Updated Search for Alternative Unit Cells

As 2nd and 3rd generation Focal Plane Arrays (FPA) become more complex, the readout integrated circuit (ROIC) has emerged as a major discriminator in system performance. The focus of development and advancement has traditionally involved the detector technology. Early ROICs were simple multiplexers that performed little if any signal processing on the detector diode signal. Advances in silicon fabrication processes for analog integrated circuits have opened a new era in IRFPAs where signal digital functions can be achieved on the focal plane. We present an overview of significant advances in the area of mixed mode ROIC designs that enable greater functionality and performance of the sensor chip assembly. Innovations, continuing progress in CMOS technology, and greater foundry access have allowed enhancements in practically every aspect of the ROIC, from sophisticated unit cells to lower noise and lower power signal paths to highly programmable digital support circuitry. Denser detector input circuits with active amplifiers (FEDI or CTIA) have been implemented in unit cells as small as 27 micrometer X 27 micrometer. In addition, multiple gain, temporal filtering, or spatial filtering capabilities have been incorporated into these small unit cells. Significant reductions in focal plane power have been fabricated and demonstrated enabling a factor of 2 increase in frame rates for very large staring FPAs and a factor of 4 increase in line rates for scanning FPAs. Other developments include, but are not limited to, alternative schemes for time-delayed integration (TDI) and breakthroughs for uncooled applications. As the chip designs increase in capability, greater systems on a chip are feasible, especially with more programmable features provided by the on-chip digital circuitry.

A key towards understanding the properties of high-temperature superconductors is knowledge about the distribution of the valence electrons and holes. Despite their complicated crystal structure, electronic structure of many cuprates has been calculated. The most successful calculation method is based on the theorem that the ground state energy is a unique functional of the electron density. Thus, experimental techniques that give information about the valence electron density are essential to test the calculations and to gain further insight into these important materials. To move beyond locating the atoms in the crystal unit cell towards addressing the details of the electron distribution requires in general extremely accurate diffraction experiments. Highly perfect crystals with small unit cells have been approached. It has been known for several decades that in electron diffraction the amplitudes of those reflections at small scattering angles that are present in crystals with large unit cells, are strongly influenced by the valence electron distribution.

With the high-resolution electron microscopes now commercially available, it is possible to obtain resolution in images of crystal lattices of 2Å or better. While this is applied most frequently for obtaining simple fringe patterns from materials having small unit cells, it may also be used to resolve detail within the unit cell of more complicated structures as in figure 1, obtained with a JEM-100B electron microscope at 100kV. We are investigating means for obtaining better images, e.g. by the use of pointed filaments, and are studying the possibility that the details of contrast in such images may be interpreted in terms of the crystal structure.Since a thin crystal is primarily a phase object, a simple, intuitive interpretation of the contrast is not possible. Calculations of contrast made by Elizabeth Chidzey (1970) using a phase-grating approximation for very thin crystals and a more complete n-beam dynamical theory for thicker crystals have confirmed the strong dependence of the contrast on crystal orientation, crystal thickness and the defocus of the microscope.

Rotational barriers in ammonium hexachlorometallates as studied by NMR, tunnelling spectroscopy and ab initio calculations

The authors are thankful for the useful comments [1] on their letter [2]. The presented artificial magnetic conductor (AMC) is not composed of square but hexagonal unit cells. The referenced first paragraph of [2, Section II], “Planar AMC Design,” makes reference to a general behavior based on a very simple LC model for the unit-cell geometry. Generally, the unit cell can be considered a frequency selective surface (FSS) backed by a dielectric slab with metallic ground plane. To increase the bandwidth of an AMC, it is necessary to reduce the total capacitance C. When the distance between unit cells increases, the gap capacitance decreases. If this is the main capacitive element contributing to the total C, the bandwidth increases. This happens, for example, for the patch unit cell in [3, Table I], a work referenced by [1]. In [3], only one capacitive element is used to model the unit cell, and the objective is to maximize the bandwidth while preserving the resonance frequency. The point is that the total C of the unit-cell model has to be expressed as a series or parallel combination of capacitive elements (gap, parallel plates, fringing effects, etc.), which obviously depends on the geometry of such a unit cell. However, the geometry of the presented unit cell (formed by dipoles) is mainly inductive due to the narrow strips. The behavior of the presented unit cell is described in the third paragraph of [2, p. 616]. It is clearly mentioned that there are two main capacitive elements that contribute to the total C: coplanar C (or gap C) and parallel plates C. Moreover, it is stated that “in sum, the wider AMC operation bandwidth for smaller unit-cell size (W) of this novel design is due to both slightly increased L and more significantly C decreased values of the parallel LC equivalent circuit.” This does not contradict [1] since, according to Fig. 3 and Table I of [2], for smaller unit cell W, the distance between cells 2d (the gap) decreases, and so the gap capacitance.

The acetone‐ and methyl ethyl ketone‐amylose V complexes have been prepared and their unit cell dimensions calculated from their x‐ray powder patterns. This work demonstrates the size‐determining effect of these two ketones on the amylose V complex. The small unit cell, with a helix diameter of 13.0 A., observed with the wet methanol and ethanol complexes, is not found with the acetone and methyl ethyl ketone complexes. The wet ketone‐amylose V complexes and the thoroughly dried ketone complexes have the larger helix diameter of 13.7 A. This is the first report of an anhydrous amylose V complex with the expanded helix diameter. Thorough extraction of the methyl ethyl ketone‐amylose complex with methyl alcohol did not change the 13.7 A. helix diameter, although methyl alcohol‐extracted acetone‐amylose complex resulted in a mixture of the 13.0 A. and 13.7 A. helix diameter types. This work demonstrates the size effect of the lower molecular weight ketones on the amylose crystalline structure.

Structure of the Alzheimer's Amyloid Precursor Protein (APP) Copper Binding Domain in 'small unit cell' form, atomic resolution

Discrete dislocation plasticity analysis of the high-temperature cyclic response of composites

Crystallization and Preliminary X-ray Analysis of Papaya Mosaic Virus Coat Protein

A linear stability analysis is performed for the homogeneous state of a monodisperse gas-fluidized bed of spherical particles undergoing hydrodynamic interactions and solid-body collisions at small particle Reynolds number and finite Stokes number. A prerequisite for the stability analysis is the determination of the particle velocity variance which controls the particle-phase pressure. In the absence of an imposed shear, this velocity variance arises solely due to the hydrodynamic interactions among the particles. Since the uniform state of these suspensions is unstable over a wide range of values of particle volume fraction φ and Stokes number St, full dynamic simulations cannot be used in general to characterize the properties of the homogeneous state. Instead, we use an asymptotic analysis for large Stokes numbers together with numerical simulations of the hydrodynamic interactions among particles with specified velocities to determine the hydrodynamic sources and sinks of particle-phase energy. In this limit, the velocity distribution to leading order is Maxwellian and therefore standard kinetic theories for granular/hard-sphere molecular systems can be used to predict the particle-phase pressure and rheology of the bed once the velocity variance of the particles is determined. The analysis is then extended to moderately large Stokes numbers for which the anisotropy of the velocity distribution is considerable by using a kinetic theory which combines the theoretical analysis of Koch (1990) for dilute suspensions (φ [Lt ] 1) with numerical simulation results for non-dilute suspensions at large Stokes numbers. A linear stability analysis of the resulting equations of motion provides the first a priori predictions of the marginal stability limits for the homogeneous state of a gas-fluidized bed. Dynamical simulations following the detailed motions of the particles in small periodic unit cells confirm the theoretical predictions for the particle velocity variance. Simulations using larger unit cells exhibit an inhomogeneous structure consistent with the predicted instability of the homogeneous gas–solid suspension.

Calibration method of atomic multipole moments (AMMs) is presented with respect to geometries of all-siliceous zeolite models obtained with X-ray diffraction (XRD) methods. Mulliken atomic charges and AMMs are calculated for all-siliceous types possessing small size elementary unit cells at the hybrid density functional theory (DFT) (B3LYP) and general gradient approximation (GGA) Perdew-Burke-Ernzerhof (PBE) levels and then used to fit the dependences versus geometry variables for the Mulliken charges and versus special coordinate for the AMMs. Fitted and exact charges and AMMs are used to compute electrostatic potential (EP) and electric field (EF) for all-siliceous zeolites with CRYSTAL. A possibility of application of the point AMMs to quantum mechanical/molecular mechanics computations or classic simulation of physical adsorption is evaluated. The considered models expand over wide range of structural parameters and could be applied even to amorphous all-siliceous systems.

Inherent negative refraction on acoustic branch of two dimensional phononic crystals

The importance of beam alignment and crystal tilt in high resolution electron microscopy

Enumeration of wurtzite derivatives and related dipolar tetrahedral structures

X-ray scattering measurements give the squared scattering amplitudes, at discrete values of the scattering factor S, which are related to the electron density in real space ρ (r) by a simple Fourier transform relationship. Thus, X-ray scattering may be analyzed in either real or reciprocal space, the choice often being made on the basis of personal preference. Much early work in electron density analysis, done on high symmetric crystals with small unit cells, employed a small number of low order structure factors. The method is still being used in comparison of theory and experiment on structures such as silicon1 and vanadium.2 Reciprocal space analysis is also extremely well suited to leastsquares adjustment of a parametrized model because of the discrete nature of the Fourier transform of a periodic (though continuous) function.