase of the luminescence decay time of the dominating transition with increasing well width by several orders of magnitude in parallel to a redshift of the emission peaks. The experimental results are consistently explained by a quantitative model based on the piezoelectric fields in strained wurtzite quantum wells. We estimate the piezoelectric constant of GaN to ${d}_{31}=\ensuremath{-}0.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm/V.

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TL;DR: In this article, the phase diagram of a staged compound is calculated from first principles for x ranging from 0 to 1 and it is shown that there is a tendency for Li ordering at $x=\frac{1}{2}$ in agreement with experiment [J. N. Reimers and J. R. Dahn, 1992].

Abstract: In this work, the phase diagram of ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ is calculated from first principles for x ranging from 0 to 1. Our calculations indicate that there is a tendency for Li ordering at $x=\frac{1}{2}$ in agreement with experiment [J. N. Reimers and J. R. Dahn, J. Electrochem. Soc. 139, 2091 (1992)]. At low Li concentration, we find that a staged compound is stable in which the Li ions selectively segregate to every other Li plane leaving the remaining Li planes vacant. We do not find the two-phase region observed at high Li concentration and speculate that this two-phase region is caused by the metal-insulator transition that occurs at concentrations slightly below $x=1.$

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TL;DR: In this paper, the Langevin-dynamics approach was used to study the dynamics of magnetic nanoparticles, and the results were compared with different analytical expressions used to model the relaxation of nanoparticle ensembles, assessing their accuracy.

Abstract: The stochastic Landau-Lifshitz-Gilbert equation of motion for a classical magnetic moment is numerically solved (properly observing the customary interpretation of it as a Stratonovich stochastic differential equation), in order to study the dynamics of magnetic nanoparticles. The corresponding Langevin-dynamics approach allows for the study of the fluctuating trajectories of individual magnetic moments, where we have encountered remarkable phenomena in the overbarrier rotation process, such as crossing-back or multiple crossing of the potential barrier, rooted in the gyromagnetic nature of the system. Concerning averaged quantities, we study the linear dynamic response of the archetypal ensemble of noninteracting classical magnetic moments with axially symmetric magnetic anisotropy. The results are compared with different analytical expressions used to model the relaxation of nanoparticle ensembles, assessing their accuracy. It has been found that, among a number of heuristic expressions for the linear dynamic susceptibility, only the simple formula proposed by Shliomis and Stepanov matches the coarse features of the susceptibility reasonably. By comparing the numerical results with the asymptotic formula of Storonkin {Sov. Phys. Crystallogr. 30, 489 (1985) [Kristallografiya 30, 841 (1985)]}, the effects of the intra-potential-well relaxation modes on the low-temperature longitudinal dynamic response have been assessed, showing their relatively small reflection in the susceptibility curves but their dramatic influence on the phase shifts. Comparison of the numerical results with the exact zero-damping expression for the transverse susceptibility by Garanin, Ishchenko, and Panina {Theor. Math. Phys. (USSR) 82, 169 (1990) [Teor. Mat. Fiz. 82, 242 (1990)]}, reveals a sizable contribution of the spread of the precession frequencies of the magnetic moment in the anisotropy field to the dynamic response at intermediate-to-high temperatures.

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IBM

^{1}TL;DR: In this article, the authors measured the electrical characteristics and the efficiencies of single-layer organic light-emitting diodes based on poly[2methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV), with Au anodes and Ca, Al, and Au cathodes.

Abstract: We have measured the electrical characteristics and the efficiencies of single-layer organic light-emitting diodes based on poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV), with Au anodes and Ca, Al, and Au cathodes. We show that proper accounting of the built-in potential leads to a consistent description of the current-voltage data. For the case of Au and Al cathodes, the current under forward bias is dominated by holes injected from the anode and is space-charge limited with a field-dependent hole mobility. The Ca cathode is capable of injecting a space-charge-limited electron current.

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TL;DR: Using nonresonant bond-polarization theory, the Raman intensity of a single-wall carbon nanotube is calculated as a function of the polarization of light and the chirality of the carbon nanoteye as mentioned in this paper.

Abstract: Using nonresonant bond-polarization theory, the Raman intensity of a single-wall carbon nanotube is calculated as a function of the polarization of light and the chirality of the carbon nanotube. The force-constant tensor for calculating phonon dispersion relations in the nanotubes is scaled from those for two-dimensional graphite. The calculated Raman spectra do not depend much on the chirality, while their frequencies clearly depend on the nanotube diameter. The polarization and sample orientation dependence of the Raman intensity shows that the symmetry of the Raman modes can be obtained by varying the direction of the nanotube axis, keeping the polarization vectors of the light fixed.

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TL;DR: In this paper, the optimal geometries of carbon allotropes related to graphite, called graphyne, graphdiyne, graphyne-3, and Graphyne-4, as well as their electronic band structures were calculated using a full-potential linear combination of atomic orbitals method in the local density approximation.

Abstract: The optimized geometries of carbon allotropes related to graphite, called graphyne, graphdiyne, graphyne-3, and graphyne-4, as well as their electronic band structures were calculated using a full-potential linear combination of atomic orbitals method in the local-density approximation. These carbon allotropes consist of hexagons connected by linear carbon chains. The bond length of a hexagon is a little longer than that of the bond that links a hexagon to the outside carbon. Furthermore, part of the linear carbon chain is composed of acetylenic linkages (---C\ensuremath{\equiv}C---) rather than cumulative linkages (=C=C=). The binding energies are 7.95 eV/atom for graphyne and 7.78 eV/atom for graphdiyne, and the optimized lattice lengths are 6.86 \AA{} for graphyne and 9.44 \AA{} for graphdiyne. These materials are semiconductors with moderate band gaps. The band gap occurs at the M point or \ensuremath{\Gamma} point depending on the number of acetylenic linkages that are contained between the nearest-neighboring hexagons. The effective masses are very small for both conduction and valence bands.

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TL;DR: In this article, a simple approach for infrequent-event systems that extends the time scale with high parallel efficiency is presented, where a replica of the system independently on each processor until the first transition occurs gives the correct transition-time distribution and hence the correct dynamics.

Abstract: Although molecular-dynamics simulations can be parallelized effectively to treat large systems (10{sup 6}{endash}10{sup 8} atoms), to date the power of parallel computers has not been harnessed to make analogous gains in {ital time} scale. I present a simple approach for infrequent-event systems that extends the time scale with high parallel efficiency. Integrating a replica of the system independently on each processor until the first transition occurs gives the correct transition-time distribution, and hence the correct dynamics. I obtain {gt}90{percent} efficiency simulating Cu(100) surface vacancy diffusion on 15 processors. {copyright} {ital 1998} {ital The American Physical Society}

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TL;DR: In this article, an integrated classical and quantum-mechanical theory of weak microcavity effects in layered media that treats both radiative and wave-guided modes is presented, with the transition probability into each mode given by Fermi's ''golden rule''.

Abstract: We present an integrated classical and quantum-mechanical theory of weak microcavity effects in layered media that treats both radiative and waveguided modes. The electromagnetic field of radiative modes is determined using classical field quantization, with the transition probability into each mode given by Fermi's ``golden rule.'' We apply this theory to model the dependence of the electroluminescence spectral intensity and polarization of organic light-emitting devices (OLED's) on emission angle, organic layer thickness, and applied voltage. Light propagation in the OLED layers and the substrate is described by both ray and wave optics. Theoretical predictions are compared to experimental observations on single heterostructure, and multiple layer stacked red-green-blue OLEDs. Analysis of the polarization, spectral shape, and intensity of the electroluminescence spectrum in the forward-scattered half plane accurately fits the experimental data. The theory predicts, and the experimental measurements confirm, that the in-plane emission from conventional OLED structures is strongly TM polarized, and can be redshifted by as much as 60 nm with respect to the peak emission in the normal direction. Measurements coupled to our analysis also indicate that the efficiency of generating singlet excitons in aluminum tris(8-hydroxyquinoline) $({\mathrm{Alq}}_{3})$-based OLED's is $5\ifmmode\pm\else\textpm\fi{}1%,$ with a \ensuremath{\sim}500-\AA{}-thick ${\mathrm{Alq}}_{3}$ layer corresponding to the highest external power efficiency.

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TL;DR: Seebeck et al. as discussed by the authors measured the impact of partial void filling on the lattice thermal conductivity of polycrystalline antimonides with the skutterudite crystal structure with La partially filling the voids.

Abstract: Polycrystalline samples of antimonides with the skutterudite crystal structure with La partially filling the voids have been prepared in an effort to quantify the impact of partial void filling on the lattice thermal conductivity of these compounds. It is observed that a relatively small concentration of La in the voids results in a relatively large decrease in the lattice thermal conductivity. In addition, the largest decrease in the lattice thermal conductivity, compared to ‘‘unfilled’’ CoSb 3 is not observed near 100% filling of the voids with La, as was previously believed. This suggests a point-defect-type phonon scattering effect due to the partial, random distribution of La in the voids as well as the ‘‘rattling’’ effect of the La ions, resulting in the scattering of a larger spectrum of phonons than in the case of 100% filling. An additional benefit of partial filling in thermoelectric materials is that it may be one way of adjusting the electronic properties of these compounds. Seebeck, resistivity, Hall effect and structural data for these skutterudite compounds are also presented. @S0163-1829~98!02926-9#