Monte Carlo Simulations of Quantum Spin Systems in the Valence Bond Basis

We discuss a projector Monte Carlo method for quantum spin models formulated in the valence bond basis, using the S=1/2 Heisenberg antiferromagnet as an example. Its singlet ground state can be projected out of an arbitrary basis state as the trial state, but a more rapid convergence can be obtained using a good variational state. As an alternative to first carrying out a time consuming variational Monte Carlo calculation, we show that a very good trial state can be generated in an iterative fashion in the course of the simulation itself. We also show how the properties of the valence bond basis enable calculations of quantities that are difficult to obtain with the standard basis of Sz eigenstates. In particular, we discuss quantities involving finite-momentum states in the triplet sector, such as the dispersion relation and the spectral weight of the lowest triplet.Comment: 15 pages, 7 figures, for the proceedings of "Computer Simulation Studies in Condensed Matter Physics XX

Spin pumping by a field-driven domain wall

We calculate the charge current in a metallic ferromagnet to first order in the time derivative of the magnetization direction. Irrespective of the microscopic details, the result can be expressed in terms of the conductivities of the majority and minority electrons and the non-adiabatic spin transfer torque parameter $\beta$. The general expression is evaluated for the specific case of a field-driven domain wall and for that case depends strongly on the ratio of $\beta$ and the Gilbert damping constant. These results may provide an experimental method to determine this ratio, which plays a crucial role for current-driven domain-wall motion.Comment: 4 pages, 1 figure v2: some typos corrected v3: published versio

Spraying practice for orchard and garden

This bulletin tells briefly how to fight the more common insect pests and plant diseases which infest Iowa orchards and gardens, especial attention being given to the apple. The directions are up-to-date and are based upon the work of scientific investigators and the experience of practical fruit growers. While they are given to meet the peculiar needs under Iowa conditions, they are nevertheless general, and the strength of the mixtures and number of sprayings will need to be varied somewhat to meet changing conditions

Iowa 403 —A New Seedling Apple

One of the promising seedling apples originated at the Iowa Experiment Station Is now offered to the nurserymen of the state under the label Iowa 403. It is hoped that the nurserymen will assist In making a thorough test of the variety, so that as soon as possible Its value for planting In this region may be determined. The distribution at this time will consist solely of clons. Since the nurserymen, as a class, are prepared to give the clons more skillful attention than other people, this distribution Is to be offered to all Iowa nurserymen whose names appear on the 1914 list of the State Inspector. Later distribution will be made to others who may be Interested In testing this apple

The analysis of reactively loaded microstrip antennas by finite difference time domain modelling

In recent years, much interest has been shown in the use of printed circuit antennas in mobile satellite and communications terminals at microwave frequencies. Although such antennas have many advantages in weight and profile size over more conventional reflector/horn configurations, they do, however, suffer from an inherently narrow bandwidth. A way of optimizing the bandwidth of such antennas by an electronic tuning technique using a loaded probe mounted within the antenna structure is examined, and the resulting far-field radiation patterns are shown. Simulation results from a 2D finite difference time domain (FDTD) model for a rectangular microstrip antenna loaded with shorting pins are given and compared to results obtained with an actual antenna. It is hoped that this work will result in a design package for the analysis of microstrip patch antenna elements

Standard-M mobile satellite terminal employing electronic beam squint tracking

In recent years, extensive experience has been built up at the University of Bristol in the use of the Electronic Beam Squint (EBS) tracking technique, applied to large earth station facilities. The current interest in land mobile satellite terminals, using small tracking antennas, has prompted the investigation of the applicability of the EBS technique to this environment. The development of an L-band mechanically steered vehicle antenna is presented. A description of the antenna is followed by a detailed investigation of the tracking environment and its implications on the error detection capability of the system. Finally, the overall hardware configuration is described along with plans for future work

High-Q Gold and Silicon Nitride Bilayer Nanostrings

Low-mass, high-Q, silicon nitride nanostrings are at the cutting edge of nanomechanical devices for sensing applications. Here we show that the addition of a chemically functionalizable gold overlayer does not adversely affect the Q of the fundamental out-of-plane mode. Instead the device retains its mechanical responsiveness while gaining sensitivity to molecular bonding. Furthermore, differences in thermal expansion within the bilayer give rise to internal stresses that can be electrically controlled. In particular, an alternating current excites resonant motion of the nanostring. This AC thermoelastic actuation is simple, robust, and provides an integrated approach to sensor actuation.Comment: 5 pages, 4 figures + supplementary materia

Cold storage for Iowa apples

Under present conditions of apple growing in Iowa temporary gluts are common in very many local markets from the time the earliest fruits begin to ripen till early autumn or mid autumn followed usually by an immediate shortage as soon as the early fruit passes out of season. This holds true to a constantly increasing extent from south central Iowa to the north boundary of the State. The result is low prices locally for much of the early fruit with high prices usually prevailing from mid autumn to the end of the season. Even though prices are high very often the available supply of winter apples is not first-class fruit. The experiments demonstrate that certain desirable fall apples which are hardy enough to be grown sucessfully even in northern Iowa, can be held in good market condition through the winter months if handled carefully and stored quickly. This makes it possible to maintain a supply of home-grown fruit till late winter or early spring even in those parts of Iowa where practically none but early apples are now grown