Formation of a stable deacagonal quasicrystalline Al-Pd-Mn surface layer

We report the in situ formation of an ordered equilibrium decagonal Al-Pd-Mn quasicrystal overlayer on the 5-fold symmetric surface of an icosahedral Al-Pd-Mn monograin. The decagonal structure of the epilayer is evidenced by x-ray photoelectron diffraction, low-energy electron diffraction and electron backscatter diffraction. This overlayer is also characterized by a reduced density of states near the Fermi edge as expected for quasicrystals. This is the first time that a millimeter-size surface of the stable decagonal Al-Pd-Mn is obtained, studied and compared to its icosahedral counterpart.Comment: Submitted to Phys. Ref. Lett. (18 July 2001

Ab initio study of ferroelectric domain walls in PbTiO3

We have investigated the atomistic structure of the 180-degree and 90-degree domain boundaries in the ferroelectric perovskite compound PbTiO3 using a first-principles ultrasoft-pseudopotential approach. For each case we have computed the position, thickness and creation energy of the domain walls, and an estimate of the barrier height for their motion has been obtained. We find both kinds of domain walls to be very narrow with a similar width of the order of one to two lattice constants. The energy of the 90-dergree domain wall is calculated to be 35 mJ/m^2, about a factor of four lower than the energy of its 180-degree counterpart, and only a miniscule barrier for its motion is found. As a surprising feature we detected a small offset of 0.15-0.2 eV in the electrostatic potential across the 90-degree domain wall.Comment: 12 pages, with 9 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/bm_dw/index.htm

Superchemistry: dynamics of coupled atomic and molecular Bose-Einstein condensates

We analyze the dynamics of a dilute, trapped Bose-condensed atomic gas coupled to a diatomic molecular Bose gas by coherent Raman transitions. This system is shown to result in a new type of `superchemistry', in which giant collective oscillations between the atomic and molecular gas can occur. The phenomenon is caused by stimulated emission of bosonic atoms or molecules into their condensate phases

QCD-like theories at nonzero temperature and density

We investigate the properties of hot and/or dense matter in QCD-like theories with quarks in a (pseudo)real representation of the gauge group using the Nambu-Jona-Lasinio model. The gauge dynamics is modeled using a simple lattice spin model with nearest-neighbor interactions. We first keep our discussion as general as possible, and only later focus on theories with adjoint quarks of two or three colors. Calculating the phase diagram in the plane of temperature and quark chemical potential, it is qualitatively confirmed that the critical temperature of the chiral phase transition is much higher than the deconfinement transition temperature. At a chemical potential equal to half of the diquark mass in the vacuum, a diquark Bose-Einstein condensation (BEC) phase transition occurs. In the two-color case, a Ginzburg-Landau expansion is used to study the tetracritical behavior around the intersection point of the deconfinement and BEC transition lines, which are both of second order. We obtain a compact expression for the expectation value of the Polyakov loop in an arbitrary representation of the gauge group (for any number of colors), which allows us to study Casimir scaling at both nonzero temperature and chemical potential.Comment: JHEP class, 31 pages, 7 eps figures; v2: error in Eq. (3.11) fixed, two references added; matches published versio

Can We Really Prevent Suicide?

Every year, suicide is among the top 20 leading causes of death globally for all ages. Unfortunately, suicide is difficult to prevent, in large part because the prevalence of risk factors is high among the general population. In this review, clinical and psychological risk factors are examined and methods for suicide prevention are discussed. Prevention strategies found to be effective in suicide prevention include means restriction, responsible media coverage, and general public education, as well identification methods such as screening, gatekeeper training, and primary care physician education. Although the treatment for preventing suicide is difficult, follow-up that includes pharmacotherapy, psychotherapy, or both may be useful. However, prevention methods cannot be restricted to the individual. Community, social, and policy interventions will also be essentia

Numerical visualization and optimization on the core penetration in multi-cavity co-injection molding with a bifurcation runner structure

[[abstract]]Co-Injection Molding and multi-cavity molding are common processes for plastic products manufacturing. These two systems are sometimes combined and applied in the manufacture of bifurcation-structure products. In the previous literature results, the dynamic behavior of the core penetration in co-injection multi-cavity molding with a bifurcation structure is quite complicated and the behavior is sensitive to injection flow rates, different materials, and other process conditions. However, how these influential factors truly affect the core penetration behavior and the detailed mechanism of core penetration behavior has not yet been fully understood. In this study, we focused on studying the multi-cavity co-injection system with a bifurcation runner structure. The results showed that when the skin-to-core ratio is fixed (say 72/28), the melt flow behavior of a co-injection system, utilizing the same material for both skin and core, is very similar to that of a single shot injection molding. Specifically, the non-symmetrical bifurcation runner structure will influence the flow behavior greatly and cause the core distribution imbalance between different cavities. Due to the geometric nature of the bifurcation runner design, this core distribution imbalance problem will still persist even if we modify the melt temperature, mold temperature, or even change the plastic material. Furthermore, when the skin-to-core ratio is fixed (say 72/28), the changes of the flow rate have very little effect on the core penetration result in the final molded product; the final molded product will still have a core distribution imbalance issue. However, we observed that when the flow rate is increased, the core material will occupy more volume space in the upstream portion of the runner and the core penetration distance will be reduced in the flow direction downstream. This feature is very useful to further manipulate the skin-core interface in a multi-cavity system. Moreover, regarding how to improve a poor inter-cavity balance of core material distribution, using a suitable adjustment of the skin-to-core ratio will be greatly helpful. However, the core break-through defect can be a common problem in co-injection molding when an unsuitable skin-to-core ratio is used. To prevent the core break-through defect, increasing the flow rate properly can be one of the good options that we can use. Hence, we concluded that a suitable adjustment of the skin-to-core ratio and a proper flow rate control can be used to optimize the core material distribution in multi-cavity co-injection molding with a bifurcation runner structure. Lastly, in order to validate our inference and the effectiveness of our proposal to improve the inter-cavity imbalance and core break-through problem, a series of experimental studies were performed. And, all experimental results are in good agreement with those of our numerical predictions to further validate the feasibility of our proposed method to gain a better control of the core material distribution with a bifurcation runner structure in multi-cavity co-injection molding.[[notice]]補正完

Iterative joint frequency offset and channel estimation for OFDM systems using first and second order approximation algorithms

[[abstract]]To implement an algorithm for joint estimation of carrier frequency offset (CFO) and channel impulse response (CIR) in orthogonal frequency division multiplexing (OFDM) systems, the maximum-likelihood criterion is commonly adopted. A major difficulty arises from the highly nonlinear nature of the log-likelihood function which renders local extrema or multiple solutions for the CFO and CIR estimators. Use of an approximation method coupled with an adaptive iteration algorithm has been a popular approach to ease problem solving. The approximation used in those existing methods is usually of the first order level. Here, in addition to a new first order approximation method, we also propose a second order approximation method. Further, for the part of the adaptive iteration algorithm, we adopt a new technique which will enable performance improvement. Our first order approximation method is found to outperform the existing ones in terms of estimation accuracies, tracking range, computation complexity, and convergence speed. As expected, our second order approximation method provides an even further improvement at the expense of higher computation complication.[[notice]]補正完畢[[journaltype]]國外[[incitationindex]]SCI[[booktype]]紙本[[booktype]]電子版[[countrycodes]]DE

Measurement of the photon+b+b-jet production differential cross section in ppˉp\bar{p} collisions at \sqrt{s}=1.96~\TeV

We present measurements of the differential cross section dsigma/dpT_gamma for the inclusive production of a photon in association with a b-quark jet for photons with rapidities |y_gamma|< 1.0 and 30<pT_gamma <300 GeV, as well as for photons with 1.5<|y_gamma|< 2.5 and 30< pT_gamma <200 GeV, where pT_gamma is the photon transverse momentum. The b-quark jets are required to have pT>15 GeV and rapidity |y_jet| < 1.5. The results are based on data corresponding to an integrated luminosity of 8.7 fb^-1, recorded with the D0 detector at the Fermilab Tevatron $p\bar{p}$ Collider at sqrt(s)=1.96 TeV. The measured cross sections are compared with next-to-leading order perturbative QCD calculations using different sets of parton distribution functions as well as to predictions based on the kT-factorization QCD approach, and those from the Sherpa and Pythia Monte Carlo event generators.Comment: 10 pages, 9 figures, submitted to Phys. Lett.

Search for the Higgs boson in lepton, tau and jets final states

We present a search for the standard model Higgs boson in final states with an electron or muon and a hadronically decaying tau lepton in association with two or more jets using 9.7 fb^{-1} of Run II Fermilab Tevatron Collider data collected with the D0 detector. The analysis is sensitive to Higgs boson production via gluon fusion, associated vector boson production, and vector boson fusion, followed by the Higgs boson decay to tau lepton pairs or to W boson pairs. The ratios of 95% C.L. upper limits on the cross section times branching ratio to those predicted by the standard model are obtained for orthogonal subsamples that are enriched in either H -> tau tau decays or H -> WW decays, and for the combination of these subsample limits. The observed and expected limit ratios for the combined subsamples at a Higgs boson mass of 125 GeV are 11.3 and 9.0 respectively

Search for a Narrow ttbar Resonance in ppbar Collisions at sqrt{s}=1.96 TeV

We report a search for a narrow ttbar resonance that decays into a lepton+jets final state based on an integrated luminosity of 5.3/fb of proton-antiproton collisions at sqrt{s}=1.96 TeV collected by the D0 Collaboration at the Fermilab Tevatron Collider. We set upper limits on the production cross section of such a resonance multiplied by its branching fraction to ttbar which we compare to predictions for a leptophobic topcolor Z' boson. We exclude such a resonance at the 95% confidence level for masses below 835 GeV.Comment: 7 pages, 3 figures, submitted to Physical Review Letter