Near-neighbor Pairs Research Articles

Boron acceptor concentration in diamond from excitonic recombination intensities

Excitonic recombinations are investigated by cathodoluminescence in a series of homoepitaxial diamond layers doped with boron in the range $(2\ifmmode\times\else\texttimes\fi{}{10}^{16})$--$(5\ifmmode\times\else\texttimes\fi{}{10}^{18}) \mathrm{at} {\mathrm{cm}}^{\ensuremath{-}3}.$ As opposed to earlier observations made on polycrystalline boron-doped diamond, we show that the ratio between the neutral-boron bound exciton and the free-exciton recombination intensities is proportional to the boron content up to $6\ifmmode\times\else\texttimes\fi{}{10}^{17}$cm${}^{\ensuremath{-}3}$ and starts to saturate above this value. The probability of an exciton in diamond being trapped by only one boron impurity is calculated and appears to be in good agreement with the observed saturation, suggesting the formation of excitons bound to near-neighbor boron pairs at high boron concentrations.

Local environment of ferromagnetically ordered Mn in epitaxial InMnAs

The magnetic properties of the ferromagnetic semiconductor In0.98Mn0.02As were characterized by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. The Mn exhibits an atomic-like L2,3 absorption spectrum that indicates that the 3d states are highly localized. In addition, a large dichroism at the Mn L2,3 edge was observed from 5 to 300 K at an applied field of 2T. A calculated spectrum assuming atomic Mn2+ yields the best agreement with the experimental InMnAs spectrum. A comparison of the dichroism spectra of MnAs and InMnAs shows clear differences suggesting that the ferromagnetism observed in InMnAs is not due to hexagonal MnAs clusters. The temperature dependence of the dichroism indicates the presence of two ferromagnetic species, one with a transition temperature of 30 K and another with a transition temperature in excess of 300 K. The dichroism spectra are consistent with the assignment of the low temperature species to random substitutional Mn and the high temperature species to Mn near-neighbor pairs.

Distinguishing step relaxation mechanisms via pair correlation functions

Theoretical predictions of coupled step motion are tested by direct STM measurement of the fluctuations of near-neighbor pairs of steps on Si(111)-root3 x root3 R30 - Al at 970K. The average magnitude of the pair-correlation function is within one standard deviation of zero, consistent with uncorrelated near-neighbor step fluctuations. The time dependence of the pair-correlation function shows no statistically significant agreement with the predicted t^1/2 growth of pair correlations via rate-limiting atomic diffusion between adjacent steps. The physical considerations governing uncorrelated step fluctuations occurring via random attachment/detachment events at the step edge are discussed.

Spectroscopic Evidence for Near-Neighbor Bonded H(D) in A-Si:N:H(D) Thin Films

AbstractIncreases in the frequency of the silicon monohydride (Si-H) bond-stretching vibration in hydrogenated silicon nitrides, over and above what can be attributed to chemical induction effects, are shown to be generated by a repulsive interaction between the H-atom of the Si-H group, and a near-neighbor SiN-H group. This interaction increases the effective bond-stretching force constant of the H-atom of the Si-H group. This effect has been verified by ab-initio quantum chemistry calculations that have been used to calculate the energy of the H-atom of the Si-H group as a function of the relative separation between near-neighbor Si-H and SiN-H groups. Decreases in the bond-stretching frequencies of SiN-H groups in heavily hydrogenated nitrides are attributed to H-bonding effects associated with near-neighbor pairs of SiN-H groups.

Short-range antiferromagnetic orientational correlations in Rb3C60.

The neutron pair distribution function (PDF) of ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ at 10 K reveals local deviations from the Stephens model of merohedral disorder. The experimental PDF in the 3--7 \AA{} range is best represented by a greater fraction of near-neighbor pairs with opposite standard orientations than would be expected from purely random decoration, indicating the presence of short-range ``antiferromagnetic'' orientational correlations.

The structure of Cu3Au above the critical temperature

The diffuse x-ray scattering intensity from a single crystal of Cu3Au was measured in a volume of reciprocal space at a temperature of 703 K, approximately 35 K above the transition temperature for ordering in this alloy. Intensity was measured in absolute units and was separated into components due to short-range order (SRO) and atomic displacements. The SRO parameters determined agree qualitatively with previous experimenters but agree best in magnitude with the original study by Cowley [J. Appl. Phys. 21, 24 (1950)]. Computer simulations of the structure, based on the SRO parameters, show that atoms with ordered environments tend to cluster into small domains that have dimensions of 1–10 unit cells in length and are surrounded by a disordered matrix. The average displacements of near-neighbor pairs were found to be contrary to what has previously been assumed: Au-Au neighbors were found to have negative displacements on average and Au-Cu pairs positive displacements, compared to the average interatomic displacements.

Subunit arrangement in beef heart complex III.

Beef heart mitochondrial complex III was separated into 12 polypeptide bands representing 11 different subunits by using the electrophoresis conditions described by Schägger et al. [(1986) Methods Enzymol. 126, 224-237]. Eight of the 12 polypeptide bands were identified from their NH2-terminal sequences as obtained by electroblotting directly from the NaDodSO4-polyacrylamide gel onto a solid support. The topology of the subunits in complex III was explored by three different approaches. (1) Protease digestion experiments of submitochrondrial particles in the presence and absence of detergent showed that subunits II and VI are on the M side of the inner membrane and subunits V and XI on the C side. (2) Labeling experiments with the membrane-intercalated probes [125I]TID and arylazidoPE indicated that cytochrome b is the predominant bilayer embedded subunit of complex III, while the non-heme iron protein appears to be peripherally located. (3) Cross-linking studies with carbodiimides and homobifunctional cleavable reagents demonstrated that near-neighbor pairs include subunits I+II, II+VI, III+VI, IV+V, V+X, and reagents demonstrated that near-neighbor pairs include subunits I+II, II+VI, III+VI, IV+V, V+X, and VI+VII. The cytochrome c binding site was found to include subunits IV, VIII, and X. The combined data are used to provide an updated model for the topology of beef heart complex III.

X-ray-absorption studies ofYBa2Cu3O7−δandGdBa2Cu3O7−δsuperconductors

X-ray-absorption measurements of the high-${T}_{c}$ superconductors $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ show that their near-edge and extended fine structures (EXAFS) are essentially identical over the temperature range of 4.2 to 688 K. The results indicate that not only are $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ very similar to each other in local bonding and electronic structure, but also remain virtually unchanged, electronically and structurally, over this wide range in temperature, from far below to well above ${T}_{c}$. The positions of the Cu near-edge spectra for these superconductors suggest a Cu valence between that of Cu in CuO, which has a formal valence of II, and that in KCu${\mathrm{O}}_{2}$, with a formal valence of III. Our EXAFS structural results agree well with those from neutron diffraction data. Recently proposed alternate structures are shown to be inconsistent with our data. The characteristic vibration frequencies of the near-neighbor atoms show that the two-dimensional Cu---O planes and the one-dimensional Cu---O chains form the most rigid parts of the structure, and these layers are bound more weakly along the $c$ axis via the Ba and Y planes. These oscillator frequencies are identified with specific near-neighbor pairs, and the values are consistent with those for some of the modes seen by inelastic neutron scattering as well as by infrared and Raman spectroscopy. No evidence of phonon softening is apparent in any of the phonon modes studied, providing evidence against a strong electron-phonon coupling mechanism.

Molecular dynamics of the water liquid-vapor interface.

The results of molecular dynamics calculations on the equilibrium interface between liquid water and its vapor at 325 K are presented. For the TIP4P model of water intermolecular pair potentials, the average surface dipole density points from the vapor to the liquid. The most common orientations of water molecules have the C2 nu molecular axis roughly parallel to the interface. The distributions are quite broad and therefore compatible with the intermolecular correlations characteristic of bulk liquid water. All near-neighbor pairs in the outermost interfacial layers are hydrogen bonded according to the common definition adopted here. The orientational preferences of water molecules near a free surface differ from those near rigidly planar walls which can be interpreted in terms of patterns found in hexagonal ice 1. The mean electric field in the interfacial region is parallel to the mean polarization which indicates that attention cannot be limited to dipolar charge distributions in macroscopic descriptions of the electrical properties of this interface. The value of the surface tension obtained is 132 +/- 46 dyn/cm, significantly different from the value for experimental water of 68 dyn/cm at 325 K.

Molecular dynamics study of the hydrophobic interaction in an aqueous solution of krypton

Molecular dynamics calculations have been performed for systems of two and several atomic solutes dissolved in a model for liquid water to study the hydrophobic association of nonpolar solutes in aqueous solutions. The parameters of the potentials were chosen to be appropriate for an aqueous solution of Kr. In accordance with previous work, it is found that at infinite dilution a pair of atoms has two preferred configurations, one in which they are near neighbors and another in which they are separated by a water molecule, and that the latter is more stable. In the simulations of systems containing several atomic solutes, the atoms were found to form clusters in which the atom-atom distances corresponded to the formation of near-neighbor and solvent-separated pairs. On the average, the effect of the solvent is to tend to keep the solutes apart rather than to cause them to associate. This result contradicts the conventional wisdom on hydrophobic interaction, and a new analysis of experimental solubility data gives support to the result.

Distribution of interatomic spacings in random alloys

In a disordered solid solution the interatomic spacings will vary because of fluctuations in the local environment. This paper extends previous theories of these variations for a system in which A and B atoms are distributed at random over the sites of a simple lattice, with atomic fractions x and (1−x), respectively, and for which the differences between A and B in regard to size and interaction characteristics are small enough to be treated in first order only. To this order exact results are derived regarding the statistics of relative positions of AA, AB, and BB near-neighbor pairs: expressions are given especially for the departures for the average spacing of each of these three types of pairs from the corresponding spacing of the mean lattice, and for the mean-square fluctuations of these relative positions about their means. The expressions can be evaluated from the elastic spectrum of the crystal and da/dx, the variation of the lattice constant with composition. For the common cubic lattices, graphs are given for their approximate numerical evaluation from da/dx and the three elastic constants. The elastic-relaxation contribution to the configuration-dependent energy can be evaluated from the same expressions.

Short-range order in structural models of metallic glass alloys containing Ti

Considerable effort has gone into determining order at an atomically local scale in metallic glass alloys. For those alloys which contain approximately 20% metalloid, the lack of metalloid-metalloid near neighbor pairs has been confirmed and this fact fits intuitive expectations. Recent evidence has been accumulating that similar kinds of order also exist in glasses containing no metalloid elements; e.g. Ti-Cu and Ti-Ni. In these cases the suggestion from neutron diffraction data is that Cu-Cu (or Ni-Ni) near-neighbor pairs occur less frequently than one would expect from a random distribution. On the other hand the measurements on Zr-Cu glasses do not seem to indicate the same kind of order as deduced from the Ti-Cu measurements. The present paper attempts to unravel some of the facts by studying computer simulated models of the atomic structure of such alloys. Detailed analyses are presented in terms of the concentration-concentration correlation statistical distribution functions. The shape of the distributions are affected by various controls on the short-range order of the models. The theoretical efforts attempt to establish the proper conclusions that may be drawn from experimental probes.

Low-temperature specific heat and magnetic susceptibility of nonmetallic vanadium bronzes

A linear $\ensuremath{\gamma}T$ term is observed in the low-temperature specific-heat behavior of the nonmetallic vanadiumoxide bronzes of sodium and copper. The low-temperature magnetic susceptibility is equally anomalous and points to a singlet or diamagnetic ground state. It is postulated that the ${\mathrm{V}}^{4+}$ centers in these bronzes form near-neighbor pairs or bipolarons through deformation-induced attraction. The tunneling of these bipolarons between pairs of sites gives rise to the linear term of the specific heat. The progessive breaking up with temperature of the singlet bipolarons provides a natural explanation of both the magnetic susceptibility as well as of the electrical conductivity.

An application of graph theory to a computer storage problem in residual analysis

The r test procedure for adequacy of the multiple regression model is extended for moderate sample sizes by approximating the moments of the distribution of the test statistic with those of the distribution of the Durbin and Watson (1950) upper-bounding variable. The computation of these moments is simplified by the application of some elementary graph theory. Recommendations con-cerning the selection of near-neighbor pairs are suggested in order to reduce computer storage and execution time.