Magnetism in parent Fe-chalcogenides: quantum fluctuations select a plaquette order

We analyze magnetic order in iron-chalcogenide Fe$_{1+y}$Te -- the parent compound of high-temperature superconductor Fe$_{1+y}$Te$_{1-x}$Se$_x$. Neutron scattering experiments show that magnetic order in this material contains components with momentum $Q_1=(\pi/2, \pi/2)$ and $Q_2 =(\pi/2, -\pi/2)$ in Fe-only Brillouin zone. The actual spin order depends on the interplay between these two components. Previous works argued that spin order is a single-$Q$ state (either $Q_1$ or $Q_2$). Such an order breaks rotational $C_4$ symmetry and order spins into a double diagonal stripe. We show that quantum fluctuations actually select another order -- a double $Q$ plaquette state with equal weight of $Q_1$ and $Q_2$ components, which preserves $C_4$ symmetry but breaks $Z_4$ translational symmetry. We argue that the plaquette state is consistent with recent neutron scattering experiments on Fe$_{1+y}$Te.Comment: 8 pages, 3 figure

Visions of the Amistad: American Public Engagement with the Amistad Story

Steven Spielberg’s movie Amistad, released in 1997, is considered to be a good movie. The New York Times published a very favorable review of the film when it came out stating that it provides “tough, sobering depictions of the captives’ ordeal.” It has all of the ingredients of a great historical motion picture; it is interesting, exciting at times, and educational. Amistad is a go to film for many high school history teachers to begin discussion on the Amistad case. While the movie follows the movements of high profile characters from the Amistad story such as Martin Van Buren, Cinque, Roger Baldwin, and John Quincy Adams, it leaves out the American public at large. There are scenes where crowds are shown gathered at the caboose of a train to hear Van Buren speak and outside the courtroom in Connecticut, but these are just fleeting images of an American public who was actively reading about and forming opinions on the Amistad case as it was occurring. Maybe it should not come as a surprise that Spielberg’s Amistad leaves out the American public when talking about different aspects of the Amistad events. Many scholarly sources that look at the Amistad focus on the main political and legal personalities involved in the case (John Quincy Adams’ speech to the Supreme Court has been thoroughly investigated), but not on the reaction of the American public. This gap in the scholarly research is understandable as the Amistad event has many important aspects that give scholars numerous choices as to where to direct their research. This oversight is where I directed my own efforts in trying to add a different way to look at the Amistad event as a whole. After analyzing existing scholarly sources on the Amistad event, I look at different newspaper articles about the Amistad event from 1839 to 1845 and investigate the American public’s reaction to the case based on how these newspapers present their arguments to their readership. I argue that region and political affiliation had a large impact on how Americans viewed the Amistad event

Foreign Workers and the Organic Farms’ Demand for Seasonal Unskilled Labor

This study analyzes farm labor management strategies of organic farms by employing Heckan selection model. Econometric results present interesting relationships between foreign worker-related variables and the hiring of non-family seasonal part-time workers. First, the extent of hiring is inversely related to the population of illegal residents. Second, the outcome variable (extent of hiring) is directly related to the number of H2A permits (foreign guest farm worker visas for temporary, contractual work) issued. The results also indicate that higher wages, corporate farms, and more educated, younger and full-time farm operators are associated with greater tendencies to hire non-family seasonal part-time workers. Family labor is used to supplement the lack of available non-family seasonal part-time workers.organic farm, unskilled labor, immigration, Heckman selection model, foreign worker, Farm Management,

On the robustness of ultra-high voltage 4H-SiC IGBTs with an optimized retrograde p-well

The robustness of ultra-high voltage (>10kV) SiC IGBTs comprising of an optimized retrograde p-well is investigated. Under extensive TCAD simulations, we show that in addition to offering a robust control on threshold voltage and eliminating punch-through, the retrograde is highly effective in terms of reducing the stress on the gate oxide of ultra-high voltage SiC IGBTs. We show that a 10 kV SiC IGBT comprising of the retrograde p-well exhibits a much-reduced peak electric field in the gate oxide when compared with the counterpart comprising of a conventional p-well. Using an optimized retrograde p-well with depth as shallow as 1 μm, the peak electric field in the gate oxide of a 10kV rated SiC IGBT can be reduced to below 2 MV.cm -1 , a prerequisite to achieve a high-degree of reliability in high-voltage power devices. We therefore propose that the retrograde p-well is highly promising for the development of>10kV SiC IGBTs

TCAD device modelling and simulation of wide bandgap power semiconductors

Technology computer-aided Design (TCAD) is essential for devices technology development, including wide bandgap power semiconductors. However, most TCAD tools were originally developed for silicon and their performance and accuracy for wide bandgap semiconductors is contentious. This chapter will deal with TCAD device modelling of wide bandgap power semiconductors. In particular, modelling and simulating 3C- and 4H-Silicon Carbide (SiC), Gallium Nitride (GaN) and Diamond devices are examined. The challenges associated with modelling the material and device physics are analyzed in detail. It also includes convergence issues and accuracy of predicted performance. Modelling and simulating defects, traps and the effect of these traps on the characteristics are also discussed

Event-driven sediment transport in a highly responsive lowland river as influenced by climate and land-use change, Haw River, North Carolina, USA

Anthropogenic activities have dramatically altered river systems from their natural state. Not only has population growth enhanced the magnitude of human impacts such as contaminants, it has also led to an increasingly direct link between a river and its basin. Land-use change has increased available material by mobilizing large quantities of sediment, while impervious surfaces and storm drains have intensified discharge events by more efficiently directed runoff to stream channels. In central North Carolina, the hydrologic response to precipitation events has intensified 22-91 percent. Yet, this change is shown to have occurred during a period when precipitation and temperature remain normal compared to historical baselines. Sediment is an efficient vehicle for transporting contaminants, which are known to decouple and become bioavailable upon deposition in these reservoirs. This is particularly problematic because of the increasing number of reservoirs built along rivers and utilized for drinking water. Sixty-one suspended sediment samples were collected between April 2008 and June 2010 in the Haw River, a lowland river located in a high-population growth area of the Piedmont of North Carolina. Radioisotope measurements identify excess sediment from human impacts (construction) present primarily during peak discharge of a hydrologic event. Such human impacts have implications not only for ecosystems and public health but also for the utilization of radioisotopes to elucidate sediment transport. While there is a global focus on climate change, this study illustrates how land-use change is already having significant consequences that will only be exacerbated by the intensified hydrologic cycle predicted to result from global warming