LEED structure analysis of a Co{001} surface

A {001} surface of face-centered-cubic cobalt was cleaned to the point of elimination of all impurities except carbon and oxygen, which were reduced to minimum terminal amounts. A LEED structure analysis of this surface, using 12 intensity spectra at 3 angles of incidence, reveals that the atomic arrangement corresponds to truncation of the bulk structure but with about 4% contraction of the first interlayer spacing along 〈001〉 with respect to the bulk.     

Surface structure analysis based on the exclusive use of the specular LEED spot – a theoretical study

The exclusive use of the specularly reflected beam (the (0,0) spot) may be a more practical way of collecting data for a LEED I–V structure analysis under certain experimental conditions. In this paper we discuss the special properties of the (0,0) spot intensity and test its sensitivity towards structural changes for the model system CO/Ni(1 1 1) within the framework of a R factor analysis. It is found that the (0,0) spot can, indeed, be used for a reliable structure determination if the energy range is increased by collecting data at different polar and azimuthal angles of incidence. The R factor contrast is, however, reduced with respect to a conventional LEED I–V analysis.     

Enhancement of histological volumes through averaging and their use for the analysis of magnetic resonance images

Magnetic resonance imaging (MRI) of small animals is routinely performed in research centers. But despite its many advantages, MR still suffers from limited spatial resolution which makes the interpretation and quantitative analysis of the images difficult, particularly for small structures of interest within areas of significant heterogeneity. One possibility to address this issue is to complement the MR images with histological data, which requires reconstructing 3D volumes from a series of 2D images. A number of methods have been proposed recently in the literature to address this issue, but deformation or tearing during the slicing process often produces reconstructed volumes with visible artifacts and imperfections. In this paper, we show that a possible solution to this problem is to work with several histological volumes, reconstruct each of these separately and then compute an average. The resulting histological atlas shows structures and substructures more clearly than any individual volume. We also propose an original approach to normalize intensity values across slices, a required preprocessing step when reconstructing histological volumes. We show that the histological atlas we have created can be used to localize structures and substructures, which cannot be seen easily in MR images. We also create an MR atlas that is associated with the histological atlas. We show that using the histological volumes to create the MR atlas is better than using the MR volumes only. Finally, we validate our approach quantitatively on MR image volumes by comparing volumetric measurements obtained manually and obtained automatically with our atlases.     

Is there a future for LEED intensities?

The application of LEED intensities within surface crystallography is illuminated with respect to its today's standard, frontier and hopes. It is shown that structure determination is possible with high accuracy in case of simple surfaces. For more complex surfaces with about a handful of atoms in the surface unit-cell structural parameters are still reliably determined, however, with tremendously increasing computational amount of work. Some approximative approaches which could show a way out of the misery are described. However, importance is attached to other and newer developments of LEED as well. So the possibility to extract structural informations from diffuse intensities in case of disordered adsorption is demonstrated as well as the use of intensities to probe the critical phenomena and kinetics of surface phase transitions. Examples are given which make the future of LEED intensities appear quite promising.     

The use of the Patterson function in surface structure determination by LEED

Recently, Clarke, Mason and Tescari have proposed a modified Patterson function for deriving surface structures from LEED intensity data. The technique is analysed in this paper by applying it to “averaged” experimental data and to data from kinematical calculations. It is found that truncation errors (associated particularly with the inner potential) prevent the technique from being generally useful, and their absence in the work of Clarke et al. was due to their use of an unrealistic inner potential. In addition, the complex form of scattering factors applicable to LEED provide a fundamental barrier to the use of Patterson functions for the analysis of LEED data in the most interesting case of adsorbate structures on surfaces.     

Design and demonstration of a novel optical CDMA platform for use in avionics applications

We designed, built, and demonstrated a highly scalable incoherent optical CDMA platform under DARPA contract which was delivered to Lockheed Martin for additional testing in avionic applications. The platform enables users to communicate with each other at ∼1.25 Gbit/s per user with raw BER of less than 10⁻¹². The system architecture uses (3, 11) fast wavelength-hopping, time-spreading prime codes with a chip size of 73 ps utilizing picosecond optical pulses allocated in the time and wavelength domains. A novel design of a “dual code” optical encoder and decoder realized a novel optical layer implementation of an XOR gate and enabled secure network connectivity using a “One-time pad” encryption approach. The testbed is also designed to conduct eavesdropping studies on testbed users. The incoherent OCDMA approach is compatible with existing DWDM optical networks and uses off-the-shelf components.     

A test of energy averaging in LEED: The coincidence lattice structure formed by Ag on Cu(111)

An analysis of LEED data from a structure believed to consist of a monolayer of silver on a clean copper (111) surface has been performed. Using a hard-sphere model as a guide for the surface structure, energy-averaged beam intensities from experiment have been compared with those from kinematical calculations as proposed by Tucker and Duke. Significant differences are found which are believed to be associated with multiple scattering. This casts doubt on the value of this technique for determining surface structures of strong-scattering adsorbates on metals.     

Angle-resolved ups measurements in a modified LEED system

A LEED chamber has been modified to include a differentially pumped discharge lamp (He or Ne) and an additional retarding grid electron energy analyser for UPS. This small analyser is located at right angles to the LEED analyser and does not interfere with normal LEED and Auger operations. The UPS signal is amplified by a channel plate multiplier and accelerated onto a phosphor-coated screen. Directional information is obtained by scanning this screen with a collimated photomultiplier detector. A phase-lock amplifier is used to differentiate the signal from the photomultipler. Alternatively the phosphor screen can be used as a collector to measure a total spectrum. The acceptance angle of the UPS analyser is 90°. In the angular resolving mode it is possible to observe emission from a (100) fcc crystal in the 〈100〉, 〈110〉 and 〈111〉 directions with a fixed incident photon angle in the range 20–40° to the normal. The acceptance angle of the detector was usually ~7° but this can be varied by changing the collimating tube on the photomultipler. The direction dependent features of the d-band spectrum of clean nickel with a (100) surface have been examined. Characteristic features were observed for each of the 〈100〉, 〈111〉 and 〈110〉 directions. These are compared with those reported for crystals with the corresponding surface orientations. The effects resulting from the chemisorption of nitric oxide on this nickel crystal have also been investigated.     

Speckle averaging in deformation analysis using video speckle interferometry

We show that speckle averaging in deformation analysis using video speckle interferometry increases the signal to noise ratio and improves the resolution in the resulting image. Speckle averaging in deformation analysis normally demands a very high degree of stability and repeatibility. Averaging phase gradient maps reduces these demands considerably. Experimentally, deformation details down to 2 × 2 pixels in the averaged pictures have been resolved.     

Experimental demonstration of a classical analog to quantum noise cancellation for use in gravitational wave detection

We present results that are a classical analog to quantum noise cancellation. It is possible to breach the standard quantum limit in an interferometer by the use of squeezing to correlate orthogonal quadratures of quantum noise, causing their effects on the resulting sensitivity to cancel. A laser beam incident on a Fabry-Perot cavity was imprinted with classical, correlated noise in the same quadratures that cause shot noise and radiation pressure noise. Couplings between these quadratures due to a movable mirror, sensitive to radiation pressure, cause the excess classical noise to cancel. This cancellation was shown to improve the signal to noise ratio of an injected signal by approximately a factor of 10.     

利用磁悬浮演示真空不传声

针对传统真空不传声实验装置的缺陷在于底座传声,提出悬浮发声体的设想,并通过磁悬浮来实现.文章介绍了磁悬浮的原理和实验装置,并做了实际测试.数据分析显示,演示效果有了显著改善.     

First demonstration of a temporal coherence analysis through a parametric interferometer

We analyze the temporal coherence of an optical infrared radiation in the visible domain by using a Mach-Zehnder interferometer and a wavelength conversion stage in each arm. We exploit a sum frequency generation process in bulk PPLN crystal to convert the infrared radiation at 1.55 μm into 0.63 μm before the interferometric mixing. The applicability of the Wiener-Kintchine theorem through up-conversion processes is here demonstrated by direct comparisons among visible and infrared measurements.     

Energy shift procedure for the inner potential in LEED crystallography

The accuracy of the assumption that LEED spectra for differing inner potentials can be found by shifting calculated spectra along the energy axis is studied. The spread in angle of refraction in typical conditions up to angle of incidence θ = 25° is shown to be small. The variation of θ with energy that minimizes the spread is found. Statistical analysis of data from three different surface structures shows the assumption to be adequate at least up to θ = 25°.     

14N pulsed nuclear quadrupole resonance. 3. Effect of a pulse train. Optimal conditions for data averaging

The calculations developed in this paper aim at determining the optimal conditions of a NQR experiment when a transition is monitored by means of a pulse train with pulses of identical duration and signal acquisition after each pulse; coherences are assumed to vanish by effective transverse relaxation prior to every new pulse. These calculations demonstrate that, as in NMR, a steady state is effectively reached for any value of the recycle time. However, by contrast with NMR, it is shown that, for optimal data averaging under steady state conditions, the recycle time T can be kept as low as possible (the only limitation is the acquisition time). Nutation curves (signal amplitude versus pulse length) calculated in the steady state case are shown to depend strongly on the ratio T/T ₁ (T ₁: longitudinal relaxation time). The signal growth as a function of T/T ₁under averaging of the first transients has been evaluated as well as the number of pulses necessary for reaching a steady state.     

一种应用于心磁噪声抑制的选择性平均方法研究

在心磁信号探测中, 抑制环境噪声是提取心磁信号的关键. 为了提高心磁信号的信噪比, 信号平均得到了广泛地使用. 然而, 由于局部干扰噪声的存在, 对整段数据进行平均的传统方法不可避免地会带来心磁信号的失真. 本文通过采取模版匹配的方式, 提出了一种选择性平均方法. 结果显示, 本方法能有效剔除带有低频波动、脉冲毛刺等干扰的数据段, 获取高质量的平均心磁信号. 关键词： 心磁 信号平均 噪声抑制 信噪比