Showing papers in "Physical Review B in 1996"
TL;DR: An efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set is presented and the application of Pulay's DIIS method to the iterative diagonalization of large matrices will be discussed.
Abstract: We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order ${\mathit{N}}_{\mathrm{atoms}}^{3}$ operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ``metric'' and a special ``preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order ${\mathit{N}}_{\mathrm{atoms}}^{2}$ scaling is found for systems containing up to 1000 electrons. If we take into account that the number of k points can be decreased linearly with the system size, the overall scaling can approach ${\mathit{N}}_{\mathrm{atoms}}$. We have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable. \textcopyright{} 1996 The American Physical Society.
81,985 citations
TL;DR: The hole model provides a more detailed test of these energy functionals, and also predicts the observable electron-electron structure factor.
Abstract: We construct a generalized gradient approximation (GGA) for the density ${\mathit{n}}_{\mathrm{xc}}$(r,r+u) at position r+u of the exchange-correlation hole surrounding an electron at r, or more precisely for its system and spherical average 〈${\mathit{n}}_{\mathrm{xc}}$(u)〉=(4\ensuremath{\pi}${)}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\int}d${\mathrm{\ensuremath{\Omega}}}_{\mathit{u}}$ ${\mathit{N}}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\int}${\mathit{d}}^{3}$r n(r)${\mathit{n}}_{\mathrm{xc}}$(r,r+u). Starting from the second-order density gradient expansion, which involves the local spin densities ${\mathit{n}}_{\mathrm{\ensuremath{\uparrow}}}$(r),${\mathit{n}}_{\mathrm{\ensuremath{\downarrow}}}$(r) and their gradients \ensuremath{
abla}${\mathit{n}}_{\mathrm{\ensuremath{\uparrow}}}$(r),\ensuremath{
abla}${\mathit{n}}_{\mathrm{\ensuremath{\downarrow}}}$(r), we cut off the spurious large-u contributions to restore those exact conditions on the hole that the local spin density (LSD) approximation respects. Our GGA hole recovers the Perdew-Wang 1991 and Perdew-Burke-Ernzerhof GGA's for the exchange-correlation energy, which therefore respect the same powerful hole constraints as LSD. When applied to real systems, our hole model provides a more detailed test of these energy functionals, and also predicts the observable electron-electron structure factor. \textcopyright{} 1996 The American Physical Society.
5,341 citations
TL;DR: The pseudopotential is of an analytic form that gives optimal efficiency in numerical calculations using plane waves as a basis set and is separable and has optimal decay properties in both real and Fourier space.
Abstract: We present pseudopotential coefficients for the first two rows of the Periodic Table. The pseudopotential is of an analytic form that gives optimal efficiency in numerical calculations using plane waves as a basis set. At most, seven coefficients are necessary to specify its analytic form. It is separable and has optimal decay properties in both real and Fourier space. Because of this property, the application of the nonlocal part of the pseudopotential to a wave function can be done efficiently on a grid in real space. Real space integration is much faster for large systems than ordinary multiplication in Fourier space, since it shows only quadratic scaling with respect to the size of the system. We systematically verify the high accuracy of these pseudopotentials by extensive atomic and molecular test calculations. \textcopyright{} 1996 The American Physical Society.
5,009 citations
TL;DR: In this paper, the interaction between spin waves and itinerant electrons is considerably enhanced in the vicinity of an interface between normal and ferromagnetic layers in metallic thin films, leading to a local increase of the Gilbert damping parameter which characterizes spin dynamics.
Abstract: The interaction between spin waves and itinerant electrons is considerably enhanced in the vicinity of an interface between normal and ferromagnetic layers in metallic thin films. This leads to a local increase of the Gilbert damping parameter which characterizes spin dynamics. When a dc current crosses this interface, stimulated emission of spin waves is predicted to take place. Beyond a certain critical current density, the spin damping becomes negative; a spontaneous precession of the magnetization is predicted to arise. This is the magnetic analog of the injection laser. An extra dc voltage appears across the interface, given by an expression similar to that for the Josephson voltage across a superconducting junction. \textcopyright{} 1996 The American Physical Society.
4,433 citations
TL;DR: It is found that a non-negligible edge state survives even in graphene ribbons with less developed zigzag edges, when the system size is on a nanometer scale.
Abstract: Finite graphite systems having a zigzag edge exhibit a special edge state. The corresponding energy bands are almost flat at the Fermi level and thereby give a sharp peak in the density of states. The charge density in the edge state is strongly localized on the zigzag edge sites. No such localized state appears in graphite systems having an armchair edge. By utilizing the graphene ribbon model, we discuss the effect of the system size and edge shape on the special edge state. By varying the width of the graphene ribbons, we find that the nanometer size effect is crucial for determining the relative importance of the edge state. We also have extended the graphene ribbon to have edges of a general shape, which is defined as a mixture of zigzag and armchair sites. Examining the relative importance of the edge state for graphene ribbons with general edges, we find that a non-negligible edge state survives even in graphene ribbons with less developed zigzag edges. We demonstrate that such an edge shape with three or four zigzag sites per sequence is sufficient to show an edge state, when the system size is on a nanometer scale. The special characteristics of the edge state play a large role in determining the density of states near the Fermi level for graphite networks on a nanometer scale.
3,834 citations
TL;DR: A method to perform fully self-consistent density-functional calculations that scales linearly with the system size and which is well suited for very large systems is presented, using strictly localized pseudoatomic orbitals as basis functions.
Abstract: We present a method to perform fully self-consistent density-functional calculations that scales linearly with the system size and which is well suited for very large systems. It uses strictly localized pseudoatomic orbitals as basis functions. The sparse Hamiltonian and overlap matrices are calculated with an $O(N)$ effort. The long-range self-consistent potential and its matrix elements are computed in a real-space grid. The other matrix elements are directly calculated and tabulated as a function of the interatomic distances. The computation of the total energy and atomic forces is also done in $O(N)$ operations using truncated, Wannier-like localized functions to describe the occupied states, and a band-energy functional which is iteratively minimized with no orthogonality constraints. We illustrate the method with several examples, including carbon and silicon supercells with up to 1000 Si atoms and supercells of $\ensuremath{\beta}$-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$. We apply the method to solve the existing controversy about the faceting of large icosahedral fullerenes by performing dynamical simulations on ${\mathrm{C}}_{60}$, ${\mathrm{C}}_{240}$, and ${\mathrm{C}}_{540}$.
2,236 citations
TL;DR: A simple model is described to explain the recombination in these devices, and how the absorption, charge separation, and transport properties of the composites can be controlled by changing the size, material, and surface ligands of the nanocrystals.
Abstract: We study the processes of charge separation and transport in composite materials formed by mixing cadmium selenide or cadmium sulfide nanocrystals with the conjugated polymer poly(2-methoxy,5-(2\ensuremath{'}-ethyl)-hexyloxy-$p$-phenylenevinylene) (MEH-PPV). When the surface of the nanocrystals is treated so as to remove the surface ligand, we find that the polymer photoluminescence is quenched, consistent with rapid charge separation at the polymer/nanocrystal interface. Transmission electron microscopy of these quantum-dot/conjugated-polymer composites shows clear evidence for phase segregation with length scales in the range 10-200 nm, providing a large area of interface for charge separation to occur. Thin-film photovoltaic devices using the composite materials show quantum efficiencies that are significantly improved over those for pure polymer devices, consistent with improved charge separation. At high concentrations of nanocrystals, where both the nanocrystal and polymer components provide continuous pathways to the electrodes, we find quantum efficiencies of up to 12%. We describe a simple model to explain the recombination in these devices, and show how the absorption, charge separation, and transport properties of the composites can be controlled by changing the size, material, and surface ligands of the nanocrystals.
1,517 citations
TL;DR: A decreasing threshold fluence is found associated with a gradual transition from the long-pulse, thermally dominated regime to an ablative regime dominated by collisional and multiphoton ionization, and plasma formation.
Abstract: We report extensive laser-induced damage threshold measurements on dielectric materials at wavelengths of 1053 and 526 nm for pulse durations $\ensuremath{\tau}$ ranging from 140 fs to 1 ns. Qualitative differences in the morphology of damage and a departure from the diffusion-dominated ${\ensuremath{\tau}}^{\frac{1}{2}}$ scaling of the damage fluence indicate that damage occurs from ablation for $\ensuremath{\tau}l~10$ ps and from conventional melting, boiling, and fracture for $\ensuremath{\tau}g50$ ps. We find a decreasing threshold fluence associated with a gradual transition from the long-pulse, thermally dominated regime to an ablative regime dominated by collisional and multiphoton ionization, and plasma formation. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in quantitative agreement with the experimental results.
1,435 citations
TL;DR: The observed shortening of the luminescence decay time in CdSe nanoncrystals in a magnetic field is in excellent agreement with the theory, giving further support to the validity of the model.
Abstract: We present a theoretical analysis of the band-edge exciton structure in nanometer-size crystallites of direct semiconductors with a cubic lattice structure or a hexagonal lattice structure which can be described within the framework of a quasicubic model. The lowest energy exciton, eightfold degenerate in spherically symmetric dots, is split into five levels by the crystal shape asymmetry, the intrinsic crystal field (in hexagonal lattice structures), and the electron-hole exchange interaction. Transition oscillator strengths and the size dependence of the splittings have been calculated. Two of the five states, including the ground state, are optically passive (dark excitons). The oscillator strengths of the other three levels (bright excitons) depend strongly on crystal size, shape, and energy band parameters. The relative ordering of the energy levels is also heavily influenced by these parameters. The distance between the first optically active state and the optically forbidden ground exciton state increases with decreasing size, leading to an increase of the Stokes shift in the luminescence. Our results are in good agreement with the size dependence of Stokes shifts obtained in fluorescence line narrowing and photoluminescence experiments in CdSe nanocrystals. Mixing of the dark and bright excitons in an external magnetic field allows the direct optical recombination of the dark exciton ground state. The observed shortening of the luminescence decay time in CdSe nanoncrystals in a magnetic field is also in excellent agreement with the theory, giving further support to the validity of our model. \textcopyright{} 1996 The American Physical Society.
1,180 citations
TL;DR: It is shown that the techniques described in this paper may be usefully applied in the field of solid state physics.
Abstract: Ab initio plane-wave electronic structure calculations are widely used in the study of bulk materials. A technique for the projection of plane-wave states onto a localized basis set is used to calculate atomic charges and bond populations by means of Mulliken analysis. We analyze a number of simple bulk crystals and find correlations of overlap population with covalency of bonding and bond strength, and effective valence charge with ionicity of bonding. Thus, we show that the techniques described in this paper may be usefully applied in the field of solid state physics. \textcopyright{}1996 The American Physical Society.
1,056 citations
TL;DR: The size evolution of ten quantum dot absorption features is followed, and the size-dependent data exhibit two strong avoided crossings, demonstrating the importance of valence-band structure in the description of the excited states.
Abstract: We use photoluminescence excitation to study the electronic spectrum of CdSe quantum dots ranging from \ensuremath{\sim}12 to \ensuremath{\sim}53 \AA{} in radius. We follow the size evolution of ten quantum dot absorption features. Comparison of the spectra with theoretical predictions allows us to confidently assign six of these transitions. We discuss the most likely assignments for the remaining four. We find that the ${\mathit{n}}_{\mathit{hS}3/2}$1${\mathit{S}}_{\mathit{e}}$ and ${\mathit{n}}_{\mathit{hS}1/2}$1${\mathit{S}}_{\mathit{e}}$ transitions dominate the spectra, accounting for half of the observed features. Our size-dependent data exhibit two strong avoided crossings, demonstrating the importance of valence-band structure in the description of the excited states. \textcopyright{} 1996 The American Physical Society.
TL;DR: The corresponding generalized Kohn-Sham eigenvalue gaps are shown to incorporate part of the discontinuity D xc of the exchange-correlation potential of standard KohnSham theory, leading to band gaps far better than those of local-density approximation.
Abstract: As an alternative to the standard Kohn-Sham procedure, other exact realizations of density-functional theory ~generalized Kohn-Sham methods! are presented. The corresponding generalized Kohn-Sham eigenvalue gaps are shown to incorporate part of the discontinuity D xc of the exchange-correlation potential of standard KohnSham theory. As an example, a generalized Kohn-Sham procedure splitting the exchange contribution to the total energy into a screened, nonlocal and a local density component is considered. This method leads to band gaps far better than those of local-density approximation and to good structural properties for the materials Si, Ge, GaAs, InP, and InSb.
TL;DR: In this paper, the authors derived the effective mass Hamiltonian for wurtzite semiconductors, including the strain effects, using the k-ensuremath{\cdot}p perturbation method, which is then checked with that derived using an invariant method based on the Pikus-Bir model.
Abstract: We derive the effective-mass Hamiltonian for wurtzite semiconductors, including the strain effects. This Hamiltonian provides a theoretical groundwork for calculating the electronic band structures and optical constants of bulk and quantum-well wurtzite semiconductors. We apply Kane's model to derive the band-edge energies and the optical momentum-matrix elements for strained wurtzite semiconductors. We then use the k\ensuremath{\cdot}p perturbation method to derive the effective-mass Hamiltonian, which is then checked with that derived using an invariant method based on the Pikus-Bir model. We obtain the band structure ${\mathit{A}}_{\mathit{i}}$ parameters in the group theoretical model explicitly in terms of the momentum-matrix elements. We also find the proper definitions of the important physical quantities used in both models and present analytical expressions for the valence-band dispersions, the effective masses, and the interband optical-transition momentum-matrix elements near the band edges, taking into account the strain effects. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this article, the results of an investigation into the grain-size dependence of lattice structure for barium titanate (BaTiO) ceramics prepared by a sol-gel method were reported.
Abstract: We report the results of an investigation into the grain-size dependence of lattice structure for barium titanate (${\mathrm{BaTiO}}_{3}$) ceramics prepared by a sol-gel method. Raman and infrared spectroscopy, x-ray diffraction, and differential scanning calorimetry were used in combination with electron microscopy to study the evolution of lattice structure and phase transformation behavior with heat treatment and grain growth from the nano scale to the micron scale for ${\mathrm{BaTiO}}_{3}$ polycrystals. Raman spectroscopy and optical second-harmonic-generation measurements indicated the onset of local room-temperature acentric crystal symmetry with heat treatment and crystallite growth, well before the observation of any tetragonal structure by x-ray diffraction. Analysis of the room-temperature Raman spectra for ultrafine grain (grain size 0.1 \ensuremath{\mu}m) polycrystals suggested that a locally orthorhombic structure preceded the globally tetragonal form with grain growth. In support of this observation, differential scanning calorimetry suggested the orthorhombic-tetragonal phase transformation shifts up through room temperature with decreasing grain size. Hot-stage transmission electron microscopy studies revealed that fine grain (grain size \ensuremath{\approxeq}0.1 \ensuremath{\mu}m) ceramics, which showed a thermal anomaly associated with the cubic-tetragonal phase transformation, were untwinned at room temperature, as well as on cycling through the normal Curie temperature, suggesting a single-domain state for individual grains. The findings are discussed in light of a number of possible causes, including the presence of processing-related hydroxyl defects and the effect of elastic constraints on phase transformation behavior for ${\mathrm{BaTiO}}_{3}$ grains in a polycrystalline microstructure. \textcopyright{} 1996 The American Physical Society.
TL;DR: Stochichiometric variations in the GaN thin films together with the design of specific buffer layers can be utilized to strain engineer the material to an extent that greatly exceeds the possibilities known from other semiconductor systems because of the largely different covalent radii of the Ga and the N atom.
Abstract: Photoluminescence (PL), Raman spectroscopy, and x-ray diffraction are employed to demonstrate the co-existence of a biaxial and a hydrostatic strain that can be present in GaN thin films. The biaxial strain originates from growth on lattice-mismatched substrates and from post-growth cooling. An additional hydrostatic strain is shown to be introduced by the presence of point defects. A consistent description of the experimental results is derived within the limits of the linear and isotropic elastic theory using a Poisson ratio $\ensuremath{
u}=0.23\ifmmode\pm\else\textpm\fi{}0.06$ and a bulk modulus $B=200\ifmmode\pm\else\textpm\fi{}20$ GPa. These isotropic elastic constants help to judge the validity of published anisotropic elastic constants that vary greatly. Calibration constants for strain-induced shifts of the near-band-edge PL lines with respect to the ${E}_{2}$ Raman mode are given for strain-free, biaxially strained, and hydrostatically contracted or expanded thin films. They allow us to extract differences between hydrostatic and biaxial stress components if present. In particular, we determine that a biaxial stress of one GPa would shift the near-band-edge PL lines by 27\ifmmode\pm\else\textpm\fi{}2 meV and the ${E}_{2}$ Raman mode by 4.2\ifmmode\pm\else\textpm\fi{}0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ by use of the listed isotropic elastic constants. It is expected from the analyses that stoichiometric variations in the GaN thin films together with the design of specific buffer layers can be utilized to strain engineer the material to an extent that greatly exceeds the possibilities known from other semiconductor systems because of the largely different covalent radii of the Ga and the N atom.
TL;DR: The ferromagnetic-metal-paramagnetic-insulator transition temperature varies as ${T}_m}={T}_{m}(0)\ensuremath{-}p{Q}^{2}$ due to strain fields resulting from ordered or disordered oxygen displacements $Q$ that are parametrized by the statistical mean and variance of the $A$ cation radius, respectively.
Abstract: Large disorder effects due to size differences between $A$-site ${R}^{3+}$ ($R=\mathrm{L}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{P}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{N}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{m}$) and ${M}^{2+} (M=\mathrm{C}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{B}\mathrm{a})$ cations have been found in magnetoresistive $({R}_{0.7}{M}_{0.3})\mathrm{Mn}{\mathrm{O}}_{3}$ perovskites. The ferromagnetic-metal-paramagnetic-insulator transition temperature ${T}_{m}$ varies as ${T}_{m}={T}_{m}(0)\ensuremath{-}p{Q}^{2}$ due to strain fields resulting from ordered or disordered oxygen displacements $Q$ that are parametrized by the statistical mean and variance of the $A$ cation radius, respectively. The value of $p$ is related to the Mn-O force constant showing that ${\mathrm{Mn}}^{3+}$ Jahn-Teller distortions assist electron localization at ${T}_{m}$. The maximum possible ${T}_{m}$ is estimated to be \ensuremath{\sim}530 K although experimentally observable values are \ensuremath{\le}360 K. A large suppression of magnetoresistance due to cation disorder is also evidenced.
TL;DR: Two different types of amorphous carbon films were deposited on Si substrates, with film hardness of 22 GPa and 40 GPa, by pulsed laser evaporation of graphite targets, and the core level of the C1{ital s} core level shifted toward lower binding energy in the films for annealing temperatures above 900 K.
Abstract: Two different types of amorphous carbon films were deposited on Si substrates, with film hardness of 22 GPa and 40 GPa, by pulsed laser evaporation of graphite targets. The x-ray photoemission spectra (XPS) of the C 1s core level in these films shown two components at 284.3\ifmmode\pm\else\textpm\fi{}0.1 eV and 285.2\ifmmode\pm\else\textpm\fi{}0.1 eV, which were identified with the ${\mathit{sp}}^{2}$ and ${\mathit{sp}}^{3}$ hybrids forms of carbon. The ${\mathit{sp}}^{3}$/${\mathit{sp}}^{2}$ concentration ratio deduced from the area of the components had a value of 2/5 for the harder amorphous carbon film and 1/4 for the softer. Upon annealing the harder film at different temperatures, the ${\mathit{sp}}^{3}$/${\mathit{sp}}^{2}$ ratio remained nearly constant up to about 900 K and then decreased until reaching a value of zero above 1100 K. The C 1s core level shifted 0.3\ifmmode\pm\else\textpm\fi{}0.1 eV toward lower binding energy in the films for annealing temperatures above 900 K. This shift was correlated with an increase in the asymmetry of the C 1s XPS spectra and of the density of states at the Fermi level, as observed by ultraviolet photoemission spectroscopy. There was no detectable \ensuremath{\pi} plasmon in the harder films below 900 K, despite the presence of ${\mathit{sp}}^{2}$ atoms and \ensuremath{\pi} bonds at those temperatures. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this article, the authors studied the dipole response of atomic clusters by solving the equations of the time-dependent local density approximation in real-time and showed that pseudopotential effects are strong in lithium and act to broaden the Mie resonance and give it a substantial redshift.
Abstract: We study the dipole response of atomic clusters by solving the equations of the time-dependent local-density approximation in real time. The method appears to be more efficient than matrix or Green's function methods for large clusters modeled with realistic ionic pseudopotentials. As applications of the method, we exhibit results for sodium and lithium clusters and for ${\mathrm{C}}_{60}$ molecules. The calculated Mie resonance in ${\mathrm{Na}}_{147}$ is practically identical to that obtained in the jellium approximation, leaving the origin of the redshift unresolved. The pseudopotential effects are strong in lithium and act to broaden the Mie resonance and give it a substantial redshift, confirming earlier studies. There is also a large broadening due to Landau damping in the calculated ${\mathrm{C}}_{60}$ response, again confirming earlier studies. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this paper, it was shown spectroscopically that electronic energy transfer in close-packed CdSe quantum-dot (QD) solids arises from dipole-dipole interdot interactions between proximal dots.
Abstract: We show spectroscopically that electronic energy transfer in close-packed CdSe quantum-dot (QD) solids arises from dipole-dipole interdot interactions between proximal dots. We use cw and time-resolved photoluminescence to study electronic energy transfer in optically thin and clear, close-packed QD solids prepared from CdSe QD samples tunable from 17 to 150 \AA{} in diameter (\ensuremath{\sigma}4.5%). High-resolution scanning electron microscopy and small-angle x-ray scattering are used to build a well-defined structural model for the QD solids. In mixed QD solids of small and large dots, we measure quenching of the luminescence (lifetime) of the small dots accompanied by enhancement of the luminescence (lifetime) of the large dots consistent with electronic energy transfer from the small to the large dots. In QD solids of single size dots, a redshifted and modified emission line shape is consistent with electronic energy transfer within the sample inhomogeneous distribution. We use F\"orster theory for long-range resonance transfer through dipole-dipole interdot interactions to explain electronic energy transfer in these close-packed QD solids. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this article, it has been shown theoretically that it may be possible to increase the thermoelectric figure of merit (Z$) of certain materials by preparing them in the form of two-dimensional quantum-well structures.
Abstract: Recently, it has been shown theoretically that it may be possible to increase the thermoelectric figure of merit ($Z$) of certain materials by preparing them in the form of two-dimensional quantum-well structures. Using $\mathrm{PbTe}/{\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Te}$ multiple-quantum-well structures grown by molecular-beam epitaxy, we have performed thermoelectric and other transport measurements as a function of quantum-well thickness and doping. Our results are found to be consistent with theoretical predictions and indicate that an increase in $Z$ over bulk values may be possible through quantum confinement effects using quantum-well structures.
TL;DR: Field-induced insulator-to-metal transitions have been found in single crystals, which are accompanied by a melting of the insulating charge-ordered state, which is argued in terms of the effect of discommensuration of the charge concentration on the charge-ordering state.
Abstract: Field-induced insulator-to-metal transitions have been found in ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}(0.3l~xl~0.5)$ single crystals, which are accompanied by a melting of the insulating charge-ordered (i.e., ${\mathrm{Mn}}^{3+}$/${\mathrm{Mn}}^{4+}$ ordered) state. The transition is of the first order with a hysteresis and is even irreversible at low temperatures. The $x$-dependent metal-insulator phase diagrams in the $H\ensuremath{-}T$ plane indicate that a deviation of $x$ from 0.5 modifies the robustness of the charge-ordering state, which is argued in terms of the effect of discommensuration of the charge concentration on the charge-ordered state.
TL;DR: In this article, the electronic structure of the perovskite LaCoO 3$ for different spin states of Co ions was calculated in the local density approximation LDA+U approach.
Abstract: The electronic structure of the perovskite ${\mathrm{LaCoO}}_{3}$ for different spin states of Co ions was calculated in the local-density approximation LDA+U approach The ground state is found to be a nonmagnetic insulator with Co ions in a low-spin state Somewhat higher in energy, we find two intermediate-spin states followed by a high-spin state at significantly higher energy The calculations show that Co 3d states of ${\mathit{t}}_{2\mathit{g}}$ symmetry form narrow bands which could easily localize, while ${\mathit{e}}_{\mathit{g}}$ orbitals, due to their strong hybridization with the oxygen 2p states, form a broad \ensuremath{\sigma}* band With temperature variation which is simulated by a corresponding change of the lattice parameters, a transition from the low- to intermediate-spin state occurs This intermediate-spin (occupation ${\mathit{t}}_{2\mathit{g}}^{5}$${\mathit{e}}_{\mathit{g}}^{1}$) can develop an orbital ordering which can account for the nonmetallic nature of ${\mathrm{LaCoO}}_{3}$ at 90 KT500 K Possible explanations of the magnetic behavior and gradual insulator-metal transition are suggested \textcopyright{} 1996 The American Physical Society
TL;DR: First-principles full-potential linear muffin-tin orbital calculations of the elastic constants and related structural and electronic properties of BN, AlN, GaN, and InN in both the zinc-blende and wurtzite structures are presented.
Abstract: The results of first-principles full-potential linear muffin-tin orbital calculations of the elastic constants and related structural and electronic properties of BN, AlN, GaN, and InN in both the zinc-blende and wurtzite structures are presented. The results include all of the equilibrium lattice constants, the bulk moduli, the TO-phonon frequencies at \ensuremath{\Gamma}, their mode Gr\"uneisen parameters, the full set of cubic elastic constants, and deformation potentials. The elastic constants for the wurtzite crystals are first obtained from those calculated for zinc blende by Martin's transformation method. The components related to strains along the c axis (${\mathit{C}}_{13}$ and ${\mathit{C}}_{33}$) are found to be less accurate than the others. An elaboration of Martin's approach utilizing first-principles calculation for distortions which maintains hexagonal symmetry but allows for a nonideal c/a ratio is implemented. As a byproduct of the relaxation calculations of the wurtzite internal parameter u we also obtain the ${\mathit{A}}_{1}$ and an estimate of the ${\mathit{E}}_{1}$ TO-phonon frequencies in the hexagonal materials. Good agreement is obtained with recent experimental results for the elastic constants of wurtzite AlN and GaN and zinc-blende BN as well as for the other properties mentioned above for all materials. Our results provide predictions for the remaining crystal structure materials combinations for which no direct experimental data are presently available. From these results and experimental LO-TO splittings, we determine the bond-stretching and bond-bending parameters \ensuremath{\alpha} and \ensuremath{\beta} of Keating's semiempirical valence-force-field model. We use this model to rationalize some of the observed trends in the behavior with the cation. The shift and splittings of the energy bands due to strains are used to obtain a complete set of deformation potentials for the zinc-blende crystals at symmetry points for several of the important eigenvalues. We also define deformation potentials for the valence-band maximum of the wurtzite structure and relate them to the corresponding [111] strain deformation and optical mode deformation potentials in zinc blende. \textcopyright{} 1996 The American Physical Society.
TL;DR: The limiting low and high temperature resistivities place a limit on the maximum possible magnetoresistance of these materials and may explain why the "colossal" magnetores resistance reported in the literature correlates with the suppression of ${T}_{C}$.
Abstract: An investigation designed to display the intrinsic properties of perovskite manganites was accomplished by comparing the behavior of bulk samples with that of thin films. Epitaxial 1500 \AA{} films of perovskite ${\mathrm{La}}_{0.67}$${\mathrm{Ca}}_{0.33}$Mn${\mathrm{O}}_{3}$ and ${\mathrm{La}}_{0.67}$${\mathrm{Sr}}_{0.33}$Mn${\mathrm{O}}_{3}$ were grown by solid source chemical vapor deposition on LaAl${\mathrm{O}}_{3}$ and post annealed in oxygen at 950 \ifmmode^\circ\else\textdegree\fi{}C. Crystals were prepared by laser heated pedestal growth. The magnetic and electrical transport properties of the polycrystalline pellets, crystals, and annealed films are essentially the same. Below $\frac{{T}_{C}}{2}$ the intrinsic magnetization decreases as ${T}^{2}$ (as can be expected for itinerant electron ferromagnets) while the intrinsic resistivity increases proportional to ${T}^{2}$. The constant and ${T}^{2}$ coefficients of the resistivity are largely independent of magnetic field and alkaline earth element (Ca, Sr, or Ba). Hall effect measurements indicate that holes are mobile carriers in the metallic state. We identify three distinct types of negative magnetoresistance. The largest effect, observed near the Curie temperature, is 25% for the Sr and 250% [$\frac{\ensuremath{\Delta}R}{R(H)}$] for the Ca compound. There is also magnetoresistance associated with the net magnetization of polycrystalline samples which is not seen in films. Finally a small magnetoresistance linear in $H$ is observed even at low temperatures. The high temperature (above ${T}_{C}$) resistivity of ${\mathrm{La}}_{0.67}$${\mathrm{Ca}}_{0.33}$Mn${\mathrm{O}}_{3}$ is consistent with small polaron hopping conductivity with a slight transition at 750 K, while ${\mathrm{La}}_{0.67}$${\mathrm{Sr}}_{0.33}$Mn${\mathrm{O}}_{3}$ does not exhibit activated conductivity until about 500 K, well above ${T}_{C}$. The limiting low and high temperature resistivities place a limit on the maximum possible magnetoresistance of these materials and may explain why the "colossal" magnetoresistance reported in the literature correlates with the suppression of ${T}_{C}$.
TL;DR: Strong local environment effects arising from neighboring cation charge differences that suggest localization of the low density of minority carriers, leading to effective half-metallic ferromagnetism in the CMR regime are identified.
Abstract: Possible origins of ``colossal magnetoresistance'' (CMR) behavior in the ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ system are studied using the local spin-density method. These calculations allow the quantification of the effects of Mn d--O p hybridization that have been largely neglected in previously published work. As regards the end-point compounds ${\mathrm{CaMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$, the very different structural and magnetic symmetries of their ground states are predicted correctly. The distortion from the cubic perovskite structure of the ${\mathrm{LaMnO}}_{3}$ lattice is necessary to produce an antiferromagnetic insulating ground state. The distortion also strengthens the Mn magnetic moments. Application to ferromagnetic and constrained ferrimagnetic phases of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ in the CMR regime x\ensuremath{\approxeq}1/4--1/3 suggests, as observed, that magnetic coupling switches from antiferromagnetic to ferromagnetic. Hybridization between Mn d states and O p states is found to be strongly spin dependent, because the majority Mn d bands overlap the O p bands while the minority Mn d bands are separated by a gap from the O p bands. Both ferromagnetic and ferrimagnetic orderings are obtained and compared. We identify strong local environment effects arising from neighboring cation charge differences (${\mathrm{La}}^{3+}$ or ${\mathrm{Ca}}^{2+}$) that suggest localization of the low density of minority carriers, leading to effective half-metallic ferromagnetism in the CMR regime. This behavior supports in some respects the popular ``double exchange'' picture of Zener but indicates the Mn d--O p hybridization is much too strong to be considered perturbatively. Half-metallic character promotes the possibility of very large magnetoresistance, and may well be an essential ingredient in the CMR effect.
TL;DR: Using molecular-dynamics computer simulations, it is demonstrated that the cooling-rate dependence of microscopic quantities is significantly more pronounced than the one of macroscopic properties, thus demonstrating that the used potential is a good model for silica glass.
Abstract: Using molecular-dynamics computer simulations we investigate how in silica the glass transition and the properties of the resulting glass depend on the cooling rate with which the sample is cooled. By coupling the system to a heat bath with temperature ${\mathit{T}}_{\mathit{b}}$(t), we cool the system linearly in time, ${\mathit{T}}_{\mathit{b}}$(t)=${\mathit{T}}_{\mathit{i}}$-\ensuremath{\gamma}t, where \ensuremath{\gamma} is the cooling rate. In qualitative accordance with experiments, the temperature dependence of the density shows a local maximum, which becomes more pronounced with decreasing cooling rate. We find that the glass transition temperature ${\mathit{T}}_{\mathit{g}}$ is in accordance with a logarithmic dependence on \ensuremath{\gamma}. The enthalpy, density, and thermal expansion coefficient for the glass at zero temperature decrease with decreasing \ensuremath{\gamma}. We show that also microscopic quantities, such as the radial distribution function, the bond-bond angle distribution function, the coordination numbers, and the distribution function for the size of the rings, depend significantly on \ensuremath{\gamma}. We demonstrate that the cooling-rate dependence of these microscopic quantities is significantly more pronounced than the one of macroscopic properties. Furthermore, we show that these microscopic quantities, as determined from our simulation, are in good agreement with the ones measured in real experiments, thus demonstrating that the used potential is a good model for silica glass. The vibrational spectrum of the system also shows a significant dependence on the cooling rate and is in qualitative accordance with the one found in experiments. Finally we investigate the properties of the system at finite temperatures in order to understand the microscopic mechanism for the density anomaly. We show that the anomaly is related to a densification and subsequent opening of the tetrahedral network when the temperature is decreased, whereas the distance between nearest neighbors, i.e., the size of the tetrahedra, does not change significantly. \textcopyright{} 1996 The American Physical Society.
TL;DR: An approach to improve metal/organic contacts in organic electronic devices by utilizing chemically tailored electrodes is demonstrated and a physical principle for manipulating the relative energy levels between two materials is established.
Abstract: We demonstrate tuning of Schottky energy barriers in organic electronic devices by utilizing chemically tailored electrodes. The Schottky energy barrier of Ag on poly[2-methoxy, 5-(2\ensuremath{'}-ethyl-hexyloxy)- 1,4-phenylene was tuned over a range of more than 1 eV by using self-assembled monolayers (SAM's) to attach oriented dipole layers to the Ag prior to device fabrication. Kelvin probe measurements were used to determine the effect of the SAM's on the Ag surface potential. Ab initio Hartree-Fock calculations of the molecular dipole moments successfully describe the surface potential changes. The chemically tailored electrodes were then incorporated in organic diode structures and changes in the metal/organic Schottky energy barriers were measured using an electroabsorption technique. These results demonstrate the use of self-assembled monolayers to control metal/organic interfacial electronic properties. They establish a physical principle for manipulating the relative energy levels between two materials and demonstrate an approach to improve metal/organic contacts in organic electronic devices. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this article, the computational complexity associated with the density-functional-based determination of infrared intensities and nonresonant Raman scattering activities is the same as that required for vibrational modes.
Abstract: We show that the computational complexity associated with the density-functional-based determination of infrared intensities and nonresonant Raman scattering activities is the same as that required for vibrational modes. Further, we use extremely large basis sets to determine the intrinsic accuracy for calculating such phenomena within the density-functional theory. We present benchmark calculations on ${\mathrm{CH}}_{4}$, ${\mathrm{H}}_{2}$O, ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$, ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$, and ${\mathrm{C}}_{2}$${\mathrm{H}}_{6}$ within both the local-density approximation (LDA) and the generalized gradient approximation (GGA). Tests of the reliability and numerical stability of the theoretical scheme are presented. We show that in order to obtain reliable results, appropriate polarization basis functions and well-converged wave functions are necessary. While most of the Raman spectra predicted by LDA agree very well with experimental data, some of the infrared intensities show substantial errors. The GGA functional overcomes most of these deficiencies, leading to an overall good agreement with experiment. \textcopyright{} 1996 The American Physical Society.
TL;DR: A detailed description of a method for calculating static linear-response functions in the problem of lattice dynamics based on density functional theory and it uses linear muffin-tin orbitals as a basis for representing first-order corrections to the one-electron wave functions.
Abstract: A detailed description of a method for calculating static linear-response functions in the problem of lattice dynamics is presented. The method is based on density-functional theory and it uses linear muffin-tin orbitals as a basis for representing first-order corrections to the one-electron wave functions. This makes it possible to greatly facilitate the treatment of the materials with localized orbitals. We derive variationally accurate expressions for the dynamical matrix. We also show that large incomplete-basis-set corrections to the first-order changes in the wave functions exist and can be explicitly calculated. Some useful hints on the k-space integration for metals and the self-consistency problem at long wavelengths are also given. As a test application we calculate bulk phonon dispersions in Si and find good agreement between our results and experiments. As another application, we calculate lattice dynamics of the transition-metal carbide NbC. The theory reproduces the major anomalies found experimentally in its phonon dispersions. The theory also predicts an anomalous behavior of the lowest transverse acoustic mode along the (\ensuremath{\xi}\ensuremath{\xi}0) direction. Most of the calculated frequencies agree within a few percent with those measured. \textcopyright{} 1996 The American Physical Society.
TL;DR: An analytic model is presented to describe the existence of photonic energy gaps in the propagation of surface plasmon polaritons on corrugated surfaces, designed to give direct access to expressions for the electromagnetic field and surface charge distributions associated with modes at the band edges thus allowing their physical character to be easily appreciated.
Abstract: We present an analytic model to describe the existence of photonic energy gaps in the propagation of surface plasmon polaritons on corrugated surfaces. We concentrate on elucidating the physical origin of the band gap, and accordingly we place strong emphasis on the physical reasoning and assumptions that we use. Our model is designed to give direct access to expressions for the electromagnetic field and surface charge distributions associated with modes at the band edges, thus allowing their physical character to be easily appreciated. Having established why a band gap occurs we then find expressions for the central position and width of the gap. We compare the results of our model for the gap width with those already in the literature, and find excellent agreement. Our results for the central position of the gap, notably the prediction that it should fall as the corrugation amplitude rises, contradicts one prediction made in the literature. We also reexamine the comparisons made in the literature between experiment and theory for the gap width, and find them inadequate because the theories have been compared to inappropriate experimental data. Consequently we present our own recent experimental data, enabling us to validate our theoretical results, in particular confirming our prediction that the central position of the gap falls as the corrugation amplitude is increased. The limitations of our model are discussed, as well as possible extensions and areas for future research.