Systems of interacting nanomagnets known as artificial spin ice 1-4 have allowed the design, realization and study of geometrically frustrated exotic collective states 5-10 that are absent in natural magnets. We have experimentally measured 11,12 the thermally induced moment fluctuations in the Shakti geometry of artificial spin ice. We show that its disordered moment configuration is a topological phase described by an emergent dimer-cover model 13 with excitations that can be characterized as topologically charged defects. Examination of the lowenergy dynamics of the system confirms that these effective topological charges have long lifetimes associated with their topological protection, that is, they can be created and annihilated only as charge pairs with opposite sign and are kinetically constrained. This manifestation of classical topological order 14-19 demonstrates that geometrical design in nanomagnetic systems can lead to emergent, topologically protected kinetics that can limit pathways to equilibration and ergodicity. Artificial spin ices are lithographically fabricated systems of interacting single-domain nanomagnets. These systems can be used to investigate the collective magnetic behaviour of interacting moments as effective models for understanding the complex phenomena of frustration. Each nanomagnet moment aligns along the edges of a lattice and points towards or away from the lattice vertices. In their low-energy collective states, the moments enter a so-called ice-manifold; an ensemble in which, at each vertex, the difference between the number of moments pointing in and out is minimized, leading to the ice-rule (2-in/2-out 20 at vertices where four moments meet or 1-in/2-out, 2-in/1-out at vertices where three moments meet). Originally inspired by rare-earth pyrochlore spin ice materials, these artificial spin ice systems evolved towards new geometries 5,6 , with exotic phases absent in natural magnets 2,3,7,8,21. Recent experimental works have characterized the thermal fluctuations of the individual magnetic moments, opening new vistas in the real-time, real-space analysis of frustration 11,12,22-25. The Shakti lattice geometry 5-7 (Fig. 1) is a decimation of the square ice lattice geometry. In Fig. 1e, we show the possible moment configurations at vertices and label them by the number of islands at each vertex (the coordination number, z) and by their relative energy hierarchy. The collective ground state is a configuration in which the z = 2 and z = 4 vertices are all in their lowest energy state (that is, type I 4 for the four-island vertices and type I 2 for the twoisland vertices) while only half of the z = 3 vertices lie in their lowest

Human immunodeficiency virus type 1 (HIV-1)-infected H9 and blood mononuclear cells (MNCs) were studied by immunogold electron microscopy for the presence of HIV-1 gag p24 protein, env gp41 and gpl20 proteins, and host cell molecules CD4, CDlla, CD25, CD54, CD63, HLA class I and HLA-DR. Uninfected H9 cells and MNC membranes labelled for CD4, HLA class I and class II, and, at low density, CDlla and CD54; lysosomal structures in the cytoplasm labelled for CD63. The infected cell surface showed immunolabelling for HIV-1 proteins, as did budding particle-like structures.Immunogold labelling of the cell membrane for CD4 was almost non-existent. The level of immunolabelling for CD1 la and CD54 on infected cells was greater than that on uninfected cells; this is presumably related to a state of activation during virus synthesis. Budding particle-like structures and free virions in the intercellular space were immunogold-labelled for all host cell markers investigated. This was confirmed by double immunogold labelling using combinations of HIV-1 gag p24 labelling and labelling for the respective host cell molecule. We conclude that virions generated in HIV-1-infected cells concentrate host-derived molecules on their envelope. Also molecules with a prime function in cellular adhesion concentrate on the virion.Infection of cells by human immunodeficiency virus type 1 (HIV-1) is followed by the disappearance of the virus receptor molecule CD4 from the cell membrane (Geleziunas et al., 1991 ;Gielen et al., 1989; Hoxie et al., 1986). This phenomenon has also been observed for other surface molecules including HLA antigens (Eales et al., 1988; Gelderblom et al., 1987b;Henderson et al., 1987;Kerkau et al., 1989;Schols et al., 1992) and the CD3, CD8 and CDll antigens (Stevenson et al., 1987). By using immuno-electron microscopy we have previously demonstrated the complete absence of CD4 antigen and the partial absence of HLA-DR and CD5 antigen on H9 cells 2 days after HIV-1 infection (Meerloo et al., 1992). The CD3 and CD25 antigens remained detectable on the cell surface at similar density, and the CD63 antigen, a t Present address:

This paper presents the results of experimental measurements of transverse thermal diffusivity for six different industrial Ceramic Matrix Composite materials under the action of in-plane uni-axial mechanical loading. Measurements have also been taken using a one-dimensional Laser Flash technique on 8 mm diameter disc specimens without mechanical load. These results have been used to benchmark data obtained using a three-dimensional Laser Flash rig developed to operate on large mechanical test specimens whilst being loaded in a test machine. The latter facility has been used to measure the degradation of transverse thermal diffusivity with uni-axial strain. For five of the composites tested the degradation of transverse thermal diffusivity was small; and the degradation of transverse thermal diffusivity was linear with strain. One composite showed evidence of fibre pullout, and a decrease of transverse thermal diffusivity to a low value. The different behaviours observed were believed to be associated with the weave type/composite lay-up, and the selection of composite constituent materials, although insufficient data is available to make general conclusions.

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The star KIC8462852 (Tabby's star) has shown anomalous drops in light flux. We perform a statistical analysis of the more numerous smaller dimming events by using methods found useful for avalanches in ferromagnetism and plastic flow. Scaling exponents for avalanche statistics and temporal profiles of the flux during the dimming events are close to mean field predictions. Scaling collapses suggest that this star may be near a nonequilibrium critical point. The large events are interpreted as avalanches marked by modified dynamics, limited by the system size, and not within the scaling regime.

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Laser melting is an important industrial activity encountered in a variety of laser manufacturing processes, e.g. selective laser melting, welding, brazing, soldering, glazing, surface alloying, cladding etc. The majority of these processes are carried out by using either circular or rectangular beams. At present, the melt pool characteristics such as melt pool geometry, thermal gradients and cooling rate are controlled by the variation of laser power, spot size or scanning speed. However, the variations in these parameters are often limited by other processing conditions. Although different laser beam modes and intensity distributions have been studied to improve the process, no other laser beam geometries have been investigated. The effect of laser beam geometry on the laser melting process has received very little attention. This paper presents an investigation of the effects of different beam geometries including circular, rectangular and diamond shapes on laser melting of metallic materials. The finite volume method has been used to simulate the transient effects of a moving beam for laser melting of mild steel (EN-43A) taking into account Marangoni and buoyancy convection. The temperature distribution, melt pool geometry, fluid flow velocities and heating/cooling rates have been calculated. Some of the results have been compared with the experimental data.

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