The electronic band structure of van der Waals (vdW) layered crystals has properties that depend on the composition, thickness and stacking of the component layers. Here we use density functional theory and high field magneto-optics to investigate the metal chalcogenide InSe, a recent addition to the family of vdW layered crystals, which transforms from a direct to an indirect band gap semiconductor as the number of layers is reduced. We investigate this direct-to-indirect bandgap crossover, demonstrate a highly tuneable optical response from the near infrared to the visible spectrum with decreasing layer thickness down to 2 layers, and report quantum dot-like optical emissions distributed over a wide range of energy. Our analysis also indicates that electron and exciton effective masses are weakly dependent on the layer thickness and are significantly smaller than in other vdW crystals. These properties are unprecedented within the large family of vdW crystals and demonstrate the potential of InSe for electronic and photonic technologies.

Cyclotron motion of charge carriers in metals and semiconductors leads to Landau quantization and magneto-oscillatory behavior in their properties. Cryogenic temperatures are usually required to observe these oscillations. We show that graphene superlattices support a different type of quantum oscillation that does not rely on Landau quantization. The oscillations are extremely robust and persist well above room temperature in magnetic fields of only a few tesla. We attribute this phenomenon to repetitive changes in the electronic structure of superlattices such that charge carriers experience effectively no magnetic field at simple fractions of the flux quantum per superlattice unit cell. Our work hints at unexplored physics in Hofstadter butterfly systems at high temperatures.

Graphene superlattices were shown to exhibit high-temperature quantum oscillations due to periodic emergence of delocalized Bloch states in high magnetic fields such that unit fractions of the flux quantum pierce a superlattice unit cell. Under these conditions, semiclassical electron trajectories become straight again, similar to the case of zero magnetic field. Here, we report magnetotransport measurements that reveal second-, third-, and fourth-order magnetic Bloch states at high electron densities and temperatures above 100 K. The recurrence of these states creates a fractal pattern intimately related to the origin of Hofstadter butterflies. The hierarchy of the fractal states is determined by the width of magnetic minibands, in qualitative agreement with our band-structure calculations.

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As a continuation of our previous work, this paper concentrates on two more complex 3D woven structures, i.e., orthogonal and angle-interlock structures, for industrial applications such as textile composites. Studying the features of these woven structures has led to the establishment of mathematical models to describe them. Further, algorithms for generating these weave structures are developed based on these models. A CAD/CAM software for these 3D woven structures is created on the MS Windows platform. Solid models of the structures are also made available using virtual reality modeling language (VRML).High performance woven textile structures have been widely used in developing various innovative materials, of which textile reinforced composites and geotextiles are examples. In order to achieve specific properties, high performance woven textiles have complicated 3D woven structures such as multilayer, orthogonal, and angle-interlock structures, which add extra difficulty to their design and manufacture. CAD/ CAM systems for woven structures such as ScotWeave and Sophis are commercially available to assist the design and manufacture of a wide range of woven structures. Unfortunately, these commercial systems, to our best knowledge, do not include facilities for the CAD/CAM of the complicated 3D structures we discuss here.Research work (including our own) attempting to model intricate weaves and develop algorithms for the CAD/CAM of complicated woven structures has been going on for years [3-5, 6, 10]. Mathematical models and algorithms have been developed to deal with different kinds of woven structures, among which are backedcloth and multilayer weaves. Based on the modeling for these woven structures, we have developed modules of a CAD/CAM software package accordingly [5,6,10].In this paper, we will focus on two other complex 3D woven structures, orthogonal and angle-interlock, in an attempt to describe the weaves and create the algorithms for the CAD/CAM of these structures, We will also apply the theory to the establishment of a software module that creates solid models for these structures using a virtual reality modelling language (VRML). These models are useful in visualizing the complicated woven structures and evaluating their physical and other properties. Orthogonal StructuresThere are three sets of yarns in an orthogonal structure, the straight warp yarn, the straight weft yarn, and the binding warp yarn. The straight warp and weft yams form the non-interlaced body structure that is integrated by the binding warp ends following the binding weave.The thickness of this structure is measured by the number of layers of the straight warp or weft yarn. According to the binding weave, there are two kinds of orthogonal structures, i.e., ordinary and enhanced [4]. An ordinary structure uses only one binding warp set, while an enhanced structure uses two binding warp sets with inverse binding weaves. Figure 1 a shows an ordinary orthogonal structure with four layers of straight weft, three layers of straigh...

In this article we report the results of time integrated and time resolved photoluminescence spectroscopy and photoluminescence time decay measurements as a function of excitation density at 6 K on high quality self-organized InAs/GaAs quantum dots. To understand the form of the experimentally observed photoluminescence transients a Monte Carlo model has been developed that allows for the effects of random capture of photo-excited carriers. By comparison with the results of our model we are able to ascribe the excitation density dependence of the overall form of the decay of the emission from the quantum dot ground states and the biexponential nature of the decay of the first excited state emission as being due to the combined effects of radiative recombination, density dependent carrier scattering, and the restriction of carrier scattering due to state blocking caused by the effects of Pauli exclusion. To successfully model the form of the biexponential decay of the highest energy excited states we have to invoke the nonsequential scattering of carriers between the quantum dot states.

At the present there exist two different types of riot helmet shells, a thermoplastic and a composite shell. The composite shell is believed to be superior in impact performance, but more expensive and complicated in manufacturing partially due to the fact that the textile reinforcement material has to be cut into pattern pieces before they can be applied to the helmet shaped mould. However, the discontinuation of fibres is likely to reduce the level of protection and shorten the lifetime of a helmet. The aim of this research is to develop a helmet shell consisting of a single-piece of fabric without creating wrinkles and without the necessity of cutting the fabric. For this purpose, a new type of woven fabric has been developed and an apparatus has been set up to drape the fabric to a helmet shell. The helmet shell was manufactured using a single piece of Kevlar woven fabric. In parallel the finite element method is applied to investigate the shock absorption behaviour of the helmet shell made from both multi-piece and single-piece fabric reinforcements. The results show clearly the advantage of the single-piece helmet shell over its existing multi-piece counterpart.

This is the first paper of a two-part series dealing with the establishment of mathematical models of two t>pes of complex woveD-fabric structures, i.e. backed cloths and multi-layer fabrics, for CAD of weaves used for such fabrics and for CAM by generating au instruction code for controlling the shedding mechanism of a loom. Part I describes the mathematical models and automatic generation of both warp-and weft-backed-cloth weaves. A backedcloth weave is mathematically expressed as a 2D binary matrix, which can be obtained from certain combinations of two single-layer weaves in the form of a 2D binary matrix. These combinations produce the backed cloth, w hich is free from interference between the component weaves. A Windows-based CAD/CAM program is written to implement these models. Part 11 deals with multi-layer fabrics.