Multiband superconductivity in the Chevrel phases SnMo6S8 and PbMo6S8.

Sub-Kelvin scanning tunneling spectroscopy in the Chevrel phases SnMo6S8 and PbMo6S8 reveals two distinct superconducting gaps with Δ1=3 meV, Δ2∼1.0 meV and Δ1=3.1 meV, Δ2∼1.4 meV, respectively. The gap distribution is strongly anisotropic, with Δ2 predominantly seen when scanning across unit-cell steps on the (001) sample surface. The spectra are well fitted by an anisotropic two-band BCS s-wave gap function. Our spectroscopic data are confirmed by electronic heat capacity measurements, which also provide evidence for a twin-gap scenario.

Ag(2.54)Tl(2)Mo(12)Se(15): a new structure type containing Mo(6) and Mo(9) clusters.

The novel structure-type Ag(2.54)Tl(2)Mo(12)Se(15) (silver thallium molybdenum selenide) is built up of Mo(6)Se(i)(8)Se(a)(6) and Mo(9)Se(i)(11)Se(a)(6) cluster units in a 1:2 ratio, which are three-dimensionally connected to form the Mo-Se network. The Ag and Tl cations are distributed in several voids within the cluster network. Three of the seven independent Se atoms and one Tl atom lie on sites with 3.. symmetry (Wyckoff sites 2c or 2d).

Real-space vortex glass imaging and the vortex phase diagram of SnMo6S8.

Using scanning tunneling microscopy at 400 mK, we have obtained maps of around 100 vortices in SnMo(6)S(8) from 2-9 T. The orientational and positional disorder at 5 and 9 T show that these are the first large-scale images of a vortex glass. At higher temperature a magnetization peak effect is observed, whose upper boundary coincides with a lambda anomaly in the specific heat. Our data favor a kinetic glass description of the vortex melting transition, indicating that vortex topological disorder persists at fields and temperatures far below the peak effect in low-T(c) superconductors.

Crystal structure and physical properties of a novel Kondo antiferromagnet: U(3)Ru(4)Al(12).

A novel ternary compound U(3)Ru(4)Al(12) has been identified in the U-Ru-Al ternary diagram. Single-crystal x-ray diffraction indicates a hexagonal Gd(3)Ru(4)Al(12)-type structure for this uranium-based intermetallic. While this structure type usually induces geometrically a spin-glass behaviour, an antiferromagnetic ordering is observed at T(N) = 8.4 K in the present case. The reduced effective magnetic moment of U atoms (µ(eff) = 2.6 µ(B)) can be explained by Kondo-like interactions and crystal field effects that have been identified by a logarithmic temperature dependence of the electrical resistivity, negative values of the magnetoresistivity and particular shape of the Seebeck coefficient.

Cr1.45Tl1.87Mo15Se19, a monoclinic variant of the hexagonal In3Mo15Se19 type.

The monoclinic compound Cr(1.45)Tl(1.87)Mo(15)Se(19) (chromium thallium pentadecamolybdenum nonadecaselenide) represents a variant of the hexagonal In(3)Mo(15)Se(19) structure type. Its crystal structure consists of an equal mixture of Mo(6)Se(8)Se(6) and Mo(9)Se(11)Se(6) cluster units. The Mo and Se atoms of the median plane of the Mo(9)Se(11)Se(6) unit, as well as three Cr ions, lie on sites with m symmetry (Wyckoff site 2e). The fourth Cr ion is in a 2b Wyckoff position with 1 site symmetry.

A disorder-enhanced quasi-one-dimensional superconductor.

A powerful approach to analysing quantum systems with dimensionality d>1 involves adding a weak coupling to an array of one-dimensional (1D) chains. The resultant quasi-1D (q1D) systems can exhibit long-range order at low temperature, but are heavily influenced by interactions and disorder due to their large anisotropies. Real q1D materials are therefore ideal candidates not only to provoke, test and refine theories of strongly correlated matter, but also to search for unusual emergent electronic phases. Here we report the unprecedented enhancement of a superconducting instability by disorder in single crystals of Na2-δMo6Se6, a q1D superconductor comprising MoSe chains weakly coupled by Na atoms. We argue that disorder-enhanced Coulomb pair-breaking (which usually destroys superconductivity) may be averted due to a screened long-range Coulomb repulsion intrinsic to disordered q1D materials. Our results illustrate the capability of disorder to tune and induce new correlated electron physics in low-dimensional materials.

Structural analysis of polymers of sickle cell hemoglobin. II. Sickle hemoglobin macrofibers.

Sickle cell hemoglobin macrofibers are an important intermediate in the low pH crystallization pathway of deoxygenated hemoglobin S that link the fiber to the crystal. Macrofibers are a class of helical particles differing primarily in their diameters but are related by a common packing of their constituent subunits. We have performed three-dimensional reconstructions of three types of macrofibers. These reconstructions show that macrofibers are composed of rows of Wishner-Love double strands in an arrangement similar to that in the crystal. We have measured the orientation and co-ordinates of double strands in macrofibers using cross-correlation techniques. In this approach, the electron density projections of double strands calculated from the known high-resolution crystal structure are compared with regions along the length of the particles in which the distinct pattern of double strands in c-axis projection may be observed. Contrary to assertions by Makinen & Sigountos (1984), our results unambigously demonstrate that adjacent rows of double strands in macrofibers are oriented in an antiparallel manner, as in the Wishner-Love crystal. Adjacent rows of antiparallel double strands are displaced along the helical axis relative to their co-ordinates in the crystal. Electron density models of macrofibers based on the crystallographic structure of the sickle hemoglobin double strand are in good agreement with the projections of macrofibers observed in electron micrographs. We have studied the structure of a closely related crystallization intermediate, the sickle hemoglobin paracrystal. The arrangement of double strands in paracrystals is similar to that in Wishner-Love crystals, except that they are displaced along the a-axis of the crystal. Measurements of the double strand co-ordinates reveal that the distribution of strand positions is bimodal. These results further establish the close structural relationship between macrofibers and paracrystals as intermediates in the crystallization of deoxygenated sickle hemoglobin.

Structural analysis of polymers of sickle cell hemoglobin. I. Sickle hemoglobin fibers.

The structure of fibers of deoxyhemoglobin S has been under investigation for several years and a number of different models have been proposed for the arrangement of molecules within the particles. We have used reconstruction and modeling techniques in our analysis of these structures. Several new approaches have been employed in this analysis in order to provide improved estimates of the co-ordinates, pairing, and polarity of the hemoglobin S molecules. Fibers have a variable pitch and, in order to minimize distortions in the reconstructed density maps associated with these variations in pitch, we have developed an iterative procedure to measure the instantaneous pitch and have modified the reconstruction algorithm to incorporate the measured values. This procedure improves the accuracy with which the hemoglobin S molecules can be located in the density maps. Furthermore, the determination of the instantaneous pitch allows us to measure directly the rotation of the individual hemoglobin molecules. These measurements are in excellent agreement with the values predicted using a random angular walk model (as originally proposed for F-actin) to describe the variable pitch. The reconstructions confirm that the fiber consists of 14 strands of hemoglobin S arranged in a hexagonally shaped cross-section. We have determined the pairing of the molecules to form double strands directly from the density maps by identifying the molecules that have intermolecular distances that conform to those of double strands in the Wishner-Love crystal. The seven double strands identified in this manner are consistent with the strand pairings proposed by Dykes et al. (1979) rather than the alternate pairings proposed by Rosen & Magdoff-Fairchild (1985). In addition, we have for the first time determined the polarity of the double strands directly from the reconstruction data. This was achieved using a procedure that amounts to essentially "dissecting" individual double strands from the reconstructed density maps so that their density distribution could be examined independently of the neighboring double strands. Knowledge of the relative polarities of the double strands is essential for determining the intermolecular interactions that stabilize the fiber.

Structural analysis of polymers of sickle cell hemoglobin. III. Fibers within fascicles.

We have examined the structure of hemoglobin S fibers, which are associated into large bundles, or fascicles. Electron micrographs of embedded and cross-sectioned fascicles provide an end-on view of the component fibers. The cross-sectional images are rotationally blurred as a result of the twist of the fiber within the finite thickness of the section. We have applied restoration techniques to recover a deblurred image of the fiber. The first step in this procedure involved correlation averaging images of cross-sections of individual fibers in order to improve the signal-to-noise ratio. The rotationally blurred image was then geometrically transformed to polar co-ordinates. In this space, the rotational blur is transformed into a linear blur. The linearly blurred image is the convolution of the unblurred image and a point spread function that can be closely approximated by a square pulse. Deconvolution in Fourier space, followed by remapping to Cartesian co-ordinates, produced a deblurred image of the original micrograph. The deblurred images indicate that the fiber is comprised of 14 strands of hemoglobin S. This result provides confirmation of the fiber structure determined using helical reconstruction techniques and indicates that the association of fibers into ordered arrays does not alter their molecular structure.

Macrofiber structure and the dynamics of sickle cell hemoglobin crystallization.

Fibers of deoxyhemoglobin S undergo spontaneous crystallization by a mechanism involving a variety of intermediate structures. These intermediate structures, in common with the fiber and crystal, consist of Wishner-Love double strands of hemoglobin S molecules arranged in different configurations. The structure of one of the key intermediates linking the fiber and crystal, called a macrofiber, has been studied by a variety of analytical procedures. The results of the analysis indicate that the intermediates involved in the fiber to crystal transition have many common structural features. Fourier analysis of electron micrographs of macrofibers confirms that they are composed of Wishner-Love double strands of hemoglobin molecules. Electron micrographs of macrofiber cross-sections reveal that the arrangement of the double strands in macrofibers resembles that seen in micrographs of the a axis projection of the crystal. This orientation provides an end-on view of the double strands which appear as paired dumb-bell-like masses. The structural detail becomes progressively less distinct towards the edge of the particle due to twisting of the double strands about the particle axis. Serial sections of macrofibers confirm that these particles do indeed rotate about their axes. The twist of the particle is right handed and its average pitch is 10,000 A. The effect of rotation on the appearance of macrofiber cross-sections 300 to 400 A thick can be simulated by a 15 degrees rotation of an a axis crystal projection. The relative polarity of the double strands in macrofibers and crystals can be determined easily by direct inspection of the micrographs. In both macrofibers and crystals they are in an anti-parallel array. On the basis of these observations we conclude that crystallization of macrofibers involves untwisting and alignment of the double strands.

Three-dimensional left ventricular wall motion in man. Coordinate systems for representing wall movement direction.

We have studied the three-dimensional (3D) motion of left ventricular (LV) epicardial points by tracking one to three dozen coronary artery bifurcations in eleven human subjects. Wall motion was analyzed using several different coordinate systems: (1) cylindrical centered about the LV long axis, (2) spherical with origin at the LV center-of-gravity (COG), and (3) spherical with origin at the LV center-of-contraction (COC), the best-fit 3D point toward which the wall moves. The coordinate systems were studied both fixed and moving with time. Three-dimensional motions were decomposed into three directional components, with high radial (in and out) percentages being regarded as the figure-of-merit of a given coordinate system. Average percentage radial motions were fixed cylindrical 16%, fixed spherical COG 35%, fixed spherical COC 47%, moving cylindrical 17%, moving spherical COG 30%, moving spherical COC 91%. Spherical systems were generally better than cylindrical systems, with the COC representing a better origin than the COG. Moving systems were appreciably better than fixed only for the COC model, indicating that the COC, which traverses up and down the LV midline, moves significantly while the other systems are more stationary. At each instant in time, almost all (91%) of the 3D motion of the entire heart wall is directed toward a single moving 3D point, the COC. Thus, there exists in principle a near-perfect 3D heart wall motion model. Approximately 25% of 3D wall motion is unseen in conventional monoplane views. Also, any model that represents 3D wall motion only along fixed straight 3D lines (eg, end-diastole to end-systole) necessarily ignores 27% of the true 3D heart wall motion.

Methods for evaluating cardiac wall motion in three dimensions using bifurcation points of the coronary arterial tree.

An accurate three-dimensional (3D) representation of heart wall motion would be an important means of evaluating cardiac function. To accomplish this, we have developed an interactive computer graphics system designed to enter the time-dependent 3D positions of bifurcations of the coronary arterial tree. These bifurcations are precise markers of the epicardial surface, and their motions accurately represent the motion of the underlying heart wall. We demonstrate techniques for calculating local wall motion, including displacement and velocity, for determining a time-dependent center-of-contraction point towards which the epicardium tends to move and for tracking the mechanical contraction wave using cross-correlation methods. We have applied these techniques to study seven patients with normal left ventriculograms and coronary arteriograms. We have found these methods to be generally applicable and to provide information not obtainable without 3D analysis.

Clinical applications of combined cerebral angiograms and brain CT scans.

This paper illustrates the clinical value of a computer technique for superimposition of two different kinds of imaging procedures. The initial application was to overlay cerebral angiograms onto computed tomograms of the brain. Case material from three patients with intracerebral abnormalities is presented. The sum of information from the combined studies exceeds that from either study alone and has value for both diagnosis and treatment of head lesions.

Schooling behavior of tadpoles: a potential indicator of ototoxicity.

Fish and tadpoles in schools use hair cells of their lateral line system to assess their position in relation to neighbors. This suggests that pharmaceutical agents that damage hair cells in the mammalian inner ear may also alter geometry in fish and tadpole schools. We used a computer-based image analysis system to examine the effect of the ototoxic aminoglycoside antibiotic, streptomycin, on school geometry for tadpoles of the African clawed frog Xenopus laevis. Tadpoles exposed to streptomycin in the surrounding water show a general tendency toward clumping, and an increase in the distance over which they orient parallel to neighbors, compared to controls. These behavioral responses appear in 18 min or less, and are evident in some tadpoles exposed to concentrations as low as 5 micrograms/ml. Results suggest that analysis of spatial relations in tadpole schools could serve as a method for rapidly detecting ototoxic potential of agents suspected of damaging hair cells.