Heat capacity jump at T_c and pressure derivatives of superconducting transition temperature in the Ba(1-x)NaxFe2As2 (0.1 <= x <= 0.9) series

We present the evolution of the initial (up to ~ 10 kbar) hydrostatic, pressure dependencies of T_c and of the ambient pressure jump in the heat capacity associated with the superconducting transition as a function of Na - doping in the Ba(1-x)NaxFe2As2 family of iron-based superconductors. For Na concentrations 0.15 <= x <= 0.9, the jump in specific heat at T_c, Delta C_p, follows the Delta C_p ~ T^3 scaling found for most BaFe2As2 - based superconductors. Pressure dependencies are non-monotonic for x = 0.2 and x = 0.24. For other Na concentrations T_c decreases under pressure in almost linear fashion. The anomalous behavior of the x = 0.2 and x = 0.24 samples under pressure are possibly due to the crossing of the phase boundaries of the narrow antiferromagnetic tetragonal phase, unique for the Ba(1-x)NaxFe2As2 series, with the application of pressure

The motif problem

Fix a choice and ordering of four pairwise non-adjacent vertices of a parallelepiped, and call a motif a sequence of four points in R^3 that coincide with these vertices for some, possibly degenerate, parallelepiped whose edges are parallel to the axes. We show that a set of r points can contain at most r^2 motifs. Generalizing the notion of motif to a sequence of L points in R^p, we show that the maximum number of motifs that can occur in a point set of a given size is related to a linear programming problem arising from hypergraph theory, and discuss some related questions.Comment: 17 pages, 1 figur

Evaluation of macrophyte control in 38 Florida lakes using triploid grass carp

Florida’s large number of shallow lakes, warm climate and long growing season have contributed to the development of excessive growths of aquatic macrophytes that have seriously interfered with many water use activities. The introduction of exotic aquatic macrophyte species such as hydrilla ( Hydrilla verticillata ) have added significantly to aquatic plant problems in Florida lakes. The use of grass carp ( Ctenopharyngodon idella ) can be an effective and economical control for aquatic vegetation such as hydrilla. Early stocking rates (24 to 74 grass carp per hectare of lake area) resulted in grass carp consumption rates that vastly exceeded the growth rates of the aquatic plants and often resulted in the total loss of all submersed vegetation. This study looked at 38 Florida lakes that had been stocked with grass carp for 3 to 10 years with stocking rates ranging from < 1 to 59 grass carp per hectare of lake and 1 to 207 grass carp per hectare of vegetation to determine the long term effects of grass carp on aquatic macrophyte communities. The median PAC (percent area coverage) value of aquatic macrophytes for the study lakes after they were stocked with grass carp was 14% and the median PVI (percent volume infested) value of aquatic macrophytes was 2%. Only lakes stocked with less than 25 to 30 fish per hectare of vegetation tended to have higher than median PAC and PVI values. When grass carp are stocked at levels of > 25 to 30 fish per hectare of vegetation the complete control of aquatic vegetation can be achieved, with the exception of a few species of plants that grass carp have extreme difficulty consuming. If the management goal for a lake is to control some of the problem aquatic plants while maintaining a small population of predominately unpalatable aquatic plants, grass carp can be stocked at approximately 25 to 30 fish per hectare of vegetation

Unconventional pairing in the iron arsenide superconductors

We use magnetic long range order as a tool to probe the Cooper pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron diffraction experiments, in particular for the unusual behavior of the magnetic order below $T_{c}$, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family.Comment: 3 figures and 4 pages; final version as published

Fish community structure in some naturally acid Florida lakes

This study investigated the status of fish communities in 12 naturally acid Florida lakes. The small, shallow lakes were located in the Ocala National Forest, the Trail Ridge, and panhandle Florida; regions where lakes have low acid neutralizing capacities and are considered sensitive to further acidification from anthropogenic sources. Fifteen species from seven families were captured during mark-recapture sampling. Warmouth (Lepomis gulosus) was the only cosmopolitan species in the study. Bluegill (Lepomis macrochirus) and largemouth bass (Micropterus salmoides), collected from 11 and 10 lakes, respectively, were also widely distributed species. Total fish abundance and biomass were not related to lake pH or total alkalinity. (Document has 202 pages

The solidification of Al–Pd–Mn studied by high-energy X-ray diffraction from electrostatically levitated samples

We report on the results of a high-energy x-ray diffraction study of Al–Pd–Mn to investigate the solidification products obtained during free-cooling using an electrostatic levitation furnace. The primary solidification product from the melt is i-Al–Pd–Mn which coexists with a significant remaining liquid component. As the sample cools further, we find that the solidification pathway is consistent with the liquidus projection and pseudo-binary cut through the ternary phase diagram reported previously. At ambient temperature we have identified the major phase to be the ξ′-phase orthorhombic approximant, along with minor phases identified as Al and, most likely, the R-phase orthorhombic approximant. We have also observed a distinct prepeak in the liquid at high temperature, signifying the presence of extended atomic order. Interestingly, this prepeak was not observed in previous neutron diffraction measurements on the Al–Pd–Mn system. No undercooling was observed preceding the solidification of the i-Al–Pd–Mn phase from the melt which may signal the close similarity of the short-range order in the solid and liquid. However, this can not be clearly determined because of the potential for heterogenous nucleation associated with the presence of an Al2O3 impurity at the surface of the sample

Effects of transition metal substitutions on the incommensurability and spin fluctuations in BaFe2As2 by elastic and inelastic neutron scattering

The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, BaFe2As2 are compared quantitatively by inelastic neutron scattering measurements and are found to be indis- tinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensu- rate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a signifi- cant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior.Comment: 5 pages, 5 figures, Major change in the manuscrip

Enhancement of interlayer exchange in an ultrathin two-dimensional magnet

Following the recent isolation of monolayer CrI3 (ref. 1), many more two-dimensional van der Waals magnetic materials have been isolated2,3,4,5,6,7,8,9,10,11,12. Their incorporation in van der Waals heterostructures offers a new platform for spintronics5,6,7,8,9, proximity magnetism13 and quantum spin liquids14. A primary question in this field is how exfoliating crystals to the few-layer limit influences their magnetism. Studies of CrI3 have shown a different magnetic ground state for ultrathin exfoliated films1,5,6 compared with the bulk, but the origin is not yet understood. Here, we use electron tunnelling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 to probe its magnetic order and find a tenfold enhancement of the interlayer exchange compared with bulk crystals. Moreover, temperature- and polarization-dependent Raman spectroscopy reveals that the crystallographic phase transition of bulk crystals does not occur in exfoliated films. This results in a different low-temperature stacking order and, we hypothesize, increased interlayer exchange. Our study provides insight into the connection between stacking order and interlayer interactions in two-dimensional magnets, which may be relevant for correlating stacking faults and mechanical deformations with the magnetic ground states of other more exotic layered magnets such as RuCl3 (ref. 14)