Leonid Electrophonic Bursters

We investigate the conditions under which Leonid meteoroids might generate short duration (burster) electrophonic sounds. A `first order' theory is employed to estimate the approximate electron number density in the meteoroid ablation column as a function of time. Using the threshold conditions discussed in an earlier communication (Beech and Foschini 1999) we find that Leonid meteoroids more massive than about 0.1 kg can potentially generate short duration electrophonic bursters.Comment: 5 pages, no figures. Accepted for the publication on Astronomy and Astrophysic

Management options for pregnant feedlot heifers

Heifers that were 167 days pregnant when slaughtered gained faster and more efficiently than open heifers, or heifers that had been aborted with a prostaglandin analog at 83 or 138 days, unless the slaughter weight was adjusted for the 1.7% lower carcass yield (dressing %). When the slaughter weights for all these management options were adjusted using the carcass yield of open heifers, there was no difference in gain except for the depressed performance associated with late abortions. However, open heifers were 6.7% more efficient than heifers pregnant when slaughtered. Heifers aborted at 138 days had substantially reduced gains and feed conversion. These results indicate that because of increased carcass yield, packers can afford to pay a premium for heifers that are open or have been aborted during the first trimester. Unless a premium is paid for open heifers, pregnant heifers (provided they are sold before calving) sold on a live weight basis might be more profitable because of the apparent increased gain and efficiency

Calcutta Botanic Garden and the colonial re-ordering of the Indian environment

This article examines three hand-painted colour maps that accompanied the annual report of the Calcutta Botanic Garden for 1846 to illustrate how the Garden’s layout, uses and functions had changed over the previous 30 years. The evolution of the Calcutta Botanic Garden in the first half of the nineteenth-century reflects a wider shift in attitudes regarding the relationship between science, empire and the natural world. On a more human level the maps result from, and illustrate, the development of a vicious personal feud between the two eminent colonial botanists charged with superintending the garden in the 1840s

Spontaneous DC Current Generation in a Resistively Shunted Semiconductor Superlattice Driven by a TeraHertz Field

We study a resistively shunted semiconductor superlattice subject to a high-frequency electric field. Using a balance equation approach that incorporates the influence of the electric circuit, we determine numerically a range of amplitude and frequency of the ac field for which a dc bias and current are generated spontaneously and show that this region is likely accessible to current experiments. Our simulations reveal that the Bloch frequency corresponding to the spontaneous dc bias is approximately an integer multiple of the ac field frequency.Comment: 8 pages, Revtex, 3 Postscript figure

Effects of site management in pine plantations on the coastal lowlands of subtropical Queensland, Australia

Biomass and nutrient distribution in a 30-year-old slash pine (Pinus elliottii) plantation in Queensland, Australia, were estimated following clear-felling in November 1995, to provide a basis for interpreting changes in the nutrient pools and tree responses to harvest residue management practices applied at the establishment of the second rotation. Total biomass at clearfall of a typical slash pine stand was 316 t ha-1, of which 206 t ha-1 was removed in logs. Nitrogen and P removed in logs accounted for 7.6 and 3.4% of total N and P, respectively, in the ecosystem. Residues remaining after logging contained 12% of the total N and 5.2% of the total P. Proper management of these residues is therefore critical for sustaining site productivity. Following clear felling, a long-term experiment was established to assess the impacts of harvesting residues and litter management regimes on soil fertility and productivity of the second crop F1 hybrid between slash pine and Honduras Caribbean pine (P. caribaea var. hondurensis). Early results from this experiment showed that tree stem volume and above ground biomass production increased by 31 and 29%, respectively, at age 39 months by retaining litter and logging residues, compared with the treatment in which logging residues and litter were removed. Further improvements in tree growth have been achieved by doubling the quantity of residues retained and by controlling weed competition. Foliar nutrient concentrations indicated that N may play an important role in the maintenance of long-term site productivity. Differences exist in the growth and foliar nutrient concentrations between the different hybrid families tested but all families responded similarly to the residue treatments. The presence of the residues increased soil moisture levels in the surface soil during a dry season. There was a marked reduction in the quantity of residue, especially the finer fraction, after 39 months. This study has contributed to an improved understanding of the soil and plant factors controlling productivity and provided a basis for more detailed studies on processes underpinning plantation sustainability

Mental health: Future challenges [Mental Capital and Wellbeing: Making the most of ourselves in the 21st century]

The aim of the Foresight Project on Mental Capital and Wellbeing (www.foresight.gov.uk) is to advise the Government on how to achieve the best possible mental development and mental wellbeing for everyone in the UK in the future. The starting point of the Project was to generate an understanding of the science of mental capital and wellbeing (MCW) and to develop a vision for how the size and nature of the challenges exposed by the Project could evolve over the next 20 years. To make this analysis tractable, the work was divided into five broad areas: Mental capital through life; Learning through life; Mental health; Wellbeing and work; and Learning difficulties. This report presents the findings for Mental health and draws upon a comprehensive assessment of the scientific state-of-the art: overall, around 80 reviews have been commissioned across the five areas

Theory of Coherent Time-dependent Transport in One-dimensional Multiband Semiconductor Superlattices

We present an analytical study of one-dimensional semiconductor superlattices in external electric fields, which may be time-dependent. A number of general results for the (quasi)energies and eigenstates are derived. An equation of motion for the density matrix is obtained for a two-band model, and the properties of the solutions are analyzed. An expression for the current is obtained. Finally, Zener-tunneling in a two-band tight-binding model is considered. The present work gives the background and an extension of the theoretical framework underlying our recent Letter [J. Rotvig {\it et al.}, Phys. Rev. Lett. {\bf 74}, 1831 (1995)], where a set of numerical simulations were presented.Comment: 15 pages, Revtex 3.0, uses epsf, 2 ps figures attache

Microwave Photoconductivity in Two-Dimensional Electron Systems due to Photon-Assisted Interaction of Electrons with Leaky Interface Phonons

We calculate the contribution of the photon-assisted interaction of electrons with leaky interface phonons to the dissipative dc photoconductivity of a two-dimensional electron system in a magnetic field. The calculated photoconductivity as a function of the frequency of microwave radiation and the magnetic field exhibits pronounced oscillations. The obtained oscillation structure is different from that in the case of photon-assisted interaction with impurities. We demonstrate that at a sufficiently strong microwave radiation in the certain ranges of its frequency (or in certain ranges of the magnetic field) this mechanism can result in the absolute negative conductivity.Comment: 3 pages, 1 figur

Linear optical absorption spectra of mesoscopic structures in intense THz fields: free particle properties

We theoretically study the effect of THz radiation on the linear optical absorption spectra of semiconductor structures. A general theoretical framework, based on non-equilibrium Green functions, is formulated, and applied to the calculation of linear optical absorption spectrum for several non-equilibrium mesoscopic structures. We show that a blue-shift occurs and sidebands appear in bulk-like structures, i.e., the dynamical Franz-Keldysh effect [A.-P. Jauho and K. Johnsen, Phys. Rev. Lett. 76, 4576 (1996)]. An analytic calculation leads to the prediction that in the case of superlattices distinct stable steps appear in the absorption spectrum when conditions for dynamical localization are met.Comment: 13 Pages, RevTex using epsf to include 8 ps figures. Submitted to Phys. Rev. B (3 April 97

Quantum metastability in a class of moving potentials

In this paper we consider quantum metastability in a class of moving potentials introduced by Berry and Klein. Potential in this class has its height and width scaled in a specific way so that it can be transformed into a stationary one. In deriving the non-decay probability of the system, we argue that the appropriate technique to use is the less known method of scattering states. This method is illustrated through two examples, namely, a moving delta-potential and a moving barrier potential. For expanding potentials, one finds that a small but finite non-decay probability persists at large times. Generalization to scaling potentials of arbitrary shape is briefly indicated.Comment: 10 pages, 1 figure