Pair Of Rows Research Articles

Characterization of Metal Alloy Systems by Scanning Tunneling Microscopy and Low‐Energy Ion Backscattering

AbstractLow‐energy ion backscattering and scanning tunneling microscopy (STM) can be used in combination to study the surface crystallography of single crystals, in particular also for alloy and ordered intermetallic compound systems. The elemental composition as well as the location of the atoms is determined by low‐energy noble gas impact collision ion scattering spectroscopy with neutral detection (NICISS). Microscopic information on an atomic scale is obtained with the help of STM. The influence of adsorption‐induced surface modifications is investigated at Cu3Au and compared with Cu surfaces. From the NICISS investigation the AuCu termination is inferred for clean Cu3Au(100) and (110). After careful preparation of the clean Cu3Au(110) surface the prominent (2 × 1) LEED superstructure changes into a pronounced (4 × 1) LEED pattern. The (4 × 1) structure can be explaining by row pairing of CuCu and AuAu rows in the topmost layer. Upon oxygen exposure of the Cu3Au compounds, Cu tends to segregate to the surface. In case of Cu3Au(110) a (2 × 1)O superstructure appears which is in part similar to the known added row structure obtained for Cu(110)(2 × 1)O. Comparison with the Cu(110)O surface shows, however, that the known periodically ordered structure of CuO stripes at undersaturation with oxygen does not show up at Cu3Au(110)‐O.

Oxygen induced missing row reconstruction of substitutionally disordered Mo0.75Re0.25(100)

Adsorption of oxygen on Mo0.75Re0.25(100) with exposures in the range 0.5–100 L and subsequent annealing at or above about 1400 K produces a 2 × 1 superstructure as observed by low energy electron diffraction. The superstructure spots are very bright indicating a strong oxygen induced substrate reconstruction. By full dynamical LEED intensity analysis this is proved to be of the missing row type in agreement with earlier results by ion scattering. As usual, the missing of rows induces row pairing and buckling in the second and third substrate layers, respectively, by amplitudes of the order of 0.1 Å. Substrate interlayer distances change drastically. Oxygen atoms reside in 3-fold coordinated sites binding to both first and second layer atoms with bond lengths of 2.02 Å and 2.04 Å (oxygen hard core radii 0.66 Å and 0.68 Å), respectively. The MoO bonds being stronger than ReO bonds seems to cause and dominate an oxygen induced surface segregation process activated by the high annealing temperature which results in a considerable enrichment of Mo in the surface. If one views the oxygen coordinated atoms as belonging to a composite first layer, both the stoichiometry and distances of deeper layers are very similar to those found for the clean surface.

Intercropped Oat and Fababean in Alaska: Dry Matter Production, Dinitrogen Fixation, Nitrogen Transfer, and Nitrogen Fertilizer Response

AbstractThere are conflicting reports on the amount of N transferred from legumes to intercropped nonlegumes and on the need for N fertilizer to obtain high total dry matter yields on soil with low N availability. This study evaluates (i) the ability of fababean (Vicia faba L.) to meet its N requirements from biological N2 fixation: (ii) the amount of N transferred to intercropped oat (Avena sativa L.); and (iii) N fertilizer management to maximize dry matter production and N2 fixation in a cold soil. A randomized complete block experimental design with four replications was used with two rows of fababean alternating with two rows of oat. Urea was applied in bands between fababean or oat rows at 90 kg N ha−1 to get the following treatments: (i) oat with added N (+N), fababean +N, inoculated with Rhizobium leguminosarum bv. viceae (+I); (ii) oat +N, fababean without added N, inoculated (−N, +I); (iii) oat −N, fababean −N,+I; (iv) oat −N, fababean +N,−I; and (v) oat −N, fababean −N, + I. A second study compared uptake of 15N‐depleted fertilizer by oat and fababean, where the fertilizer was broadcast over the plot or band‐applied between rows of oat and fababean planted in alternate rows. Less than 1% of the uninoculated fababean plants nodulated in either experiment; thus, they were a good check for estimating N2 fixation using the difference method. Nitrogen fertilizer increased dry matter production of fababean in some years, but did not increase N uptake. There was little evidence of N transfer from fababean to oat. Oat depended on N fertilizer for maximum dry matter production 3 of the 4 yr. Maximum total dry matter production was achieved by fertilizing the oat and not the fababean in 3 of the 4 yr. The amount of N2 fixed by fababean averaged 50 kg N ha−1. Fababean plants obtained >50% of their N from broadcast urea, but <10% when urea was banded between oat and fababean rows. Configuring a commercial drill to band fertilizer between pairs of oat rows alternating with fababean is a practical way to maximize both N2 fixation and total dry matter production.

Development of an intense negative hydrogen ion source with an external magnetic filter

An intense negative hydrogen ion source has been developed, which has a strong external magnetic filter field in the wide area of 35 cm×62 cm produced by a pair of permanent magnet rows located at 35.4 cm separation. The filter strength is 70 G in the center and the line-integrated filter strength is 850 G cm, which keeps the low electron temperature in the extraction region. Strong cusp magnetic field, 1.8 kG on the chamber surface, is generated for improvement of the plasma confinement. These resulted in the high arc efficiency at the low operational gas pressure. 16.2 A of H− ion current with the energy of 47 keV was obtained at the arc efficiency of 0.1 A/kW at the gas pressure of 3.8 mTorr in the cesium-mode operation. The magnetic field in the extraction gap is also strong, 450 G, for the electron suppression. The ratio of the extraction current to the negative ion current was less than 2.2 at the gas pressure of 3 mTorr. The two-stage acceleration was tested, and 13.6 A of H− ion beam was accelerated to 125 keV.

Charge-dependent atomic-scale structures of high-index and ( 110) gold electrode surfaces as revealed by scanning tunneling microscopy

The atomic and nanoscale structures of high-index gold surfaces in aqueous perchloric acid electrolyte as revealed by in-situ scanning tunneling microscopy (STM) under electrode potential control are reported with the objective of ascertaining the terrace-step morphology and superstructures as a function of the crystallographic orientation. Six faces, Au(221), (331), (533), (311), (210), and (410), two each lying in the three main zones of the unit projected stereographic triangle, were selected to investigate the role of the step orientation and terrace width for non-vicinal faces. Data for the low-index surface Au(llO) are included for comparison with Au(331) and (221), since all three feature formally a n(111)−(111) terrace-step structure. Measurements of the double-layer capacitance as a function of the electrode potential, E, in dilute perchloric acid were also undertaken in order to evaluate the potential of zero charge (Epzc) for each surface and to check the potential-dependent surface stability. The two surfaces in the (111)−(100) zone, Au(533) and (311), both display essentially (1 × 1) (i.e. bulk-termination) atomic structures at positive electrode charges (i.e. for E >E ), yet exhibit significant surface relaxation at negative charges involving edge-atom depression and row buckling. For Au(221) and (331), lying in the (111)−(110) zone, however, such surface relaxation is seen even at positive electrode charges. This behavioral distinction can be understood on the basis of the differing step structures. Moreover, Au(331) undergoes a reversible (1 × 2) reconstruction at negative charges, involving row pairing. This reconstruction is compared with that observed for Au(110), which also involves “row pairing” but for which several distinct local microstructures can be distinguished. The faces in the (100)−(110) zone, Au(210) and (410), exhibit locally ordered atomic arrangements indicative of an essentially bulk-termination structure. The longer-range superstructures for the high-index faces, specifically involving domain-edge propagation across monoatomic steps, exhibit systematic trends consistent with an effectively attractive step-step interaction at distances within ca. 5 Å.

Row Ordering for a Sparse QR Decomposition

A new row ordering strategy based on pairing rows to minimize local fill-in is presented. The row ordering can be combined with most column ordering strategies to reduce computation, maintain sparsity, and solve rank deficient problems. Comparison of the new row pairing algorithm with Duff’s fixed pivot row ordering on a collection of sparse matrix test problems shows a median 47–71% reduction, depending on the column ordering, in floating point operations (flops) required for the QR decomposition. On a finite element application using nested domain decomposition for the column ordering, the new row ordering is competitive with the row ordering from nested domain decomposition.

Analyses for a planar variably-polarizing undulator

A new undulator for generating variably-polarized radiation was proposed in an earlier paper. This device consists of two pairs of planar permanent magnet rows above and below the electron orbit plane. The magnetic field generated with this undulator, stronger than any other existing planar helical device, induces various types of electron motion such as vertically or horizontally sinusoidal motion and helical motion. The analyses of magnetic field and undulator radiation spectra are made on this undulator. For calculating the spectra and angular distributions in various modes, the SPring-8 storage ring parameters are assumed. This undulator generates brilliant circularly-polarized undulator radiation comparable in intensity with linearly-polarized radiation from a conventional undulator.

An elliptically-polarizing undulator with phase adjustable energy and polarization

We present a planar helical undulator designed to produce elliptically-polarized light. Helical magnetic fields may be produced by a variety of undulators with four parallel cassettes of magnets. In our design, all cassettes are mounted in two planes on slides so that they may be moved parallel to the electron beam. This allows us to produce X-rays of left- or right-handed elliptical or circular polarization as well as horizontal or vertical linear polarization. In model calculations, we have found that by sliding the top pair of rows with respect to the bottom pair, or the left pair with respect to the right pair, we retain the polarization setting but change the magnetic field strength, and hence the X-ray energy. This allows us to select both energy and polarization by independent phase adjustments alone, without changing the gap between the rows. Such a design may be simpler to construct than an adjustable-gap machine. We present calculations that model its operation and its effects on an electron beam.

Linear operators which preserve combinatorial orthogonality

A (1, -1, 0) matrix is said to be a potentially sign-orthogonal sign pattern if the inner product of any pair of distinct rows or columns equals zero or contains terms that are positive and terms that are negative. A potentially orthogonal matrix is a matrix such that some matrix with the same (+, -, 0) sign pattern is an orthogonal matrix. Evidently all potentially orthogonal (1, -1, 0) matrices are potentially sign-orthogonal sign patterns, but not conversely. In this article we characterize the linear operators on the n × n real matrices that map the set X of matrices with potentially sign-orthogonal sign patterns to itself and the complement of X to itself. We also show that the same operators are the only ones that map the set Y of potentially orthogonal matrices to itself and the complement of Y to itself.

Binary Cube Emulation of Butterfly Networks Encoded by Gray Code

We present algorithms for binary cube networks that emulate butterfly network computations on binary-reflected Gray-coded data in the same time complexity as that required for binary-coded data. The code conversion required for the emulation with binary-reflected Gray-coded data is either performed in local memories or through concurrent exchanges. The emulation of a butterfly network with one or two rows mapped to each binary cube node requires n communication cycles on an n-cube. For more than two rows per node, one additional communication cycle is required for every pair of rows, with concurrent communication on all channels of every node. The encoding upon completion can be either binary, or binary-reflected Gray code, or any combination thereof, without affecting the communication complexity.

Surface structures of GaAs passivated by chalcogen atoms

When a GaAs(001) surface with Se adsorbates was flash-heated under a low chemical potential condition, a 2 × 3 RHEED pattern, previously reported as an intermediate structure, remained even after the sample was cooled. The atomic structure observed by STM is in good agreement with the dimer model proposed to explain the chalcogen-passivated GaAs(001) surfaces. Se dimers were found to be buckled, but the 2 × -periodicity was maintained in the [ 1 10] direction, unlike the previously observed 4 × -structure forming the dimer row pairs. Some other structures, the axes of which are in directions different from [110], were also observed on the Se/GaAs(001) surface. The S-passivation effect was studied by measuring current-voltage properties for the S/GaAs(001) surface.

Positional variation in the soil energy balance beneath a row-crop canopy

When crops are grown in a row configuration, differential shading of the soil, coupled with other soil-canopy micrometeorological interactions, may result in large positional variations in the soil energy balance. Experiments were conducted near Manhattan, KS, to examine positional variations in the soil energy balance beneath a soybean ( Glycine max) canopy during periods of partial cover. Continuous measurements of net radiation ( Rn) and soil heat flux ( G) were obtained at four equally spaced positions between one pair of plant rows. Net radiation was calculated from radiation, temperature, and emissivity measurements of the soil, canopy, and sky. Soil heat flux was determined at each location from detailed heat flux and subsurface temperature measurements. When the soil surface was dry, sensible heat flux ( H) was estimated as a residual of the heat balance by assuming that latent heat flux was negligible. Measurements of H and soil-air temperature gradients were used to estimate aerodynamic transport coefficients at each position. Results indicate that the magnitude and pattern of the soil heat balances are strongly dependent on location beneath the canopy. The patterns of G, Rn, and surface temperature at each measurement position were associated with the patterns of shortwave soil irradiance, which were functions of sun-canopy geometry. Soil irradiance between the plant rows sometimes exceeded global irradiance because of reflected radiation from the canopy. Daily Rn between the rows was five to ten times greater than that measured directly beneath the canopy, depending on environmental conditions. Large positional variations in G were also documented. Temperature differences between the sunlit and shaded portions of the surface exceeded 25°C when the surface was dry. Air temperatures, measured at 5 cm above each position, were also dependent on position with respect to the plant rows. When the surface was dry, the majority of sensible heat flux occurred from the soil directly between the plant rows, exceeding 400 W m −2 under certain conditions. Estimates of local aerodynamic transfer coefficients for the soil surface ranged from 1 to 50 mm s −1, but were highly variable and not correlated with above-canopy windspeeds or position beneath the canopy. Results suggest that positional variation in aerodynamic transport from the soil cannot be discerned with the methods used in this study.

Effect of water regime on yield of drip irrigated first ratoon cane intercropped with maize and groundnut

Irrigating an inter-row cropping system particularly by the drip method offers a possible means of improving sugar yield with a concurrent increase in food crop production in the drier areas of Mauritius. In the present study, sole cane and cane intercropped with maize or groundnut were either rainfed or received full or half of their water requirements via driplines buried in between each pair of cane rows. It has been shown that maize reduces cane growth and yield and the effect is more pronounced under stress conditions. There are indications that cane can tolerate a certain amount of stress during the presence of the intercrop and recuperation is possible in the absence of water stress during the stalk elongation phase.

Iodine adlayer structures on Au(111) as discerned by atomic-resolution scanning tunnelling microscopy: relation to iodide electrochemical adsorption

Coverage-dependent adlayer structures of iodine on Au(111) in air or organic solvents obtained by using a high-precision “beetle” — type scanning tunnelling microscope are reported in comparison with those obtained under potential control in aqueous electrochemical environments. The well-known (√3 × √3)R30° structure ( Θ I = 0.33) was only observed at the electrochemical interface at low potentials. Instead, two higher coverage adlattices are evident from the present STM images. The first comprises a (5 × √3) structure ( Θ I = 0.40), with a pair of iodine rows compressed by 20% in the R30° substrate direction. The second high-coverage phase (Θ I ≈ 0.44) consists of a hexagonal iodine overlayer compressed and rotated a few degrees from the R30° direction, giving rise to a long-range (~ 20 Å) corrugation in the STM image. The virtues of quantitative atomic-resolution STM for deducing such complex adiayer structures are pointed out.

Eigenvalues of a class of (0,±1) symmetric matrices

Let G( n) denote the class of all symmetric matrices of order n with zero diagonal and off-diagonal entries ±1. For any C ϵ G( n), let f( C) denote a maximum eigenvalue of C 2, and define g( n) = min{ f;( C): C ϵ G( n)}. Let n≡2 (mod 4). The following two results are proved: (i) Suppose there exists an orthogonal matrix in G( n+4) but not in G( n). Then g( n) = g( n−1 = g( n−2) = n+3. (ii) Let C ϵ G( n) be a matrix for which the bound g( n)= n+3 is attained. Then, if C has a pair of rows whose inner product is ±2, C can be embedded as a principal submatrix of an orthogonal matrix in G( n+4). The case when an orthogonal matrix exists in G( n) has already been investigated by Cameron, Delsarte, and Goethals.

Segregation and ordering at the (1 x 2) reconstructed Pt80Fe20(110) surface determined by low-energy electron diffraction.

The surface of an ordered Pt80Fe20(110) crystal exhibits (1×2) and (1×3) reconstructions depending on the annealing treatment after ion bombardment. The (1×3) structure occurs after annealing in the range 750 to 900 K. Annealing above 1000 K leads to the (1×2) structure, which is, from the present result, unambiguously attributed to the same geometrical reconstruction as Pt(110) but with smaller relaxation amplitudes: a detailed low-energy electron-diffraction analysis concludes to a missing-row structure with row pairing in layers 2 and 4 accompanied by a buckling in layers 3 and 5. The top layer spacing is contracted by 13%, and further relaxations are detectable down to the fifth layer. The specific diffraction spots associated with the bulk chemical ordering along the dense [1¯10] rows are very weak: The I(V) analysis shows that this chemical ordering is absent in the outermost ‘‘visible’’ rows but gradually recovers over five to six layers deep. General Pt enrichment is found in the surface ‘‘visible’’ rows (in layers 1–3), but segregation and order yield a subtle redistribution of Pt and Fe atoms in deeper rows: For example, in layer 2, the visible row is Pt rich, whereas the other row (buried under layer 1) is enriched with Fe. Because of the many parameters considered, a fit procedure was applied to a large data basis to solve the structure; the results were confirmed and illustrated subsequently by a standard I(V) analysis for the most relevant parameters. The final r factors are RDE=0.36, RP=0.34, and RZJ=0.14 for two beam sets at normal and oblique incidence consisting of 26 and 21 beams, respectively.

CO and O 2 adsorption on Rh(110)

The adsorption of CO and oxygen on Rh(110) has been studied using a thermal molecular beam, thermal desorption and LEED. CO adsorbs with an initial sticking coefficient of 0.68 and shows a coverage dependence which is well described by the Kisliuk formalism for precursor kinetics. The desorption shows a main peak at 485 K, which shifts to lower temperature with increasing coverage up to ~ 0.4 monolayers. Above this coverage a shoulder appears at ~ 425 K and a further shoulder at 390 K above about 0.75 monolayers. These effects are due to repulsive lateral interactions in the adlayer, though the only clear ordered pattern seen in the LEED at 320 K is a (2 × 1)p1g1 pattern above 0.9 monolayers. For oxygen the initial sticking coefficient is 0.62 (±0.01) at 310 K which diminishes relatively slowly with increasing oxygen coverage. We believe that this is due to row pairing in the adlayer which produces stretched” rows of higher reactivity adjacent to them. The row pairing is seen in a wide sequence of LEED patterns with increasing coverage — (1 × 3), (2 × 2) plgl, (1 × 2) (1 × 3) , (2 × 3) p1 g1, c(2 × 6), c(2 × 8). Much higher exposures are require latter 3 structures since the sticking coefficient has diminished to a low value by then. The apparent “saturation” point in the sticking curves is 0.65 monolayers, but oxygen can continue to adsorb slowly. We propose that the last three structures are due to completely restructured surfaces, possibly with subsurface oxygen, though this requires further work. Desorption from these high coverage states begins at the relatively low temperature of 750 K.

Oxygen on Cu(100) surface structure studied by scanning tunneling microscopy and by low-energy-electron-diffraction multiple-scattering calculations.

A scanning-tunneling-microscopy study for a surface formed by the chemisorption of oxygen on Cu(100) and displaying a (2 \ensuremath{\surd}2 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 )R45\ifmmode^\circ\else\textdegree\fi{} low-energy-electron-diffraction (LEED) pattern shows the expected rotationally related domains but no unambiguous evidence for coexisting regions of c(2\ifmmode\times\else\texttimes\fi{}2) symmetry. There is a clear pairing of atomic rows, but a low corrugation (\ensuremath{\lesssim}0.2 \AA{}). These features are consistent with results from a new LEED multiple-scattering analysis applied to six diffracted beams for normal incidence. The missing-row model recently proposed [Zeng et al., Surf. Sci. 208, L7 (1989)] is extended to include a 0.30-\AA{} lateral relaxation for top-layer copper atoms adjacent to the missing rows, and small (around 0.10-\AA{}) vertical relaxations in both the first and second copper layers. These features yield the best correspondence between experimental and calculated LEED intensities yet reached for this surface structure.

A review of potato intercropping practices in western Hubei, China

Abstract The historical development of potato: maize intercropping practices is described, indicating how the desire to increase total yield, and the government decision to achieve self-sufficiency in grain production, have been accommodated. The present systems essentially consist of alternate pairs of maize rows planted one month after emergence of single or pair rows of potato. Following potato harvest, another crop is planted in its place between maize, and following the maize harvest winter wheat is frequently planted. Much yield compensation exists between potato, maize, and soybean, within this system. Research results have shown that total productivity can only be marginally increased by choice of short-season potato varieties, increase of total plant population, use of plastic mulches, and alteration of maize sowing dates, and future research concentrates on these efforts.

Affine and projective planes

The aim of this work is to suggest a setting for the discussion and classification of finite projective planes. Our approach rests heavily on the results and techniques of algebraic coding theroy. The approach is to study a finite projective plane II via its various affine parts and, to this end, we introduce the notion of the hull of an affine plane π: the hull turns out to be the code generated, over an appropriate finite field F p , by all differences of those pairs of rows of an indicence matrix that represent parallel lines of π