Classifying Novel Phases of Spinor Atoms

We consider many-body states of bosonic spinor atoms which, at the mean-field level, can be characterized by a single-particle wave function. Such states include BEC phases and insulating Mott states with one atom per site. We describe and apply a classification scheme that makes explicit spin symmetries of such states and enables one to naturally analyze their collective modes and topological excitations. Quite generally, the method allows classification of a spin F system as a polyhedron with 2F vertices. After discussing the general formalism we apply it to the many-body states of bosons with hyperfine spins two and three. For spin-two atoms we find the ferromagnetic state, a continuum of nematic states, and a state having the symmetry of the point group of the regular tetrahedron. For spin-three atoms we obtain similar ferromagnetic and nematic phases as well as states having symmetries of various types of polyhedra with six vertices: the hexagon, the pyramid with pentagonal base, the prism, and the octahedron.Comment: Added references, corrected typos, minor changes in tex

Microstructure and conductance-slope of InAs/GaSb tunnel diodes

InAs/GaSb and similar materials systems have generated great interest as a heterojunction for tunnel field effect transistors (TFETs) due to favorable band alignment. However, little is currently understood about how such TFETs are affected by materials defects and nonidealities. We present measurements of the conductance slope for various InAs/GaSb heterojunctions via two-terminal electrical measurements, which removes three-terminal parasitics and enables direct study on the effect of microstructure on tunnelling. Using this, we can predict how subthreshold swings in TFETs can depend on microstructure. We also demonstrate growth and electrical characterization for structures grown by metalorganic chemical vapor deposition (MOCVD)—a generally more scalable process compared with molecular beam epitaxy (MBE). We determine that misfit dislocations and point defects near the interface can lead to energy states in the band-gap and local band bending that result in trap-assisted leakage routes and nonuniform band alignment across the junction area that lower the steepness of the conductance slope. Despite the small lattice mismatch, misfit dislocations still form in InAs on GaSb due to relaxation as a result of large strain from intermixed compositions. This can be circumvented by growing GaSb on InAs, straining the GaSb underlayer, or lowering the InAs growth temperature in the region of the interface. The conductance slope can also be improved by annealing the samples at higher temperatures, which we believe acts to annihilate point defects and average out major fluctuations in band alignment across the interface. Using a combination of these techniques, we can greatly improve the steepness of the conductance slope which could result in steeper subthreshold swings in TFETs in the future.National Science Foundation (U.S.). Center for Energy Efficient Electronics Science (Award 0939514)Natural Sciences and Engineering Research Council of Canada (Postgraduate M Scholarship

Nematic Order by Disorder in Spin-2 BECs

The effect of quantum and thermal fluctuations on the phase diagram of spin-2 BECs is examined. They are found to play an important role in the nematic part of the phase diagram, where a mean-field treatment of two-body interactions is unable to lift the accidental degeneracy between nematic states. Quantum and thermal fluctuations resolve this degeneracy, selecting the uniaxial nematic state, for scattering lengths $a_4>a_2$, and the square biaxial nematic state for $a_4<a_2$. Paradoxically, the fluctuation induced order is stronger at higher temperatures, for a range of temperatures below $T_c$. For the experimentally relevant cases of spin-2 $^{87}$Rb and $^{23}$Na, we argue that such fluctuations could successfully compete against other effects like the quadratic Zeeman field, and stabilize the uniaxial phase for experimentally realistic conditions. A continuous transition of the Ising type from uniaxial to square biaxial order is predicted on raising the magnetic field. These systems present a promising experimental opportunity to realize the `order by disorder' phenomenon.Comment: 5 pages, 4 figures; 1 reference and 1 minor correctio

Status of the Major Oyster Diseases in Virginia 2004 A Summary of the Annual Monitoring Program

2004 was the second very wet year in a row. While rainfall and streamflows were normal in winter and early spring, and just slightly above average during the summer, the fall of 2004 was nearly as wet as the year before. Salinities were again depressed throughout the lower Bay. Water temperatures were below normal during the winter, but typical otherwise. Low salinities and, in the winter, temperatures brought continued abatement in the oyster diseases caused by Perkinsus marinus (Dermo) and Haplosporidium nelsoni (MSX). Among quarterly James River Survey sites, maximum annual P. marinus prevalences were the lowest they had been since before 1992. In summer and fall, when P. marinus is normally most prevalent, it was found in a maximum of 68% of oysters at Wreck Shoal and in no more than 20% of the oysters anywhere else. Advanced infections were rare, and limited to Wreck Shoal. H. nelsoni was found in a single oyster, at Wreck Shoal in July

Status of the Major Oyster Diseases in Virginia 2005 A Summary of the Annual Monitoring Program

More normal riverflows and salinities returned in 2005 after two very wet years. Temperatures were somewhat colder than normal during the winter, and warmer during the summer. The physical environment was generally more favorable for parasite activity, and thus brought a slight increase in prevalence and intensity of the oyster diseases caused by Perkinsus marinus (Dermo) and Haplosporidium nelsoni (MSX). Among quarterly James River Survey sites, maximum annual P. marinus prevalences returned to levels typical of the mid- 1990s, before the years of drought. P. marinus prevalence reached 92% at Wreck Shoal, 56% at Point of Shoal, 68% at Horsehead Rock, and 8% at Deepwater Shoal, where P. marinus was observed for the first time since early 2003. More advanced, heavy infections became more numerous, but were limited to Wreck Shoal. H. nelsoni was restricted to Wreck Shoal, where it increased in maximum prevalence from 4% to 20%