New Symmetries in Crystals and Handed Structures

For over a century, the structure of materials has been described by a combination of rotations, rotation-inversions and translational symmetries. By recognizing the reversal of static structural rotations between clockwise and counterclockwise directions as a distinct symmetry operation, here we show that there are many more structural symmetries than are currently recognized in right- or left-handed handed helices, spirals, and in antidistorted structures composed equally of rotations of both handedness. For example, though a helix or spiral cannot possess conventional mirror or inversion symmetries, they can possess them in combination with the rotation reversal symmetry. Similarly, we show that many antidistorted perovskites possess twice the number of symmetry elements as conventionally identified. These new symmetries predict new forms for "roto" properties that relate to static rotations, such as rotoelectricity, piezorotation, and rotomagnetism. They also enable symmetry-based search for new phenomena, such as multiferroicity involving a coupling of spins, electric polarization and static rotations. This work is relevant to structure-property relationships in all material structures with static rotations such as minerals, polymers, proteins, and engineered structures.Comment: 15 Pages, 4 figures, 3 Tables; Fig. 2b has error

Vortices in polariton OPO superfluids

This chapter reviews the occurrence of quantised vortices in polariton fluids, primarily when polaritons are driven in the optical parametric oscillator (OPO) regime. We first review the OPO physics, together with both its analytical and numerical modelling, the latter being necessary for the description of finite size systems. Pattern formation is typical in systems driven away from equilibrium. Similarly, we find that uniform OPO solutions can be unstable to the spontaneous formation of quantised vortices. However, metastable vortices can only be injected externally into an otherwise stable symmetric state, and their persistence is due to the OPO superfluid properties. We discuss how the currents charactering an OPO play a crucial role in the occurrence and dynamics of both metastable and spontaneous vortices.Comment: 40 pages, 16 figure

Dynamic Evolution of Microscopic Wet Cracking Noises

Characterizing the interaction between water and microscopic defects is one of the long-standing challenges in understanding a broad range of cracking processes. Different physical aspects of microscopic events, driven or influenced by water, have been extensively discussed in atomistic calculations but have not been accessible in microscale experiments. Through the analysis of the emitted noises during the evolution of individual, dynamic microcracking events, we show that the onset of a secondary instability known as hybrid events occurs during the fast healing phase of microcracking, which leads to (local) sudden increase of pore water pressure in the process zone, inducing a secondary instability, which is followed by a fast-locking phase on the microscopic faults (pulse-like rupture)

Thermo-Electric Detection of Early Fatigue Damage in Metals

There are numerous nondestructive inspection methods which can be used to detect and quantitatively characterize advanced fatigue damage following crack initiation. However, crack nucleation occurs at a much smaller microstructural scale following a more or less extended period of gradual material degradation which remains beyond the reach of known eddy current, ultrasonic, and other inspection methods. Before crack initiation, fatigue degradation remains an elusive process leading to distributed crystal defects on the scale of individual grains and grain boundary imperfections. This gradual evolution of early fatigue damage first results in increasing dislocation density, formation of slip bands, microplasticity, cold work, etc., then leads to crack nucleation at multiple sites. Following crack nucleation the growing microcracks ultimately coalesce into larger detectable fatigue cracks, but current NDE methods cannot detect the often quite serious preexisting fatigue damage in the material before this point

Laser-directed hierarchical assembly of liquid crystal defects and control of optical phase singularities

Topological defect lines are ubiquitous and important in a wide variety of fascinating phenomena and theories in many fields ranging from materials science to early-universe cosmology, and to engineering of laser beams. However, they are typically hard to control in a reliable manner. Here we describe facile erasable “optical drawing” of self-assembled defect clusters in liquid crystals. These quadrupolar defect clusters, stabilized by the medium's chirality and the tendency to form twisted configurations, are shaped into arbitrary two-dimensional patterns, including reconfigurable phase gratings capable of generating and controlling optical phase singularities in laser beams. Our findings bridge the studies of defects in condensed matter physics and optics and may enable applications in data storage, singular optics, displays, electro-optic devices, diffraction gratings, as well as in both optically- and electrically-addressed pixel-free spatial light modulators

Subglacial lake drainage detected beneath the Greenland ice sheet

The contribution of the Greenland ice sheet to sea-level rise has accelerated in recent decades. Subglacial lake drainage events can induce an ice sheet dynamic response—a process that has been observed in Antarctica, but not yet in Greenland, where the presence of subglacial lakes has only recently been discovered. Here we investigate the water flow paths from a subglacial lake, which drained beneath the Greenland ice sheet in 2011. Our observations suggest that the lake was fed by surface meltwater flowing down a nearby moulin, and that the draining water reached the ice margin via a subglacial tunnel. Interferometric synthetic aperture radar-derived measurements of ice surface motion acquired in 1995 suggest that a similar event may have occurred 16 years earlier, and we propose that, as the climate warms, increasing volumes of surface meltwater routed to the bed will cause such events to become more common in the future

Light-induced spiral mass transport in azo-polymer films under vortex-beam illumination

When an azobenzene-containing polymer film is exposed to a non-uniform illumination, a light-induced mass migration process may be induced, leading to the formation of relief patterns on the polymer free surface. Despite a research effort of many years and several proposed models many aspects of this phenomenon remain not well understood. Here we report the appearance of spiral-shaped relief patterns on the polymer under the illumination of focused Laguerre-Gauss beams, having helical wavefront and an optical vortex at their axis. The induced spiral reliefs are sensitive to the vortex topological charge and to the wavefront handedness. These findings are unexpected, because the "doughnut"-shaped intensity profile of Laguerre- Gauss beams contains no information about the wavefront handedness. We propose a model that explains the main features of this phenomenon from the surface-mediated interference of the longitudinal and the transverse components of the optical field. These results may find applications in optical micro- and nanolithography and optical-field.Comment: Open Access at Journal Reference homepag

Maternal cadmium, iron and zinc levels, DNA methylation and birth weight

Background Cadmium (Cd) is a ubiquitous and environmentally persistent toxic metal that has been implicated in neurotoxicity, carcinogenesis and obesity and essential metals including zinc (Zn) and iron (Fe) may alter these outcomes. However mechanisms underlying these relationships remain limited. Methods We examined whether maternal Cd levels during early pregnancy were associated with offspring DNA methylation at regulatory sequences of genomically imprinted genes and weight at birth, and whether Fe and Zn altered these associations. Cd, Fe and Zn were measured in maternal blood of 319 women ≤12 weeks gestation. Offspring umbilical cord blood leukocyte DNA methylation at regulatory differentially methylated regions (DMRs) of 8 imprinted genes was measured using bisulfite pyrosequencing. Regression models were used to examine the relationships among Cd, Fe, Zn, and DMR methylation and birth weight. Results Elevated maternal blood Cd levels were associated with lower birth weight (p = 0.03). Higher maternal blood Cd levels were also associated with lower offspring methylation at the PEG3 DMR in females (β = 0.55, se = 0.17, p = 0.05), and at the MEG3 DMR in males (β = 0.72, se = 0.3, p = 0.08), however the latter association was not statistically significant. Associations between Cd and PEG3 and PLAGL1 DNA methylation were stronger in infants born to women with low concentrations of Fe (p < 0.05). Conclusions Our data suggest the association between pre-natal Cd and offspring DNA methylation at regulatory sequences of imprinted genes may be sex- and gene-specific. Essential metals such as Zn may mitigate DNA methylation response to Cd exposure. Larger studies are required