Current biodiversity and biogeography trends of reef-building corals in Qatar

Coral reefs represent one of the most biologically and socio-economically important natural assets in the Arabian region, particularly in Qatar. Stress through increasing sea temperatures and salinities, growing development, and other pressures, has resulted in shifts in reef communities across the region in recent years. Branching corals have disappeared from most reef systems while previously thriving inshore reefs are being reduced to rubble with little signs of life remaining. We here conduct a thorough re-assessment of coral reefs around Qatar, focusing on coral cover and biodiversity using photoquadrat surveys, and we investigate potential environmental drivers of coral communities in the region. Results show higher coral cover on offshore reefs than on inshore reefs. Additionally, among previously documented inshore corals reefs, several have lost their coral cover and have shifted to non-coral ecosystems composed of rubble, turf algae, and oysters. Coral diversity is higher on offshore reefs than inshore reefs, with previously reported species now absent on inshore reefs. Environmental factors such as salinity and sea surface temperatures could play an important role in driving current patterns of coral biodiversity and biogeography and in predicting their shift as a function of future regional climate scenarios. We expect inshore reefs to be generally more fragile and susceptible to stress due the higher variability in environmental conditions. These same inshore reefs are also the most exposed to human interactions and coastal development. To ensure the future of coral reefs in the region and to sustain their ecosystem services, coral reefs in Qatar will need to be effectively regulated (i.e. through the development and implementation of an ecosystem-based management plan). In addition, recovery of impacted reefs could be assisted through the creation of a network of marine protected areas (MPAs), allowing for natural replenishment of coral communities through connectivity. In the meantime, local reefs are likely to continue their decline in coral cover and diversity, particularly the inshore environments.qscienc

Enhanced high-harmonic generation from chromium-doped magnesium oxide

High-order harmonic generation (HHG) from crystals offers a new source of coherent extreme ultraviolet (XUV) attosecond radiation.Comment: Significant change of conten

Transport in topological insulator nanowires

In this chapter we review our work on the theory of quantum transport in topological insulator nanowires. We discuss both normal state properties and superconducting proximity effects, including the effects of magnetic fields and disorder. Throughout we assume that the bulk is insulating and inert, and work with a surface-only theory. The essential transport properties are understood in terms of three special modes: in the normal state, half a flux quantum along the length of the wire induces a perfectly transmitted mode protected by an effective time reversal symmetry; a transverse magnetic field induces chiral modes at the sides of the wire, with different chiralities residing on different sides protecting them from backscattering; and, finally, Majorana zero modes are obtained at the ends of a wire in a proximity to a superconductor, when combined with a flux along the wire. Some parts of our discussion have a small overlap with the discussion in the review [Bardarson and Moore, Rep. Prog. Phys., 76, 056501, (2013)]. We do not aim to give a complete review of the published literature, instead the focus is mainly on our own and directly related work.Comment: 22 pages, 8 figures; Chapter in "Topological Matter. Springer Series in Solid-State Sciences, vol 190. Springer

Enhanced high-harmonic generation (HHG) from Cr-doped MgO crystals

International audienc

Enhanced extreme ultraviolet high-harmonic generation from chromium-doped magnesium oxide

International audienceHigh-order harmonic generation (HHG) from crystals is emerging as a new ultrashort source of coherent extreme ultraviolet (XUV) light. Doping the crystal structure can offer a new way to control the source properties. Here, we present a study of HHG enhancement in the XUV spectral region from an ionic crystal, using dopant-induced vacancy defects, driven by a laser centered at a wavelength of 1.55 lm. Our numerical simulations based on solutions of the semiconductor Bloch equations and density-functional theory are supported by our experimental observations and demonstrate an increase in the XUV high harmonic yield from doped bulk magnesium oxide (MgO) compared to undoped MgO, even at a low defect concentration. The anisotropy of the harmonic emission as a function of the laser polarization shows that the pristine crystal's symmetry is preserved. Our study paves the way toward the control of HHG in solids with complex defects caused by transition-metal doping

Thermal and Electrical Conduction of Single-crystal Bi2Te3 Nanostructures grown using a one step process

Single-crystal Bi2Te3 nanowires (NWs) and nanoribbons (NRs) were synthesized by a vapor-liquid-solid (VLS) method from Bi2Te3 powder. To investigate the thermal properties of the Bi2Te3 nanostructure, a nondestructive technique based on temperature dependent Raman mapping was carried out. The Raman peaks were red shifted with increasing temperature. In addition, the fraction of the laser power absorbed inside the Bi2Te3 nanostructures was estimated by optical simulation and used to calculate the thermal conductivity value (kappa). The thermal conductivity value obtained for the Bi2Te3 NW and NR was 1.47 Wm(-1)K(-1) and 1.81 Wm(-1)K(-1) at 300 K, respectively. The electrical conductivity of the Bi2Te3 nanostructure was also measured. In particular, an excellent electrical conductivity value of 1.22 star 10(3) Omega(-1) cm(-1) was obtained for the Bi2Te3 NW at 300 K. This result can be attributed to topological insulator surface states. As a result of our study, the figure of merit (ZT) for the Bi2Te3 NW and NR can be significantly improved