Showing papers in "Physical Review B in 1997"

Maximally localized generalized Wannier functions for composite energy bands

Abstract: We discuss a method for determining the optimally localized set of generalized Wannier functions associated with a set of Bloch bands in a crystalline solid. By ''generalized Wannier functions'' we mean a set of localized orthonormal orbitals spanning the same space as the specified set of Bloch bands. Although we minimize a functional that represents the total spread Sigma(n)(r(2))(n) - (r)(n)(2) of the Wannier functions in real space, our method proceeds directly from the Bloch functions as represented on a mesh of k points, and carries out the minimization in a space of unitary matrices U-mn((k)) describing the rotation among the Bloch bands at each k point. The method is thus suitable for use in connection with conventional electronic-structure codes. The procedure also returns the total electric polarization as well as the location of each Wannier center. Sample results for Si, GaAs, molecular C2H4, and LiCl will be presented.

Spontaneous polarization and piezoelectric constants of III-V nitrides

Abstract: The spontaneous polarization, dynamical Born charges, and piezoelectric constants of the III-V nitrides AlN, GaN, and InN are studied ab initio using the Berry-phase approach to polarization in solids. The piezoelectric constants are found to be up to ten times larger than in conventional III-V and II-VI semiconductor compounds, and comparable to those of ZnO. Further properties at variance with those of conventional III-V compounds are the sign of the piezoelectric constants (positive as in II-VI compounds) and the very large spontaneous polarization.

Large thermoelectric power in NaCo 2 O 4 single crystals

Abstract: We measured and analyzed the transport properties of single-crystal ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}$, which is a metallic transition-metal oxide consisting of a two-dimensional triangle lattice of Co. Reflecting the crystal structure, the resistivity is highly anisotropic between in- and out-of-plane directions, and the in-plane resistivity is as low as 200 $\ensuremath{\mu}\ensuremath{\Omega}$ cm at 300 K. Most strikingly, the in-plane thermoelectric power of ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}$ is about 100 $\ensuremath{\mu}$V/K at 300 K, which is nearly ten times larger than that of typical metals. The large thermoelectric power and the low resistivity suggest that ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}$ is a potential thermoelectric material.

Dynamical matrices, born effective charges, dielectric permittivity tensors, and interatomic force constants from density-functional perturbation theory

Abstract: Starting from the knowledge of first-order changes of wave functions and density with respect to small atomic displacements or infinitesimal homogeneous electric fields within the density-functional theory, we write the expressions for the diagonal or mixed second-order derivatives of the total energy with respect to these perturbations: dynamical matrices for different wave vectors, Born effective-charge tensors and electronic dielectric permittivity tensors. Interatomic force constants and the phonon-band structure are then obtained by computing the Fourier transform of dynamical matrices on a regular mesh of wave vectors, with an eventual, separate treatment of the long-range dipole-dipole interaction. The same ingredients also allow one to compute the low-frequency response of the crystal to homogeneous electric fields.

Nonstandard symmetry classes in mesoscopic normal-superconducting hybrid structures

Abstract: Normal-conducting mesoscopic systems in contact with a superconductor are classified by the symmetry operations of time reversal and rotation of the electron's spin. Four symmetry classes are identified, which correspond to Cartan's symmetric spaces of type C, CI, D, and DIII. A detailed study is made of the systems where the phase shift due to Andreev reflection averages to zero along a typical semiclassical single-electron trajectory. Such systems are particularly interesting because they do not have a genuine excitation gap but support quasiparticle states close to the chemical potential. Disorder or dynamically generated chaos mixes the states and produces forms of universal level statistics different from Wigner-Dyson. For two of the four universality classes, the n-level correlation functions are calculated by the mapping on a free one-dimensional Fermi gas with a boundary. The remaining two classes are related to the Laguerre orthogonal and symplectic random-matrix ensembles. For a quantum dot with a normal-metal--superconducting geometry, the weak-localization correction to the conductance is calculated as a function of sticking probability and two perturbations breaking time-reversal symmetry and spin-rotation invariance. The universal conductance fluctuations are computed from a maximum-entropy S-matrix ensemble. They are larger by a factor of 2 than what is naively expected from the analogy with normal-conducting systems. This enhancement is explained by the doubling of the number of slow modes: owing to the coupling of particles and holes by the proximity to the superconductor, every cooperon and diffusion mode in the advanced-retarded channel entails a corresponding mode in the advanced-advanced (or retarded-retarded) channel.

Ab initio study of lithium intercalation in metal oxides and metal dichalcogenides

Abstract: A study of the average voltage to intercalate lithium in various metal oxides is presented. By combining the ab initio pseudopotential method with basic thermodynamics the average intercalation voltage can be predicted without the need for experimental data. This procedure is used to systematically study the effect of metal chemistry, anion chemistry, and structure. It is found that Li is fully ionized in the intercalated compounds with its charge transferred to the anion and to the metal. The substantial charge transfer to the anion is responsible for the large voltage difference between oxides, sulfides, and selenides. Ionic relaxation, as a result of Li intercalation, causes nonrigid-band effects in the density of states of these materials. Suggestions for compounds that may have a substantially larger voltage than currently used materials are also presented. @S0163-1829~97!01028-X#

First-principles responses of solids to atomic displacements and homogeneous electric fields: Implementation of a conjugate-gradient algorithm

Abstract: The changes in density, wave functions, and self-consistent potentials of solids, in response to small atomic displacements or infinitesimal homogeneous electric fields, are considered in the framework of the density-functional theory. A variational: principle for second-order derivatives of the energy provides a basis for efficient algorithmic approaches to these linear responses, such as the state-by-state conjugate-gradient algorithm presented here in detail. The phase of incommensurate perturbations of periodic systems, that are, like phonons, characterized by some wave vector, can be factorized: the incommensurate problem is mapped on an equivalent one presenting the periodicity of the unperturbed ground state. The singularity of the potential change associated with an homogeneous field is treated by the long-wave method. The efficient implementation of these theoretical ideas using plane waves, separable pseudopotentials, and a nonlinear exchange-correlation core correction is described in detail, as well as other technical issues.

Filled skutterudite antimonides : electron crystals and phonon glasses

Abstract: Crystallographic data, electrical and thermal transport measurements, and magnetic susceptibility values are reported for several compounds and alloys with the filled skutterudite structure, R{sub 1{minus}y}Fe{sub 4{minus}x}Co{sub x}Sb{sub 12} (R=La, Ce, or Th; 0{lt}y{lt}1; x=0,1). Room-temperature velocity of sound data is also reported. These materials are of interest because of their potential in thermoelectric power generation and refrigeration applications. The transport properties of both filled and unfilled skutterudite compounds are analyzed using standard semiconductor transport models. Filled skutterudite antimonides appear to be a good approximation of an idealized solid with the good electrical transport properties of a crystal but the poor heat conduction characteristics of a glass. The incoherent rattling of the weakly bound rare-earth atoms in these materials lowers the thermal conductivity at room temperature to values comparable to that of vitreous silica. Relative to the analogous unfilled compounds, the filled skutterudites exhibit larger effective masses and smaller mobilities. Good overall electrical transport is maintained, however, as evidenced by values for the figure of merit (ZT) greater than 1 at elevated temperatures (700{endash}1000 K). Above room temperature, there is very little difference in the electrical and thermal transport behavior between the La and Ce filled compounds. The effects of themore » hybridization caused by the proximity of the Ce 4f level to the Fermi energy, however, are evident at temperatures below 300 K. {copyright} {ital 1997} {ital The American Physical Society}« less

Electric-field and temperature dependence of the hole mobility in poly(p-phenylene vinylene)

Abstract: The current-voltage characteristics of poly(dialkoxy p-phenylene vinylene)-based hole-only devices are measured as a function of temperature. The hole current is space-charge limited, which provides a direct measurement of the hole mobility ${\mathrm{\ensuremath{\mu}}}_{\mathrm{p}}$ as a function of electric field E and temperature. The hole mobility exhibits a field dependence ln ${\mathrm{\ensuremath{\mu}}}_{\mathrm{p}}$\ensuremath{\propto}$\sqrt{E}$ as has also been observed from time-of-flight experiments in many molecularly doped polymers and amorphous glasses. For the zero-field hole mobility an activation energy of 0.48 eV is obtained. The combination of a field-dependent mobility and space-charge effects provides a consistent description of the hole conduction in conjugated polymer films as a function of voltage, temperature, and layer thickness.

Structural effects on the magnetic and transport properties of perovskite A 1-x A' x MnO 3 (x=0.25, 0.30)

Abstract: The evolution of the structural properties of ${A}_{1\ensuremath{-}x}{A}_{x}^{\ensuremath{'}}{\mathrm{MnO}}_{3}$ was determined as a function of temperature, average $A$-site radius $〈{r}_{A}〉,$ and applied pressure for the ``optimal'' doping range $x=0.25,$ 0.30, by using high-resolution neutron powder diffraction. The metal-insulator transition, which can be induced both as a function of temperature and of $〈{r}_{A}〉,$ was found to be accompanied by significant structural changes. Both the paramagnetic charge-localized phase, which exists at high temperatures for all values of $〈{r}_{A}〉,$ and the spin-canted ferromagnetic charge-ordered phase, which is found at low temperatures for low values of $〈{r}_{A}〉,$ are characterized by large metric distortions of the ${\mathrm{MnO}}_{6}$ octahedra. These structural distortions are mainly incoherent with respect to the space-group symmetry, with a significant coherent component only at low $〈{r}_{A}〉.$ These distortions decrease abruptly at the transition into the ferromagnetic metal phase. These observations are consistent with the hypothesis that, in the insulating phases, lattice distortions of the Jahn-Teller type, in addition to spin scattering, provide a charge-localization mechanism. The evolution of the average structural parameters indicates that the variation of the electronic bandwidth is the driving force for the evolution of the insulator-to-metal transition at ${T}_{C}$ as a function of ``chemical'' and applied pressure.

Charge, orbital, and magnetic ordering in La 0.5 Ca 0.5 MnO 3 s

Abstract: The unusual magnetic properties of ${\mathrm{La}}_{0.5}$ ${\mathrm{Ca}}_{0.5}$ ${\mathrm{MnO}}_{3}$ were found to be associated with structural and magnetic ordering phenomena, resulting from the close interplay between charge, orbital, and magnetic ordering. Analysis of synchrotron x-ray and neutron powder diffraction data indicates that the anomalous and hysteretic behavior of the lattice parameters occurring between ${\mathrm{T}}_{\mathrm{C}}$ \ensuremath{\sim}225 K and ${\mathrm{T}}_{\mathrm{N}}$ \ensuremath{\sim}155 K is due to the development of a Jahn-Teller (J-T) distortion of the ${\mathrm{MnO}}_{6}$ octahedra, the ${\mathrm{d}}_{\mathrm{z}}^{2g}$ orbitals being oriented perpendicular to the orthorhombic b axis. We observed an unusual broadening of the x-ray Bragg reflections throughout this temperature region, suggesting that this process occurs in stages. Below ${\mathrm{T}}_{\mathrm{N}}$ , the development of well-defined satellite peaks in the x-ray patterns, associated with a transverse modulation with q=[1/2-\ensuremath{\varepsilon},0,0], indicates that quasicommensurate (\ensuremath{\varepsilon}\ensuremath{\sim}0) orbital ordering occurs within the a-c plane as well. The basic structural features of the charge-ordered low-temperature phase were determined from these satellite peaks. The low-temperature magnetic structure is characterized by systematic broadening of the magnetic peaks associated with the ``${\mathrm{Mn}}^{+3}$ '' magnetic sublattice. This phenomenon can be explained by the presence of magnetic domain boundaries, which break the coherence of the spin ordering on the ${\mathrm{Mn}}^{+3}$ sites while preserving the coherence of the spin ordering on the ${\mathrm{Mn}}^{+4}$ sublattice as well as the identity of the two sublattices. The striking resemblance between these structures and the structural ``charge ordering'' and ``discommensuration'' domain boundaries, which were recently observed by electron diffraction and real-space imaging, strongly suggests that these two types of structures are the same and implies that, in this system, commensurate long-range charge ordering coexists with quasicommensurate orbital ordering.

Forces and frequency shifts in atomic-resolution dynamic-force microscopy

Abstract: True atomic resolution in vacuum with a force microscope is now obtained routinely by using the frequency shift of an oscillating cantilever as the imaging signal. Here, a calculation is presented that relates the frequency shift to the forces between tip and sample for both large and small oscillation amplitudes. Also, the frequency versus distance data for van der Waals dominated tip-sample interactions is related to the geometry of the tip apex. Published frequency versus distance data are used to show that the apex of tips providing atomic resolution is faceted and not rounded. Further, an extended jump-to-contact criterion for large amplitudes is established.

Ti k-edge xanes studies of ti coordination and disorder in oxide compounds: comparison between theory and experiment

Abstract: Experimental Ti $K$-edge x-ray-absorption near-edge structure (XANES) spectra for a variety of Ti(IV)-bearing crystalline oxide model compounds are compared with those calculated using the ab initio multiple-scattering code FEFF7. A scattering-theoretic interpretation of various features in the experimental spectra, including pre-edge and main-edge peaks, is presented together with an interpretation of the effects of disorder. The observed pre-edge features are found to vary in both position (by $\ensuremath{\approx}2\ifmmode\pm\else\textpm\fi{}0.1\mathrm{eV}$) and normalized height (from $\ensuremath{\approx}0.04$ to $1.0\ifmmode\pm\else\textpm\fi{}0.05$) as a function of Ti coordination (4, 5, or 6 oxygen nearest neighbors), in agreement with calculations. In aperiodic oxide compounds where the Ti coordination is unknown (e.g., titanosilicate glasses and melts), pre-edge position and height can be used to derive reliable information on Ti coordination chemistry. For example, one can distinguish between fivefold coordinated Ti (i.e., ${\mathrm{TiO}}_{5}$) and a 50:50 mixture of fourfold- and sixfold-coordinated Ti (i.e., ${\mathrm{TiO}}_{4}$ vs ${\mathrm{TiO}}_{6}$). Finally, it is proposed that the intensity of the main-edge features can be used as a probe of disorder in the short- and medium-range environment of Ti. This is exemplified by Ti XANES studies of the effect of radiation damage on ${\mathrm{CaTiSiO}}_{5}$ and the melting of ${\mathrm{K}}_{2}{\mathrm{TiSi}}_{2}{\mathrm{O}}_{7}$ glass at high temperature.

Relativistic calculations of spin-dependent x-ray-absorption spectra

Abstract: An efficient interpolative approach is presented for relativistic calculations of polarized x-ray-absorption spectra (XAS) including spin and spin-orbit interactions. The method is based on a spinor-relativistic Dirac-Fock treatment of atomic densities and dipole matrix elements, and a nonrelativistic treatment of propagation using high-order multiple scattering theory. This approach is implemented in an automated code FEFF7 which gives quantitative agreement with experiment for x-ray magnetic circular dichroism of Gd and Fe, and for polarized XAS of Cd, including both scr(l){r_arrow}scr(l){plus_minus}1 dipole transitions. {copyright} {ital 1997} {ital The American Physical Society}

Size effect on the crystal phase of cobalt fine particles

Abstract: We have synthesized Co fine particles with the average diameter $(D)$ of less than 500 \AA{} by sputtering Co in a somewhat high inert-gas pressure. It has been found that there is a close relationship between the particle size and the crystal phase; that is, pure fcc (\ensuremath{\beta}) phase for $Dl~200\AA{},$ a mixture of hcp (\ensuremath{\alpha}) and \ensuremath{\beta} phases for $D\ensuremath{\sim}300\AA{},$ and \ensuremath{\alpha} phase with inclusion of a very small amount of \ensuremath{\beta} phase for $Dg~400\AA{}.$ Precise structural characterizations have revealed that the \ensuremath{\beta} particles are multiply twinned icosahedrons and the \ensuremath{\alpha} particles are perfect single crystals with external shape of a Wulff polyhedron. In order to explain the size effect on the crystal phase of Co fine particles, we have performed theoretical calculations for total free energies of an \ensuremath{\alpha} single crystal, a \ensuremath{\beta} single crystal, and a multiply twinned \ensuremath{\beta} icosahedron. The present calculations well explain the size dependence of the crystal phase of the Co fine particles, and have revealed that the stabilization of \ensuremath{\beta} phase, confirmed by previous studies, is the intrinsic effect caused by the small dimensionality of fine particles. Moreover, the phase transformations that occurred in annealing experiments can also be explained by the theory.

Non-Fermi-liquid states and pairing instability of a general model of copper oxide metals

Abstract: A model of copper-oxygen bonding and antibonding bands with the most general two-body interactions allowable by symmetry is considered. The model has a continuous transition as a function of hole density x and temperature T to a phase in which a current circulates in each unit cell. This phase preserves the translational symmetry of the lattice while breaking time-reversal invariance and fourfold rotational symmetry. The product of time reversal and fourfold rotation is preserved. The circulating current phase terminates at a critical point at x=${\mathrm{x}}_{\mathrm{c}}$, T=0. In the quantum critical region about this point the logarithm of the frequency of the current fluctuations scales with their momentum. The microscopic basis for the marginal Fermi-liquid phenemenology and the observed long-wavelength transport anomalies near x=${\mathrm{x}}_{\mathrm{c}}$ are derived from such fluctuations. The symmetry of the current fluctuations is such that the associated magnetic field fluctuations are absent at oxygen sites and have the correct form to explain the anomalous copper nuclear relaxation rate. Crossovers to the Fermi-liquid phase on either side of ${\mathrm{x}}_{\mathrm{c}}$ and the role of disorder are briefly considered. The current fluctuations promote superconductive instability with a propensity towards ``D-wave'' symmetry or ``extended S-wave''symmetry depending on details of the band structure. Several experiments are proposed to test the theory.

Vortex pinning and non-Hermitian quantum mechanics

Abstract: A delocalization phenomenon is studied in a class of non-Hermitian random quantum-mechanical problems. Delocalization arises in response to a sufficiently large constant imaginary vector potential. The transition is related to depinning of flux lines from extended defects in type-II superconductors subject to a tilted external magnetic field. The physical meaning of the complex eigenvalues and currents of the non-Hermitian system is elucidated in terms of properties of tilted vortex lines. The singular behavior of the penetration length describing stretched exponential screening of a perpendicular magnetic field (transverse Meissner effect), the surface transverse magnetization, and the trapping length is determined near the flux-line depinning point.

Coupling between the ferroelectric and antiferromagnetic orders in YMnO 3

Abstract: Anomalies in the dielectric constant and loss tangent have been observed in the ferroelectromagnet YMnO{sub 3} near its N{acute e}el temperature of {approximately}80 K and below its ferroelectric Curie temperature of {approximately}914 K. These anomalies are indicative of coupling between the ferroelectric and antiferromagnetic orders in this compound. A small but distinct magnetoelectric effect and a magnetoresistive effect up to {approximately}15{percent} were also detected in a magnetic field at 5 T. The results will be contrasted with previous theoretical predictions. {copyright} {ital 1997} {ital The American Physical Society}

Transport and conservation laws

Abstract: We study the effect of conservation laws on the finite-temperature transport properties in one-dimensional integrable quantum many-body systems. We show that the energy current is closely related to the first conservation law in these systems and therefore the thermal transport coefficients are anomalous. Using an inequality on the time decay of current correlations we show how the existence of conserved quantities implies a finite charge stiffness (weight of the zero-frequency component of the conductivity) and so ideal conductivity at finite temperatures.

Spin-Gap Proximity Effect Mechanism of High Temperature Superconductivity

Abstract: When holes are doped into an antiferromagnetic insulator they form a slowly fluctuating array of {open_quotes}topological defects{close_quotes} (metallic stripes) in which the motion of the holes exhibits a self-organized quasi-one-dimensional electronic character. The accompanying lateral confinement of the intervening Mott-insulating regions induces a spin gap or pseudogap in the environment of the stripes. We present a theory of underdoped high-temperature superconductors and show that there is a {ital local} separation of spin and charge and that the mobile holes on an individual stripe acquire a spin gap via pair hopping between the stripe and its environment, i.e., via a magnetic analog of the usual superconducting proximity effect. In this way a high pairing scale without a large mass renormalization is established despite the strong Coulomb repulsion between the holes. Thus the {ital mechanism} of pairing is the generation of a spin gap in spatially confined {ital Mott-insulating} regions of the material in the proximity of the metallic stripes. At nonvanishing stripe densities, Josephson coupling between stripes produces a dimensional crossover to a state with long-range superconducting phase coherence. This picture is established by obtaining exact and well-controlled approximate solutions of a model of a one-dimensional electron gas in an activemore » environment. An extended discussion of the experimental evidence supporting the relevance of these results to the cuprate superconductors is given. {copyright} {ital 1997} {ital The American Physical Society}« less

Systematic ab initio investigation of bare boron clusters:mDetermination of the geometryand electronic structures of B n (n=2–14)

Abstract: Based on {ital ab initio} quantum-chemical methods, accurate calculations on small boron clusters B{sub n} (n=2{endash}14) were carried out to determine their electronic and geometric structures. The geometry optimization with a linear search of local minima on the potential-energy surface was performed using analytical gradients in the framework of the restricted Hartree-Fock self-consistent-field approach. Most of the final structures of the boron clusters (n{gt}9) are composed of two fundamental units: either of hexagonal or of pentagonal pyramids. Proposing an {open_quotes}Aufbau principle{close_quotes} one can easily construct various highly stable boron species. The resulting quasiplanar and convex structures can be considered as fragments of planar surfaces and as segments of nanotubes or hollow spheres, respectively. {copyright} {ital 1997} {ital The American Physical Society}

Magnetic localization in mixed-valence manganites

Abstract: The metal-insulator transition is mixed-valence manganites of the ~La0.7Ca0.3!MnO3 type is ascribed to a modification of the spin-dependent potential J HsnS associated with the onset of magnetic order at TC . Here JH is the on-site Hund’s-rule exchange coupling of an e g electron with s51/2 to the t 2g ion core with S 53/2. Above TC, the e g electrons are localized by the random spin-dependent potential and conduction is by variable-range hopping. Over the whole temperature range, the resistivity varies as ln( r/r ‘) 5@T0$12( M/ MS) 2 %/T# 1/4 , where M/ MS is the reduced magnetization. The temperature and field dependence of the resistivity deduced from the molecular-field theory of the magnetization reproduces the experimental data over a wide range of temperature and field. @S0163-1829~97!04513-X# Interest in mixed-valence manganites of the ~La0.7Ca0.3!MnO3 type has revived 1 with the observations of large negative magnetoresistive effects, 2,3 especially in suitably annealed thin films. 4 The magnetoresistance is greatest in the vicinity of the Curie point TC of ferromagnetic compositions which exhibit ‘‘metallic’’ ~temperatureindependent! conduction at low temperatures and thermally activated conduction above TC . These compositions have a structure which is a variant of the cubic perovskite cell where the Mn-O bond lengths are unequal and Mn-O-Mn bond angles differ from 180 °. 5 Their electronic properties are re

Recombination dynamics of localized excitons in In 0.20 Ga 0.80 N- In 0.05 Ga 0.95 N multiple quantum wells

Abstract: Dynamical behavior of radiative recombination has been assessed in the ${\mathrm{In}}_{0.20}$${\mathrm{Ga}}_{0.80}$N (2.5 nm)/ ${\mathrm{In}}_{0.05}$${\mathrm{Ga}}_{0.95}$N (6.0 nm) multiple-quantum-well structure by means of transmittance, electroreflectance (ER), photoluminescence excitation (PLE), and time-resolved photoluminescence (TRPL) spectroscopy. The PL at 20 K was mainly composed of two emission bands whose peaks are located at 2.920 eV and 3.155 eV. Although the peak at 3.155 eV was weak under low photoexcitation, it grew superlinearly with increasing excitation intensity. The ER and PLE revealed that the transition at 3.155 eV is due to the excitons at quantized levels between n=1 conduction and n=1 A(${\mathrm{\ensuremath{\Gamma}}}_{9\mathrm{v}}$) valence bands, while the main PL peak at 2.920 eV is attributed to the excitons localized at the trap centers within the well. The TRPL features were well understood as the effect of localization where photogenerated excitons are transferred from the n=1 band to the localized centers, and then are localized further to the tail state.

Thermal, magnetic, and transport properties of single-crystal Sr 1 − x Ca x RuO 3 ( 0 x 1 . 0 )

Abstract: ${\mathrm{SrRuO}}_{3}$ is a highly correlated, narrow $d$-band metal which undergoes a ferromagnetic transition at ${T}_{c}=165\mathrm{K}.$ ${\mathrm{CaRuO}}_{3},$ which is also a highly correlated metal, has the same crystal structure, comparable electrical resistivity and similar effective Ru moment, but it remains paramagnetic at least down to 1 K. High- and low-field magnetization and susceptibility, thermoremanent magnetization, low-temperature heat capacity, electrical resistivity, and Hall effect measurements are presented on as-grown, untwinned, orthorhombic single-crystal samples of ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{RuO}}_{3}$ for the entire concentration range $0l~xl~1.0.$ ${T}_{c}$ is depressed uniformly with increasing $x,$ all the way to $x=1.0,$ with possible spin-glass-type ordering for $x$ close to 1.0. The critical Sr doping of paramagnetic ${\mathrm{CaRuO}}_{3}$ required to cause magnetic correlations among the Ru moments is $\ensuremath{\cong}1\mathrm{at}.%.$ Magnetization to 7 T shows strong hysteresis for mixed $(xg0)$ crystals only, with evidence for a rotation of the easy magnetic axis out of the $\mathrm{ab}$ plane. Low-temperature magnetization in dc fields to 30 T for $x=0$ shows a lack of saturation to the full $S=1$ moment, $2{\ensuremath{\mu}}_{B}/\mathrm{Ru}\mathrm{}\mathrm{atom},$ underscoring the itinerant character of the ferromagnetism. Similar data for $x=1.0$ show it to be a highly exchange enhanced paramagnet, a borderline antiferromagnet or ferromagnet. This is consistent with previous Ru-O in-plane and out-of-plane doping studies. Low-temperature heat capacity $(1lTl20\mathrm{K})$ shows that the mass enhancement ($\ensuremath{\gamma}=29{\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^{2}$ and ${m}^{*}\ensuremath{\approx}3$ for $x=0$) and the Debye temperature (${\ensuremath{\Theta}}_{D}=390\mathrm{K}$ for $x=0$) are nonmonotonically varying with increasing $x.$ The large electrical resistivity suggests these materials are ``bad'' metals, with a mean free path at room temperature $\ensuremath{\approx}10\mathrm{A}$ for $x=0.$ The Hall effect shows a sign reversal for $x=0$ and $x=1.0,$ but not for mixed crystals. The data are compared where it is appropriate to data derived from comparable experiments from polycrystalline samples and from epitaxially grown thin films. The results support the highly electron-correlated nature of ordered magnetism in Ru-based oxides and the results should help to advance our understanding of the transport, magnetic, and thermodynamic properties of bad metals.

Experimental and theoretical approach to spin splitting in modulation-doped In x Ga 1 − x As/InP quantum wells for B→0

Abstract: Spin splitting of conduction-band energy levels in a modulation-doped InP/In{sub 0.77}Ga{sub 0.23}As/InP quantum well has been studied by Shubnikov{endash}de Haas oscillations. By analyzing the characteristic beating pattern of the oscillations the coupling constant {alpha} for spin-orbit interaction was determined. Biasing a gate on top of a Hall bar was used to modify the strength of the spin-orbit coupling. The measured spin-orbit coupling parameter {alpha} is quantitatively explained by utilizing a refined envelope-function-approximation theory for heterostructures. {copyright} {ital 1997} {ital The American Physical Society}

Point-defect complexes and broadband luminescence in GaN and AlN

Abstract: We have employed the plane-wave pseudopotential method to study point defect complexes in GaN and AlN. The results reveal that defect complexes consisting of dominant donors bound to cation vacancies are likely to be formed in both materials. The position of the electronic levels in the band gap due to these defect complexes is shown to correlate well with the experimentally commonly observed broadband luminescence both in GaN and in AlN. The origin of the large bandwidth of the luminescence spectrum is discussed.

Energy relaxation by multiphonon processes in InAs/GaAs quantum dots

Abstract: Carrier relaxation and recombination in self-organized InAs/GaAs quantum dots (QD's) is investigated by photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy We demonstrate inelastic phonon scattering to be the dominant intradot carrier-relaxation mechanism Multiphonon processes involving up to four LO phonons from either the InAs QD's, the InAs wetting layer, or the GaAs barrier are resolved The observation of multiphonon resonances in the PLE spectra of the QD's is discussed in analogy to hot exciton relaxation in higher-dimensional semiconductor systems and proposed to be intricately bound to the inhomogeneity of the QD ensemble in conjunction with a competing nonradiative recombination channel observed for the excited hole states Carrier capture is found to be a cascade process with the initial capture into excited states taking less than a few picoseconds and the multiphonon (involving three LO phonons) relaxation time of the first excited hole state being 40 ps The |001〉 hole state presents a relaxation bottleneck that determines the ground-state population time after nonresonant excitation For the small self-organized InAs/GaAs QD's the intradot carrier relaxation is shown to be faster than radiative (g1 ns) and nonradiative (\ensuremath{\approx}100 ps) recombination explaining the absence of a ``phonon bottleneck'' effect in the PL spectra

Melting, freezing, and coalescence of gold nanoclusters

Abstract: We present a detailed molecular-dynamics study of the melting, freezing, and coalescence of gold nanoclusters within the framework of the embedded-atom method. Concerning melting, we find the process first to affect the surface (``premelting''), then to proceed inwards. The curve for the melting temperature vs cluster size is found to agree reasonably well with predictions of phenomenological models based on macroscopic concepts, in spite of the fact that the clusters exhibit polymorphism and structural transitions. Upon quenching, we observe a large hysterisis of the transition temperature, consistent with recent experiments on lead. In contrast, we find macroscopic sintering theories to be totally unable to describe the coalescing behavior of two small clusters. We attribute this failure to the fact that the nanocrystals are faceted, while the sintering theories are formulated for macroscopically smooth crystallites. The time for coalescence from our calculations is predicted to be much longer than expected from the macroscopic theory. This has important consequences for the morphology of cluster-assembled materials.

Comment on ``Significance of the highest occupied Kohn-Sham eigenvalue''

Abstract: With more explanation than usual and without appeal to Janak's theorem, we review the statement and proof of the ionization potential theorems for the exact Kohn-Sham density-functional theory of a many-electron system: (1) For any average electron number $N$ between the integers $Z\ensuremath{-}1$ and $Z,$ and thus for $N\ensuremath{\rightarrow}Z$ from below, the highest occupied or partly occupied Kohn-Sham orbital energy is minus the ionization energy of the $Z$-electron system. (2) For $Z\ensuremath{-}1lNlZ,$ the exact Kohn-Sham effective potential ${v}_{s}(\mathbf{r})$ tends to zero as $|\mathbf{r}|\ensuremath{\rightarrow}\ensuremath{\infty}.$ We then argue that an objection to these theorems. [L. Kleinman, Phys. Rev. B 56, 12 042 (1997)] overlooks a crucial step in the proof of theorem (2): The asymptotic exponential decay of the exact electron density of the $Z$-electron system is controlled by the exact ionization energy, but the decay of an approximate density is not controlled by the approximate ionization energy. We review relevant evidence from the numerical construction of the exact Kohn-Sham potential. In particular, we point out a model two-electron problem for which the ionization potential theorems are exactly confirmed. Finally, we comment on related issues: the self-interaction correction, the discontinuity of the exact Kohn-Sham potential as $N$ passes through the integer $Z,$ and the generalized sum rule on the exchange-correlation hole.

Environment-dependent interatomic potential for bulk silicon

Abstract: We use recent theoretical advances to develop a functional form for interatomic forces in bulk silicon. The theoretical results underlying the model include an analysis of elastic properties for the diamond and graphitic structures and inversions of ab initio cohesive energy curves. The interaction model includes two-body and three-body terms which depend on the local atomic environment through an effective coordination number. This formulation is able to capture successfully (i) the energetics and elastic properties of the ground-state diamond lattice, (ii) the covalent rehybridization of undercoordinated atoms, and (iii) a smooth transition to metallic bonding for overcoordinated atoms. Because the essential features of chemical bonding in the bulk are built into the functional form, this model promises to be useful for describing interatomic forces in silicon bulk phases and defects. Although this functional form is remarkably realistic by the usual standards, it contains a small number of fitting parameters and requires computational effort comparable to the most efficient existing models. In a companion paper, a complete parametrization of the model is given, and excellent performance for condensed phases and bulk defects is demonstrated.