The sail wing windmill and its adaptation for use in rural India

An 8 meter-diameter prototype sail wing windmill is reported that uses a one meter-diameter bullock cartwheel to which three bamboo poles are latched in a triangular pattern with overlapping ends, to form the airframe for cloth sails. This device lifts 300 pounds to a height of 20 feet in one minute in a 10 mph wind

Magnetic properties of the doped two-dimensional antiferromagnet

The variety of the normal-state magnetic properties of cuprate high-Tc superconductors is interpreted based on the self-consistent solution of the self-energy equations for the two-dimensional t-J model. The observed variations of the spin correlation length with the hole concentration x, of the spin susceptibility with x and temperature T and the scaling of the static uniform susceptibility are well reproduced by the calculated results. The nonmonotonic temperature dependence of the Cu spin-lattice relaxation rate is connected with two competing tendencies in the low-frequency susceptibility: its temperature decrease due to the increasing spin gap and the growth of the susceptibility in this frequency region with the temperature broadening of the maximum in the susceptibility.Comment: 6 pages, 5 figures, Proc. Int. Conf. "Modern Problems of Superconductivity", 9-14 Sept. 2002, Yalta, Ukrain

Two-dimensional t-J model at moderate doping

Using the method which retains the rotation symmetry of spin components in the paramagnetic state and has no preset magnetic ordering, spectral and magnetic properties of the two-dimensional t-J model in the normal state are investigated for the ranges of hole concentrations 0 <= x <= 0.16 and temperatures 0.01t <= T <= 0.2t. The used hopping t and exchange J parameters of the model correspond to hole-doped cuprates. The obtained solutions are homogeneous which indicates that stripes and other types of phase separation are not connected with the strong electron correlations described by the model. A series of nearly equidistant maxima in the hole spectral function calculated for low T and x is connected with hole vibrations in the region of the perturbed short-range antiferromagnetic order. The hole spectrum has a pseudogap in the vicinity of (0,\pi) and (\pi,0). For x \approx 0.05 the shape of the hole Fermi surface is transformed from four small ellipses around (\pm\pi/2,\pm\pi/2) to two large rhombuses centered at (0,0) and (\pi,\pi). The calculated temperature and concentration dependencies of the spin correlation length and the magnetic susceptibility are close to those observed in cuprate perovskites. These results offer explanations for the observed scaling of the static uniform susceptibility and for the changes in the spin-lattice relaxation and spin-echo decay rates in terms of the temperature and doping variations in the spin excitation spectrum of the model.Comment: 12 pages, 14 figure

The spin-1 two-dimensional J1-J2 Heisenberg antiferromagnet on a triangular lattice

The spin-1 Heisenberg antiferromagnet on a triangular lattice with the nearest- and next-nearest-neighbor couplings, $J_1=(1-p)J$ and $J_2=pJ$, $J>0$, is studied in the entire range of the parameter $p$. Mori's projection operator technique is used as a method which retains the rotation symmetry of spin components and does not anticipate any magnetic ordering. For zero temperature four second-order phase transitions are observed. At $p\approx 0.038$ the ground state is transformed from the long-range ordered $120^\circ$ spin structure into a state with short-range ordering, which in its turn is changed to a long-range ordered state with the ordering vector ${\bf Q^\prime}=\left(0,-\frac{2\pi}{\sqrt{3}}\right)$ at $p\approx 0.2$. For $p\approx 0.5$ a new transition to a state with a short-range order occurs. This state has a large correlation length which continuously grows with $p$ until the establishment of a long-range order happens at $p \approx 0.65$. In the range $0.5<p<0.96$, the ordering vector is incommensurate. With growing $p$ it moves along the line ${\bf Q'-Q}_1$ to the point ${\bf Q}_1=\left(0,-\frac{4\pi}{3\sqrt{3}}\right)$ which is reached at $p\approx 0.96$. The obtained state with a long-range order can be conceived as three interpenetrating sublattices with the $120^\circ$ spin structure on each of them.Comment: 13 pages, 5 figures, accepted for publication in Physics Letters

High temperature dispersion strengthening of NiAl

A potential high temperature strengthening mechanism for alloys based on the intermetallic compound NiAl was investigated. This study forms part of an overall program at NASA Lewis Research Center for exploring the potential of alloys based on NiAl for high temperature applications. An alloy containing 2.26 at% Nb and produced by hot extrusion of blended powders was examined in detail using optical and electron microscopy. Interdiffusion between the blended Nb and NiAl powders results in the formation of intermediate phases. A fine dispersion of precipitates of a hexagonal, ordered NiAlNb phases in a matrix of NiAl can be produced and this results in strengthening of the alloy by interfering with dislocation motion at high temperature. These precipitates are, however, found to coarsen during the high temperature (1300 K) deformation at slow strain rates and this may impose some limitatioins on the use of this strengthening mechanism