Line Defects in Molybdenum Disulfide Layers

Layered molecular materials and especially MoS2 are already accepted as promising candidates for nanoelectronics. In contrast to the bulk material, the observed electron mobility in single-layer MoS2 is unexpectedly low. Here we reveal the occurrence of intrinsic defects in MoS2 layers, known as inversion domains, where the layer changes its direction through a line defect. The line defects are observed experimentally by atomic resolution TEM. The structures were modeled and the stability and electronic properties of the defects were calculated using quantum-mechanical calculations based on the Density-Functional Tight-Binding method. The results of these calculations indicate the occurrence of new states within the band gap of the semiconducting MoS2. The most stable non-stoichiometric defect structures are observed experimentally, one of which contains metallic Mo-Mo bonds and another one bridging S atoms

Modulated structures in electroconvection in nematic liquid crystals

Motivated by experiments in electroconvection in nematic liquid crystals with homeotropic alignment we study the coupled amplitude equations describing the formation of a stationary roll pattern in the presence of a weakly-damped mode that breaks isotropy. The equations can be generalized to describe the planarly aligned case if the orienting effect of the boundaries is small, which can be achieved by a destabilizing magnetic field. The slow mode represents the in-plane director at the center of the cell. The simplest uniform states are normal rolls which may undergo a pitchfork bifurcation to abnormal rolls with a misaligned in-plane director.We present a new class of defect-free solutions with spatial modulations perpendicular to the rolls. In a parameter range where the zig-zag instability is not relevant these solutions are stable attractors, as observed in experiments. We also present two-dimensionally modulated states with and without defects which result from the destabilization of the one-dimensionally modulated structures. Finally, for no (or very small) damping, and away from the rotationally symmetric case, we find static chevrons made up of a periodic arrangement of defect chains (or bands of defects) separating homogeneous regions of oblique rolls with very small amplitude. These states may provide a model for a class of poorly understood stationary structures observed in various highly-conducting materials ("prechevrons" or "broad domains").Comment: 13 pages, 13 figure

Antiferroelectric Smectic Ordering as a Prelude to the Ferroelectric Nematic:Introducing the Smectic ZAZ_A Phase

We have structurally characterized the liquid crystal phase that appears as an intermediate state when a dielectric nematic, having polar disorder of its molecular dipoles, transitions to the almost perfectly polar-ordered ferroelectric nematic. This intermediate phase, which fills a 100-year-old void in the taxonomy of smectics and which we term the "smectic $Z_A$", is antiferroelectric, with the nematic director and polarization oriented parallel to smectic layer planes, and the polarization alternating in sign from layer to layer. The period of this polarization wave ($\sim$180 A) is mesoscopic, corresponding to $\sim$40 molecules side-by-side, indicating that this lamellar structure is collectively stabilized. A Landau free energy, originally formulated to model incommensurate antiferroelectricity in crystals, describes the key features of the nematic-Sm$Z_A$-ferroelectric nematic phase sequence.Comment: main paper (with 7 figures) and supplement (with 12 figures) comprise 65 page

Three-dimensional soliton-like distortions in flexoelectric nematic liquid crystals: modeling and linear analysis

This article models experimentally observed three dimensional particle-like waves that develop in nematic liquid crystals, with negative dielectric and conductive anisotropy, when subject to an applied alternating electric field. The liquid crystal is confined in a thin region between two plates, perpendicular to the applied field. The horizontal, uniformly aligned director field is at equilibrium due to the negative anisotropy of the media. However, such a state is unstable to perturbations that manifest themselves as confined, bullet-like, director distortions traveling up and down the sample at a speed of several hundred microns per second. It is experimentally predicted that flexoelectricity plays a key role in generating the soliton-like behavior. We develop a variational model that accounts for ansiostropic dielectric, conductive, flexolectric, elastic and viscous forces. We perform a stability analysis of the uniformly aligned equilibrium state to determine the threshold wave numbers, size, phase-shift and speed of the soliton-like disturbance. We show that the model predictions are in very good agreement with the experimentally measured values. The work models and analyzes a three-dimensional soliton-like instability reported, for the first time in flexoelectric liquid crystals, pointing towards a potential application as a new type of nanotransport device

Atmospheric conditions during the Visean (Carboniferous), and post-depositional hydrocarbon generation in the Shubenacadie Basin, Nova Scotia

Halite has become of particular interest recently due to its potential to preserve ancient air and provide the first paleoatmospheric composition data that is not reliant on proxies. As halite precipitates, fluid inclusions become trapped within the crystal structure. Some of which also capture miniscule gas bubbles of the air present during precipitation. This study presents the first results of Visean (Carboniferous) paleoatmosphere captured in the Shubenacadie Basin, Nova Scotia. To ensure the gas-hosting fluid inclusions are of primary origin, strict screening parameters must be applied. Those deemed adequate from visual and petrographic examinations, had reasonable homogenization temperatures, argon concentration between 0.8 and 1.0 mol %, contained a sum of methane and carbon dioxide below 10 mol %, and displayed minimal gas variation within samples were used for paleoatmospheric interpretation. Trace element concentrations (Mg, K, Ca) of halite were also examined to determine if they could provide further confirmation of primary origin but further work is required to determine its viability. The few halite samples that met all the screening parameters originated from the upper MacDonald Road formation and yielded an O2 window of 11 to 16 mol %. The failed samples in the upper formations contained visible and olfactible hydrocarbons while the lower formations contained elevated methane. This hydrocarbon generation is consistent with and increasing thermal alteration index with depth due to heat generated by the underlying Mississippi Valley-Type deposit in the basin

A fluid inclusion study of acidity in bedded halite of the Larne Halite Member, Triassic Mercia Mudstone Group from the Carnduff 2 Core, County Antrim, Northern Ireland

The Carnduff 2 core, drilled in 2014 by the Gaelectric Storage Limited Energy Company in County Antrim, Northern Ireland contains Triassic red mudstones and evaporites of the Mercia Mudstone Group. These rocks have been poorly studied in Northern Ireland. However, recent work suggests that they were deposited in an ancient perennial saline lake system. This thesis describes a 591.6 meter core with 92.7% recovery. Fluid inclusion petrography, microthermometry, and laser Raman spectroscopy were conducted on 12 halite beds of the Larne Halite Member to characterize Triassic lake waters.;Chevron and cumulate halite crystals contain abundant unaltered primary fluid inclusions. Most primary fluid inclusions are all liquid, but there are also fluid inclusions that contain daughter crystals. Petrographic studies showed that primary inclusions have a double rim. This double rim is not seen on fluid inclusions with neutral pH. This suggests that double rims may be a new criterion for determining acidity. Fluid inclusions failed to freeze during freezing-melting microthermometry, even after being exposed to -190º C for several minutes. The failure to freeze during microthermometry runs is likely due to low pH and/or extremely high salinity. Laser Raman spectroscopy of 45 inclusions showed peaks for bisulfate and/or aluminum sulfate compounds in 10 of the 12 beds, indicating high sulfate and aluminum in solution. More importantly, such peaks are characteristic of waters having pH values \u3c ~3.;This study provides evidence for the first time that bedded halite of the Mercia Mudstone Group of Northern Ireland formed in acid, saline lakes. Additionally, new criteria for determining low pH in the rock record has been established. Finally, this study suggests that, when compared to Permian acid brine deposits of North America, the red beds and evaporites of the Triassic Mercia Mudstone Group of Northern Ireland may be part of spatially extensive and long-lasting, acid saline environments of Pangea