Showing papers in "Physical Review B in 2002"

Density-functional method for nonequilibrium electron transport

Abstract: We describe an ab initio method for calculating the electronic structure, electronic transport, and forces acting on the atoms, for atomic scale systems connected to semi-infinite electrodes and with an applied voltage bias. Our method is based on the density-functional theory (DFT) as implemented in the well tested SIESTA approach (which uses nonlocal norm-conserving pseudopotentials to describe the effect of the core electrons, and linear combination of finite-range numerical atomic orbitals to describe the valence states). We fully deal with the atomistic structure of the whole system, treating both the contact and the electrodes on the same footing. The effect of the finite bias (including self-consistency and the solution of the electrostatic problem) is taken into account using nonequilibrium Green's functions. We relate the nonequilibrium Green's function expressions to the more transparent scheme involving the scattering states. As an illustration, the method is applied to three systems where we are able to compare our results to earlier ab initio DFT calculations or experiments, and we point out differences between this method and existing schemes. The systems considered are: (i) single atom carbon wires connected to aluminum electrodes with extended or finite cross section, (ii) single atom gold wires, and finally (iii) large carbon nanotube systems with point defects.

Determination of effective permittivity and permeability of metamaterials from reflection and transmission coefficients

Abstract: We analyze the reflection and transmission coefficients calculated from transfer matrix simulations on finite lengths of electromagnetic metamaterials, to determine the effective permittivity ~«! and permeability ~m! .W e perform this analysis on structures composed of periodic arrangements of wires, split ring resonators ~SRRs!, and both wires and SRRs. We find the recovered frequency-dependent« and m are entirely consistent with analytic expressions predicted by effective medium arguments. Of particular relevance are that a wire medium exhibits a frequency region in which the real part of « is negative, and SRRs produce a frequency region in which the real part of m is negative. In the combination structure, at frequencies where both the recovered real parts of « and m are simultaneously negative, the real part of the index of refraction is also found to be unambiguously negative.

Slater-Pauling behavior and origin of the half-metallicity of the full-Heusler alloys

Abstract: Using the full-potential screened Korringa-Kohn-Rostoker method we study the full-Heusler alloys based on Co, Fe, Rh, and Ru. We show that many of these compounds show a half-metallic behavior; however, in contrast to the half-Heusler alloys the energy gap in the minority band is extremely small due to states localized only at the Co (Fe, Rh, or Ru) sites which are not present in the half-Heusler compounds. The full-Heusler alloys show a Slater-Pauling behavior and the total spin magnetic moment per unit cell ${(M}_{t})$ scales with the total number of valence electrons ${(Z}_{t})$ following the rule ${M}_{t}{=Z}_{t}\ensuremath{-}24.$ We explain why the spin-down band contains exactly 12 electrons using arguments based on group theory and show that this rule holds also for compounds with less than 24 valence electrons. Finally we discuss the deviations from this rule and the differences compared to the half-Heusler alloys.

Comparison of atomic-level simulation methods for computing thermal conductivity

Abstract: We compare the results of equilibrium and nonequilibrium methods to compute thermal conductivity. Using Sillinger-Weber silicon as a model system, we address issues related to nonlinear response, thermal equilibration, and statistical averaging. In addition, we present an analysis of finite-size effects and demonstrate how reliable results can be obtained when using nonequilibrium methods by extrapolation to an infinite system size. For the equilibrium Green-Kubo method, we show that results for the thermal conductivity are insensitive to the choice of the definition of local energy from the many-body part of the potential. Finally, we show that the results obtained by the equilibrium and nonequilibrium methods are consistent with each other and for the case of Si are in reasonable agreement with experimental results.

Hardness conserving semilocal pseudopotentials

Abstract: A new type of pseudopotentials for local orbital methods is presented. Hardness conserving semilocal pseudopotentials have been generated for all elements from H to Am. The construction is based on a minimization of errors with the norm conservation conditions for 2--3 relevant ionic configurations of the atom. Besides the transferability between atomic states, the portability among density functionals is also of interest. This paper explores if the norm-conservation errors can be kept reasonably small when minimized for two functionals, e.g., the generalized gradient approximation (GGA) and local density approximation, simultaneously. It is found that the errors can be kept at roughly the same low level as for a single functional. Since these pseudopotentials are mainly designed for use with local orbital methods, semicore functions may be treated as valence functions, helping to increase the accuracy and portability. Therefore the name density functional semicore pseudopotential or DSPP is suggested. To further improve portability and, importantly, also aid numerical stability with GGA's, a core density (nonlinear core correction) is used. As with other pseudopotentials, scalar relativistic corrections to atomic scattering properties can easily be incorporated into this PP. Finally performance DSPP's versus all electron DSPP's with the same method, will be shown for an extensive set of test calculations. It is found that the DSPP is a very well behaved pseudo-potential.

Analysis of guided resonances in photonic crystal slabs

Abstract: We present a three-dimensional analysis of guided resonances in photonic crystal slab structures that leads to a new understanding of the complex spectral properties of such systems. Specifically, we calculate the dispersion diagrams, the modal patterns, and transmission and reflection spectra of these resonances. From these calculations, a key observation emerges involving the presence of two temporal pathways for transmission and reflection processes. Using this insight, we introduce a general physical model that explains the essential features of complex spectral properties. Finally, we show that the quality factors of these resonances are strongly influenced by the symmetry of the modes and the strength of the index modulation.

Origin of p -type doping difficulty in ZnO: The impurity perspective

Abstract: We investigate the p-type doping difficulty in ZnO by first-principles total-energy calculations. The dopants being considered are group-I elements Li, Na, and K and group-V elements N, P, and As. We find that substitutional group-I elements are shallow acceptors, while substitutional group-V elements such as P and As are deep acceptors. The AX centers that convert acceptors into deep donors are found to be unstable except for P and As. Without compensation by intrinsic defects, the most likely cause for doping difficulty is the formation of interstitials for group-I elements and antisites for group-V elements. Among all the dopants studied here, N is a relatively better candidate for p-type ZnO.

String method for the study of rare events

Abstract: We present an efficient method for computing the transition pathways, free energy barriers, and transition rates in complex systems with relatively smooth energy landscapes. The method proceeds by evolving strings, i.e., smooth curves with intrinsic parametrization whose dynamics takes them to the most probable transition path between two metastable regions in configuration space. Free energy barriers and transition rates can then be determined by a standard umbrella sampling around the string. Applications to Lennard-Jones cluster rearrangement and thermally induced switching of a magnetic film are presented.

Symmetry-general least-squares extraction of elastic data for strained materials from ab initio calculations of stress

Abstract: A symmetry-general approach for the least-squares, therefore precise, extraction of elastic coefficients for strained materials is reported. It analyzes stresses calculated ab initio for properly selected strains. The problem, its implementation, and its solution strategy all differ radically from a previous energy-strain approach that we published last year, but the normal equations turn out to be amenable to the same constrainment scheme that makes both approaches symmetry general. The symmetry considerations governing the automated selection of appropriately strained models and their Cartesian systems are detailed. The extension to materials under general stress is discussed and implemented. VASP was used for ab initio calculation of stresses. A comprehensive range of examples includes a triclinic material (kyanite) and simple materials with a range of symmetries at zero pressure, MgO under hydrostatic pressure, ${\mathrm{Ti}}_{4}{\mathrm{As}}_{3}$ under [001] uniaxial strain, and Si under [001] uniaxial stress. The MgO case agrees with recent experimental work including elastic coefficients as well as their first and second derivatives. The curves of elastic coefficients for Si show a gradual increase in the 33 compliance coefficient, leading to a collapse of the material at -11.7 GPa, compared with -12.0 GPa experimentally. Interpretation of results for Be using two approximations [local density (LDA), generalized gradient (GGA)], two approaches (stress strain and energy strain), two potential types (projector augmented wave and ultrasoft), and two quantum engines (VASP and ORESTES) expose the utmost importance of the cell data used for the elastic calculations and the lesser importance of the other factors. For stiffness at relaxed cell data, differences are shown to originate mostly in the considerable overestimation of the residual compressive stresses at x-ray cell data by LDA, resulting in a smaller relaxed cell, thus larger values for diagonal stiffness coefficients. The symmetry generality of the approach described here enabled the creation of a robust user interface going seamlessly from the database search to the printout of the elastic coefficients. With it, even nonspecialist users can reliably produce technologically relevant results like those discussed here in a simple point-and-click fashion from corresponding entries in the CRYSTMET\textregistered{} and ICSD\textregistered{} structure databases, i.e., for all pure-phase nonorganic materials with known crystal structure. The case of ${\mathrm{Ti}}_{4}{\mathrm{As}}_{3}$ exposes, on a first cluster of properties, stiffness, compliance, and the isotropic properties that can be derived from them, the current reality of mining crystal structure databases with ab initio software for technological properties that were never measured before. Further developments in that direction are currently underway.

Tight-binding description of graphene

Abstract: We investigate the tight-binding approximation for the dispersion of the $\ensuremath{\pi}$ and ${\ensuremath{\pi}}^{*}$ electronic bands in graphene and carbon nanotubes. The nearest-neighbor tight-binding approximation with a fixed ${\ensuremath{\gamma}}_{0}$ applies only to a very limited range of wave vectors. We derive an analytic expression for the tight-binding dispersion including up to third-nearest neighbors. Interaction with more distant neighbors qualitatively improves the tight-binding picture, as we show for graphene and three selected carbon nanotubes.

Fine structure of neutral and charged excitons in self-assembled In(Ga)As/(Al)GaAs quantum dots

Abstract: The fine structure of excitons is studied by magnetophotoluminescence spectroscopy of single self-assembled In(Ga)As/(Al)GaAs quantum dots. Both strength and orientation of the magnetic field are varied. In a combination with a detailed theoretical analysis, these studies allow us to develop a comprehensive picture of the exciton fine structure. Symmetry of the dot structures as well as its breaking cause characteristic features in the optical spectra, which are determined by the electron-hole exchange and the Zeeman interaction of the carriers. The symmetry breaking is either inherent to the dot due to geometry asymmetries, or it can be obtained by applying a magnetic field with an orientation different from the dot symmetry axis. From data on spin splitting and on polarization of the emission we can identify neutral as well as charged exciton complexes. For dots with weakly broken symmetry, the angular momentum of the neutral exciton is no longer a good quantum number and the exchange interaction lifts degeneracies within the fine-structure manifold. The symmetry can be restored by a magnetic field due to the comparatively strong Zeeman interactions of electron and hole. For dots with a strongly broken symmetry, bright and dark excitons undergo a strong hybridization, as evidenced by pronounced anticrossings when states within the manifold are brought into resonance. The fine structure can no longer be described within the frame developed for structures of higher dimensionality. In particular, the hybridization cannot be broken magnetically. For charged excitons, the exchange interaction vanishes, demonstrating that the exchange splitting of a neutral exciton can be switched off by injecting an additional carrier.

All-angle negative refraction without negative effective index

Abstract: We describe an all-angle negative refraction effect that does not employ a negative effective index of refraction and involves photonic crystals. A few simple criteria sufficient to achieve this behavior are presented. To illustrate this phenomenon, a microsuperlens is designed and numerically demonstrated.

Atomistic simulations of nanotube fracture

Abstract: The fracture of carbon nanotubes is studied by molecular mechanics simulations. The fracture behavior is found to be almost independent of the separation energy and to depend primarily on the inflection point in the interatomic potential. The fracture strain of a zigzag nanotube is predicted to be between 10% and 15%, which compares reasonably well with experimental results. The predicted range of fracture stresses is 65--93 GPa and is markedly higher than observed. The computed fracture strengths of chiral and armchair nanotubes are above these values. Various plausible small-scale defects do not suffice to bring the failure stresses into agreement with available experimental results. As in the experiments, the fracture of carbon nanotubes is predicted to be brittle.

Role of bianisotropy in negative permeability and left-handed metamaterials

Abstract: The recently proposed artificial media with negative magnetic permeability and left-handed metamaterials are revisited at the light of the theory of artificial bi(iso/aniso)tropic media. In particular, the existence of bianisotropic effects in those materials is investigated, making use of an approximate model. Some unexplained properties of the electromagnetic-wave propagation through these media, revealed by closer inspection of previous numerical simulations and experimental work, are highlighted. It is shown that these peculiarities are properly explained if the bianisotropy is explicitly accounted for. The bianisotropy is related to the existence of magnetoelectric coupling in the artificial constituents (artificial atoms) of the medium. A simple modification of the artificial atom that precludes the bianisotropy is also proposed.

Origin of apparent colossal dielectric constants

Abstract: Experimental evidence is provided that colossal dielectric constants ${\ensuremath{\varepsilon}}^{\ensuremath{'}}g~1000,$ sometimes reported to exist in a broad temperature range, can often be explained by Maxwell-Wagner-type contributions of depletion layers at the interface between sample and contacts or at grain boundaries. We demonstrate this on a variety of different materials. We speculate that the largest intrinsic dielectric constant observed so far in nonferroelectric materials is of order ${10}^{2}.$

Electron spin relaxation by nuclei in semiconductor quantum dots

Abstract: We have studied theoretically electron spin relaxation in semiconductor quantum dots via interaction with nuclear spins. The relaxation is shown to be determined by three processes: (i) the precession of the electron spin in the hyperfine field of the frozen fluctuation of the nuclear spins; (ii) the precession of the nuclear spins in the hyperfine field of the electron; and (iii) the precession of the nuclear spin in the dipole field of its nuclear neighbors. In external magnetic fields the relaxation of electron spins directed along the magnetic field is suppressed. Electron spins directed transverse to the magnetic field relax completely in a time on the order of the precession period of its spin in the field of the frozen fluctuation of the nuclear spins. Comparison with experiment shows that the hyperfine interaction with nuclei may be the dominant mechanism of electron spin relaxation in quantum dots.

Phase diagram of the ferroelectric relaxor (1-x)PbMg1/3Nb2/3O3-xPbTiO3

Abstract: Synchrotron x-ray powder diffraction measurements have been performed on unpoled ceramic samples of $(1\ensuremath{-}x)\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}\ensuremath{-}x{\mathrm{PbTiO}}_{3}$ (PMN-xPT) with $30%l~xl~39%$ as a function of temperature around the morphotropic phase boundary, which is the line separating the rhombohedral and tetragonal phases in the phase diagram. The experiments have revealed very interesting features previously unknown in this or related systems. The sharp and well-defined diffraction profiles observed at high and intermediate temperatures in the cubic and tetragonal phases, respectively, are in contrast to the broad features encountered at low temperatures. These peculiar characteristics, which are associated with the monoclinic phase of ${\mathrm{M}}_{C}$-type previously reported by Kiat et al. [Phys. Rev. B 65, 064106 (2000)] and Singh and Pandey [J. Phys. Condens Matter 13, L931 (2001)], can only be interpreted as multiple coexisting structures with ${\mathrm{M}}_{C}$ as the major component. An analysis of the diffraction profiles has allowed us to properly characterize the PMN-xPT phase diagram and to determine the stability region of the monoclinic phase, which extends from $x=31%$ to $x=37%$ at 20 K. The complex lansdcape of observed phases points to an energy balance between the different PMN-xPT phases which is intrinsically much more delicate than that of related systems such as ${\mathrm{PbZr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{3}$ or $(1\ensuremath{-}x)\mathrm{Pb}({\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{1/3}){\mathrm{O}}_{3}\ensuremath{-}x{\mathrm{PbTiO}}_{3}.$ These observations are in good accord with an optical study of $x=33%$ by Xu et al. [Phys. Rev. B 64, 020102 (2001)], who observed monoclinic domains with several different polar directions coexisting with rhombohedral domains, in the same single crystal.

Spin pumping and magnetization dynamics in metallic multilayers

Abstract: We study the magnetization dynamics in thin ferromagnetic films and small ferromagnetic particles in contact with paramagnetic conductors. A moving magnetization vector causes ‘‘pumping’’ of spins into adjacent nonmagnetic layers. This spin transfer affects the magnetization dynamics similar to the Landau-LifshitzGilbert phenomenology. The additional Gilbert damping is significant for small ferromagnets, when the nonmagnetic layers efficiently relax the injected spins, but the effect is reduced when a spin accumulation build-up in the normal metal opposes the spin pumping. The damping enhancement is governed by ~and, in turn, can be used to measure! the mixing conductance or spin-torque parameter of the ferromagnet‐normal-metal interface. Our theoretical findings are confirmed by agreement with recent experiments in a variety of multilayer systems.

Retarded field calculation of electron energy loss in inhomogeneous dielectrics

Abstract: The exact solution of Maxwell's equations in the presence of arbitrarily shaped dielectrics is expressed in terms of surface-integral equations evaluated at the interfaces. The electromagnetic field induced by the passage of an external electron is then calculated in terms of self-consistently obtained boundary charges and currents. This procedure is shown to be suitable for the simulation of electron energy loss spectra when the materials under consideration are described by local frequency-dependent response functions. The particular cases of translationally invariant interfaces and axially symmetric interfaces are discussed in detail. The versatility of this method is emphasized by examples of energy loss spectra for electrons passing near metallic and dielectric wedges, coupled cylinders, spheres, and tori, and other complex geometries, where retardation aspects and Cherenkov losses can sometimes be significant.

Anatomy of spin-transfer torque

Abstract: Spin-transfer torques occur in magnetic heterostructures because the transverse component of a spin current that flows from a nonmagnet into a ferromagnet is absorbed at the interface. We demonstrate this fact explicitly using free-electron models and first-principles electronic structure calculations for real material interfaces. Three distinct processes contribute to the absorption: (1) spin-dependent reflection and transmission, (2) rotation of reflected and transmitted spins, and (3) spatial precession of spins in the ferromagnet. When summed over all Fermi surface electrons, these processes reduce the transverse component of the transmitted and reflected spin currents to nearly zero for most systems of interest. Therefore, to a good approximation, the torque on the magnetization is proportional to the transverse piece of the incoming spin current.

First-principles study of structural, vibrational, and lattice dielectric properties of hafnium oxide

Abstract: Crystalline structures, zone-center phonon modes, and the related dielectric response of the three low-pressure phases of ${\mathrm{HfO}}_{2}$ have been investigated in density-functional theory using ultrasoft pseudopotentials and a plane-wave basis. The structures of low-pressure ${\mathrm{HfO}}_{2}$ polymorphs are carefully studied with both the local-density approximation (LDA) and the generalized gradient approximation. The fully relaxed structures obtained with either exchange-correlation scheme agree reasonably well with experiment, although LDA yields better overall agreement. After calculating the Born effective charge tensors and the force-constant matrices by finite-difference methods, the lattice dielectric susceptibility tensors for the three ${\mathrm{HfO}}_{2}$ phases are computed by decomposing the tensors into the contributions from individual infrared-active phonon modes.

Phonons and electron-phonon scattering in carbon nanotubes

Abstract: Electron-phonon scattering is studied within an effective-mass theory. A continuum model for acoustic phonons is introduced and electron-phonon interaction due to modification of band structure is derived as well as a normal deformation potential. In a metallic nanotube, the deformation potential does not participate in electron scattering and a metallic nanotube becomes nearly a one-dimensional ballistic conductor even at room temperature. A resistivity determined by small band-structure interaction depends on the chirality at low temperatures. A magnetic field perpendicular to the axis induces electron scattering by the deformation potential, giving rise to huge positive magnetoresistance.

Origin and properties of the gap in the half-ferromagnetic Heusler alloys

Abstract: We study the origin of the gap and the role of chemical composition in the half-ferromagnetic Heusler alloys using the full-potential screened Korringa-Kohn-Rostoker method. In the paramagnetic phase the ${C1}_{b}$ compounds, like NiMnSb, present a gap. Systems with 18 valence electrons, ${Z}_{t},$ per unit cell, like CoTiSb, are semiconductors, but when ${Z}_{t}g18,$ antibonding states are also populated, thus the paramagnetic phase becomes unstable and the half-ferromagnetic one is stabilized. The minority occupied bands accommodate a total of nine electrons and the total magnetic moment per unit cell in ${\ensuremath{\mu}}_{B}$ is just the difference between ${Z}_{t}$ and $2\ifmmode\times\else\texttimes\fi{}9.$ While the substitution of the transition metal atoms may preserve the half-ferromagnetic character, substituting the sp atom results in a practically rigid shift of the bands and the loss of half-metallicity. Finally we show that expanding or contracting the lattice parameter by 2% preserves the minority-spin gap.

Superconductivity and heavy fermion behavior in PrOs 4 Sb 12

Abstract: Superconductivity has been observed in ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$ at ${T}_{C}=1.85 \mathrm{K}$ and appears to involve heavy fermion quasiparticles with an effective mass ${m}^{*}\ensuremath{\sim}50 {m}_{e}$ as inferred from the jump in the specific heat at ${T}_{C},$ the upper critical field near ${T}_{C},$ and the normal state electronic specific heat. Thermodynamic and transport measurements suggest that the heavy fermion state has a quadrupolar origin, although a magnetic origin cannot be completely ruled out.

Quantum orders and symmetric spin liquids

Abstract: A concept---quantum order---is introduced to describe a new kind of orders that generally appear in quantum states at zero temperature. Quantum orders that characterize the universality classes of quantum states (described by complex ground-state wave functions) are much richer than classical orders that characterize the universality classes of finite-temperature classical states (described by positive probability distribution functions). Landau's theory for orders and phase transitions does not apply to quantum orders since they cannot be described by broken symmetries and the associated order parameters. We introduced a mathematical object---projective symmetry group---to characterize quantum orders. With the help of quantum orders and projective symmetry groups, we construct hundreds of symmetric spin liquids, which have SU(2), U(1), or ${Z}_{2}$ gauge structures at low energies. We found that various spin liquids can be divided into four classes: (a) Rigid spin liquid---spinons (and all other excitations) are fully gapped and may have bosonic, fermionic, or fractional statistics. (b) Fermi spin liquid---spinons are gapless and are described by a Fermi liquid theory. (c) Algebraic spin liquid---spinons are gapless, but they are not described by free fermionic-bosonic quasiparticles. (d) Bose spin liquid---low-lying gapless excitations are described by a free-boson theory. The stability of those spin liquids is discussed in detail. We find that stable two-dimensional spin liquids exist in the first three classes (a)--(c). Those stable spin liquids occupy a finite region in phase space and represent quantum phases. Remarkably, some of the stable quantum phases support gapless excitations even without any spontaneous symmetry breaking. In particular, the gapless excitations in algebraic spin liquids interact down to zero energy and the interaction does not open any energy gap. We propose that it is the quantum orders (instead of symmetries) that protect the gapless excitations and make algebraic spin liquids and Fermi spin liquids stable. Since high-${T}_{c}$ superconductors are likely to be described by a gapless spin liquid, the quantum orders and their projective symmetry group descriptions lay the foundation for a spin liquid approach to high-${T}_{c}$ superconductors.

Hall conductivity of a two-dimensional graphite system

Abstract: Within a self-consistent Born approximation, the Hall conductivity of a two-dimensional graphite system in the presence of a magnetic field is studied by quantum transport theory. The Hall conductivity is calculated for short- and long-range scatterers. It is calculated analytically in the limit of strong magnetic fields and in the Boltzmann limit in weak magnetic fields. The numerical calculation shows that the Hall conductivity displays the quantum Hall effect when the Fermi energy is in low-lying Landau levels and the scattering is weak. When the Fermi energy becomes away from $\ensuremath{\varepsilon}=0,$ it tends to the Boltzmann result.

Vacancy and interstitial defects in hafnia

Abstract: We have performed plane wave density functional theory calculations of atomic and molecular interstitial defects and oxygen vacancies in monoclinic hafnia $({\mathrm{HfO}}_{2})$ The atomic structures of singly and doubly positively charged oxygen vacancies, and singly and doubly negatively charged interstitial oxygen atoms and molecules are investigated We also consider hafnium vacancies, substitutional zirconium, and an oxygen vacancy paired with substitutional zirconium in hafnia Our results predict that atomic oxygen incorporation is energetically favored over molecular incorporation, and that charged defect species are more stable than neutral species when electrons are available from the hafnia conduction band The calculated positions of defect levels with respect to the bottom of the silicon conduction band demonstrate that interstitial oxygen atoms and molecules and positively charged oxygen vacancies can trap electrons from silicon

Quasiguided modes and optical properties of photonic crystal slabs

Abstract: We formulate a scattering-matrix-based numerical method to calculate the optical transmission properties and quasiguided eigenmodes in a two-dimensionally periodic photonic crystal slab (PCS) of finite thickness. The square symmetry (point group C4v) is taken for the illustration of the method, but it is quite general and works for any point group symmetry for one-dimensional (1D) and 2D PCS’s. We show that the appearance of well-pronounced dips in the transmission spectra of a PCS is due to the interaction with resonant waveguide eigenmodes in the slab. The energy position and width of the dips in transmission provide information on the frequency and inverse radiative lifetime of the quasiguided eigenmodes. We calculate the energies, linewidths, and electromagnetic fields of such quasiguided eigenmodes, and analyze their symmetry and optical activity. The electromagnetic field in such modes is resonantly enhanced, which opens possibilities for use in creating resonant enhancement of different nonlinear effects.

Co2MnX (X=Si, Ge, Sn) Heusler compounds: An ab initio study of their structural, electronic, and magnetic properties at zero and elevated pressure

Abstract: The structural, electronic, and magnetic properties of ${\mathrm{Co}}_{2}\mathrm{Mn}X$ $(X=\mathrm{Si},$ Ge, Sn) Heusler compounds have been determined by means of all-electron full-potential linearized augmented plane wave (FLAPW) calculations. We focus on the effects on the electronic and magnetic properties induced by: (i) substitution of the X atom, (ii) applied pressure, and (iii) the use of the local spin density approximation (LSDA) vs the generalized gradient approximation (GGA) in density functional theory. A comparison between LSDA and GGA for the exchange-correlation functional shows that GGA is essential for an accurate description of the equilibrium volumes and of the electronic and magnetic properties of these systems. We find that both the energy gap and the spin gap increase as the X atomic number decreases. As a result of the semiconducting (metallic) character found in the minority (majority) spin band structure, the Si and Ge based alloys are predicted to be half-metallic. In contrast, ${\mathrm{Co}}_{2}\mathrm{MnSn}$ is found to be a ``nearly half-metallic'' compound, since the minority valence band maximum crosses the Fermi level. The calculated total magnetization of $5{\ensuremath{\mu}}_{B}$ is in excellent agreement with recent experiments. By including a fully self-consistent treatment of spin-orbit coupling, the GGA calculated orbital moments are shown to be very small (about $0.008{\ensuremath{\mu}}_{B}$ for Mn and about $0.02{\ensuremath{\mu}}_{B}$ for Co), showing that the quenching of the orbital magnetic moment is nearly complete. The calculated hyperfine fields, both at zero and elevated pressure, are compared with available experimental data, and show general agreement, except for Mn. Finally, the calculated Mn $2p$ exchange splittings, found to be in good agreement with experiment, are proportional to the Mn magnetic moments, suggesting a localized nature of ferromagnetism in these Heusler compounds.

Estimation of the isotope effect on the lattice thermal conductivity of group IV and group III-V semiconductors

Abstract: The isotope effect on the lattice thermal conductivity for group IV and group III-V semiconductors is calculated using the Debye-Callaway model modified to include both transverse and longitudinal phonon modes explicitly. The frequency and temperature dependences of the normal and umklapp phonon-scattering rates are kept the same for all compounds. The model requires as adjustable parameters only the longitudinal and transverse phonon Gr\"uneisen constants and the effective sample diameter. The model can quantitatively account for the observed isotope effect in diamond and germanium but not in silicon. The magnitude of the isotope effect is predicted for silicon carbide, boron nitride, and gallium nitride. In the case of boron nitride the predicted increase in the room-temperature thermal conductivity with isotopic enrichment is in excess of 100%. Finally, a more general method of estimating normal phonon-scattering rate coefficients for other types of solids is presented.