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.

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).

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.

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.

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.

Cs2Mo15S19: a novel ternary reduced molybdenum sulfide containing Mo6 and Mo9 clusters.

The crystal structure of dicaesium pentadecamolybdenum nonadecasulfide, Cs(2)Mo(15)S(19), consists of a mixture of Mo(6)S(8)S(6) and Mo(9)S(11)S(6) cluster units in a 1:1 ratio. Both units are interconnected via inter-unit Mo-S bonds. The Cs(+) cations occupy large voids between the different cluster units. The Cs and two inner S atoms lie on sites with 3 symmetry (Wyckoff site 12c) and the Mo and S atoms of the median plane of the Mo(9)S(11)S(6) cluster unit on sites with 2 symmetry (Wyckoff site 18e).

Cs(6)Mo(27)S(31): a novel ternary reduced molybdenum sulfide containing Mo(9) and Mo(18) clusters.

The crystal structure of hexacaesium heptacosamolybdenum hentriacontasulfide, Cs(6)Mo(27)S(31), consists of a mixture of Mo(9)S(11)S(6) and Mo(18)S(20)S(6) cluster units in a 1:1 ratio. The units are connected through Mo-S bonds. Cs(+) cations occupy large voids between the different cluster units.

Rb5Mo27Se31, a novel ternary reduced molybdenum selenide containing Mo12 and Mo15 clusters.

The crystal structure of Rb5Mo27Se31, pentarubidium heptacosamolybdenum hentriacontaselenium, consists of a mixture of Mo12Se14Se6 and Mo15Se17Se6 cluster units in a 1:1 ratio. Both types of cluster are interconnected through inter-unit Mo-Se bonds. Rb+ cations occupy large voids between the different cluster units.

Rb4Mo21Se24 containing Mo12 and Mo15 clusters.

Rubidium molybdenum selenide, Rb(4)Mo(21)Se(24), crystallizes in the trigonal space group R-3. Its crystal structure consists of a mixture of Mo(12)Se(14)Se(6) and Mo(15)Se(17)Se(6) cluster units in a 1:2 ratio. Both units are interconnected through Mo-Se bonds. The Rb(+) cations occupy large voids between the different cluster units.

Relative magnitude and asynchrony of directional components of contraction in sedated dog left ventricle.

The purpose of this study was to quantitate the temporal relationships and the extent and speed of shortening in segments of myocardium responsive to contraction in circumferential, longitudinal, and oblique fiber groups. Measurements were made in five sedated dogs (morphine, diazepam) with and without alterations in preload and afterload (nitroprusside, phenylephrine). The measurement interval was the phase of rapid contraction, determined by differentiation of the segment length vs. time. In the control state, percentage segment shortening was greater in circumferential than in longitudinal [15.2 +/- 0.24 (SE) vs. 10.5 +/- 0.80%; P = 0.0020] and in the subepicardial oblique than in the subendocardial oblique fiber directions (16.6 +/- 0.65 vs. 9.7 +/- 0.36%; P = 0.0010). Shortening was proportional to both maximum speed and duration of shortening (r = 0.735 +/- 0.015 and 0.757 +/- 0.017, respectively). Duration of shortening was significantly longer in circumferential than in longitudinal (mean difference 39.3 +/- 6.6 ms; P = 0.0039) and in subepicardial oblique than in subendocardial oblique directions (mean difference 27.7 +/- 5.5 ms; P = 0.0072). Velocities of up to 3.0 segment lengths/s were attained in response to nitroprusside. These data reveal the local anisotropy and asynchrony of contraction in the myocardium; however, they also support the concept of the myocardium as a functional continuum. The dominance of circumferential over longitudinal and subepicardial over subendocardial oblique contractile components indicates their relative contributions to the constriction of the midmyocardial shell.

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.