3,948 research outputs found
Thermal expansion in carbon nanotubes and graphene: nonequilibrium Green's function approach
The nonequilibrium Green's function method is applied to investigate the
coefficient of thermal expansion (CTE) in single-walled carbon nanotubes
(SWCNT) and graphene. It is found that atoms deviate about 1% from equilibrium
positions at T=0 K, resulting from the interplay between quantum zero-point
motion and nonlinear interaction. The CTE in SWCNT of different sizes is
studied and analyzed in terms of the competition between various vibration
modes. As a result of this competition, the axial CTE is positive in the whole
temperature range, while the radial CTE is negative at low temperatures. In
graphene, the CTE is very sensitive to the substrate. Without substrate, CTE
has large negative region at low temperature and very small value at high
temperature limit, and the value of CTE at T=300 K is
K which is very close to recent experimental result,
K (Nat. Nanotechnol. \textbf{10}, 1038 (2009)). A very weak substrate
interaction (about 0.06% of the in-plane interaction) can largely reduce the
negative CTE region and greatly enhance the value of CTE. If the substrate
interaction is strong enough, the CTE will be positive in whole temperature
range and the saturate value at high temperature reaches
K.Comment: final version, to appear in PR
Anisotropic thermal expansion of Fe1.06Te and FeTe0.5Se0.5 single crystals
Heat capacity and anisotropic thermal expansion was measured for Fe1.06Te and
FeTe0.5Se0.5 single crystals. Previously reported phase transitions are clearly
seen in both measurements. In both cases the thermal expansion is anisotropic.
The uniaxial pressure derivatives of the superconducting transition temperature
in FeTe0.5Se0.5 inferred from the Ehrenfest relation have opposite signs for
in-plane and c-axis pressures. Whereas the Gruneisen parameters for both
materials are similar and only weakly temperature-dependent above ~ 80 K, at
low temperatures (in the magnetically ordered phase) the magnetic contribution
to the Gruneisen parameter in Fe1.06Te is significantly larger than electron
and phonon contributions combined
Ab initio parametrised model of strain-dependent solubility of H in alpha-iron
The calculated effects of interstitial hydrogen on the elastic properties of
alpha-iron from our earlier work are used to describe the H interactions with
homogeneous strain fields using ab initio methods. In particular we calculate
the H solublility in Fe subject to hydrostatic, uniaxial, and shear strain. For
comparison, these interactions are parametrised successfully using a simple
model with parameters entirely derived from ab initio methods. The results are
used to predict the solubility of H in spatially-varying elastic strain fields,
representative of realistic dislocations outside their core. We find a strong
directional dependence of the H-dislocation interaction, leading to strong
attraction of H by the axial strain components of edge dislocations and by
screw dislocations oriented along the critical slip direction. We
further find a H concentration enhancement around dislocation cores, consistent
with experimental observations.Comment: part 2/2 from splitting of 1009.3784 (first part was 1102.0187),
minor changes from previous version
On the accuracy of the melting curves drawn from modelling a solid as an elastic medium
An ongoing problem in the study of a classical many-body system is the
characterization of its equilibrium behaviour by theory or numerical
simulation. For purely repulsive particles, locating the melting line in the
pressure-temperature plane can be especially hard if the interparticle
potential has a softened core or contains some adjustable parameters. A method
is hereby presented that yields reliable melting-curve topologies with
negligible computational effort. It is obtained by combining the Lindemann
melting criterion with a description of the solid phase as an elastic
continuum. A number of examples are given in order to illustrate the scope of
the method and possible shortcomings. For a two-body repulsion of Gaussian
shape, the outcome of the present approach compares favourably with the more
accurate but also more computationally demanding self-consistent harmonic
approximation.Comment: 25 pages, 7 figure
Structural, electronic, magnetic, and thermal properties of single-crystalline UNi0.5Sb2
We studied the properties of the antiferromagnetic (AFM) UNi0.5Sb2 (TN
\approx 161 K) compound in Sb-flux grown single crystals by means of
measurements of neutron diffraction, magnetic susceptibility ({\chi}), specific
heat (Cp), thermopower (S), thermal conductivity ({\kappa}), linear thermal
expansion ({\Delta}L/L), and electrical resistivity ({\rho}) under hydrostatic
pressures (P) up to 22 kbar. The neutron diffraction measurements revealed that
the compound crystallizes in the tetragonal P42/nmc structure, and the value of
the U-moments yielded by the histograms at 25 K is \approx 1.85 \pm 0.12
{\mu}B/U-ion. In addition to the features in the bulk properties observed at
TN, two other hysteretic features centered near 40 and 85 K were observed in
the measurements of {\chi}, S, {\rho}, and {\Delta}L/L. Hydrostatic pressure
was found to raise TN at the rate of \approx 0.76 K/kbar, while suppressing the
two low temperature features. These features are discussed in the context of
Fermi surface and hybridization effects.Comment: 17 pages, 8 figure
Relaxation of classical many-body hamiltonians in one dimension
The relaxation of Fourier modes of hamiltonian chains close to equilibrium is
studied in the framework of a simple mode-coupling theory. Explicit estimates
of the dependence of relevant time scales on the energy density (or
temperature) and on the wavenumber of the initial excitation are given. They
are in agreement with previous numerical findings on the approach to
equilibrium and turn out to be also useful in the qualitative interpretation of
them. The theory is compared with molecular dynamics results in the case of the
quartic Fermi-Pasta-Ulam potential.Comment: 9 pag. 6 figs. To appear in Phys.Rev.
Scaling-up water management interventions for rainfed agriculture in the Ethiopian Highlands: status, issues, and opportunities
Ethiopia is the second most populous country in Africa with more than 110 million people. The capacity to feed its rapidly growing population largely depends on rainfed agricultural production systems, in a range of agro climatic regions from arid and semiarid lowlands to temperate
highlands. Agriculture is undermined by both severe land degradation and high inter- and intra-seasonal rainfall variability. As a result, the current average productivity of rainfed farming remains low (1.7 t ha-1 for pulses and 2.7 t ha-1 for cereals). This is despite a slow yield increase (e.g. about 1.5 t ha-1 for cereals and 1 t ha-1 for pulses) due to the introduction of new crop cultivars,
fertilizers and management practices. Recognising the large yield gap in rainfed systems, the Ethiopian government has, since 1970, initiated a number of public welfare programs. These have involved various natural resource management
programs with a special focus on agricultural water management (AWM) in Sustainable Land Management Projects (SLMP). SLMPs, centered around rainfed production systems, have been implemented to address land degradation, enhance crop and livestock productivity, and improve household incomes. Integrated resource management approaches have helped local communities obtain tangible benefits from AWM,
and strengthened a number of ecosystem services, when compared to a sectoral approach. In the last 15 years, through SLMP 1 and 2, more than 2% of agricultural fields, and communal rainfed land, in Ethiopia, has been subject to AWM and sustainable land management. This has benefitted
around 1.4 million households and supported environmental sustainability. Over 430,000 people have also benefited from related income generating activities. However, systematic data on various aspects of AWM is required to obtain a clear understanding of the overall impact of these interventions. This study proposes following a landscape approach, in order to realize the full potential of diverse AWM interventions, and a consortium approach to capacity building to achieve large scale, system level outcomes
Whole genome sequencing of one complex pedigree illustrates challenges with genomic medicine
BACKGROUND: Human Phenotype Ontology (HPO) has risen as a useful tool for precision medicine by providing a standardized vocabulary of phenotypic abnormalities to describe presentations of human pathologies; however, there have been relatively few reports combining whole genome sequencing (WGS) and HPO, especially in the context of structural variants. METHODS: We illustrate an integrative analysis of WGS and HPO using an extended pedigree, which involves Prader-Willi Syndrome (PWS), hereditary hemochromatosis (HH), and dysautonomia-like symptoms. A comprehensive WGS pipeline was used to ensure reliable detection of genomic variants. Beyond variant filtering, we pursued phenotypic prioritization of candidate genes using Phenolyzer. RESULTS: Regarding PWS, WGS confirmed a 5.5 Mb de novo deletion of the parental allele at 15q11.2 to 15q13.1. Phenolyzer successfully returned the diagnosis of PWS, and pinpointed clinically relevant genes in the deletion. Further, Phenolyzer revealed how each of the genes is linked with the phenotypes represented by HPO terms. For HH, WGS identified a known disease variant (p.C282Y) in HFE of an affected female. Analysis of HPO terms alone fails to provide a correct diagnosis, but Phenolyzer successfully revealed the phenotype-genotype relationship using a disease-centric approach. Finally, Phenolyzer also revealed the complexity behind dysautonomia-like symptoms, and seven variants that might be associated with the phenotypes were identified by manual filtering based on a dominant inheritance model. CONCLUSIONS: The integration of WGS and HPO can inform comprehensive molecular diagnosis for patients, eliminate false positives and reveal novel insights into undiagnosed diseases. Due to extreme heterogeneity and insufficient knowledge of human diseases, it is also important that phenotypic and genomic data are standardized and shared simultaneously
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