514 research outputs found
Atomistically-informed continuum modeling and isogeometric analysis of 2D materials over holey substrates
This work develops, discretizes, and validates a continuum model of a
molybdenum disulfide (MoS) monolayer interacting with a periodic holey
silicon nitride substrate via van der Waals (vdW) forces. The MoS layer is
modeled as a geometrically nonlinear Kirchhoff-Love shell, and vdW forces are
modeled by a Lennard-Jones potential, simplified using approximations for a
smooth substrate topography. The material parameters of the shell model are
calibrated by comparing small-strain tensile and bending tests with atomistic
simulations. This model is efficiently discretized using isogeometric analysis
(IGA) for the shell structure and a pseudo-time continuation method for energy
minimization. The IGA shell model is validated against fully-atomistic
calculations for several benchmark problems with different substrate
geometries. The continuum simulations reproduce deflections, strains and
curvatures predicted by atomistic simulations, which are known to strongly
affect the electronic properties of MoS, with deviations well below the
modeling errors suggested by differences between the widely-used reactive
empirical bond order and Stillinger-Weber interatomic potentials. Agreement
with atomistic results depends on geometric nonlinearity in some cases, but a
simple isotropic St. Venant-Kirchhoff model is found to be sufficient to
represent material behavior. We find that the IGA discretization of the
continuum model has a much lower computational cost than atomistic simulations,
and expect that it will enable efficient design space exploration in strain
engineering applications. This is demonstrated by studying the dependence of
strain and curvature in MoS over a holey substrate as a function of the
hole spacing on scales inaccessible to atomistic calculations. The results show
an unexpected qualitative change in the deformation pattern below a critical
hole separation
The responsibility to protect and the use of force: remaking the procrustean bed?
The emergence of the Responsibility to Protect (R2P) owed much to the need to enhance the UN’s ability to act forcibly in the face of the most extreme cases of gross human suffering. Too often in the past such responses were emasculated or thwarted by the necessity to successfully navigate the UN Charter’s prescriptions over the use of force, by the unwillingness of member states to provide military forces, or by a combination of the two. In accepting that certain types of inhuman activity can lead to the legitimate use of force within the UN Charter framework, the adoption of R2P appeared to resolve at least some of these problems, and as such it offered hope to those wishing to see the UN adopt a more assertive response to the grossest of human rights abuses. But, using stalemate over Syria as its backdrop, this article demonstrates the dubiousness of the claim that such a normative development can ever trump the hard edged political and strategic factors which determine when states will accept and/or participate in the use of force, and it suggests a radical solution to the dangers inherent in R2P’s intimate association with military intervention
Atomistically-informed continuum modeling and isogeometric analysis of 2D materials over holey substrates
This work develops, discretizes, and validates a continuum model of a molybdenum disulfide (MoS2) monolayer interacting with a periodic holey silicon nitride (Si3N4) substrate via van der Waals (vdW) forces. The MoS2 layer is modeled as a geometrically nonlinear Kirchhoff–Love shell, and vdW forces are modeled by a Lennard-Jones (LJ) potential, simplified using approximations for a smooth substrate topography. Both the shell model and LJ interactions include novel extensions informed by close comparison with fully-atomistic calculations. The material parameters of the shell model are calibrated by comparing small-strain tensile and bending tests with atomistic simulations. This model is efficiently discretized using isogeometric analysis (IGA) for the shell structure and a pseudo-time continuation method for energy minimization. The IGA shell model is validated against fully-atomistic calculations for several benchmark problems with different substrate geometries. Agreement with atomistic results depends on geometric nonlinearity in some cases, but a simple isotropic St.Venant–Kirchhoff model is found to be sufficient to represent material behavior. We find that the IGA discretization of the continuum model has a much lower computational cost than atomistic simulations, and expect that it will enable efficient design space exploration in strain engineering applications. This is demonstrated by studying the dependence of strain and curvature in MoS2 over a holey substrate as a function of the hole spacing on scales inaccessible to atomistic calculations. The results show an unexpected qualitative change in the deformation pattern below a critical hole separation
Innovation and Access to Medicines for Neglected Populations: Could a Treaty Address a Broken Pharmaceutical R&D System?
As part of a cluster of articles leading up to the 2012 World Health Report and critically reflecting on the theme of “no health without research,” Suerie Moon and colleagues argue for a global health R&D treaty to improve innovation in new medicines and strengthening affordability, sustainable financing, efficiency in innovation, and equitable health-centered governance
Big Food, Food Systems, and Global Health
In an article that forms part of the PLoS Medicine series on Big Food, guest editors David Stuckler and Marion Nestle lay out why more examination of the food industry is necessary, and offer three competing views on how public health professionals might engage with Big Food
Risk factors for progression from severe maternal morbidity to death: a national cohort study.
Women continue to die unnecessarily during or after pregnancy in the developed world. The aim of this analysis was to compare women with severe maternal morbidities who survived with those who died, to quantify the risk associated with identified factors to inform policy and practice to improve survival
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
The active metabolite of leflunomide, A77 1726, attenuates inflammatory arthritis in mice with spontaneous arthritis via induction of heme oxygenase-1
Upregulation of the Wnt Co-Receptor LRP6 Promotes Hepatocarcinogenesis and Enhances Cell Invasion
Background: Activation of the Wnt/b-catenin signaling pathway plays a crucial role in hepatocellular carcinoma (HCC). Lowdensity lipoprotein (LDL) receptor-related protein-6 (LRP6) is one of the co-receptors of the Wnt/b-catenin pathway and forms a signaling complex with Wnt ligand and Frizzled receptor to activate downstream signaling. However, the role of LRP6 in hepatocarcinogenesis is unclear. In this study, we examined its expression and roles in human HCC. Methodology/Principal Findings: Using real-time quantitative RT-PCR, we found that LRP6 was frequently (45%) overexpressed in human HCCs (P = 0.003). In vitro studies showed that ectopic expression of LRP6 increased the protein level of b-catenin. Moreover, overexpression of the full-length and constitutively active LRP6, respectively, activated the WNT/b-catenin signaling pathway, as shown by the TCF/b-catenin reporter assay. With regard to the effects of LRP6 overexpression in HCC cells, stable overexpression of the constitutively active LRP6 in BEL-7402 HCC cells enhanced cell proliferation, cell migration, and invasion in vitro as well as tumorigenicity in nude mice. Conclusions/Significance: Our findings indicate that overexpression of LRP6 contributes to the hyperactivation of the Wnt
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