Large Flat Regions Research Articles

Registration of combined range–intensity scans: Initialization through verification

This paper presents an automatic registration system for aligning combined range–intensity scan pairs. The overall approach is designed to handle several challenges including extensive structural changes, large viewpoint differences, repetitive structure, illumination differences, and flat regions. The technique is split into three stages: initialization, refinement, and verification. During initialization, intensity keypoints are backprojected into the scans and matched to form candidate transformations, each based on a single match. We explore methods of improving this image-based matching using the range data. For refinement, we extend the Dual-Bootstrap ICP algorithm for alignment of range data and introduce novel geometric constraints formed by backprojected image-based edgel features. The verification stage determines if a refined transformation is correct. We treat verification as a classification problem based on accuracy, stability, and a novel boundary alignment measure. Experiments with 14 scan pairs show both the overall effectiveness of the algorithm and the importance of its component techniques.

Equilibrium structure and energetics of CHNO isomers: Steps towards ab initio rovibrational spectra of quasi-linear molecules

We report large-scale electronic structure calculations for fulminic acid, HCNO, isocyanic acid, HNCO, and cyanic acid, HOCN, in their ground electronic states. The coupled cluster CCSD(T) method including all single and double excitations and a perturbative term for connected triple substitutions is used in conjunction with large correlation consistent polarized valence basis sets of the form cc-pVXZ ( X = 2–6) and cc-pCVXZ ( X = 2–5). Our results show the importance of including all electrons in the correlation treatment to obtain a converged molecular structure for the extremely floppy HCNO molecule and the correct energetics of the three isomers. All-electron correlation calculations and frozen core calculations with very large basis sets clearly converge towards a linear electronic minimum for HCNO surrounded by a very large flat region of the potential energy surface for hydrogen bending motions. For each of the three isomers we have computed the barrier to linearity along the respective minimum energy path and several spectroscopic parameters and equilibrium rotational constants.

Pseudogap and central peak in the Emery model

The effect of antiferromagnetic (AF) correlations is studied in the framework of the three-band (Emery) model, with respect to experiments in BSCCO. We study the pseudogap regime with a central peak. Detailed dispersions of quasiparticle peaks show that one can simultaneously fit Fermi surfaces and ARPES leading-edge energy scales. The band parameter regime is a strong-coupling one: marked renormalization of the copper–oxygen overlap, making it smaller than the oxygen–oxygen overlap, while the copper–oxygen energy splitting is the largest of the three. The same regime was found previously in a zeroth-order fit of Fermi surfaces. The inclusion of AF correlations in a weak-coupling approach resolves the only qualitative discrepancy of the zeroth-order mean-field slave-boson calculation with experiment: it is argued that the observed large flat region of the dispersion around the vH point is due to the very non-dispersive central peak in the X– M direction. The sudden increase of the experimental one-particle dispersion in the X– M direction is explained by the quasiparticle strength shifting to the upper wing of the magnetic pseudogap, as one moves further away from the X (van Hove) point. Near it, the lower wing is predicted to be observed in the X– M direction, in addition to the narrow central peak, giving rise to a two-peaked structure below the Fermi level, as found experimentally.

Carving and adaptive drainage enforcement of grid digital elevation models

An effective and widely used method for removing spurious pits in digital elevation models consists of filling them until they overflow. However, this method sometimes creates large flat regions which in turn pose a problem for the determination of accurate flow directions. In this study, we propose to suppress each pit by creating a descending path from it to the nearest point having a lower elevation value. This is achieved by carving, i.e., lowering, the terrain elevations along the detected path. Carving paths are identified through a flooding simulation starting from the river outlets. The proposed approach allows for adaptive drainage enforcement whereby river networks coming from other data sources are imposed to the digital elevation model only in places where the automatic river network extraction deviates substantially from the known networks. An improvement to methods for routing flow over flat regions is also introduced. Detailed results are presented over test areas of the Danube basin.

Formation of the wetting layer in Ge/Si(111) studied by STM and XAFS

We have followed by scanning tunneling microscopy (STM) the Stranski–Krastanov (SK) growth of thin Ge films obtained by reactive deposition epitaxy on a Si(111) surface. For Ge thickness smaller than 1.2 ML, STM images show large flat regions (reconstructed 7×7) without protrusions. With increasing thickness Ge-Si 2D islands (reconstructed 5×5) start nucleating, while the Si substrate retains its original 7×7 reconstruction. The islands are flat and have a triangular shape with a lateral size which increases progressively with deposition. We have studied the growth law of the average dimensions of the islands and of the average number of islands per unit surface as a function of coverage. STM images suggest a sizeable process of intermixing at the Ge/Si interface, which has been directly confirmed by XAFS (X-ray absorption fine structure) spectra on Ge/Si(111) samples. We have followed the evolution of the wetting layer up to its completion, both after and during deposition. The appearance of a percolated structure is observed when a critical fraction of the surface (about 70%) is covered.

Flat Spots: Topological Signatures of an Open Universe in Cosmic Background Explorer Sky Maps

We investigate the behavior of light rays in an open universe with a partly periodic horn topology. The geodesics can be solved exactly and the periodic topology creates characteristic new effects in temperature maps of the microwave background. Large flat regions appear in Cosmic Background Explorer sky maps even when multiple images of astronomical sources are unobservable.

Atomic step distributions on annealed periodic Si(001) gratings

One-dimensional (1D) and two-dimensional (2D) periodic square-wave gratings fabricated on Si(001) with repeat spacings of 4 and 5 μm and amplitudes of 0.1–0.25 μm were annealed in ultrahigh vacuum at temperatures between 900 and 1200 °C. The surface profiles of the gratings were determined by atomic force microscopy in air after cooling to room temperature. The microscopic structures of the steps and terraces of the gratings were investigated in situ using low-energy electron microscopy. The morphology of the gratings changes with annealing temperature. At 950 °C, both 1D and 2D gratings have surface profiles resembling truncated sinusoids, with large flat regions at the grating extrema. Between 1050 and 1100 °C, the flats at the minima of the 2D gratings expand whereas the flats at the maxima shrink. This asymmetry does not occur for the 1D gratings in this temperature range. At 1200 °C, the profiles of both the 1D and 2D gratings become sinusoidal, with no flats at the extrema. The different morphologies and their temperature dependence can be explained on the basis of the thermodynamics of the surface and the role of surface diffusion and evaporation.

Scanning force microscopy and polymerization studies on cast thin films of hectorite and montmorillonite

Thin films of the smectite clays Na+ exchanged montmorillonite, Ca2+ exchanged montmorillonite, Na2+ and Cu2+ exchanged hectorite, Cu2+ exchanged sintered hectorite, and Cu2+ and Na2+ exchanged hectorite exposed to benzene, aniline, and thiophene were studied using the techniques of scanning force microscopy (SFM), electron spin resonance (ESR), and x-ray diffraction. The microstructure of the two montmorillonite clays was markedly different, with the Ca-montmorillonite exhibiting a sturdy, close packed array of crystallites of average dimension 0.4 μm. The Na-montmorillonite clays consisted of smaller (0.1 μm), more poorly defined crystallites with an overall cauliflowerlike appearance. The pure Na-hectorite and Cu-hectorite clays exhibited large flat regions composed on interlocking platelets. Heat sintering as well as exposure to benzene resulted in measurable shifts in the platelets perpendicular to the surface for the Cu-hectorite. It is proposed that in the sintered case these shifts are due to interlayer water loss accompanied by Cu reactions, while in the benzene case interlayer benzene polymerization causes inhomogeneous vertical expansion of the clay. Exposure of Cu-hectorite films to both aniline and thiophene results in large-scale polymerization of these monomers. The microstructure of both of these polymers is similar to that observed in electrochemically prepared films on Pt substrates. Na-hectorite samples did not exhibit surface polymerization upon thiophene, aniline, or benzene exposure.

Anomalous Fermi liquid and strong-coupling superconductivity in cuprates

The hole spectrum of the two-dimensionalt-J model is shown to change drastically due to transitions from the long-range to short-range antiferromagnetic order at the hole concentrationx≈0.04 and from the short-range order to a paramagnetic state atx≈0.19. At 0.05≲x≲0.19 the hole band has large flat regions similar to those observed in photoemission. These regions lead to an anomalous 2D Fermi liquid with a 2D Fermi surface. In spite of a sharp maximum produced by the flat regions in the density of states near the Fermi level, the hole-magnon interaction alone was found to be unable to give rise to superconductivity. By adding a moderate interaction with apex oxygen vibrations highTc are obtained fordx2−y2 pairing. Strong odd-frequency s-wave superconducting fluctuations appear near the transition into the paramagnetic state.

Radar Observations of Asteroids 7 Iris, 9 Metis, 12 Victoria, 216 Kleopatra, and 654 Zelinda

We report 13-cm wavelength radar observations of the mainbelt asteroids 7 Iris, 9 Metis, 12 Victoria, 216 Kleopatra, and 654 Zelinda obtained at Arecibo between 1980 and 1989. The echoes are highly polarized yet broadly distributed in Doppler frequency, indicating that our targets are smooth on decimeter scales but very rough on some scale(s) larger than about 1 m. The echo spectra are generally consistent with existing size, shape, and spin information based on radiometric, lightcurve, and occultation data. All of our targets possess distinctive radar signatures that reveal largescale topography. Reflectivity spikes within narrow ranges of rotation phase suggest large flat regions on Iris, Metis, and Zelinda, while bimodal spectra imply nonconvex, possibly bifurcated shapes for Kleopatra and Victoria. Kleopatra has the highest radar albedo yet measured for a main-belt asteroid, indicating a high metal concentration and making Kleopatra the best main-belt candidate for a core remnant of a differentiated and subsequently disrupted parent body. Upon completion of the Arecibo telescope upgrade, there will be several opportunities per year to resolve main-belt asteroids with hundreds of delay-Doppler cells, which can be inverted to provide estimates of both three-dimensional shape and radar scattering properties.

Tunneling spectroscopy on graphite defects: implications for biological STM

Scanning-tunneling-microscope images are obtained for freshly cleaved graphite in air. Large flat regions of graphite are observed, as well as steps between graphite layers. Tunneling spectroscopy (TS) shows a lower surface density of states for graphite near surface defects at the bias voltages tested. This indicates that TS is sensitive to the state of the substrate and this may have implications regarding the use of TS to reveal detailed information about the presence of molecular adsorbates on surfaces with defects.

NiSi 2 on Si(111) : II. Effects of substrate temperature and defect structure

Using LEED and AES, the growth of NiSi 2 has been investigated on stepped Si(111) surfaces misoriented by 4° toward the [1 1 0] direction. The reaction of Ni with Si(111) was followed as a function of the amount of Ni deposited and the deposition temperature on surfaces with two types of step arrangements. The first is the clean surface which undergoes a structural phase separation at T ≲ 820° C resulting in large flat regions separated by regions of high step density. The second surface contains the Ni-induced ( 19 × 19 ) R23.4° reconstruction which can be pinned in a single rotational domain with corresponding changes in the step structure: the step edges rotate to follow the 19 unit cell, the step-step separation increases to match the width of the 19 unit cell, and the 19 reconstruction is correlated across the steps. On both types of stepped surfaces, even at the extremely slow Ni-deposition rate of ∼ 0.4 ML/min, the previously reported reversal in silicide orientation with increasing Ni-coverage occurs. This change from B-type to A-type occurs at a slightly lower coverage on the clean, stepped surface than on the clean planar surface. On the 19 -reconstructed stepped surface the quality of the B-type silicide is improved and the formation of A-type is suppressed, although no such changes occur on the planar 19 -reconstructed surface. Deposition of Ni at increasing substrate temperatures results in increased diffusion of Ni into the substrate. This is accompanied by broadening of the LEED beams indicating structural damage to the surface. Growth following deposition at 250° C gives poor quality B-type silicide, but no change in the quality of A-type. At a substrate temperature of 450° C, Ni deposition results in no surface Ni observable by AES. The stages of the Ni-Si reaction were monitored during step-wise annealing. When the Ni-Si reaction is mostly complete as determined by AES, the I– V curves first become characteristic of B-type silicide. At low coverages, further annealing improves the quality of the B-type. At high coverages, further annealing causes the I– V curves to change to those characteristic of A-type. Finally, at even higher annealing temperatures, the thin silicide films aggregate to form NiSi 2 islands. The temperature of aggregation is much lower on the stepped surfaces than on planar surfaces.

Long-term average spectrum of Spanish

The long-term average spectrum of Spanish, as spoken in Mexico, was obtained for thirty subjects—fifteen men and fifteen women. To obtain the average spectrum, use was made of a digital system comprised of an FFT analyzer interfaced to a microcomputer. All data acquisition, storage and processing was thus digital. The main features of the average spectrum are: peaks in the region of 240 Hz and 500 Hz, a −20 dB/octave slope from 0·5 kHz to 1 kHz; a flat region between 1 kHz and 2 kHz, a further decline of −10 dB/octave to ≈3 kHz and, finally, a large flat region (within 5 dB) from 3 kHz up to 10 kHz. These results agree reasonably well with some other studies for English. The technique employed here is thus adequate for this kind of study, being relatively fast and easy to implement.