8,357 research outputs found
Strong mobility degradation in ideal graphene nanoribbons due to phonon scattering
We investigate the low-field phonon-limited mobility in armchair graphene
nanoribbons (GNRs) using full-band electron and phonon dispersion relations. We
show that lateral confinement suppresses the intrinsic mobility of GNRs to
values typical of common bulk semiconductors, and very far from the impressive
experiments on 2D graphene. Suspended GNRs with a width of 1 nm exhibit a
mobility close to 500 cm^2/Vs at room temperature, whereas if the same GNRs are
deposited on HfO2 mobility is further reduced to about 60 cm^2/Vs due to
surface phonons. We also show the occurrence of polaron formation, leading to
band gap renormalization of ~118 meV for 1 nm-wide armchair GNRs.Comment: 11 pages, 4 figure
Seroepidemiology of <i>Trichomonas vaginalis</i> in rural women in Zimbabwe and patterns of association with HIV infection
Serological assays using dried blood spots from 5221 women in rural areas of eastern Zimbabwe
were used to assess the epidemiology of Trichomonas vaginalis infection, and its association with
HIV. Antibodies to T. vaginalis and to HIV were detected by enzyme immunoassays. Behavioural
and demographic data were collected by confidential questionnaires. In total, 516 (9.9%) women
were seropositive for T. vaginalis and seroprevalence increased with age among younger women.
Divorced, widowed and single women were more likely to be seropositive. After controlling for
age, seropositivity was significantly associated with being sexually active, having multiple sex
partners, having a partner who had multiple sex partners, and having a new sex partner in the
past year. Seropositivity was associated with a recent history of genital discharge. Overall, 208
(40.3%) T. vaginalis-positive samples were also positive for HIV, compared with 1106 (23.5%)
T. vaginalis-negative samples (age and sex adjusted OR 2.11, 95% CI 1.74–2.55, P<0.001). There
was increased risk for being HIV-positive amongst T. vaginalis-seropositive women regardless of
residence, employment or education. In a logistic regression controlling for common risk factors,
the association remained significant. T. vaginalis-seropositive young women with a history of
genital discharge were much more likely to be HIV-positive than women who were T. vaginalis-seronegative
and had no history of discharge (OR 6.08, 95% CI 2.95–12.53). Although a causal
relationship cannot be assumed, detection and treatment of trichomoniasis may be important in
strategies to reduce HIV transmission through sexually transmitted infection control
Effective one-body dynamics in multiple-quantum NMR experiments
A suitable NMR experiment in a one-dimensional dipolar coupled spin system
allows one to reduce the natural many-body dynamics into effective one-body
dynamics. We verify this in a polycrystalline sample of hydroxyapatite (HAp) by
monitoring the excitation of NMR many-body superposition states: the
multiple-quantum coherences. The observed effective one-dimensionality of HAp
relies on the quasi 1d structure of the dipolar coupled network that, as we
show here, is dynamically enhanced by the quantum Zeno effect. Decoherence is
also probed through a Loschmidt echo experiment, where the time reversal is
implemented on the double-quantum Hamiltonian, I_{i,+}I_{j,+} + I_{i,-}I_{j,-}.
We contrast the decoherence of adamantane, a standard 3d system, with that of
HAp. While the first shows an abrupt Fermi-type decay, HAp presents a smooth
exponential law.Comment: 8 pages, 6 figure
Evaluation of roasting effect on selected green tea volatile flavor compound and pyrazine content by hs-spme gc-ms
The present study aims at the development and validation of a quali-quantitative headspace solid-phase microextraction gas chromatography–mass spectrometry (HS-SPME GC-MS) method for the analysis of odorants in different Chinese and Japanese green teas. A 65 µm PDMS/DVB fiber was used, and GC-MS was performed on a ZB-5 (30 m × 0.25 mm ID, 0.25 µm film) column. The main odorants in green tea samples were identified and the method was fully validated. Linearity (r2 0.981–0.999), sensitivity (LOQ 0.005–0.04 µg/mL), reproducibility (CV% 3.12–10.37), accuracy (recovery% 79.08–99.17) and matrix effect (ME% −9.5 to +4.5) were determined. Quantitation of 2,4-heptadienal, β-damascenone, β-ionone, linalool, indole, 2-ethyl-6-methyl-pyrazine, 2-ethyl-5-methyl-pyrazine, 2-ethyl-3,6-dimethylpirazine, 2-ethyl-3,5-dimethyl-pyrazine and 2-acetyl-pyrazine was carried out in the presence of isotopically labeled compounds as the internal standards. The proposed method was applied to the comparison of the profile of the volatile flavor compounds (VFCs) of green tea subjected to roasting treatment at three different temperatures (160, 180 and 200◦C for 30 min). In particular, the roasting process was monitored by following the quantitative variations of the selected odorant content, considered as the most important contributory components to the Hojicha or black tea (roasted tea) flavor. A temperature of 160◦C was found to be the best roasting temperature
Phenomenological study of the atypical heavy flavor production observed at the Fermilab Tevatron
We address known discrepancies between the heavy flavor properties of jets
produced at the Tevatron collider and the prediction of conventional-QCD
simulations. In this study, we entertain the possibility that these effects are
real and due to new physics. We show that all anomalies can be simultaneously
fitted by postulating the additional pair production of light bottom squarks
with a 100% semileptonic branching fraction.Comment: 30 pages, 13 figures, 3 tables. Submitted to Phys. Rev.
Field-amplified sample injection and sweeping micellar electrokinetic chromatography in analysis of glyphosate and aminomethylphosphonic acid in wheat
Glyphosate, a widely used herbicide, has been classified as probably carcinogenic to humans by the International Agency for Research on Cancer (IARC). In the present study a method based on Field-Amplified Sample Injection and Sweeping Micellar Electrokinetic Chromatography (FASI sweep-MEKC) has been developed and validated for determination of glyphosate and its microbial metabolite aminomethylphosphonic acid (AMPA) in wheat flour. The method involved a preliminary solid phase extraction for cleanup of the aqueous extracts from wheat flour, based sequentially on C18 and strong anion exchange cartridges, followed by derivatization using 9-fluorenylmethylchloroformate. Optimization of sample cleanup and derivatization procedure was carried out by a HPLC-UV method, whereas FASI sweep-MEKC was applied for achieving the sensitivity necessary for analysis of real samples. To this regard, optimum conditions involved the use of an extended path fused-silica capillary (80 cm total length, 50 μm, i.d.) filled with a high concentration buffer (sodium phosphate 100 mM, pH 2.2). Electrokinetic sampling was carried out at −10 kV with injection time of 700 s and the separation of the loaded analytes was performed under MEKC conditions using sodium phosphate buffer 50 mM at pH 2.2, supplemented with sodium dodecyl sulfate, 100 mM. The method was validated for linearity, precision, accuracy and sensitivity, showing that using conventional UV detection (210 nm) the achieved limit of quantitation (LOQ) values for both the analytes were widely lower than those set by Authorities. In particular, LOQ for glyphosate and AMPA were found to be 5 and 2.5 ng/mL, respectively, corresponding to 0.1 and 0.05 mg/kg, in wheat flour. The method, applied to commercially available real samples (wheat flour from different manufacturers) and to an experimental sample obtained by cv. Svevo wheat, can be considered as a convenient alternative to the existing approaches in analysis of complex matrices
Variations in the occurrence of SuperDARN F region echoes
The occurrence of F region ionospheric echoes observed by a number of
SuperDARN HF radars is analyzed statistically in order to infer solar cycle,
seasonal, and diurnal trends. The major focus is on Saskatoon radar data for
1994–2012. The distribution of the echo occurrence rate is presented in terms of
month of observation and magnetic local time. Clear repetitive patterns are
identified during periods of solar maximum and solar minimum. For years near
solar maximum, echoes are most frequent near midnight during winter. For
years near solar minimum, echoes occur more frequently near noon during
winter, near dusk and dawn during equinoxes and near midnight during summer.
Similar features are identified for the Hankasalmi and Prince George radars
in the northern hemisphere and the Bruny Island TIGER radar in the southern
hemisphere. Echo occurrence for the entire SuperDARN network demonstrates
patterns similar to patterns in the echo occurrence for the Saskatoon radar
and for other radars considered individually. In terms of the solar cycle,
the occurrence rate of nightside echoes is shown to increase by a factor of
at least 3 toward solar maximum while occurrence of the near-noon echoes
does not significantly change with the exception of a clear depression during
the declining phase of the solar cycle
Spatially resolved spectral determination of polysaccharides in hydrothermally carbonized biomass
Reliable information on the decomposition of polysaccharides is important to evaluate the evolution and properties of hydrothermally carbonized chars. Hyperspectral imaging offers a quick and robust alternative to expensive and time-consuming laboratory methods to determine the polysaccharide contents of biomass and biomass-derived chars. Here, we show that the decomposition of hemicellulose and cellulose were visible in the acquired hyperspectral images even without image calibration. Image regression based on sample holocellulose, glucan and the sum of xylan, galactan, arabinan and mannan provided good calibration models and enabled visualizing the decomposition of polysaccharides based on carbonization temperature. Hyperspectral imaging thus provides a non-destructive alternative to traditional polysaccharide analyses of hydrochars for laboratory and potential future industrial applications.Peer reviewe
Reactivity of cellulose during hydrothermal carbonization of lignocellulosic biomass.
Hydrothermal carbonization (HTC) of pure cellulose (CE) and birchwood (BW) samples was carried out at temperatures between 160 and 280 °C, 0.5 h residence time and biomass-to-water ratio 1:5, to investigate the reactivity of cellulose in lignocellulosic biomass. Thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) showed that the CE samples remained unaltered at temperatures up to 220 °C, but were significantly decomposed at 230 °C producing a thermal recalcitrant aromatic and high energy-dense material. FTIR showed that dehydration and aromatization reactions occurred at temperature equal or higher than 230 °C for the CE samples while a similar increase in aromatization for the BW hydrochars was evident only at temperatures equal or higher than 260 °C. Acid hydrolysis, TGA and FTIR suggested that a higher thermal resistance of naturally occurring cellulose in BW (when compared to CE sample) could be related to a ‘protecting shield’ offered by interlinked lignin in the plant matrix
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