Impact of Flower Harvesting on the Salt Marsh Plant \u3cem\u3eLimonium carolinianum\u3c/em\u3e

Because of the potentially detrimental effects of seed production on adult survivorship and growth, moderate flower harvesting may have little negative impact on population growth of long-lived perennial plants such as Limonium carolinianum (Walter) Britton. We examined this by collecting data on survivorship, growth, and fecundity of an unharvested population over a period of 5 years and conducted a controlled experiment to examine the effect of harvesting on adult survivorship and growth over a 3-year period. Data were summarized in the form of a stage structured matrix population model with a stochastic element that incorporated year-to-year variation in transition probabilities. Contrary to our original hypothesis, we found that preventing seed set through removal of flowers did not increase adult survivorship or growth. By determining the harvest level that reduced population growth rate to 1.0, we estimated the maximum sustainable harvest level to be 16%, a value that is approximately half that of reported harvest levels on accessible marshes in the study area. In spite of this, the reported harvest levels are unlikely to drive local populations to extinction in the foreseeable future. Providing the adult population size is \u3e100 and harvest levels are \u3c90%, time to local extinction will exceed 100 years. This is a function of the very high survivorship of adults in this species and the fact that harvesting has no negative impact on adult survivorship or growth. However, because of the long preadult phase in this species (8–9 years) and the fact that fecundity of young adults is low, recovery from overharvesting is extremely slow. Adult population size can be reduced to 25% of its original value in 7 years at high harvest levels, but it will take 34 years on average to recover once harvesting is terminated

Nonextensive hydrodynamics for relativistic heavy-ion collisions

The nonextensive one-dimensional version of a hydrodynamical model for multiparticle production processes is proposed and discussed. It is based on nonextensive statistics assumed in the form proposed by Tsallis and characterized by a nonextensivity parameter $q$. In this formulation the parameter $q$ characterizes some specific form of local equilibrium which is characteristic for the nonextensive thermodynamics and which replaces the usual local thermal equilibrium assumption of the usual hydrodynamical models. We argue that there is correspondence between the perfect nonextensive hydrodynamics and the usual dissipative hydrodynamics. It leads to simple expression for dissipative entropy current and allows for predictions for the ratio of bulk and shear viscosities to entropy density, $\zeta/s$ and $\eta/s$, to be made.Comment: Final version accepted for publication in Phys. Rev.

Approximate Analytic Solution for the Spatiotemporal Evolution of Wave Packets undergoing Arbitrary Dispersion

We apply expansion methods to obtain an approximate expression in terms of elementary functions for the space and time dependence of wave packets in a dispersive medium. The specific application to pulses in a cold plasma is considered in detail, and the explicit analytic formula that results is provided. When certain general initial conditions are satisfied, these expressions describe the packet evolution quite well. We conclude by employing the method to exhibit aspects of dispersive pulse propagation in a cold plasma, and suggest how predicted and experimental effects may be compared to improve the theoretical description of a medium's dispersive properties.Comment: 17 pages, 4 figures, RevTe

One-loop approximation for the Heisenberg antiferromagnet

We use the diagram technique for spin operators to calculate Green's functions and observables of the spin-1/2 quantum Heisenberg antiferromagnet on a square lattice. The first corrections to the self-energy and interaction are taken into account in the chain diagrams. The approximation reproduces main results of Takahashi's modified spin-wave theory [Phys. Rev. B 40, 2494 (1989)] and is applicable in a wider temperature range. The energy per spin calculated in this approximation is in good agreement with the Monte Carlo and small-cluster exact-diagonalization calculations in the range 0 <= T < 1.2J where J is the exchange constant. For the static uniform susceptibility the agreement is good for T < 0.6J and becomes somewhat worse for higher temperatures. Nevertheless the approximation is able to reproduce the maximum in the temperature dependence of the susceptibility near T = 0.9J.Comment: 15 pages, 6 ps figure

Wiki Justice, Social Ergonomics, and Ethical Collaborations

In this essay we explore various theoretical, pedagogical, and historical aspects of wikis focusing on three questions as points of departure—"What is a wiki?"; "How do you teach with a wiki?" and finally "What is the point of a wiki?

Modelling and analysing cargo screening processes: a project outline

The efficiency of current cargo screening processes at sea and air ports is unknown as no benchmarks exists against which they could be measured. Some manufacturer benchmarks exist for individual sensors but we have not found any benchmarks that take a holistic view of the screening procedures assessing a combination of sensors and also taking operator variability into account. Just adding up resources and manpower used is not an effective way for assessing systems where human decision-making and operator compliance to rules play a vital role. For such systems more advanced assessment methods need to be used, taking into account that the cargo screening process is of a dynamic and stochastic nature. Our project aim is to develop a decision support tool (cargo-screening system simulator) that will map the right technology and manpower to the right commodity-threat combination in order to maximize detection rates. In this paper we present a project outline and highlight the research challenges we have identified so far. In addition we introduce our first case study, where we investigate the cargo screening process at the ferry port in Calais

Cold collision shift cancelation and inelastic scattering in a Yb optical lattice clock

Recently, p-wave cold collisions were shown to dominate the density-dependent shift of the clock transition frequency in a 171Yb optical lattice clock. Here we demonstrate that by operating such a system at the proper excitation fraction, the cold collision shift is canceled below the 5x10^{-18} fractional frequency level. We report inelastic two-body loss rates for 3P0-3P0 and 1S0-3P0 scattering. We also measure interaction shifts in an unpolarized atomic sample. Collision measurements for this spin-1/2 171Yb system are relevant for high performance optical clocks as well as strongly-interacting systems for quantum information and quantum simulation applications

Polaron band formation in the Holstein model

We present numerical exact results for the polaronic band structure of the Holstein molecular crystal model in one and two dimensions. The use of direct Lanczos diagonalization technique, preserving the full dynamics and quantum nature of phonons, allows us to analyze in detail the renormalization of both quasiparticle bandwidth and dispersion by the electron-phonon interaction. For the two-dimensional case some of our exact data are compared with the results obtained in the framework of a recently developed finite cluster strong-coupling perturbation theory.Comment: 10 pages (LaTeX), 6 figures (ps), submitted to Phys. Rev.

Optical absorption and single-particle excitations in the 2D Holstein t-J model

To discuss the interplay of electronic and lattice degrees of freedom in systems with strong Coulomb correlations we have performed an extensive numerical study of the two-dimensional Holstein t-J model. The model describes the interaction of holes, doped in a quantum antiferromagnet, with a dispersionsless optical phonon mode. We apply finite-lattice Lanczos diagonalization, combined with a well-controlled phonon Hilbert space truncation, to the Hamiltonian. The focus is on the dynamical properties. In particular we have evaluated the single-particle spectral function and the optical conductivity for characteristic hole-phonon couplings, spin exchange interactions and phonon frequencies. The results are used to analyze the formation of hole polarons in great detail. Links with experiments on layered perovskites are made. Supplementary we compare the Chebyshev recursion and maximum entropy algorithms, used for calculating spectral functions, with standard Lanczos methods.Comment: 32 pages, 12 figures, submitted to Phys. Rev.