452 research outputs found
Scaling of the conductance in gold nanotubes
A new form of gold nanobridges has been recently observed in ultrahigh-vacuum
experiments, where the gold atoms rearrange to build helical nanotubes, akin in
some respects to carbon nanotubes. The good reproducibility of these wires and
their unexpected stability will allow for conductance measurements and make
them promising candidates for future applications . We present here a study of
the transport properties of these nanotubes in order to understand the role of
chirality and of the different orbitals in quantum transport observables. The
conductance per atomic row shows a light decreasing trend as the diameter
grows, which is also shown through an analytical formula based on a one-orbital
model.Comment: 5 pages, 6 figure
Hofstadter butterflies of carbon nanotubes: Pseudofractality of the magnetoelectronic spectrum
The electronic spectrum of a two-dimensional square lattice in a
perpendicular magnetic field has become known as the Hofstadter butterfly
[Hofstadter, Phys. Rev. B 14, 2239 (1976).]. We have calculated
quasi-one-dimensional analogs of the Hofstadter butterfly for carbon nanotubes
(CNTs). For the case of single-wall CNTs, it is straightforward to implement
magnetic fields parallel to the tube axis by means of zone folding in the
graphene reciprocal lattice. We have also studied perpendicular magnetic fields
which, in contrast to the parallel case, lead to a much richer, pseudofractal
spectrum. Moreover, we have investigated magnetic fields piercing double-wall
CNTs and found strong signatures of interwall interaction in the resulting
Hofstadter butterfly spectrum, which can be understood with the help of a
minimal model. Ubiquitous to all perpendicular magnetic field spectra is the
presence of cusp catastrophes at specific values of energy and magnetic field.
Resolving the density of states along the tube circumference allows recognition
of the snake states already predicted for nonuniform magnetic fields in the
two-dimensional electron gas. An analytic model of the magnetic spectrum of
electrons on a cylindrical surface is used to explain some of the results.Comment: 14 pages, 12 figures update to published versio
Vibrational modes and low-temperature thermal properties of graphene and carbon nanotubes: A minimal force-constant model
We present a phenomenological force-constant model developed for the
description of lattice dynamics of sp2 hybridized carbon networks. Within this
model approach, we introduce a new set of parameters to calculate the phonon
dispersion of graphene by fitting the ab initio dispersion. Vibrational modes
of carbon nanotubes are obtained by folding the 2D dispersion of graphene and
applying special corrections for the low-frequency modes. Particular attention
is paid to the exact dispersion law of the acoustic modes, which determine the
low-frequency thermal properties and reveal quantum size effects in carbon
nanotubes. On the basis of the resulting phonon spectra, we calculate the
specific heat and the thermal conductance for several achiral nanotubes of
different diameter. Through the temperature dependence of the specific heat we
demonstrate that phonon spectra of carbon nanotubes show one-dimensional
behavior and that the phonon subbands are quantized at low temperatures.
Consequently, we prove the quantization of the phonon thermal conductance by
means of an analysis based on the Landauer theory of heat transport.Comment: 14 pages, 12 figure
17-beta-Estradiol in relation to age at menarche and adult obesity in premenopausal women
BACKGROUND: We hypothesize that premenopausal endogenous estradiol may be associated with age at menarche and adult overweight and obesity, potentially contributing to breast cancer risk. METHODS: We assessed age at menarche by questionnaire among 204 healthy Norwegian women, aged 25 â 35 years. Measures of body composition included body mass index (BMI, kg/m2), waist circumference (WC, cm), waist-to-hip ratio (WHR) and fat percentage dual energy X-ray absorptiometry, (DEXA). Daily salivary 17-b-estradiol (E2) concentrations were collected throughout one entire menstrual cycle and assessed by radioimmunoassay (RIA). Linear regression analyses and linear mixed models for repeated measures were used and potential confounding factors and effect modiïŹers were tested.
RESULTS: Among women with an early age at menarche (12 years), the overall mean salivary E2 concentration increased by 3.7 pmol/l (95% conïŹdence interval, 1.8 â 5.7 pmol/l) with each 9.8 cm (1 SD) increase in WC, which represents a 20.7% change in the mean for the total group. Among the same early maturers, a 1 SD (0.06) change in WHR was directly associated with a 24.0% change in mean E2 concentration for the total group.
CONCLUSIONS: Our ïŹndings support the hypothesis that early age at menarche, together with adult overweight and obesity, result in high levels of 17-b-estradiol throughout the menstrual cycle.AnthropologyHuman Evolutionary Biolog
Acceptability of a theory-based sedentary behaviour reduction intervention for older adults ('On Your Feet to Earn Your Seat')
Background: Adults aged 60 years and over spend most time sedentary and are the least physically active of all
age groups. This early-phase study explored acceptability of a theory-based intervention to reduce sitting time and
increase activity in older adults, as part of the intervention development process.
Methods: An 8-week uncontrolled trial was run among two independent samples of UK adults aged 60â75 years.
Sample 1, recruited from sheltered housing on the assumption that they were sedentary and insufficiently active,
participated between December 2013 and March 2014. Sample 2, recruited through community and faith centres
and a newsletter, on the basis of self-reported inactivity (<150 weekly minutes of moderate-to-vigorous activity)
and sedentary behaviour (â„6 h mean daily sitting), participated between March and August 2014. Participants
received a booklet offering 16 tips for displacing sitting with light-intensity activity and forming activity habits,
and self-monitoring âtick-sheetsâ. At baseline, 4-week, and 8-week follow-ups, quantitative measures were taken
of physical activity, sedentary behaviour, and habit. At 8 weeks, tick-sheets were collected and a semi-structured
interview conducted. Acceptability was assessed for each sample separately, through attrition and adherence to
tips, ANOVAs for behaviour and habit changes, and, for both samples combined, thematic analysis of interviews.
Results: In Sample 1, 12 of 16 intervention recipients completed the study (25 % attrition), mean adherence was
40 % (per-tip range: 15â61 %), and there were no clear patterns of changes in sedentary or physical activity behaviour
or habit. In Sample 2, 23 of 27 intervention recipients completed (15 % attrition), and mean adherence was 58 %
(per-tip range: 39â82 %). Sample 2 decreased mean sitting time and sitting habit, and increased walking, moderate
activity, and activity habit. Qualitative data indicated that both samples viewed the intervention positively, found the
tips easy to follow, and reported health and wellbeing gains.
Conclusions: Low attrition, moderate adherence, and favourability in both samples, and positive changes in Sample 2,
indicate the intervention was acceptable. Higher attrition, lower adherence, and no apparent behavioural impact
among Sample 1 could perhaps be attributable to seasonal influences. The intervention has been refined to address
emergent acceptability problems. An exploratory controlled trial is underway
The use and interpretation of anthropometric measures in cancer epidemiology: A perspective from the world cancer research fund international continuous update project
Anthropometric measures relating to body size, weight and composition are increasingly being associated with cancer risk and progression. Whilst practical in epidemiologic research, where population-level associations with disease are revealed, it is important to be aware that such measures are imperfect markers of the internal physiological processes that are the actual correlates of cancer development. Body mass index (BMI), the most commonly used marker for adiposity, may mask differences between lean and adipose tissue, or fat distribution, which varies across individuals, ethnicities, and stage in the lifespan. Other measures, such as weight gain in adulthood, waist circumference and waist-to-hip ratio, contribute information on adipose tissue distribution and insulin sensitivity. Single anthropometric measures do not capture maturational events, including the presence of critical windows of susceptibility (i.e., age of menarche and menopause), which presents a challenge in epidemiologic work. Integration of experimental research on underlying dynamic genetic, hormonal, and other non-nutritional mechanisms is necessary for a confident conclusion of the overall evidence in cancer development and progression. This article discusses the challenges confronted in evaluating and interpreting the current evidence linking anthropometric factors and cancer risk as a basis for issuing recommendations for cancer prevention
Early Gadolinium Enhancement for Area at Risk Determination: A Preclinical Validation Study
ObjectivesâThe aim of this study was to determine if early gadolinium enhancement (EGE) by cardiovascular magnetic resonance (CMR) imaging in a canine model of reperfused myocardial infarction depicts the area at risk (AAR) as determined by microsphere blood flow analysis.
BackgroundâIt remains controversial whether only the irreversibly injured myocardium enhances when performing CMR imaging in the setting of acute myocardial infarction. Recently, EGE has been proposed as a measure of the AAR in acute myocardial infarction as it correlates well with T2-weighted imaging of the AAR, but still requires pathological validation.
MethodsâEleven dogs underwent 2 hours of coronary artery occlusion and 48 hours of reperfusion prior to imaging at 1.5T. EGE imaging was performed 3 minutes after contrast administration with coverage of the entire left ventricle. Late gadolinium enhancement (LGE) imaging was performed between 10 and 15 minutes after contrast injection. AAR was defined as myocardium with blood flow (mL/min/g) \u3c 2SD from remote myocardium determined by microspheres during occlusion. The size of infarction was determined using triphenyltetrazolium chloride (TTC).
ResultsâThere was no significant difference in the size of enhancement by EGE compared to the size of AAR by microspheres (44.1± 15.8% vs. 42.7± 9.2%, p=0.61) with good correlation (r=0.88, p \u3c 0.001) and good agreement by Bland-Altman analysis (mean bias 1.4± 17.4%). There was no difference in the size of enhancement by EGE compared to enhancement on native T1 and T2 maps. The size of EGE was significantly greater than the infarct by TTC, (44.1± 15.8% vs. 20.7± 14.4%, p \u3c 0.001) and LGE (44.1± 15.8% vs. 23.5± 12.7%, p \u3c 0.001).
ConclusionâAt three minutes post-contrast, EGE correlated well with the AAR by microspheres and CMR, and was greater than infarct size. Thus, EGE enhances both reversibly and irreversibly injured myocardium
Why do academics engage locally? Insights from the University of Stavanger
The role of individual actors in knowledge-exchange collaborations has been accorded much importance. Through their involvement with industry, innovation is especially enacted in their regions. Motivations for academic engagement have been fairly researched but academicsâ motivations for local collaborations remain to be properly understood. The aim of this paper is therefore, to explore the motivations of academics for regional engagement. This exploration is done by drawing on empirical data collected through interviews with 16 academics in the Engineering Faculty of the University of Stavanger (UiS), Norway. It is evident that in addition to personal motivations to collaborate, academics are driven to engage locally by certain incentives that may be embedded in regionally ânon-constructedâ advantages. In general, the presence of regional advantages that are relevant for advancing the academicâs research provide the motivation to engage locally
Risk of testicular germ-cell tumours in relation to childhood physical activity
The US Servicemen's Testicular Tumor Environmental and Endocrine Determinants (STEED) caseâcontrol study of testicular germ-cell tumours (TGCTs) enrolled participants and their mothers in 2002â2005. Hours of sports or vigorous childhood physical activity per week were ascertained for three time periods; 1stâ5th grades, 6thâ8th grades and 9thâ12th grades. Son- and mother-reports were analysed separately and included 539 control sonâmother pairs and 499 case sonâmother pairs. Odds ratios and 95% confidence intervals were produced. The analysis of the sons' responses found no relationship between childhood physical activity and TGCT, while the mothers' analysis found an inverse association, which was solely due to nonseminoma. Future studies should seek to validate responses further using recorded information sources such as school records
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