Oscillator Strength of Metallic Carbon Nanotubes

Based on the tight binding method with hopping integral between the nearest-neighbor atoms, an oscillator strength \int_0^{\infty} \d \omega {\rm Re} \sigma (\omega) is discussed for armchair and metallic zigzag carbon nanotubes. The formulae of the oscillator strength are derived for both types of nanotubes and are compared with the result obtained by a linear chain model. In addition, the doping dependence is investigated in the absence of Coulomb interaction. It is shown that the oscillator strength of each carbon nanotube shows qualitatively the same doping dependence, but the fine structure is different due to it's own peculiar band structure. Some relations independent of the radius of the tube are derived, and a useful formula for determining the amount of doping is proposed.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jpn. at June 30, 200

Enhancement of the Electron Spin Resonance of Single-Walled Carbon Nanotubes by Oxygen Removal

We have observed a nearly fourfold increase in the electron spin resonance (ESR) signal from an ensemble of single-walled carbon nanotubes (SWCNTs) due to oxygen desorption. By performing temperature-dependent ESR spectroscopy both before and after thermal annealing, we found that the ESR in SWCNTs can be reversibly altered via the molecular oxygen content in the samples. Independent of the presence of adsorbed oxygen, a Curie-law (spin susceptibility $\propto 1/T$) is seen from $\sim$4 K to 300 K, indicating that the probed spins are finite-level species. For both the pre-annealed and post-annealed sample conditions, the ESR linewidth decreased as the temperature was increased, a phenomenon we identify as motional narrowing. From the temperature dependence of the linewidth, we extracted an estimate of the intertube hopping frequency; for both sample conditions, we found this hopping frequency to be $\sim$100 GHz. Since the spin hopping frequency changes only slightly when oxygen is desorbed, we conclude that only the spin susceptibility, not spin transport, is affected by the presence of physisorbed molecular oxygen in SWCNT ensembles. Surprisingly, no linewidth change is observed when the amount of oxygen in the SWCNT sample is altered, contrary to other carbonaceous systems and certain 1D conducting polymers. We hypothesize that physisorbed molecular oxygen acts as an acceptor ($p$-type), compensating the donor-like ($n$-type) defects that are responsible for the ESR signal in bulk SWCNTs.Comment: 14 pages, 7 figure

Non-structural carbohydrates in woody plants compared among laboratories

Non-structural carbohydrates (NSC) in plant tissue are frequently quantified to make inferences about plant responses to environmental conditions. Laboratories publishing estimates of NSC of woody plants use many different methods to evaluate NSC. We asked whether NSC estimates in the recent literature could be quantitatively compared among studies. We also asked whether any differences among laboratories were related to the extraction and quantification methods used to determine starch and sugar concentrations. These questions were addressed by sending sub-samples collected from five woody plant tissues, which varied in NSC content and chemical composition, to 29 laboratories. Each laboratory analyzed the samples with their laboratory-specific protocols, based on recent publications, to determine concentrations of soluble sugars, starch and their sum, total NSC. Laboratory estimates differed substantially for all samples. For example, estimates for Eucalyptus globulus leaves (EGL) varied from 23 to 116 (mean = 56) mg g⁻¹ for soluble sugars, 6–533 (mean = 94) mg g⁻¹ for starch and 53–649 (mean = 153) mg g⁻¹ for total NSC. Mixed model analysis of variance showed that much of the variability among laboratories was unrelated to the categories we used for extraction and quantification methods (method category R² = 0.05–0.12 for soluble sugars, 0.10–0.33 for starch and 0.01–0.09 for total NSC). For EGL, the difference between the highest and lowest least squares means for categories in the mixed model analysis was 33 mg g⁻¹ for total NSC, compared with the range of laboratory estimates of 596 mg g⁻¹. Laboratories were reasonably consistent in their ranks of estimates among tissues for starch (r = 0.41–0.91), but less so for total NSC (r = 0.45–0.84) and soluble sugars (r = 0.11–0.83). Our results show that NSC estimates for woody plant tissues cannot be compared among laboratories. The relative changes in NSC between treatments measured within a laboratory may be comparable within and between laboratories, especially for starch. To obtain comparable NSC estimates, we suggest that users can either adopt the reference method given in this publication, or report estimates for a portion of samples using the reference method, and report estimates for a standard reference material. Researchers interested in NSC estimates should work to identify and adopt standard methods.This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Oxford University Press. The published article can be found at: http://treephys.oxfordjournals.org/Keywords: soluble sugars, starch, particle size, reference method, standardization, non-structural carbohydrate chemical analysis, extraction and quantification consistenc

Quantification of heavy metal resistant bacteria adjacent to a wastewater treatment plant

The objective of this study was to characterize metal resistant bacteria around a wastewater treatment plant. Sediment samples were collected from the Town Branch wastewater treatment plant near Lexington Kentucky, both where the effluent discharges and approximately 10m upstream. Plate counts for heterotrophic bacteria resistant to zinc, cobalt and cadmium were performed. Preliminary results suggest that there are more bacteria resistant to zinc than either cadmium or cobalt at both sampling sites. However, no differences were found between sites. While requiring confirmation, the latter suggests that wastewater treatment plants may not be a source of heavy metal resistant bacteria