Order by disorder and spiral spin liquid in frustrated diamond lattice antiferromagnets

Frustration refers to competition between different interactions that cannot be simultaneously satisfied, a familiar feature in many magnetic solids. Strong frustration results in highly degenerate ground states, and a large suppression of ordering by fluctuations. Key challenges in frustrated magnetism are characterizing the fluctuating spin-liquid regime and determining the mechanism of eventual order at lower temperature. Here, we study a model of a diamond lattice antiferromagnet appropriate for numerous spinel materials. With sufficiently strong frustration a massive ground state degeneracy develops amongst spirals whose propagation wavevectors reside on a continuous two-dimensional ``spiral surface'' in momentum space. We argue that an important ordering mechanism is entropic splitting of the degenerate ground states, an elusive phenomena called order-by-disorder. A broad ``spiral spin-liquid'' regime emerges at higher temperatures, where the underlying spiral surface can be directly revealed via spin correlations. We discuss the agreement between these predictions and the well characterized spinel MnSc2S4

Cardiovascular Dynamics in Crocodylus porosus Breathing Air and During Voluntary Aerobic Dives

Pressure records from the heart and outflow vessels of the heart of Crocodylus porosus resolve previously conflicting results, showing that left aortic filling via the foramen of Panizza may occur during both cardiac diastole and systole. Filling of the left aorta during diastole, identified by the asynchrony and comparative shape of pressure events in the left and right aortae, is reconciled more easily with the anatomy, which suggests that the foramen would be occluded by opening of the pocket valves at the base of the right aorta during systole. Filling during systole, indicated when pressure traces in the left and right aortae could be superimposed, was associated with lower systemic pressures, which may occur at the end of a voluntary aerobic dive or can be induced by lowering water temperature or during a long forced dive. To explain this flexibility, we propose that the foramen of Panizza is of variable calibre. The presence of a 'right-left' shunt, in which increased right ventricular pressure leads to blood being diverted from the lungs and exiting the right ventricle via the left aorta, was found to be a frequent though not obligate correlate of voluntary aerobic dives. This contrasts with the previous concept of the shunt as a correlate of diving bradycardia. The magnitude of the shunt is difficult to assess but is likely to be relatively small. This information has allowed some new insights into the functional significance of the complex anatomy of the crocodilian heart and major blood vessels

Seed morphogenesis and effect of pretreatments on seed germination of Persian shallot (Allium hirtifolium Boiss.), an endangered medicinal plant

This study was conducted to evaluate seed morphogenesis and effect of pretreatments on seed germination of Persian shallot or mooseer (Allium hirtifolium Boiss.). In a first experiment, seed developmental stages of Persian shallot (Mooseer) were assessed. Results showed that number of ovules varied among locules and flowers. In most cases 3-4 ovules occurred per locule and developed into 1-2 seeds and therefore, a ripe capsule usually contained 3-6 seeds. The seeds were black in color and ranged from 3.73-3.98 mm in length and 2.75-3.16 mm in width, and the average weight of 1,000 seeds varied between 7.80-8.20 grams. In a second experiment four factors, scarification, stratification, potassium nitrate, and GA(3), were evaluated for seed germination. Results showed that seed germination was not observed without sandpaper scarification or stratification treatment. It could be concluded that the best treatment for overcoming mooseer seed dormancy and getting the longest seedling length was scarification with sandpaper, GA(3) (500 mg L-1 for 12 hours) application, and moist stratification. The results obtained in the present study revealed that mooseer seed has mechanical dormancy and only cold stratification is needed for endosperm weakening