David P. Shoemaker

A ternary alloy with PbCl2-type structure: TiNiSi(E)

Abstract : Beck and coworkers have found E phases in several ternary systems of transition elements with either silicon or germanium at the composition 1:1:1. The crystal structure of TiNiSi(E) has been determined and refined by least squares with (limited) three-dimensional single-crystal data to a final R value of 0.086 (excluding 002 due to apparent extinction, and all nonobserved reflexions). The lattice parameters for the primitive orthorhombic cell are: a sub 0 = 6.1484=0.0012, b sub 0 = 7.0173=0.0014, c sub 0 = 3.6698=0.0007 A. The E phase is isotypic with PbCl2(C23), space group Pnam. All near-neighbor distances are within 0.06 A of the following average values: Ti-Ti 3.18, Ti-Ni 2.83, Ti-Si 2.61, Ni-Ni 2.67, Ni-Si 2.33 A. The numbers of nearneighbors are compared with those in Co2Si, theta Ni2Si and U3Si2. (Author)

Concerning atomic sites and capacities for hydrogen absorption in the AB2 Friauf-Laves phases

Abstract There is considerable confusion in the literature concerning the maximum number of hydrogen atoms per AB2 that can be absorbed by the C15 and C14 Friauf-Laves phases (variously reported as 5.125 and 7). If all the tetrahedral sites are independently available to hydrogen, the number could approach 17; since, in practice, it rarely exceeds five, we assume that one or more limiting principles operate. The one that we discuss here is assumed to be mainly electrostatic and to be due to electron transfer resulting from electronegativity differences between the hydrogen and metal atoms. Assuming the operation of an exclusion rule which states that two tetrahedra with a face in common may not both contain hydrogen atoms at their centers, after an exhaustive model study we find a maximum occupancy of six hydrogen atoms per AB2 in C15 and of 6 1 3 in C14. Two attractive models accommodating six hydrogen atoms (one for C15, the other for C14) are examined in some detail and the possible implications of these and related models with respect to the step-function absorption model of Jacob, Shaltiel and coworkers is discussed. The possible occupation of triangle sites is discussed briefly.

Neutron diffraction studies of the cubic ZrCr2Dx and ZrV2Dx (Hx) PHASES

Abstract Deutendes of ZrCr 2 and both hydrides and deutendes of ZrV 2 were studied by neutron diffraction in order to determine the relative occupancies of the different types of tetrahedral sites existing in the C15 Friauf-Laves phases. In ZrV 2 , within the range of stoichiometry used, both Zr 2 V 2 and ZrV 3 sites are occupied by deuterium with the relative occupancies depending on the total absorption. In ZrCr 2 only the Zr 2 Cr 2 site is progressively filled until the formula approaches ZrCr 2 D 3.5 , near the experimental limit of absorption, at which point a minor amount of deuterium may enter ZrCr 3 sites.

Interatomic distances and atomic valences in NaZn13

The crystal structure of NaZn_(13) and of several homologous compounds AB_(13) was reported by Ketelaar and by Zintl & Hauke to be based on space group O_h^6-Fm3c, with 8 :Na in 8(a): ¼, ¼:, ¼; ... ; 8 Zn_I in 8(b): 0, 0, 0; .... ; and 96 Zn_(II) in 96(i): 0, y, z; ... . Approximate values were reported for the parameters a_0, y, and z; for NaZn_(13) Zintl & Hauke reported 12.27 A, 0.178, and 0.122 for these three parameters. Each Zn_I is surrounded by twelve Zn_(II) at the vertices of a nearly regular icosahedron, and each Na by twenty-four Zn_(II) at the vertices of a snub cube. Our interest in the structure was largely concerned with the valences of the two different kinds of Zn atoms, it being presumptive that Zn_I has a larger valence than Zn_(II) because its icosahedral coordination requires it to be smaller than Zn_(II). Lines on new powder photographs of NaZn_(13) were measured and the intensities were estimated visually with as much precision as possible. Least-squares treatments were employed in order to obtain the best possible values for the three parameters; the values obtained are a_0 = 12.2836 ± 0.0003A, y = 0.1806 ± 0.0003, and z = 0.1192 ± 0.0003. The uncertainties given are calculated standard deviations. Analysis of the interatomic distances yields a selfconsistent interpretation in which Zn_I is assumed to be quinquevalent and Zn_(II) quadrivalent, while Na may have a valence of unity or one as high as 1¼, the excess over unity being suggested by the interatomic distances and being, if real, presumably a consequence of electron transfer. A valence electron number of approximately 432 per unit cell is obtained, which is in good agreement with the value 428.48 predicted on the basis of a filled Brillouin polyhedron defined by the forms {444}, {640}, and {800}.

A passive vibration isolation stack for LIGO: Design, modeling, and testing

Multiple‐stage seismic vibration isolation stacks, which consist of alternating layers of stiff masses and compliant springs, can provide significant passive filtering of ground vibration for experiments and equipment that are sensitive to mechanical noise. We describe the design, modeling and testing of a prototype of a stack suitable for use in the Laser Interferometer Gravitational‐wave Observatory (LIGO). This is a four‐stage elastomer (spring) and stainless steel (mass) stack, consisting of a table resting on three separate legs of three layers each. The viscoelastic properties of elastomer springs are exploited to damp the stack’s normal modes while providing rapid roll‐off of stack transmission above these modal frequencies. The stack’s transmission of base motion to top motion was measured in vacuum and compared with three‐dimensional finite‐element models. In one tested configuration, at 100 Hz, horizontal transmission was 10−7, vertical transmission was 3×10−6, and the cross‐coupling terms were between these values.

Alignment of an interferometric gravitational wave detector

Interferometric gravitational wave detectors are designed to detect small perturbations in the relative lengths of their kilometer-scale arms that are induced by passing gravitational radiation. An analysis of the effects of imperfect optical alignment on the strain sensitivity of such an interferometer shows that to achieve maximum strain sensitivity at the Laser Interferometer Gravitational Wave Observatory requires that the angular orientations of the optics be within 10(-8) rad rms of the optical axis, and the beam must be kept centered on the mirrors within 1 mm. In addition, fluctuations in the input laser beam direction must be less than 1.5 x 10(-14) rad/ radicalHz in angle and less than 2.8 x 10(-10) m/ radicalHz in transverse displacement for frequencies f > 150 Hz in order that they not produce spurious noise in the gravitational wave readout channel. We show that seismic disturbances limit the use of local reference frames for angular alignment at a level approximately an order of magnitude worse than required. A wave-front sensing scheme that uses the input laser beam as the reference axis is presented that successfully discriminates among all angular degrees of freedom and permits the implementation of a closed-loop servo control to suppress the environmentally driven angular fluctuations sufficiently.

Adaptive thermal compensation of test masses in advanced LIGO

As the first generation of laser interferometric gravitational wave detectors nears operation, research and development has begun on increasing the sensitivity of the instrument while utilizing the existing infrastructure. In the laser interferometer gravitational wave observatory (LIGO), significant improvements are being planned for installation around 2007, increasing strain sensitivity through improved suspensions and test mass substrates, active seismic isolation and higher input laser power. Even with the highest quality optics available today, however, finite absorption of laser power within transmissive optics, coupled with the tremendous amount of optical power circulating in various parts of the interferometer, results in critical wavefront deformations which would cripple the performance of the instrument. A method of active wavefront correction via direct thermal actuation on optical elements of the interferometer is discussed. A simple nichrome heating element suspended off the face of an affected optic will, through radiative heating, remove the gross axisymmetric part of the original thermal distortion. A scanning heating laser will then be used to remove any remaining non-axisymmetric wavefront distortion, generated by inhomogeneities in absorption of the substrate, thermal conductivity, etc. A proof-of-principle experiment has been constructed at MIT, selected data of which are presented.

NuMesh: an architecture optimized for scheduled communication

The NuMesh system defines a high-speed communication substrate optimized for off-line routing. By determining possible communication paths at compile time, highly efficient hardware and software constructs can be exploited to yield superior network performance. These communication paths can be independently tuned to allow more utilized paths greater bandwidth. Although communication paths are scheduled, data need not be sent during every scheduled cycle. Flow-control protocols allow for empty communication cycles as well as for data backup in the network. Limited gate delays between NuMesh registers as well as single-cycle message transfers allow for a high clock frequency and low network latency. A highly pipelined architecture for this communication is presented and a mechanism for efficient flow-controlled communication is discussed. A unique communication protocol is presented and shown to provide single-cycle transfers between nodes. An overview of the necessary compiler support is also provided. Preliminary results and a description of the current hardware and software status are listed.

Prototype Michelson interferometer with Fabry–Perotcavities

We describe a rigid, internally modulated Michelson interferometer with Fabry-Perot cavities in the interferometer arms. The high contrast (0.986) and the small cavity losses (2.7%) permit efficient use of the light power available. The measured shot-noise-limited displacement sensitivity for 35mW of light power is 2.5 x 10(-17) m radicalHz, in good agreement with the calculated signal-to-noise ratio.

The crystal structure of NbCoB

(i) two strong water-thiosulphate oxygen hydrogen bonds, O(4)-H(1) . . .0(3) (2.73 A) and O(5)-H(3) . . .0(3) (2.77 ,~), (ii) two weak water-thiosulphate oxygen hydrogen bonds, O(4)-H(2) . . .0(2) (2.94 A) and O(5)-H(4) . . .0(3*) (2.81 A) , (iii) two water-water oxygen hydrogen bonds O(6)-H(5) . . .0(4*) (2.73 A) and O(7)-H(8) . . .0(8*) (2.79 A) , (iv) three water-sulphur hydrogen bonds, O(6)-H(6). . .S(I*) (3.30 A) O(7)-H(7): . . S(I*) (3.13 A) and O(8)-H(10).. .S(1) (3.40 A) .