Diagnosis for topological semimetals in the absence of spin-orbital coupling

Topological semimetals are under intensive theoretical and experimental studies. The first step of these studies is always the theoretical (numerical) predication of one of several candidate materials, starting from first principles. In these calculations, it is crucial that all topological band crossings, including their types and positions in the Brillouin zone, are found. While band crossings along high-symmetry lines, which are routinely scanned in numerics, are simple to locate, the ones at generic momenta are notoriously time-consuming to find, and may be easily missed. In this paper, we establish a theoretical scheme of diagnosis for topological semimetals where all band crossings are at generic momenta in systems with time-reversal symmetry and negligible spin-orbital coupling. The scheme only uses the symmetry (inversion and rotation) eigenvalues of the valence bands at high-symmetry points in the BZ as input, and provides the types, numbers and configurations of all topological band crossings, if any, at generic momenta. The nature of new diagnosis scheme allows for full automation and parallelizations, and paves way to high throughput numerical predictions of topological materials.Comment: 21 pages, 5 figures, 1 table; v4: accepted in PRX, a "PRELIMINARIES" section adde

Importance of cryobanking in aquatic species conservation and aquaculture

Successful cryopreservation and cryobanking of reproductive cells of aquatic species offers many benefits to the fields of aquaculture, conservation of biodiversity and biomedicine. Global growth of intensive aquaculture has increased the needs for efficient and effective means, such as cryobanks, of conserving reproductive cells for greater flexibility in broodstock management, genetic improvement programs and preservation of genetic diversity. According to the Red List of the International Union for Conservation of Nature and Natural Resources, over 5000 aquatic animal species worldwide are threatened, including fishes, molluscs, crustaceans and corals. This has also lead to an increased interest in the creation cryobanks for conservation purposes. Cryopreservation of many different cellular types has been studied eg sperm, oocytes, embryos, somatic cells and primordial germ cells or early spermatogonia. Cryobanks have been established for a range of aquatic species worldwide including Europe, USA, Brazil, Australia and New Zealand. The management of these banks requires expertise and technical capacity in genetics, reproductive physiology, cryobiology and data management. Cryopreservation protocols need to be carefully designed for each species and each type of cells to ensure optimal survival. The discussion will be focused on cryobanking in aquatic species including freshwater and marine fish species as well as invertebrates

Investigations into the cryopreservation of zebrafish (Brachydanio Rerio) embryos

A thesis submitted for the degree of Doctor of Philosophy of the University of LutonCryopreservation of fish embryos was studied using zebrafish embryo as a model system. Cryoprotectant toxicity, chilling sensitivity and embryo membrane permeability were investigated with a view to developing optimum protocols for cryopreservation of the embryos using controlled slow cooling, non-freezing storage and vitrification. Methanol was found to be the most effective cryoprotectant for controlled slow cooling and non-freezing storage of zebrafish embryos with 11 %heart beat stage embryos surviving after controlled slow cooling to -25°C. Zebrafish embryos were found to be very chilling sensitive with early developmental stages being the most sensitive to chilling injury. Embryo developmental stages after closure of the blastopore (>12-h), especially post heart beat stages were much more resistant to cryoprotectant toxicity and chilling injury. Heart beat stage (27-h) embryos proved to be the best embryo developmental stage for controlled slow cooling and non-freezing storage. Dechorionated embryos are more sensitive to cryoprotectant toxicity and chilling injury. The sensitivity to chilling injury of zebrafish embryos limited the use of controlled slow cooling and non-freezing storage for long term cryopreservation. The attempts at cryopreservation of zebrafish embryos using vitrification produced no embryo survival, although up to 32 % embryos remained morphologically intact immediately after vitrification. Poor cryoprotectant permeation, dehydration and consequently ice formation within the egg are probably the main factors on effecting embryo survival. The results of zebrafish embryo permeability studies demonstrated that the chorion of the embryos was permeable to water and cryoprotectants, whilst the vitelline (plasma) membrane was an effective permeability barrier. The inability to achieve sufficient penetration of the vitelline membrane by cryoprotectants poses severe problems for long term cryopreservation, which need to be overcome, possibly by permeabilisation of the vitelline membrane, before successful cryopreservation can be achieve

Chiral phonons and pseudo-angular momentum in non-symmorphic systems

Chiral phonons are the ones with nonzero polarization and can be observed only via a selective coupling with valley electrons and circularly polarized photons. In such process, a new physical quantity, i.e., pseudo-angular momentum (PAM), is required to meet the selection rule. However, phonon PAM was thought to be quantized and can be only defined in the symmorphic systems. In this work, we generalized the definition of PAM to three-dimensional non-symmorphic systems, which show distinct different properties compared with the one in symmorphic systems, e.g., PAM can be non-quantized and q-dependent but still be an observable quantity by experiments. Such new definitions and discoveries can help us to obtain chiral phonons in a broader class of systems with a nonzero group velocity and to convey information like chirality and angular momentum in solids as expected. Materials are also offered to understand the new definition and for further experimental detection.Comment: 7 pages, 4 figures and 1 tabl