Spontaneous and Superfluid Chiral Edge States in Exciton-Polariton Condensates

We present a scheme of interaction-induced topological bandstructures based on the spin anisotropy of exciton-polaritons in semiconductor microcavities. We predict theoretically that this scheme allows the engineering of topological gaps, without requiring a magnetic field or strong spin-orbit interaction (transverse electric-transverse magnetic splitting). Under non-resonant pumping, we find that an initially topologically trivial system undergoes a topological transition upon the spontaneous breaking of phase symmetry associated with polariton condensation. Under resonant coherent pumping, we find that it is also possible to engineer a topological dispersion that is linear in wavevector -- a property associated with polariton superfluidity.Comment: 6 pages, 4 figure

Time-Delay Polaritonics

Non-linearity and finite signal propagation speeds are omnipresent in nature, technologies, and real-world problems, where efficient ways of describing and predicting the effects of these elements are in high demand. Advances in engineering condensed matter systems, such as lattices of trapped condensates, have enabled studies on non-linear effects in many-body systems where exchange of particles between lattice nodes is effectively instantaneous. Here, we demonstrate a regime of macroscopic matter-wave systems, in which ballistically expanding condensates of microcavity exciton-polaritons act as picosecond, microscale non-linear oscillators subject to time-delayed interaction. The ease of optical control and readout of polariton condensates enables us to explore the phase space of two interacting condensates up to macroscopic distances highlighting its potential in extended configurations. We demonstrate deterministic tuning of the coupled-condensate system between fixed point and limit cycle regimes, which is fully reproduced by time-delayed coupled equations of motion similar to the Lang-Kobayashi equation

Avalanche defences for Flateyri, Iceland. From hazard evaluation to construction of defences.

The village of Flateyri was hit by two major avalanches in 1995, with the latter one killing 20 of the 45 people caught in it. These, together with the catastrophic avalanche in the nearby Súôavík in January the same year, initiated a complete review of all government actions and regulations regarding avalanche risk and avalanche defences in Iceland. The government set a long term goal for the acceptable risk to people living in avalanche prone areas and increased funding for the construction of avalanche defences. The avalanche situation in Flateyri is quite serious and the risk the inhabitants live with is very high. The town is threatened by avalanches from two gullies, Innra-Bæjargil in the north-west and Skollahvilft in the north-east, with several records of avalanches reaching well into the present residential area. To fulfil the safety requirements, an appraisal study was carried out and a proposal made for avalanche defences for the community, consisting of two earthfill deflecting dams and an earthfill catching dam. The proposal was accepted by the local authorities and the Icelandic government in the spring of 1996, design work was carried out in that summer and construction work started in September 1996. The construction of the dams is now close to completion, two years after the accident

Metal Cooling in Simulations of Cosmic Structure Formation

The addition of metals to any gas can significantly alter its evolution by increasing the rate of radiative cooling. In star-forming environments, enhanced cooling can potentially lead to fragmentation and the formation of low-mass stars, where metal-free gas-clouds have been shown not to fragment. Adding metal cooling to numerical simulations has traditionally required a choice between speed and accuracy. We introduce a method that uses the sophisticated chemical network of the photoionization software, Cloudy, to include radiative cooling from a complete set of metals up to atomic number 30 (Zn) that can be used with large-scale three-dimensional hydrodynamic simulations. Our method is valid over an extremely large temperature range (10 K < T < 10^8 K), up to hydrogen number densities of 10^12 cm^-3. At this density, a sphere of 1 Msun has a radius of roughly 40 AU. We implement our method in the adaptive mesh refinement (AMR) hydrodynamic/N-body code, Enzo. Using cooling rates generated with this method, we study the physical conditions that led to the transition from Population III to Population II star formation. While C, O, Fe, and Si have been previously shown to make the strongest contribution to the cooling in low-metallicity gas, we find that up to 40% of the metal cooling comes from fine-structure emission by S, when solar abundance patterns are present. At metallicities, Z > 10^-4 Zsun, regions of density and temperature exist where gas is both thermally unstable and has a cooling time less than its dynamical time. We identify these doubly unstable regions as the most inducive to fragmentation. At high redshifts, the CMB inhibits efficient cooling at low temperatures and, thus, reduces the size of the doubly unstable regions, making fragmentation more difficult.Comment: 19 pages, 12 figures, significant revision, including new figure

The avalanche situation in Neskaupstadur, Iceland : A preliminary defensive plan

The avalanche situation in the town of Neskaupstaôur is rather unique and in many respects more difficult than in any other place in Iceland. The avalanche risk is high in a large portion of the 2,7 km long residential area, with similar snow accumulation conditions throughout the entire area. Additionally, conditions for the construction of avalanche defences are rather difficult. The avalanche starting zones are extensive and many of these have difficult geological conditions. The uppermost houses are located close to the mountain, with limited space to construct avalanche defences above them, especially in the western part of the town. After the avalanche accidents in Súàavík and Flateyri in 1995 the Icelandic government has set a long term goal for the acceptable risk to people living in avalanche prone areas. To fulfil this goal, a risk analysis has to be made in connection with avalanche defence planning. The risk analysis for Neskaupstaôur indicates that the risk level is far above the acceptable limits in certain parts of the town. To improve the situation, a preliminary protection plan has been set up for the whole town. The plan is based on a combination of supporting structures, deflecting dams, catching dams and breaking mounds

Forging links between distant lands ASÍS - The Icelandic Centre for Asian Studies

Asian studies is not usually the first thing that comes to mind when considering Iceland. In fact, in the second half of the 20th century Iceland was too pre-occupied with its own European-American identity problem to be able to pay serious attention to a distant region such as Asia. But as Geir Sigurdsson, director of the recently opened Icelandic Centre for Asian Studies explains, the situation has changed rapidly during the last two decades