Joule heating induced negative differential resistance in free standing metallic carbon nanotubes

The features of the $IV$ characteristics of metallic carbon nanotubes (m-NTs) in different experimental setups are studied using semi-classical Boltzmann transport equation together with the heat dissipation equation to account for significant thermal effects at high electric bias. Our model predicts that the shape of the m-NT characteristics is basically controlled by heat removal mechanisms. In particular we show that the onset of negative differential resistance in free standing nanotubes finds its origins in strong transport nonlinearities associated with poor heat removal unlike in substrate-supported nanotubes.Comment: 3 pages, 3 figure

Semi-analytical approach for the Vaidya metric in double-null coordinates

We reexamine here a problem considered in detail before by Waugh and Lake: the solutions of spherically symmetric Einstein's equations with a radial flow of unpolarized radiation (the Vaidya metric) in double-null coordinates. This problem is known to be not analytically solvable, the only known explicit solutions correspond to the constant mass case (Schwarzschild solution in Kruskal-Szekeres form) and the linear and exponential mass functions originally discovered by Waugh and Lake. We present here a semi-analytical approach that can be used to discuss some qualitative and quantitative aspects of the Vaidya metric in double-null coordinates for generic mass functions. We present also a new analytical solution corresponding to $(1/v)$-mass function.Comment: 5 pages, 6 figure

The Pacific Basin in World Trade: Part III, An Analysis of Changing Trade Patterns, 1955-1975

This is the third of a sequence of papers on international flows of trade among fifteen Pacific Basin (PB) countries and between them and eleven regions in the Rest of the World (ROW). In Part I of the sequence (Hickman, Kuroda and Lau, 1977a) we presented and documented annual data on bilateral flows of total exports valued f.o.b. in current dollars among the twenty-six countries and regions for the years 1948 through1975. The primary data source is the Direction of Trade computer tape of the International Monetary Fund, but these data were supplemented from other sources, especially as regards the international trade of the socialist countries. The second report (1977b) extended the data base to include unit value export price indexes and the corresponding constant dollar trade flow matrices for the period 1955-1975. In this third report we analyze the changing pattern of PB trade over the same period, using as tools export growth decomposition indexes, trend analysis, and regression analysis of the price elasticity of import market shares. The present paper is organized as follows. In Section 2 we describe the trends in the export performance of the PB countries and ROW regions, as measured by the cumulative percentage change in each country's share of world exports between 1955 and 1975 and for selected sub periods. In Section 3 and Appendices B and C these export share changes are decomposed into three sources: changes in the degree of penetration of the various import markets, changes in the size of the import markets themselves, and an interaction effect. The decomposition indexes are shown in Section 4 to be dominated by the market penetration or competitiveness effect, so that a country gains or loses in world trade according to whether or not it can increase its shares of the markets in which it sells rather than as a passive result of changes in the size of the markets themselves. This leads to a descriptive analysis in Section 5 of the secular growth rates of the market shares of each country or region in the import markets of the twenty-five remaining countries and regions. Finally, we conclude the paper in Section 6 with art exploratory regression analysis of the responsiveness of the market shares to changes in the relative prices of the various exporting countries competing in each import market, leading to the general conclusion that relative prices do matter and presenting estimates of share or substitution elasticities in the various import markets.

Restricted Wiedemann-Franz law and vanishing thermoelectric power in one-dimensional conductors

In one-dimensional (1D) conductors with linear E-k dispersion (Dirac systems) intrabranch thermalization is favored by elastic electron-electron interaction in contrast to electron systems with a nonlinear (parabolic) dispersion. We show that under external electric fields or thermal gradients the carrier populations of different branches, treated as Fermi gases, have different temperatures as a consequence of self-consistent carrier-heat transport. Specifically, in the presence of elastic phonon scattering, the Wiedemann-Franz law is restricted to each branch with its specific temperature and is characterized by twice the Lorenz number. In addition thermoelectric power vanishes due to electron-hole symmetry, which is validated by experiment.Comment: 10 pages, 2 figure

The importance of diazotrophic cyanobacteria as primary producers during Cretaceous Oceanic Anoxic Event 2

In Livello Bonarelli black shale deposited during Cretaceous Oceanic Anoxic Event 2 (OAE-2, ca.&nbsp;94 Ma), nitrogen isotopic compositions of bulk sediments are mostly in a narrow range from &ndash;2.7 to &ndash;0.7&permil;. We also determined molecular distribution and nitrogen isotopic compositions of geoporphyrins extracted from the black shale. The nitrogen isotopic compositions of C₃₂ Ni deoxophylloerythroetioporphyrin (DPEP) and total Ni porphyrins are &ndash;3.5 and &ndash;3.3&permil;, respectively, leading us to the estimation that the mean nitrogen isotopic composition of photoautotrophic cells were around +1&permil; during the formation of Bonarelli black shale. This value is suggestive of N₂-fixation, a dominant process for these photoautotrophs when assimilating nitrogen. Furthermore, Ni-chelated C₃₂ DPEP, derived mainly from chlorophyll <i>a</i> had the highest concentration. Based on this evidence, we conclude that diazotrophic cyanobacteria were major primary producers during that time. Cyanobacteria may be key photoautotrophs during the formation of black shale type sediments intermittently observed throughout the later half of the Earth&apos;s history, and hence may have played a crucial role in the evolution of geochemical cycles even in the later half of the Earth&apos;s history

Evidence of Nonlinear Flow In Softwoods From Wood Permeability Measurements

A series of air permeability measurements of softwoods and hardwoods was conducted with flow rates from 0.015 to 32 cm3/sec to examine nonlinear flow. The permeability of loblolly pine, Douglas-fir, and white spruce was found to decrease with flow rate. The critical Reynolds numbers obtained from the decrease in permeability and also from the increase in the applied pressure difference with flow rate, based on Tompkins' equation, were between 0.41 and 1.62. These were in good agreement with the values calculated from the length-to-radius ratio of pit openings in accordance with Siau and Petty's study of short capillaries. This indicated the presence of nonlinearity owing to kinetic energy losses at the pit openings. Nonlinear flow could not be detected for paper birch and basswood