Showing papers in "Physical Review B in 2003"

Itinerant-electron metamagnetic transition and large magnetocaloric effects in La(FexSi1-x)13 compounds and their hydrides

Abstract: The itinerant-electron metamagnetic (IEM) transition and magnetocaloric effects (MCE's) in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ and $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds have been investigated. The $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ compounds exhibit large values of both the isothermal entropy change $\ensuremath{\Delta}{S}_{\mathrm{m}}$ and the adiabatic temperature change $\ensuremath{\Delta}{T}_{\mathrm{ad}}$ around the Curie temperature ${T}_{\mathrm{C}}$ in relatively low magnetic fields. Such large MCE's are explained by a large magnetization change at ${T}_{\mathrm{C}}$ and a strong temperature dependence of the critical field ${B}_{\mathrm{C}}$ for the IEM transition. By hydrogen absorption into the compounds, ${T}_{\mathrm{C}}$ is increased up to about 330 K, keeping the metamagnetic transition properties. Accordingly, the extension of the working temperature range having the large MCE's in relatively low magnetic fields is demonstrated by controlling y in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds. The correlation between the increase of ${T}_{\mathrm{C}}$ and the large MCE's in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds is discussed by taking the magnetovolume effects into consideration.

Variationally optimized atomic orbitals for large-scale electronic structures

Abstract: A simple and practical method for variationally optimizing numerical atomic orbitals used in density functional calculations is presented based on the force theorem. The derived equation provides the same procedure for the optimization of atomic orbitals as that for the geometry optimization. The optimized orbitals well reproduce convergent results calculated by a larger number of unoptimized orbitals. In addition, we demonstrate that the optimized orbitals significantly reduce the computational effort in the geometry optimization, while keeping a high degree of accuracy.

Effect of interchain interactions on the absorption and emission of poly(3-hexylthiophene)

Abstract: The absorption spectrum of polythiophene and its derivative poly(3-hexylthiophene) (P3HT) is usually described in terms of an intrachain exciton coupled to a single phonon mode. We show that this model is too simplistic for highly ordered, regioregular P3HT and that, analogous to the case of charged polarons in this material, interchain interactions must be taken into account to correctly describe the absorption spectrum. We show that the lowest energy feature in the $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ region of the absorption spectrum is associated with an interchain absorption, the intensity of which is correlated with the degree of order in the polymer. Correspondingly, we show that the emission from P3HT also exhibits contributions from both interchain and intrachain states, in a manner similar to that recently shown for poly(phenylenevinylene). Having reinterpreted the physical origin of the features in the absorption and emission spectra of P3HT, we then model these spectra and show how they evolve as the degree of order in the polymer is changed by varying several physical parameters including temperature and regioregularity of the polymer.

Magnetocapacitance effect in multiferroic BiMnO 3

Abstract: We have investigated the structural, magnetic, and electric properties of ferromagnetic ${\mathrm{BiMnO}}_{3}$ with a highly distorted perovskite structure. At ${T}_{E}=750--770\mathrm{K},$ a centrosymmetric--to--non-centrosymmetric structural transition takes place, which describes of the ferroelectricity in the system. The changes in the dielectric constant were induced by the magnetic ordering ${(T}_{M}\ensuremath{\approx}100\mathrm{K})$ as well as by the application of magnetic fields near ${T}_{M}.$ These features are attributed to the inherent coupling between the ferroelectric and ferromagnetic orders in the multiferroic system.

X-ray image reconstruction from a diffraction pattern alone

Abstract: A solution to the inversion problem of scattering would offer aberration-free diffraction-limited three-dimensional images without the resolution and depth-of-field limitations of lens-based tomographic systems. Powerful algorithms are increasingly being used to act as lenses to form such images. Current image reconstruction methods, however, require the knowledge of the shape of the object and the low spatial frequencies unavoidably lost in experiments. Diffractive imaging has thus previously been used to increase the resolution of images obtained by other means. Here we experimentally demonstrate an inversion method, which reconstructs the image of the object without the need for any such prior knowledge.

Interpretation of infrared and Raman spectra of amorphous carbon nitrides

Abstract: A general framework for the interpretation of infrared and Raman spectra of amorphous carbon nitrides is presented. In the first part of this paper we examine the infrared spectra. The peaks around 1350 and 1550 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ found in the infrared spectrum of amorphous carbon nitride or hydrogenated and hydrogen-free amorphous carbon are shown to originate from the large dynamic charge of the more delocalized \ensuremath{\pi} bonding which occurs in more ${\mathrm{sp}}^{2}$ bonded networks. The IR absorption decreases strongly when the \ensuremath{\pi} bonding becomes localized, as in tetrahedral amorphous carbon. Isotopic substitution is used to assign the modes to $\mathrm{C}=\mathrm{C}$ skeleton modes, even those modes around 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which become strongly enhanced by the presence of hydrogen. The infrared spectrum of carbon nitride may resemble the Raman spectrum at some excitation energy, but the infrared activity does not primarily result from nitrogen breaking the symmetry. In the second part we examine the Raman spectra. A general model is presented for the interpretation of the Raman spectra of amorphous carbon nitrides measured at any excitation energy. The Raman spectra can be explained in terms of an amorphous carbon based model, without need of extra peaks due to CN, NN, or NH modes. We classify amorphous carbon nitride films in four classes, according to the corresponding N-free film: $a\ensuremath{-}\mathrm{C}:\mathrm{N},$ $a\ensuremath{-}\mathrm{C}:\mathrm{H}:\mathrm{N},$ $ta\ensuremath{-}\mathrm{C}:\mathrm{H}:\mathrm{N},$ and $ta\ensuremath{-}\mathrm{C}:\mathrm{N}.$ We analyze a wide variety of samples for the four classes and present the Raman spectra as a function of N content, ${\mathrm{sp}}^{3}$ content, and band gap. In all cases, a multiwavelength Raman study allows a direct correlation of the Raman parameters with the N content, which is not generally possible for single wavelength excitation. The G peak dispersion emerges as a most informative parameter for Raman analysis. UV Raman enhances the ${\mathrm{sp}}^{1}$ CN peak, which is usually too faint to be seen in visible excitation. As for N-free samples, UV Raman also enhances the C-C ${\mathrm{sp}}^{3}$ bonds vibrations, allowing the ${\mathrm{sp}}^{3}$ content to be quantified.

Transport coefficients from first-principles calculations

Abstract: We present a method of modeling transport coefficients from first-principles calculations. We introduce the transport distribution that contains all electronic information and from which transport coefficients can easily be calculated. We use this method to analyze ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and calculate its transport coefficients for a comparison with experiment. The transport distribution gives an improved insight into the relationship between transport properties and electronic structure and is a valuable tool in the search for improved thermoelectric materials.

Combined atomistic-continuum modeling of short-pulse laser melting and disintegration of metal films

Abstract: The kinetics and microscopic mechanisms of laser melting and disintegration of thin Ni and Au films irradiated by a short, from 200 fs to 150 ps, laser pulse are investigated in a coupled atomistic-continuum computational model. The model provides a detailed atomic-level description of fast nonequilibrium processes of laser melting and film disintegration and, at the same time, ensures an adequate description of the laser light absorption by the conduction band electrons, the energy transfer to the lattice due to the electron-phonon coupling, and the fast electron heat conduction in metals. The interplay of two competing processes, the propagation of the liquid-crystal interfaces (melting fronts) from the external surfaces of the film and homogeneous nucleation and growth of liquid regions inside the crystal, is found to be responsible for melting of metal films irradiated by laser pulses at fluences close to the melting threshold. The relative contributions of the homogeneous and heterogeneous melting mechanisms are defined by the laser fluence, pulse duration, and the strength of the electron-phonon coupling. At high laser fluences, significantly exceeding the threshold for the melting onset, a collapse of the crystal structure overheated above the limit of crystal stability takes place simultaneously in the whole overheated region within \ensuremath{\sim}2 ps, skipping the intermediate liquid-crystal coexistence stage. Under conditions of the inertial stress confinement, realized in the case of short $\ensuremath{\tau}l~10\mathrm{ps}$ laser pulses and strong electron-phonon coupling (Ni films), the dynamics of the relaxation of the laser-induced pressure has a profound effect on the temperature distribution in the irradiated films as well as on both homogeneous and heterogeneous melting processes. Anisotropic lattice distortions and stress gradients associated with the relaxation of the laser-induced pressure destabilize the crystal lattice, reduce the overheating required for the initiation of homogeneous melting down to $T\ensuremath{\approx}{1.05T}_{m},$ and expand the range of pulse durations for which homogeneous melting is observed in 50 nm Ni films up to \ensuremath{\sim}150 ps. High tensile stresses generated in the middle of an irradiated film can also lead to the mechanical disintegration of the film.

Continuum generation from single gold nanostructures through near-field mediated intraband transitions

Abstract: A broad visible and infrared photoluminescence continuum is detected from surface-plasmon-enhanced transitions in gold nanostructures. We find that the ratio of generated infrared to visible emission is much stronger for gold nanostructures than for smooth gold films. While visible emission is well explained by interband transitions of d-band electrons into the conduction band and subsequent radiative recombination, the strong infrared emission cannot be accounted for by the same mechanism. We propose that the infrared emission is generated by intraband transitions mediated by the strongly confined fields near metal nanostructures (localized surface plasmons). These fields possess wave numbers that are comparable to the wave numbers of electrons in the metal, and the associated field gradients give rise to higher-order multipolar transitions. We compare photoluminescence spectra for single gold spheres, smooth and rough gold films, and sharp gold tips and demonstrate that the infrared signal is only present for surfaces with nanometer-scale roughness.

Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors

Abstract: Quasiclassic Usadel equations for two-band superconductors in the dirty limit are derived with the account of both intraband and interband scattering by nonmagnetic impurities. From these equations, the Ginzburg-Landau equations, and the equations for the critical temperature ${T}_{c}$ and the upper critical field ${H}_{c2}$ are obtained. The equation for ${H}_{c2}$ determines both the temperature dependence of ${H}_{c2}(T)$ and the orientational dependence of ${H}_{c2}(\ensuremath{\theta})$ as a function of the angle $\ensuremath{\theta}$ between $\mathbf{H}$ and the c axis. It is shown that the shape of the ${H}_{c2}(T)$ curve essentially depends on the ratio of the intraband electron diffusivities ${D}_{1}$ and ${D}_{2},$ and can be very different from the one-gap dirty-limit theory. In particular, value ${H}_{c2}(0)$ can considerably exceed ${0.7T}_{c}{\mathrm{dH}}_{c2}{/dT}_{c},$ which can have important consequences for applications of ${\mathrm{MgB}}_{2}.$ Based on a general equation for ${H}_{c2}(T,\ensuremath{\theta}),$ a scaling relation, which enables one to get the angular dependence of ${H}_{c2}(\ensuremath{\theta})$ from the equation for ${H}_{c2}$ at $\mathbf{H}\ensuremath{\Vert}c,$ is proposed. The behavior of ${H}_{c2}(\ensuremath{\theta})$ and ${H}_{c1}(\ensuremath{\theta})$ can deviate markedly from the results of the anisotropic Ginzburg-Landau theory, and the ratio of the upper critical field ${H}_{c2}^{\ensuremath{\Vert}}{/H}_{c2}^{\ensuremath{\perp}}$ for $\mathbf{H}\ensuremath{\Vert}\mathrm{ab}$ and $\mathbf{H}\ensuremath{\perp}\mathrm{ab}$ can both increase and decrease with temperature, depending on the relation between ${D}_{1}$ and ${D}_{2}.$ Implications of the obtained results for ${\mathrm{MgB}}_{2}$ are discussed.

Mechanism of electron conduction in self-assembled alkanethiol monolayer devices

Abstract: Electron tunneling through self-assembled monolayers (SAM's) of alkanethiols is investigated using nanometer-scale devices. Temperature-dependent current-voltage measurements are performed on alkanethiol SAM's to distinguish between different conduction mechanisms. Temperature-independent electron transport is observed, proving that tunneling is the dominant conduction mechanism of alkanethiols, as well as exhibiting an exponential dependence of tunneling current on the molecule length with a decay coefficient \ensuremath{\beta}. From the bias dependence of \ensuremath{\beta}, a barrier height ${\ensuremath{\Phi}}_{B}$ of $1.39\ifmmode\pm\else\textpm\fi{}0.01\mathrm{eV}$ and a zero-field decay coefficient ${\ensuremath{\beta}}_{0}$ of $0.79\ifmmode\pm\else\textpm\fi{}0.01{\AA{}}^{\ensuremath{-}1}$ are determined for alkanethiols.

Density-functional study of the structure and stability of ZnO surfaces

Abstract: An extensive theoretical investigation of the nonpolar $(101\ifmmode\bar\else\textasciimacron\fi{}0)$ and $(112\ifmmode\bar\else\textasciimacron\fi{}0)$ surfaces as well as the polar zinc-terminated (0001)-Zn and oxygen-terminated $(0001\ifmmode\bar\else\textasciimacron\fi{})$-O surfaces of ZnO is presented. Particular attention is given to the convergence properties of various parameters such as basis set, k-point mesh, slab thickness, or relaxation constraints within local-density and generalized-gradient approximation pseudopotential calculations using both plane-wave and mixed-basis sets. The pros and cons of different approaches to deal with the stability problem of the polar surfaces are discussed. Reliable results for the structural relaxations and the energetics of these surfaces are presented and compared to previous theoretical and experimental data, which are also concisely reviewed and commented.

Strong spatial dispersion in wire media in the very large wavelength limit

Abstract: It is found that there exist composite media that exhibit strong spatial dispersion even in the very large wavelength limit. This follows from the study of lattices of ideally conducting parallel thin wires (wire media). In fact, our analysis reveals that the description of this medium by means of a local dispersive uniaxial dielectric tensor is not complete, leading to unphysical results for the propagation of electromagnetic waves at any frequencies. Since nonlocal constitutive relations have been usually considered in the past as a second-order approximation, meaningful in the short-wavelength limit, the aforementioned result presents a relevant theoretical interest. In addition, since such wire media have been recently used as a constituent of some discrete artificial media (or metamaterials), the reported results open the question of the relevance of the spatial dispersion in the characterization of these artificial media.

Interatomic potentials for atomistic simulations of the Ti-Al system

Abstract: Semiempirical interatomic potentials have been developed for Al, $\ensuremath{\alpha}\ensuremath{-}\mathrm{Ti},$ and $\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$ within the embedded atom method (EAM) formalism by fitting to a large database of experimental as well as ab initio data. The ab initio calculations were performed by the linearized augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, and planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide a reasonable description of the lattice thermal expansion, demonstrating their usefulness for molecular-dynamics and Monte Carlo simulations at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in $\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$ calculated with the EAM potential is in fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in $\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$ are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that $\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$ is an antisite disorder compound, in agreement with experimental data.

Structural and electronic properties of h-BN

Abstract: Effects of stacking behavior of hexagonal basal layers to the structural stability and electronic properties of h-BN were investigated thoroughly using first-principles calculations based on the density-functional theory local-density approximation. Three of five possible h-BN structures with ``good'' stacking were found to be stable or substable. Considering that intrinsic stacking fault exist in real h-BN crystals which results in mixed stacking behavior, the experimentally observed large interlayer spacing of structures with stacking disorder such as PBN and t-BN can be understood. A substable structure with a direct band gap of about 3.395 eV was predicted. The existence of this substable structure and related intrinsic stacking fault in real h-BN explains the discrepancy in the nature of the band gap and the large variation in the observed band-gap values of h-BN.

Structure and energetics of the vacancy in graphite

Abstract: We determine properties of the vacancy in graphite from first principles calculations. The ground-state structure is associated with a formation energy of 7.4 eV and arises through a combination of symmetric relaxation and symmetry-breaking Jahn-Teller distortion to one of three degenerate, symmetry-related structures. The distortion results in a weak reconstructed bond and small out-of-plane atomic displacements. Dynamic switching between degenerate structures is activated by a barrier of 0.1 eV and we interpret scanning tunneling microscopy observations on the basis of thermal averaging between structures. The calculated migration energy of 1.7 eV is lower than that widely accepted from experiment, and we propose that the discrepancy is explained by a revised picture of trapping during vacancy transport, dependent on concentration. We discuss the significance of these findings in understanding defect behavior in irradiated graphite and related graphitic materials, in particular single-walled nanotubes.

Theory of low-frequency magnetoelectric coupling in magnetostrictive-piezoelectric bilayers

Abstract: A theoretical model is presented for low-frequency magnetoelectric (ME) effects in bilayers of magnetostrictive and piezoelectric phases. A novel approach, the introduction of an interface coupling parameter k, is proposed for the consideration of actual boundary conditions at the interface. An averaging method is used to estimate effective material parameters. Expressions for ME voltage coefficients ${\ensuremath{\alpha}}_{E}^{\ensuremath{'}}=\ensuremath{\delta}E/\ensuremath{\delta}H,$ where $\ensuremath{\delta}E$ is the induced electric field for an applied ac magnetic field $\ensuremath{\delta}H,$ are obtained by solving elastostatic and electrostatic equations. We consider both unclamped and rigidly clamped bilayers and three different field orientations of importance: (i) longitudinal fields $({\ensuremath{\alpha}}_{E,L}^{\ensuremath{'}})$ in which the poling field E, bias field H, and ac fields $\ensuremath{\delta}E$ and $\ensuremath{\delta}H$ are all parallel to each other and perpendicular to the sample plane, (ii) transverse fields $({\ensuremath{\alpha}}_{E,T}^{\ensuremath{'}})$ for in-plane H and $\ensuremath{\delta}H$ parallel to each other and perpendicular to out-of-plane E and $\ensuremath{\delta}E,$ and (iii) in-plane longitudinal fields $({\ensuremath{\alpha}}_{E,IL}^{\ensuremath{'}})$ for all the fields parallel to each other and to the sample plane. The theory predicts a giant ME coupling for bilayers with cobalt ferrite (CFO), nickel ferrite (NFO), or lanthanum strontium manganite (LSMO) for the magnetostrictive phase and barium titanate (BTO) or lead zirconate titanate (PZT) for the piezoelectric phase. Estimates of ${\ensuremath{\alpha}}_{E}^{\ensuremath{'}}$ are carried out as a function of the interface coupling k and volume fraction \ensuremath{ u} for the piezoelectric phase. In unclamped samples, ${\ensuremath{\alpha}}_{E}^{\ensuremath{'}}$ increases with increasing k. The strongest coupling occurs for equal volume of the two phases for transverse and longitudinal cases, but a maximum occurs at $\ensuremath{ u}=0.1$ for the in-plane longitudinal case. Upon clamping the bilayer, the ME effect is strengthened for the longitudinal case and is weakened for the transverse case. Other important results of the theory are as follows. (i) The strongest ME coupling is expected for the in-plane longitudinal fields and the weakest coupling for the (out-of-plane) longitudinal case. (ii) In ferrite-based composites, ${\ensuremath{\alpha}}_{E,T}^{\ensuremath{'}}$ and ${\ensuremath{\alpha}}_{E,IL}^{\ensuremath{'}}$ are a factor of 2--10 higher than ${\ensuremath{\alpha}}_{E,L}.$ (iii) The highest ME voltage coefficients are expected for CFO-PZT and the lowest values are for LSMO-PZT. Results of the present model are compared with available data on the volume and static magnetic field dependence of ${\ensuremath{\alpha}}_{E}^{\ensuremath{'}}.$ We infer, from the comparison, ideal interface conditions in NFO-PZT and poor interface coupling for CFO-PZT and LSMO-PZT.

Glassy ferromagnetism and magnetic phase separation in La 1 − x Sr x CoO 3

Abstract: We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and magnetotransport properties of the glassy ferromagnet ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}.$ The compositions studied span the range from the end-member ${\mathrm{LaCoO}}_{3}$ $(x=0.0)$ through to $x=0.7.$ These materials have attracted attention recently, primarily due to the spin-state transition phenomena in ${\mathrm{LaCoO}}_{3}$ and the unusual nature of the magnetic ground state for finite x. In this paper we present a consistent picture of the magnetic behavior of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ in terms of short-range ferromagnetic ordering and intrinsic phase separation. At high Sr doping $(xg0.2)$ the system exhibits unconventional ferromagnetism (with a Curie temperature up to 250 K), which is interpreted in terms of the coalescence of short-range-ordered ferromagnetic clusters. Brillouin function fits to the temperature dependence of the magnetization as well as high-temperature Curie-Weiss behavior suggest that the ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions are both in the intermediate spin state. At lower Sr doping $(xl0.18)$ the system enters a mixed phase that displays the characteristics of both a spin glass and a ferromagnet. A cusp in the zero-field-cooled dc magnetization, a frequency-dependent peak in the ac susceptibility and time-dependent effects in both dc and ac magnetic properties all point towards glassy behavior. On the other hand, field cooling results in a relatively large ferromagneticlike moment, with zero-field-cooled and field-cooled magnetizations bifurcating at an irreversibility point. Even in the region above $x=0.2$ the out-of-phase component of the ac susceptibility shows frequency-dependent peaks below the Curie temperature (indicative of glassy behavior) which have previously been interpreted in terms of the freezing of clusters. All of the results are consistent with the existence of a strong tendency towards magnetic phase separation in this material, a conclusion which is further reinforced by consideration of the electronic properties. The metal-insulator transition is observed to be coincident with the onset of ferromagnetic ordering $(x=0.18)$ and has a behavior in the doping dependence of the low-temperature conductivity which is strongly suggestive of percolation. This can be interpreted as a percolation transition within the simple ferromagnetic cluster model. On the metallic side of the transition the system exhibits colossal magnetoresistance-type behavior with a peak in the negative magnetoresistance (\ensuremath{\sim}10% in 90 kOe) in the vicinity of the Curie temperature. As the transition is approached from the metallic side we observe the onset of a negative magnetoresistance that increases in magnitude with decreasing temperature, reaching values as large as 90% in a 90-kOe field. This magnetoresistance is enhanced at the metal-insulator transition, where it persists even to room temperature.

Electronic structures of lead iodide based low-dimensional crystals

Abstract: The electronic structures of three-dimensional and two-dimensional lead-halide-based crystals ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ and $({\mathrm{C}}_{4}{\mathrm{H}}_{9}{\mathrm{NH}}_{3}{)}_{2}{\mathrm{PbI}}_{4}$ are investigated by photoelectron spectroscopy and band calculations using the linear combination of atomic orbitals within the density-functional theory. For both crystals, the top of the valence band is found to consist mainly of the $\ensuremath{\sigma}$-antibonding states of Pb $6s$ and I $5p$ orbitals, and the bottom of the conduction band to be composed primarily of the $\ensuremath{\sigma}$-antibonding states of Pb $6p$ and I $5s$ orbitals. Photoelectron spectra of the valence-band region indicate that the electronic structures change depending on the dimensionality of the crystals. Based on the calculation results, the differences observed in the spectra are rationalized in terms of narrowing bandwidth as the dimensionality decreases from three to two dimensions. It is shown that the bandwidth narrowing of the two-dimensional crystal is due to zero dispersion in the vertical direction and the Jahn-Teller effect in the layered structure. These effects lead to a wideband gap and high exciton stability in $({\mathrm{C}}_{4}{\mathrm{H}}_{9}{\mathrm{NH}}_{3}{)}_{2}{\mathrm{PbI}}_{4}.$

Semiempirical atomic potentials for the fcc metals Cu, Ag, Au, Ni, Pd, Pt, Al, and Pb based on first and second nearest-neighbor modified embedded atom method

Abstract: Modified embedded atom method (MEAM) potentials for fcc elements Cu, Ag, Au, Ni, Pd, Pt, Al, and Pb have been newly developed using the original first nearest-neighbor MEAM and the recently developed second nearest-neighbor MEAM formalisms. It was found that the original MEAM potentials for fcc elements show some critical shortcomings such as structural instability and incorrect surface reconstructions on (100), (110), and/or (111) surfaces. The newly developed MEAM potentials solve most of the problems and describe the bulk properties (elastic constants, structural energy differences), point defect properties (vacancy and interstitial formation energy and formation volume, activation energy of vacancy diffusion), planar defect properties (stacking fault energy, surface energy, surface relaxation and reconstruction), and thermal properties (thermal expansion coefficients, specific heat, melting point, heat of melting) of the fcc elements considered, in good agreement with relevant experimental information. It has been shown that in the MEAM the degree of many-body screening ${(C}_{\mathrm{min}})$ is an important material property and that structural stability at finite temperatures should be included as a checkpoint during development of semiempirical potentials.

Spin injection and detection in magnetic nanostructures

Abstract: We study theoretically the spin transport in a nonmagnetic metal connected to ferromagnetic injector and detector electrodes. We derive a general expression for the spin accumulation signal which covers from the metallic to the tunneling regime. This enables us to discuss recent controversy on spin injection and detection experiments. Extending the result to a superconducting device, we find that the spin accumulation signal is strongly enhanced by opening of the superconducting gap since a gapped superconductor is a low carrier system for spin transport but not for charge. The enhancement is also expected in semiconductor devices.

Optical pulse propagation in metal nanoparticle chain waveguides

Abstract: Finite-difference time-domain simulations show direct evidence of optical pulse propagation below the diffraction limit of light along linear arrays of spherical noble metal nanoparticles with group velocities up to 0.06c. The calculated dispersion relation and group velocities correlate remarkably well with predictions from a simple point-dipole model. A change in particle shape to spheroidal particles shows up to a threefold increase in group velocity. Pulses with transverse polarization are shown to propagate with negative phase velocities antiparallel to the energy flow.

Thermal conductance of epitaxial interfaces

Abstract: The thermal conductance of interfaces between epitaxial TiN and single crystal oxides is measured at temperatures between 79.4 and 294 K using time-domain thermoreflectance. The analysis method relies on the ratio of the in-phase and out-of-phase signals of the lock-in amplifier for more accurate data analysis. The validity of this approach is tested by measurements on 6.5, 11.8, and 25 nm thick thermally oxidized ${\mathrm{SiO}}_{2}$ on Si. The thermal conductances G of TiN/MgO(001), TiN/MgO(111), and ${\mathrm{T}\mathrm{i}\mathrm{N}/\mathrm{A}\mathrm{l}}_{2}{\mathrm{O}}_{3}(0001)$ interfaces are essentially identical and in good agreement with the predictions of lattice dynamics models and the diffuse mismatch model with a four-atom fcc unit cell. Near room temperature, $G\ensuremath{\approx}700{\phantom{\rule{0ex}{0ex}}\mathrm{M}\mathrm{W}\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0ex}{0ex}}{\mathrm{K}}^{\ensuremath{-}1},$ $\ensuremath{\approx}5$ times larger than the highest values reported previously for any individual interface.

Distorted perovskite with e g 1 configuration as a frustrated spin system

Abstract: The evolution of spin- and orbital-ordered states has been investigated for a series of insulating perovskites $R{\mathrm{MnO}}_{3}$ $(R=\mathrm{L}\mathrm{a},\mathrm{P}\mathrm{r},\mathrm{N}\mathrm{d},\dots{}).$ $R{\mathrm{MnO}}_{3}$ with a large ${\mathrm{GdFeO}}_{3}$-type distortion is regarded as a frustrated spin system having ferromagnetic nearest-neighbor and antiferromagnetic (AF) next-nearest-neighbor (NNN) interactions within a ${\mathrm{MnO}}_{2}$ plane. The staggered orbital order associated with the ${\mathrm{GdFeO}}_{3}$-type distortion induces the anisotropic NNN interaction, and yields unique sinusoidal and up-up-down-down AF ordered states in the distorted perovskites with ${e}_{g}^{1}$ configuration.

Subwavelength imaging in photonic crystals

Abstract: We investigate the transmission of evanescent waves through a slab of photonic crystal and explore the recently suggested possibility of focusing light with subwavelength resolution. The amplification of near-field waves is shown to rely on resonant coupling mechanisms to surface photon bound states, and the negative refractive index is only one way of realizing this effect. It is found that the periodicity of the photonic crystal imposes an upper cutoff to the transverse wave vector of evanescent waves that can be amplified, and thus a photonic-crystal superlens is free of divergences even in the lossless case. A detailed numerical study of the optical image of such a superlens in two dimensions reveals a subtle and very important interplay between propagating waves and evanescent waves on the final image formation. Particular features that arise due to the presence of near-field light are discussed.

Composition and structure of the RuO2(110) surface in an O2 and CO environment: Implications for the catalytic formation of CO2

Abstract: The phase diagram of surface structures for the model catalyst ${\mathrm{RuO}}_{2}(110)$ in contact with a gas environment of ${\mathrm{O}}_{2}$ and CO is calculated by density-functional theory and atomistic thermodynamics. Adsorption of the reactants is found to depend crucially on temperature and partial pressures in the gas phase. Assuming that a catalyst surface under steady-state operation conditions is close to a constrained thermodynamic equilibrium, we are able to rationalize a number of experimental findings on the CO oxidation over ${\mathrm{RuO}}_{2}(110).$ We also calculated reaction pathways and energy barriers. Based on the various results the importance of phase coexistence conditions is emphasized as these will lead to an enhanced dynamics at the catalyst surface. Such conditions may actuate an additional, kinetically controlled reaction mechanism on ${\mathrm{RuO}}_{2}(110).$

Calculation of Si nanowire thermal conductivity using complete phonon dispersion relations

Abstract: The lattice thermal conductivity of crystalline Si nanowires is calculated. The calculation uses complete phonon dispersions, and does not require any externally imposed frequency cutoffs. No adjustment to nanowire thermal conductivity measurements is required. Good agreement with experimental results for nanowires wider than 35 nm is obtained. A formulation in terms of the transmission function is given. Also, the use of a simpler, nondispersive ``Callaway formula,'' is discussed from the complete dispersions perspective.

Resonance magnetoelectric effects in layered magnetostrictive-piezoelectric composites

Abstract: Magnetoelectric interactions in bilayers of magnetostrictive and piezoelectric phases are mediated by mechanical deformation. This work is concerned with the theory and companion data for magnetoelectric (ME) coupling at electromechanical resonance (EMR) in the layered samples. Estimated ME voltage coefficient versus frequency profiles for nickel, cobalt, or lithium ferrite and lead zirconate titanate (PZT) predict a giant ME effect at EMR with the highest coupling expected for cobalt ferrite-PZT. There is excellent agreement between the theory and data for layered nickel ferrite-PZT; the ME voltage coefficient at resonance increases by a factor of 40 compared to low frequency values. Similar measurements on layered ferromagnetic alloy-PZT and bulk ferrite-PZT reveal even a stronger EMR assisted enhancement in ME coupling.

Electron spin relaxation times of phosphorus donors in silicon

Abstract: Donor electron spins in phosphorus-doped silicon (Si:P) are a candidate two-level system (qubit) for quantum information processing. Spin echo measurements of isotopically purified ${}^{28}\mathrm{S}\mathrm{i}:\mathrm{P}$ are presented that show exceptionally long transverse relaxation (decoherence) times, ${T}_{2},$ at low temperature. Below $\ensuremath{\sim}10\mathrm{K}$ the spin decoherence is shown to be controlled by instantaneous diffusion and at higher temperatures by an Orbach process. ${T}_{2}$ for small pulse turning angles is 14 ms at 7 K and extrapolates to $\ensuremath{\sim}60\mathrm{ms}$ for an isolated spin, over 2 orders of magnitude longer than previously demonstrated.

Full counting statistics of charge transfer in Coulomb blockade systems

Abstract: Full counting statistics (FCS) of charge transfer in mesoscopic systems has recently become a subject of significant interest, since it proves to reveal an important information about the system which can be hardly assessed by other means. While the previous research mostly addressed the FCS of noninteracting systems, the present paper deals with the FCS in the limit of strong interaction. In this Coulomb blockade limit the electron dynamics is known to be governed by a master equation. We develop a general scheme to evaluate the FCS in such case, this being the main result of the work presented. We illustrate the scheme, by applying it to concrete systems. For generic case of a single resonant level we establish the equivalence of scattering and master equation approach to FCS. Further we study a single Coulomb blockade island with two and three leads attached and compare the FCS in this case with our recent results concerning an open dot either with two and three terminals. We demonstrate that Coulomb interaction suppresses the relative probabilities of large current fluctuations.