Diala Salloum

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.

Reentrant Phase Coherence in Superconducting Nanowire Composites

The short coherence lengths characteristic of low-dimensional superconductors are associated with usefully high critical fields or temperatures. Unfortunately, such materials are often sensitive to disorder and suffer from phase fluctuations in the superconducting order parameter which diverge with temperature T, magnetic field H, or current I. We propose an approach to overcome synthesis and fluctuation problems: building superconductors from inhomogeneous composites of nanofilaments. Macroscopic crystals of quasi-one-dimensional Na2-δMo6Se6 featuring Na vacancy disorder (δ ≈ 0.2) are shown to behave as percolative networks of superconducting nanowires. Long-range order is established via transverse coupling between individual one-dimensional filaments, yet phase coherence remains unstable to fluctuations and localization in the zero (T,H,I) limit. However, a region of reentrant phase coherence develops upon raising (T,H,I). We attribute this phenomenon to an enhancement of the transverse coupling due to electron delocalization. Our observations of reentrant phase coherence coincide with a peak in the Josephson energy EJ at nonzero (T,H,I), which we estimate using a simple analytical model for a disordered anisotropic superconductor. Na2-δMo6Se6 is therefore a blueprint for a future generation of nanofilamentary superconductors with inbuilt resilience to phase fluctuations at elevated (T,H,I).

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.

Synthesis, electronic and crystal structures, and physical studies of the superconductor Cs(~1)Mo12S14, final step of the condensation of the Mo6L8(i)L6(a) unit.

The ternary reduced molybdenum sulphide Cs(~1)Mo12S14 has been synthesized by solid-state reaction at 1400 degrees C for 96 h in sealed molybdenum crucibles. The compound crystallizes in the trigonal space group P31c with the following lattice parameters: a = 9.9793 (2) A, c = 6.3730 (2) A, Z = 1. Its crystal structure was determined from single crystal X-ray diffraction data and consists of interconnected Mo6S8(i)S6(a) units forming an original three-dimensional framework in which large tunnels are occupied randomly by the Cs+ ions. 133Cs static NMR studies are in favor of a static cesium disorder. Unlike Ba4Mo12S18 where some Mo6S8(i)S6(a) units are also connected through S(i-i) ligands, this connection mode does not lead to significant interactions in the title compound. Single-crystal resistivity measurements show that Cs(~1)Mo12S14 presents a metallic behavior with a superconducting transition at 7.7 K as confirmed by magnetic measurements.

Cs6Mo27S31: a novel ternary reduced molybdenum sulfide containing Mo9 and Mo18 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).

V1.42In1.83Mo15Se19

The structure of the title compound, vanadium indium pentadecamolybdenum nonadecaselenide, V1.42In1.83Mo15Se19, is isotypic with In2.9Mo15Se19 [Grüttner et al. (1979 ▶). Acta Cryst. B35, 285–292]. It is characterized by two cluster units Mo6Sei 8Sea 6 and Mo9Sei 11Sea 6 (where i represents inner and a apical atoms) that are present in a 1:1 ratio. The cluster units are centered at Wyckoff positions 2b and 2c and have point-group symmetry and , respectively. The clusters are interconnected through additional Mo—Se bonds. In the title compound, the V3+ cations replace the trivalent indium atoms present in In2.9Mo15Se19, and a deficiency is observed on the monovalent indium site. One Mo, one Se and the V atom are situated on mirror planes, and two other Se atoms and the In atom are situated on threefold rotation axes.

Na2.9KMo12S14: a novel quaternary reduced molybdenum sulfide containing Mo12 clusters with a channel structure.

The crystal structure of trisodium potassium dodecamolybdenum tetradecasulfide, Na2.9 (2)KMo12S14, consists of Mo12S14S6 cluster units interconnected through interunit Mo—S bonds and delimiting channels in which the Na+ cations are disordered. The cluster units are centered at Wyckoff positions 2d and have point-group symmetry 3.2. The K atom lies on sites with 3.2 symmetry (Wyckoff site 2c) between two consecutive Mo12S14S6 units. One of the three independent S atoms and one Na atom lie on sites with 3.. symmetry (Wyckoff sites 4e and 4f). The other Na atom occupies a 2b position with -3.. symmetry. The crystal studied was a merohedral twin with refined components of 0.4951 (13) and 0.5049 (13).

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.

Na4.25Mo15S19: a novel ternary reduced molybdenum sulfide containing Mo6 and Mo9 clusters.

The structure of tetrasodium pentadecamolybdenum nonadecasulfide, Na4.25Mo15S19, is isotypic with Na3.9Mo15Se19 [Salloum et al. (2013 ▶). Acta Cryst. E69, i67–i68]. It is characterized by Mo6Si 8Sa 6 and Mo9Si 11Sa 6 (where i represents inner and a apical atoms) cluster units that are present in a 1:1 ratio. The cluster units are centered at Wyckoff positions 2b and 2c, and have point-group symmetry -3 and -6, respectively. The clusters are interconnected through additional Mo—S bonds. The Na+ cations occupy interunit voids formed by six or seven S atoms. One Mo, one S and one Na site [occupancy 0.751 (12)] are situated on mirror planes, and two other S atoms and one Na site (full occupancy) are situated on threefold rotation axes.