Pairing Symmetry in Iron-Pnictide Superconductor KFe2_2As2_2

The pairing symmetry is one of the major issues in the study of iron-based superconductors. We adopt a low-energy effective kinetic model based on the first-principles band structure calculations combined with the $J_1$-$J_2$ model for KFe$_2$As$_2$, the phase diagram of pairing symmetries is constructed. Putting the values of $J_1$ and $J_2$ of the $J_1$-$J_2$ model obtained by the first-principles calculations into this phase diagram, we find that the pairing symmetry for KFe$_2$As$_2$ is a nodal $d_{xy}$-wave in the folded Brillouin zone with two iron atoms per unit cell. This is in good agreement with experiments observed a nodal order parameter.Comment: 5 pages, 4 figures (The pairing symmetry is dependent on choosing an effective tight-binding model. In the publication version, we adopt a ten-orbital model by using the maximally localized Wannier functions based on the first-principles band structure calculations, and give an s-wave pairing for KFe$_2$As$_2$

Asymptotic behaviour of a semilinear elliptic system with a large exponent

Consider the problem \begin{eqnarray*} -\Delta u &=& v^{\frac 2{N-2}},\quad v>0\quad {in}\quad \Omega, -\Delta v &=& u^{p},\:\:\:\quad u>0\quad {in}\quad \Omega, u&=&v\:\:=\:\:0 \quad {on}\quad \partial \Omega, \end{eqnarray*} where $\Omega$ is a bounded convex domain in $\R^N,$ $N>2,$ with smooth boundary $\partial \Omega.$ We study the asymptotic behaviour of the least energy solutions of this system as $p\to \infty.$ We show that the solution remain bounded for $p$ large and have one or two peaks away form the boundary. When one peak occurs we characterize its location.Comment: 16 pages, submmited for publicatio

Evolution of Iron Kα_{\alpha} Line Emission in the Black Hole Candidate GX 339-4

GX 339-4 was regularly monitored with RXTE during a period (in 1999) when its X-ray flux decreased significantly (from 4.2$\times 10^{-10}$ erg cm$^{-2} s^{-1}$ to 7.6$\times 10^{-12}$ erg cm$^{-2}$s$^{-1}$ in the 3--20 keV band), as the source settled into the ``off state''. Our spectral analysis revealed the presence of a prominent iron K$_{\alpha}$ line in the observed spectrum of the source for all observations. The line shows an interesting evolution: it is centered at $\sim$6.4 keV when the measured flux is above 5$\times 10^{-11}$ erg cm$^{-2} s^{-1}$, but is shifted to $\sim$6.7 keV at lower fluxes. The equivalent width of the line appears to increase significantly toward lower fluxes, although it is likely to be sensitive to calibration uncertainties. While the fluorescent emission of neutral or mildly ionized iron atoms in the accretion disk can perhaps account for the 6.4 keV line, as is often invoked for black hole candidates, it seems difficult to understand the 6.7 keV line with this mechanism, because the disk should be less ionized at lower fluxes (unless its density changes drastically). On the other hand, the 6.7 keV line might be due to recombination cascade of hydrogen or helium like iron ions in an optically thin, highly ionized plasma. We discuss the results in the context of proposed accretion models.Comment: 18 pages, 2 figures, accepted for publication in the ApJ in v552n2p May 10, 2001 issu

A new critical curve for the Lane-Emden system

We study stable positive radially symmetric solutions for the Lane-Emden system $-\Delta u=v^p$ in $\R^N$, $-\Delta v=u^q$ in $\R^N$, where $p,q\geq 1$. We obtain a new critical curve that optimally describes the existence of such solutions.Comment: 13 pages, 1 figur

Phonons and related properties of extended systems from density-functional perturbation theory

This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudo-potential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long wave-length vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic

Mathematical modeling of the interaction between two-phase environmental flow and protective hydraulic structures

In August 2005, Hurricane Katrina struck the Gulf Coast of the United States. Over a thousand people lost their lives and the total damage was about 108 billion USD. It was the costliest United States hurricane. Two-thirds of the deaths and majority of economical loss were related to the protection system failure. This drives the study of fluid structure interaction to properly design the levees and floodwalls in the future for flooding vulnerable areas. Fluid-structure interaction is the interaction between a deformable structure and the surrounding flow. The fluid causes the deformation of the solid, and the solid reacts to the fluid. This thesis will focus on the interactions between two-phase environmental flow (air and water), and hydraulic structures (e.g. floodwalls, etc.) which are partially submerged in the water to disrupt the flow. Hydrodynamic and hydrostatic forces and impact loads from high water levels and velocities applied to the interface must be carefully monitored, as well as their impact on the structural stability. The main purpose of this work is to give a deeper understanding to the interaction processes and the coupling effects, and to determine the possible deformation or critical values of overturning moments for more robust future designs of floodwalls and levees. There are two main approaches to simulate fluid-solid interactions: the monolithic approach and the partitioned approach. In this work we use the partitioned approach by looking into the separate flow and structure models and simulating the interaction process. For the two-phase flow subproblem, the interface of air and water is treated as a material discontinuity in modeling, and is tracked by the level set method. The coupled system consists of Navier-Stokes Equation, level set method and the volume of fraction method, solved by the splitting method with residual-based variational multi-scale methods for stabilization. The structural mechanics is modeled by linear elasticity. Different types of floodwalls and two factors of safety against sliding and overturning are studied. In the Galveston area, the soil and floodwall properties determine the necessity to include soil as a part of the model. Hyperelastic and plastic models are discussed in simulating the soil behavior. The interaction process is modeled by imposing the matching conditions on the common fluid-structure boundary. Both one-way and two-way interaction models under synthetic waves are discussed and compared. One-way interaction is saving in computation and used widely in engineering design. Two-way interaction is formulated under the Arbitrary Lagrangian-Eulerian(ALE) framework. The operator splitting technique is developed for the coupled system to reduce computing cost while remain high accuracy.Computational Science, Engineering, and Mathematic

Hydrophilic polymer foam and microsphere templates for fabrication of microcellular nickel and graphene foams with energy storage applications

Hydrophilic polymer foam and microsphere templates have attracted tremendous attentions in the past decade due to their applicability in numerous areas such as catalyst carriers and mini-reactors, filtration media, carbon foam fabrication templates, thermal and electrical insulators, and tissue engineering scaffolds. Hydrophilic polymer sphere and foam templates can be used to fabricate microcellular nickel foams and graphene foams that are finding unique opportunities in energy storage applications, including battery electrodes and matrices for solar energy storage. In this study open celled hydrophilic polymer foams and microsphere templates with controllable pore size and porosity were fabricated via solid state foaming and vacuum-assisted assembling methods. Hydrophilic polymer foams were fabricated with disulfonated poly(arylene ether sulfone) (BPS) and poly(ethylene glycol) (PEG) miscible blends. Polymer microsphere templates made with PMMA, paraffin, and EAA spheres were used as templates for fabricating bulk nickel foams, which were further used as a template to fabricate graphene foams. In order to achieve bulk microcellular nickel and graphene foams, a novel electro-polishing-assisted electroless nickel (Ni) deposition process was developed to mitigate the diffusion limitation problem. Fundamental mechanisms of the proposed process were studied using a finite difference model considering both ion diffusion and chemical reaction inside the porous media. The fabricated microcellular Ni foams exhibited sufficient thermal stability and were used to fabricate three dimensional (3D) few-layer-graphene (FLG) foams using a chemical vapor deposition (CVD) method. The resulting graphene foams had a pore size less than 100 μm, density of 0.0020 g·cm⁻³, and strut wall thickness of 5 nm. The surface-to-volume ratio of the foam was 2.5×10⁵ m²·m⁻³.Materials Science and Engineerin

Single photon quantum non-demolition in the presence of inhomogeneous broadening

Electromagnetically induced transparency (EIT) has been often proposed for generating nonlinear optical effects at the single photon level; in particular, as a means to effect a quantum non-demolition measurement of a single photon field. Previous treatments have usually considered homogeneously broadened samples, but realisations in any medium will have to contend with inhomogeneous broadening. Here we reappraise an earlier scheme [Munro \textit{et al.} Phys. Rev. A \textbf{71}, 033819 (2005)] with respect to inhomogeneities and show an alternative mode of operation that is preferred in an inhomogeneous environment. We further show the implications of these results on a potential implementation in diamond containing nitrogen-vacancy colour centres. Our modelling shows that single mode waveguide structures of length $200 \mu\mathrm{m}$ in single-crystal diamond containing a dilute ensemble of NV$^-$ of only 200 centres are sufficient for quantum non-demolition measurements using EIT-based weak nonlinear interactions.Comment: 21 pages, 9 figures (some in colour) at low resolution for arXiv purpose

Stalking influenza by vaccination with pre-fusion headless HA mini-stem.

Inaccuracies in prediction of circulating viral strain genotypes and the possibility of novel reassortants causing a pandemic outbreak necessitate the development of an anti-influenza vaccine with increased breadth of protection and potential for rapid production and deployment. The hemagglutinin (HA) stem is a promising target for universal influenza vaccine as stem-specific antibodies have the potential to be broadly cross-reactive towards different HA subtypes. Here, we report the design of a bacterially expressed polypeptide that mimics a H5 HA stem by protein minimization to focus the antibody response towards the HA stem. The HA mini-stem folds as a trimer mimicking the HA prefusion conformation. It is resistant to thermal/chemical stress, and it binds to conformation-specific, HA stem-directed broadly neutralizing antibodies with high affinity. Mice vaccinated with the group 1 HA mini-stems are protected from morbidity and mortality against lethal challenge by both group 1 (H5 and H1) and group 2 (H3) influenza viruses, the first report of cross-group protection. Passive transfer of immune serum demonstrates the protection is mediated by stem-specific antibodies. Furthermore, antibodies indudced by these HA stems have broad HA reactivity, yet they do not have antibody-dependent enhancement activity

Orbital state and magnetic properties of LiV_2 O_4

LiV_2 O_4 is one of the most puzzling compounds among transition metal oxides because of its heavy fermion like behavior at low temperatures. In this paper we present results for the orbital state and magnetic properties of LiV_2 O_4 obtained from a combination of density functional theory within the local density approximation and dynamical mean-field theory (DMFT). The DMFT equations are solved by quantum Monte Carlo simulations. The trigonal crystal field splits the V 3d orbitals such that the a_{1g} and e_{g}^{pi} orbitals cross the Fermi level, with the former being slightly lower in energy and narrower in bandwidth. In this situation, the d-d Coulomb interaction leads to an almost localization of one electron per V ion in the a_{1g} orbital, while the e_{g}^{pi} orbitals form relatively broad bands with 1/8 filling. 2The theoretical high-temperature paramagnetic susceptibility chi(T) follows a Curie-Weiss law with an effective paramagnetic moment p_{eff}=1.65 in agreement with the experimental results.Comment: 11 pages, 10 figures, 2 table