Showing papers in "Physical Review B in 1995"

Density-functional theory and strong interactions: Orbital ordering in Mott-Hubbard insulators

Abstract: Evidence is presented that within the density-functional theory orbital polarization has to be treated on an equal footing with spin polarization and charge density for strongly interacting electron systems. Using a basis-set independent generalization of the LDA+U functional, we show that electronic orbital ordering is a necessary condition to obtain the correct crystal structure and parameters of the exchange interaction for the Mott-Hubbard insulator ${\mathrm{KCuF}}_{3}$.

Multiple-scattering calculations of x-ray-absorption spectra

Abstract: A high-order multiple-scattering (MS) approach to the calculation of polarized x-ray-absorption spectra, which includes both x-ray-absorption fine structure and x-ray-absorption near-edge structure, is presented. Efficient calculations in arbitrary systems are carried out by using a curved-wave MS path formalism that ignores negligible paths, and has an energy-dependent self-energy and MS Debye-Waller factors. Embedded-atom background absorption calculations on an absolute energy scale are included. The theory is illustrated for metallic Cu, Cd, and Pt. For these cases the MS expansion is found to converge to within typical experimental accuracy, both to experiment and to full MS theories (e.g., band structure), by using only a few dozen important paths, which are primarily single-scattering, focusing, linear, and triangular.

Insulator-metal transition and giant magnetoresistance in La 1-x Sr x MnO 3

Abstract: Insulator-metal phenomena depending on band filling (doping degree), temperature, and external magnetic field have been investigated for prototypical double-exchange ferromagnets, namely, crystals of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ (0\ensuremath{\le}x\ensuremath{\le}0.6). The electronic phase diagram in the plane of the temperature vs nominal hole concentration (x) has been deduced from the magnetic and electrical measurements on the melt-grown crystals. Around the ferromagnetic transition temperature ${\mathit{T}}_{\mathit{C}}$, large negative magnetoresistance was observed. Irrespective of temperature, reduction of the resistivity is scaled with the field-induced magnetization (M) as -\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}=C(M/${\mathit{M}}_{\mathit{s}}$${)}^{2}$ for M/${\mathit{M}}_{\mathit{s}}$\ensuremath{\lesssim}0.3, where ${\mathit{M}}_{\mathit{s}}$ is the saturated magnetization. The coefficient C strongly depends on x, i.e., C\ensuremath{\approxeq}4 near the compositional insulator-metal phase boundary (${\mathit{x}}_{\mathit{c}}$\ensuremath{\sim}0.17), but decreases down to \ensuremath{\approxeq}1 for xg=0.4, indicating the critical change of the electronic state.

Periodic boundary conditions in ab initio calculations.

Abstract: The convergence of the electrostatic energy in calculations using periodic boundary conditions is considered in the context of periodic solids and localized aperiodic systems in the gas and condensed phases. Conditions for the absolute convergence of the total energy in periodic boundary conditions are obtained, and their implications for calculations of the properties of polarized solids under the zero-field assumption are discussed. For aperiodic systems the exact electrostatic energy functional in periodic boundary conditions is obtained. The convergence in such systems is considered in the limit of large supercells, where, in the gas phase, the computational effort is proportional to the volume. It is shown that for neutral localized aperiodic systems in either the gas or condensed phases, the energy can always be made to converge as O(${\mathit{L}}^{\mathrm{\ensuremath{-}}5}$) where L is the linear dimension of the supercell. For charged systems, convergence at this rate can be achieved after adding correction terms to the energy to account for spurious interactions induced by the periodic boundary conditions. These terms are derived exactly for the gas phase and heuristically for the condensed phase.

Construction of tight-binding-like potentials on the basis of density-functional theory: Application to carbon.

Abstract: We present a density-functional-based scheme for determining the necessary parameters of common nonorthogonal tight-binding (TB) models within the framework of the linear-combination-of-atomic-orbitals formalism using the local-density approximation (LDA). By only considering two-center integrals the Hamiltonian and overlap matrix elements are calculated out of suitable input densities and potentials rather than fitted to experimental data. We can derive analytical functions for the C-C, C-H, and H-H Hamiltonian and overlap matrix elements. The usual short-range repulsive potential appearing in most TB models is fitted to self-consistent calculations performed within the LDA. The calculation of forces is easy and allows an application of the method to molecular-dynamics simulations. Despite its extreme simplicity, the method is transferable to complex carbon and hydrocarbon systems. The determination of equilibrium geometries, total energies, and vibrational modes of carbon clusters, hydrocarbon molecules, and solid-state modifications of carbon yield results showing an overall good agreement with more sophisticated methods.

Inas/gaas pyramidal quantum dots: strain distribution, optical phonons, and electronic structure

Abstract: The strain distribution in and around pyramidal InAs/GaAs quantum dots (QD's) on a thin wetting layer fabricated recently with molecular-beam epitaxy, is simulated numerically. For comparison analytical solutions for the strain distribution in and around a pseudomorphic slab, cylinder, and sphere are given for isotropic materials, representing a guideline for the understanding of strain distribution in two-, one-, and zero-dimensional pseudomorphic nanostructures. For the pyramidal dots we find that the hydrostatic strain is mostly confined in the QD; in contrast part of the anisotropic strain is transferred from the QD into the barrier. The optical-phonon energies in the QD are estimated and agree perfectly with recent experimental findings. From the variation of the strain tensor the local band-gap modification is calculated. Piezoelectric effects are additionally taken into account. The three-dimensional effective-mass single-particle Schr\"odinger equation is solved for electrons and holes using the realistic confinement potentials. Since the QD's are in the strong confinement regime, the Coulomb interaction can be treated as a perturbation. The thus obtained electronic structure agrees with luminescence data. Additionally AlAs barriers are considered.

Electronic and optical properties of three phases of titanium dioxide: Rutile, anatase, and brookite

Abstract: Using the self-consistent orthogonalized linear-combination-of-atomic-orbitals method in the local-density approximation, the electronic structure and the optical properties of three phases of titanium dioxide have been studied. For rutile, the calculated band structure, equilibrium lattice constant, and bulk modulus are in good agreement with other recent calculations and with experimental data. The results on the ground-state properties of anatase and brookite are reported. Compared with the rutile phase, anatase has similar ground-state properties except for a larger band gap, whereas brookite has relatively smaller bulk modulus. The optical properties of these three phases are also calculated using the band-structure results and compared with the available measurements. For the rutile phase, the anisotropic properties of the dielectric function are in good agreement with the reflectance spectroscopy. For the anatase phase, there are very limited experimental optical data for comparison. For the brookite phase, no experimental data are available. Our calculations show subtle differences in the optical properties of these three phases.

Theory of interlayer magnetic coupling

Abstract: A theory of interlayer exchange coupling is presented. A detailed and comprehensive discussion of the various aspects of the problem is given. The interlayer exchange coupling is described in terms of quantum interferences due to confinement in ultrathin layers. This approach provides both a physically transparent picture of the coupling mechanism, and a suitable scheme for discussing the case of a realistic system. This is illustrated for the Co/Cu/Co(001) system. The cases of metallic and insulating spacers are treated in a unified manner by introducing the concept of the complex Fermi surface.

Femtosecond spectroscopy of electron-electron and electron-phonon energy relaxation in ag and au

Abstract: We show experimentally that the electron distribution of a laser-heated metal is a nonthermal distribution on the time scale of the electron-phonon (e-ph) energy relaxation time ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$. We measured ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$ in 45-nm Ag and 30-nm Au thin films as a function of lattice temperature (${\mathit{T}}_{\mathit{i}}$=10--300 K) and laser-energy density (${\mathit{U}}_{\mathit{l}}$=0.3--1.3 J ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$), combining femtosecond optical transient-reflection techniques with the surface-plasmon polariton resonance. The experimental effective e-ph energy relaxation time decreased from 710--530 fs and 830--530 fs for Ag and Au, respectively, when temperature is lowered from 300 to 10 K. At various temperatures we varied ${\mathit{U}}_{\mathit{l}}$ between 0.3--1.3 J ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and observed that ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$ is independent from ${\mathit{U}}_{\mathit{l}}$ within the given range. The results were first compared to theoretical predictions of the two-temperature model (TTM). The TTM is the generally accepted model for e-ph energy relaxation and is based on the assumption that electrons and lattice can be described by two different time-dependent temperatures ${\mathit{T}}_{\mathit{e}}$ and ${\mathit{T}}_{\mathit{i}}$, implying that the two subsystems each have a thermal distribution. The TTM predicts a quasiproportional relation between ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$ and ${\mathit{T}}_{\mathit{i}}$ in the perturbative regime where ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$ is not affected by ${\mathit{U}}_{\mathit{l}}$.Hence, it is shown that the measured dependencies of ${\mathrm{\ensuremath{\tau}}}_{\mathit{E}}$ on lattice temperature and energy density are incompatible with the TTM. It is proven that the TTM assumption of a thermal electron distribution does not hold especially under our experimental conditions of low laser power and lattice temperature. The electron distribution is a nonthermal distribution on the picosecond time scale of e-ph energy relaxation. We developed a new model, the nonthermal electron model (NEM), in which we account for the (finite) electron-electron (e-e) and electron-phonon dynamics simultaneously. It is demonstrated that incomplete electron thermalization yields a slower e-ph energy relaxation in comparison to the thermalized limit. With the NEM we are able to give a consistent description of our data and obtain values for the e-e scattering rate K=0.10\ifmmode\pm\else\textpm\fi{}0.05 ${\mathrm{fs}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{eV}}^{\mathrm{\ensuremath{-}}2}$ for Ag and Au and for the e-ph coupling ${\mathit{g}}_{\mathrm{\ensuremath{\infty}}}$=3.5\ifmmode\pm\else\textpm\fi{}0.5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{Wm}}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ for Ag and 3.0\ifmmode\pm\else\textpm\fi{}0.5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{Wm}}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ for Au.

Nonlinear optical susceptibilities of semiconductors: Results with a length-gauge analysis.

Abstract: We present a simple prescription for the derivation of electronic contributions to the nonlinear optical response of crystals in the independent particle approximation. Semiconductor Bloch equations are found that include previously neglected effects of intraband motion. Applying perturbation theory to clean, cold semiconductors we find expressions for the susceptibilities lacking the unphysical divergences at zero frequency that have plagued other calculations. For these materials we present well-behaved, general expressions for ${\mathrm{\ensuremath{\chi}}}^{(2)}$ and ${\mathrm{\ensuremath{\chi}}}^{(3)}$ for arbitrary frequency mixing and give an explicit demonstration of the finite zero-frequency value of ${\mathrm{\ensuremath{\chi}}}^{(3)}$. We further show how second-order photogalvanic effects are contained in certain physical zero-frequency divergences of ${\mathrm{\ensuremath{\chi}}}^{(2)}$, and consider the corresponding physical zero-frequency divergences of ${\mathrm{\ensuremath{\chi}}}^{(3)}$.

Urbach tail of anatase TiO2.

Abstract: The fundamental absorption edge of the anatase phase of ${\mathrm{TiO}}_{2}$ has been studied by performing polarized optical transmission measurements on single crystals at temperatures ranging from 4.2 to 300 K. An Urbach tail has been found that shows an exponential spectral dependence down to liquid-helium temperature. The optical gap of anatase has been estimated to be 3.420 eV in polarization E\ensuremath{\perp}c, and 3.460 eV in polarization E\ensuremath{\parallel}c. Our experimental results can be accounted for in terms of the theory of Toyozawa and co-workers, which ascribes the Urbach tail to the momentary localization of excitons due to phonon interaction. Comparing, in this case, the measured abosrption spectra of anatase and rutile, we conclude that the excitons in anatase are self-trapped while those in rutile are free. This opposite nature of exciton states in anatase and rutile is consistent with the results of previous photoluminescence studies.

First-principles theory of ferroelectric phase transitions for perovskites: The case of BaTiO3

Abstract: We carry out a completely first-principles study of the ferroelectric phase transitions in ${\mathrm{BaTiO}}_{3}$. Our approach takes advantage of two features of these transitions: the structural changes are small, and only low-energy distortions are important. Based on these observations, we make systematically improvable approximations which enable the parametrization of the complicated energy surface. The parameters are determined from first-principles total-energy calculations using ultrasoft pseudopotentials and a preconditioned conjugate-gradient scheme. The resulting effective Hamiltonian is then solved by Monte Carlo simulation. The calculated phase sequence, transition temperatures, latent heats, and spontaneous polarizations are all in good agreement with experiment. We find the transitions to be intermediate between order-disorder and displacive character. We find all three phase transitions to be of first order. The roles of different interactions are discussed.

Quasiparticle band structure of bulk hexagonal boron nitride and related systems

Abstract: The quasiparticle band structure of bulk hexagonal boron nitride is studied within the GW approximation for the self-energy operator. The influence of the interlayer distance on the band structure is investigated both within the local density approximation and the quasiparticle approach, and the importance of an interlayer state in determining the gap is demonstrated. Also, the quasiparticle band structure for an isolated sheet of boron nitride is calculated. We show that the equivalent of the interlayer state in the case of the isolated boron nitride sheet plays the same role as in the bulk case in determing the band gap.

Elastic constants of hexagonal transition metals: Theory

Abstract: The five different elastic constants of all the hexagonal 4d transition metals (Y, Zr, Tc, and Ru) and the 5d transition metals Re and Os have been calculated by means of first-principles electronic-structure calculations using the full-potential linear muffin-tin orbital method. The calculated data agree with the experimental values within \ensuremath{\sim}30%. We demonstrate, using experimental data, that the hexagonal transition metals obey the Cauchy relations much better than the cubic ones. This is due to the fact that the shape of the density of states for the hexagonal materials retains its form to a larger extent, for all types of shears, than it does for the cubic metals. We introduce normalized elastic constants ${\mathrm{C}}_{\mathrm{ij}}^{\ensuremath{'}}$=${\mathrm{C}}_{\mathrm{ij}}$/B, where B is the bulk modulus, which show a regular behavior for the hexagonal transition metals, in contrast to the cubic transition metals, where large irregularities are observed. These regular as well as irregular behaviors are well reproduced by the full-potential calculations.

First-principles calculations of effective-mass parameters of AlN and GaN

Abstract: The electronic band structures of the wurtzite-type AlN and GaN are calculated by using a self-consistent full-potential linearized augmented plane-wave method within the local-density-functional approximation. In order to clarify the electronic properties near the Brillouin-zone (BZ) center and to give an important guideline on the material designs for short-wavelength optical devices, we link the first-principles band calculations with the effective-mass approximation. The electronic properties are analytically studied on the basis of the effective-mass Hamiltonian, where we consider the hexagonal symmetry of the wurtzite structure. The effective-mass parameters, such as electron effective mass, hole effective masses, or, equivalently, the Luttinger-like parameters, crystal-field splitting and spin-orbit splitting, are determined by reproducing the calculated band structures near the BZ center. The obtained results show that the cubic approximation is fairly successful in the analysis for the valence-band structures of the wurtzite-type nitrides. Further, the calculated parameters for GaN are consistent with the observed ones.

Generic superconducting phase behavior in high- T c cuprates: T c variation with hole concentration in YBa 2 Cu 3 O 7 − δ

Abstract: A direct determination of the relationship between ${\mathit{T}}_{\mathit{c}}$ and hole concentration p for ${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ is obtained by investigating the properties of the fully oxygen-deficient (\ensuremath{\delta}\ensuremath{\approxeq}1.0) compound for which p=x/2. Measurements of ${\mathit{T}}_{\mathit{c}}$, the thermoelectric power S, and bond-valence sums calculated from neutron-diffraction refinements for various values of x and \ensuremath{\delta} allow the full determination of the relations p=p(\ensuremath{\delta}), ${\mathit{T}}_{\mathit{c}}$=${\mathit{T}}_{\mathit{c}}$(p), and S=S(T,p) confirming that ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ satisfies the same universal relations in these quantities as the other high-${\mathit{T}}_{\mathit{c}}$ superconducting cuprates.

Dependence of giant magnetoresistance on oxygen stoichiometry and magnetization in polycrystalline La0.67Ba0.33MnOz

Abstract: We have studied resistivity, magnetization, and magnetoresistance in polycrystalline La0.67Ba0.33MnOz by reducing the oxygen stoichiometry from z=2.99 to 2.80. As the oxygen content decreases, the resistivity of La0.67Ba0.33 MnOz increases and the magnetic transition temperature shifts to lower temperature. A large magnetoresistance effect was observed over a wide temperature range for all samples except the insulating z=2.80 sample. The similarity between our results on oxygen-deficient polycrystalline La0.67 Ba0.33MnOz and films previously reported to have a very large intrinsic magnetoresistance is discussed. At low temperature the magnetoresistance was observed to be strongly dependent on the magnetization. A possible mechanism for this effect is discussed.

Visible photoluminescence from nanocrystallite Ge embedded in a glassy SiO 2 matrix: Evidence in support of the quantum-confinement mechanism

Abstract: Nanocrystallite Ge (nc-Ge) embedded in a glassy ${\mathrm{SiO}}_{2}$ matrix is fabricated and examined by x-ray photoelectron spectrometry, Raman spectrometry, and high-resolution transmission-electron microscopy. The precipitation and growth of nc-Ge are found to be related to a thermodynamical reduction of ${\mathrm{GeO}}_{2}$, the diffusion of Si atoms from the Si substrate into the glassy matrix, and an aggregation of small-sized nc-Ge. The size inhomogeneity can be precisely controlled by a double annealing, and the average size can be changed in the range of 2\char21{}6 nm. Broadband photoluminescence (PL) spectra are observed in the visible wavelength range at room temperature, and they exhibit pronounced blueshifts of the peak energies and broadening of the PL spectra, which can be correlated to the change in the size. The PL excitation spectra show a Stokes energy smaller than 0.1 eV and dependence on the measurement energy. The visible PL also shows a strong correlation to the presence and actual condition (i.e., size) of nc-Ge. Possible origins of the visible PL such as a quantum-confinement model in quantum dots, the presence of luminescent centers (Ge:E') in silica glass, or a structural transition of nc-Ge are discussed. The present experimental data are concluded to be more consistent with a quantum-confinement model than with the other models.

Native defects in gallium nitride

Abstract: The results of an extensive theoretical study of native defects in hexagonal GaN are presented. We have considered cation and anion vacancies, antisites, and interstitials. The computations were carried out using ab initio molecular dynamics in supercells containing 72 atoms. N vacancy introduces a shallow donor level, and may be responsible for the n-type character of as-grown GaN. Due to the wide gap of nitrides, self-compensation effects strongly reduce both n-type and p-type doping efficiencies due to the formation of gallium vacancy and interstitial Ga, respectively.

Electronic structure of La1-xSrxMnO3 studied by photoemission and x-ray-absorption spectroscopy

Abstract: The electronic structure of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ has been studied by photoemission and O 1s x-ray-absorption spectroscopy. Spectra of the Mn 2p core levels and the valence bands for ${\mathrm{LaMnO}}_{3}$ and ${\mathrm{SrMnO}}_{3}$ have been analyzed using a configuration-interaction cluster model. The ground state of ${\mathrm{LaMnO}}_{3}$ is found to be mixed ${\mathit{d}}^{4}$ and ${\mathit{d}}^{5}$L states and that of ${\mathrm{SrMnO}}_{3}$ to be heavily mixed ${\mathit{d}}^{3}$ and ${\mathit{d}}^{4}$L states, reflecting their strong covalency. The character of the band gap of ${\mathrm{LaMnO}}_{3}$ is of the p-to-d charge-transfer type while that of ${\mathrm{SrMnO}}_{3}$ has considerable p-p character as well as p-d character. Holes doped into ${\mathrm{LaMnO}}_{3}$ mainly of oxygen p character are coupled antiferromagnetically with the ${\mathit{d}}^{4}$ local moments of the ${\mathrm{Mn}}^{3+}$ ions and become itinerant, thus aligning the Mn moments ferromagnetically. The changes in the electronic structure with carrier doping are not of the rigid band type: By La substitution for ${\mathrm{SrMnO}}_{3}$, the so-called in-gap spectral weight (of ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ symmetry) appears with its peak located 1--2 eV below the Fermi level and grows in intensity with increasing La concentration, while the spectral intensity of the ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ states above the Fermi level decreases, showing a transfer of spectral weight from the unoccupied to the occupied ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ states with electron doping. Meanwhile, the intensity at the Fermi level remains low even in the metallic phase (0.2\ensuremath{\lesssim}x\ensuremath{\lesssim}0.6). The energy shifts of core-level peaks and valence-band features with x suggest a downward shift of the Fermi level with hole doping, but the shift is found to be very small in the metallic phase. The importance of the orbital degeneracy of the ${\mathit{e}}_{\mathit{g}\mathrm{\ensuremath{\uparrow}}}$ band and possible orbital fluctuations in the ferromagnetic phase are pointed out.

Mechanical instabilities of homogeneous crystals

Abstract: Elastic stability criteria are derived for homogeneous lattices under arbitrary but uniform external load. These conditions depend explicitly on the applied stress and reduce, in the limit of vanishing load, to the criteria due to Born, involving only the elastic constants of the crystal. By demonstrating the validity of our results through a comparison of the analysis of an fcc lattice under hydrostatic tension with direct molecular-dynamics simulation, we show that crystal stability under stress (ideal strength) is not a question only of material property, and that even qualitative predictions require the inclusion of the effects of applied stress. General implications of our findings, as well as relevance to stability phenomena in melting, polymorphism, crack nucleation, and solid-state amorphization, are discussed.

Static polarizabilities of single-wall carbon nanotubes

Abstract: The static electric polarizability tensor of single-wall carbon nanotubes is calculated within the random-phase approximation using a simple tight-binding model and a classical correction to include local fields. We find that the polarizability for constant fields parallel to the cylindrical axis is highly dependent on the details of the tube`s electronic structure. In contrast, the polarizability for fields perpendicular to the axis only depends on the tube radius. The relative magnitudes of these two quantities suggests that under the application of a randomly oriented electric field, nanotubes acquire dipole moments pointing mainly along their axes, with the size of the dipole inversely proportional to the square of the minimum direct band gap.

Atomic clusters : building blocks for a class of solids

Abstract: Atomic clusters with suitable size and composition can be designed to mimic the chemistry of atoms in the Periodic Table. These clusters which can be viewed as ``super atoms'' could then form the building blocks for a class of solids with unique structural, electronic, optical, magnetic, and thermodynamic properties. Using density-functional calculations, we outline the design principles for these clusters and describe the role of geometry and electronic shell structure on cluster-cluster interaction.

Elastic theory of flux lattices in the presence of weak disorder.

Abstract: The effect of weak impurity disorder on flux lattices at equilibrium is studied quantitatively in the absence of free dislocations using both the Gaussian variational method and the renormalization group. Our results for the mean-square relative displacements B\ifmmode \tilde{}\else \~{}\fi{}(x)=〈u(x)-u(0)${\mathrm{〉}}^{2}$\ifmmode\bar\else\textasciimacron\fi{} clarify the nature of the crossovers with distance. We find three regimes: (i) a short distance regime (``Larkin regime'') whre elasticity holds, (ii) an intermediate reigme (``random manifold'') where vortices are pinned independently, and (iii) a large distance, quasiordered regime where the periodicity of the lattice becomes important. In the last regime we find universal logarithmic growth of displacements for 2d4: B\ifmmode \tilde{}\else \~{}\fi{}(x)\ensuremath{\sim}${\mathit{A}}_{\mathit{d}}$ln\ensuremath{\Vert}x\ensuremath{\Vert} and persistence of algebraic quasi-long-range translational order. The functional renormalization group to O(\ensuremath{\epsilon}=4-d) and the variational method, when they can be compared, agree within 10% on the value of ${\mathit{A}}_{\mathit{d}}$. In d=3 we compute the function describing the crossover between the three reigmes. We discuss the observable signature of this crossover in decoration experiments and in neutron-diffraction experiments on flux lattices. Qualitative arguments are given suggesting the existence for weak disorder in d=3 of a ``Bragg glass'' phase without free dislocations and with algebraically divergent Bragg peaks. In d=1+1 both the variational method and the Cardy-Ostlund renormalization group predict a glassy state below the same transition temperature T=${\mathit{T}}_{\mathit{c}}$, but with different B\ifmmode \tilde{}\else \~{}\fi{}(x) behaviors. Applications to d=2+0 systems and experiments on magnetic bubbles are discussed.

Spontaneous interlayer coherence in double-layer quantum Hall systems: Charged vortices and Kosterlitz-Thouless phase transitions

Abstract: At strong magnetic fields, double-layer two-dimensional electron-gas systems can form an unusual broken-symmetry state with spontaneous interlayer phase coherence. In this paper we explore the rich variety of quantum and finite-temperature phase transitions associated with this broken symmetry. We describe the system using a pseudospin language in which the layer degree of freedom is mapped to a fictional spin 1/2 degree of freedom. With this mapping the spontaneous symmetry breaking is equivalent to that of a spin 1/2 easy-plane ferromagnet. In this language, spin textures can carry a charge. In particular, vortices carry \ifmmode\pm\else\textpm\fi{}e/2 electrical charge and vortex-antivortex pairs can be neutral or carry charge \ifmmode\pm\else\textpm\fi{}e. We derive an effective low-energy action and use it to discuss the charged and collective neutral excitations of the system. We have obtained the parameters of the Landau-Ginzburg functional from first-principles estimates and from finite-size exact diagonalization studies. We use these results to estimate the dependence of the critical temperature for the Kosterlitz-Thouless phase transition on layer separation.

X-ray-absorption spectroscopy and n-body distribution functions in condensed matter. II. Data analysis and applications.

Abstract: The practical and theoretical aspects of the GNXAS method for multiple-scattering extended x-ray-absorption fine-structure (EXAFS) data analysis are treated in a comprehensive account. The model function used to fit the raw absorption coefficient is described and details on the least-squares fitting procedure and parameter definition are reported. Large emphasis is given, in EXAFS analysis, to the description of criteria for a complete statistical evaluation of the results, including error estimate and model evaluation. An extensive set of applications to prototypical molecular (${\mathrm{Br}}_{2}$, ${\mathrm{CS}}_{2}$), and crystalline [c-Ge and Pd (fcc)] systems is reported. Structural parameters always coincide with the known values within statistical accuracy indicating that systematic errors due to approximations in the theory are negligible. The present results also demonstrate that x-ray absorption spectroscopy can provide information on three-body atomic arrangements like average geometrical and vibrational parameters with statistical significance. In all examples, despite the inclusion of several three-body contributions, the total number of fitting parameters does not exceed the information content of the spectra; details on error evaluation and correlation plots in the parameter space are reported.

Synthesis of B x C y N z nanotubules

Abstract: We report the successful synthesis of B{sub {ital x}}C{sub {ital y}}N{sub {ital z}} nanotubes. Arc-discharge methods were used to produce stable nanotubule structures identified by high-resolution transmission-electron microscopy. Local electron-energy-loss spectroscopy of {ital K}-edge absorptions for B, C, and N atoms was used to determine the atomic compositions of individual tubules. Tubes of stoichiometry BC{sub 2}N and BC{sub 3} have been observed, in agreement with theoretical predictions.

Resistance of atomic wires.

Abstract: The resistance of wires consisting of 1--3 atoms connecting two semi-infinite metallic electrodes is calculated for both small and large bias. The wires discussed consist of Al atoms, with one of the Al atoms substituted by S in certain cases. The resistances obtained are in the range 7--\ensuremath{\sim}30 k\ensuremath{\Omega}. For the three-atom wire, the value of the resistance when the S atom is present depends on the order of the atoms in the wire. When the S is an end atom, the resistance at larger bias also shows a dependence on polarity (diode behavior). These studies involve a self-consistent calculation of the electron density distribution for the entire electrode/wire system.