31,576 research outputs found

    Two Aspects of the Mott-Hubbard Transition in Cr-doped V_2O_3

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    The combination of bandstructure theory in the local density approximation with dynamical mean field theory was recently successfully applied to V2_2O3_3 -- a material which undergoes the f amous Mott-Hubbard metal-insulator transition upon Cr doping. The aim of this sh ort paper is to emphasize two aspects of our recent results: (i) the filling of the Mott-Hubbard gap with increasing temperature, and (ii) the peculiarities of the Mott-Hubbard transition in this system which is not characterized by a diver gence of the effective mass for the a1ga_{1g}-orbital.Comment: 2 pages, 3 figures, SCES'04 conference proceeding

    The Small Scale Velocity Dispersion of Galaxies: A Comparison of Cosmological Simulations

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    The velocity dispersion of galaxies on small scales (r∼1h−1r\sim1h^{-1} Mpc), σ12(r)\sigma_{12}(r), can be estimated from the anisotropy of the galaxy-galaxy correlation function in redshift space. We apply this technique to ``mock-catalogs'' extracted from N-body simulations of several different variants of Cold Dark Matter dominated cosmological models to obtain results which may be consistently compared to similar results from observations. We find a large variation in the value of σ12(1h−1Mpc)\sigma_{12}(1 h^{-1} Mpc) in different regions of the same simulation. We conclude that this statistic should not be considered to conclusively rule out any of the cosmological models we have studied. We attempt to make the statistic more robust by removing clusters from the simulations using an automated cluster-removing routine, but this appears to reduce the discriminatory power of the statistic. However, studying σ12\sigma_{12} as clusters with different internal velocity dispersions are removed leads to interesting information about the amount of power on cluster and subcluster scales. We also compute the pairwise velocity dispersion directly and compare this to the values obtained using the Davis-Peebles method, and find that the agreement is fairly good. We evaluate the models used for the mean streaming velocity and the pairwise peculiar velocity distribution in the original Davis-Peebles method by comparing the models with the results from the simulations.Comment: 20 pages, uuencoded (Latex file + 8 Postscript figures), uses AAS macro

    Independent Orbiter Assessment (IOA): Analysis of the life support and airlock support subsystems

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    The results of the Independent Orbiter Assessment (IOA) of the Failure Modes and Effects Analysis (FMEA) and Critical Items List (CIL) are presented. The IOA approach features a top-down analysis of the hardware to determine failure modes, criticality, and potential critical items. To preserve independence, this analysis was accomplished without reliance upon the results contained within the NASA FMEA/CIL documentation. This report documents the independent analysis results corresponding to the Orbiter Life Support System (LSS) and Airlock Support System (ALSS). Each level of hardware was evaluated and analyzed for possible failure modes and effects. Criticality was assigned based upon the severity of the effect for each failure mode. The LSS provides for the management of the supply water, collection of metabolic waste, management of waste water, smoke detection, and fire suppression. The ALSS provides water, oxygen, and electricity to support an extravehicular activity in the airlock

    On the Distribution of Haloes, Galaxies and Mass

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    The stochasticity in the distribution of dark haloes in the cosmic density field is reflected in the distribution function PV(Nh∣δm)P_V(N_h|\delta_m) which gives the probability of finding NhN_h haloes in a volume VV with mass density contrast δm\delta_m. We study the properties of this function using high-resolution NN-body simulations, and find that PV(Nn∣δm)P_V(N_n|\delta_m) is significantly non-Poisson. The ratio between the variance and the mean goes from ∼1\sim 1 (Poisson) at 1+δm≪11+\delta_m\ll 1 to <1<1 (sub-Poisson) at 1+δm∼11+\delta_m\sim 1 to >1>1 (super-Poisson) at 1+δm≫11+\delta_m\gg 1. The mean bias relation is found to be well described by halo bias models based on the Press-Schechter formalism. The sub-Poisson variance can be explained as a result of halo-exclusion while the super-Poisson variance at high δm\delta_m may be explained as a result of halo clustering. A simple phenomenological model is proposed to describe the behavior of the variance as a function of δm\delta_m. Galaxy distribution in the cosmic density field predicted by semi-analytic models of galaxy formation shows similar stochastic behavior. We discuss the implications of the stochasticity in halo bias to the modelling of higher-order moments of dark haloes and of galaxies.Comment: 10 pages, 6 figures, Latex using MN2e style. Minor changes. Accepted for publication in MNRA

    Scaling properties of the redshift power spectrum: theoretical models

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    We report the results of an analysis of the redshift power spectrum PS(k,μ)P^S(k,\mu) in three typical Cold Dark Matter (CDM) cosmological models, where μ\mu is the cosine of the angle between the wave vector and the line-of-sight. Two distinct biased tracers derived from the primordial density peaks of Bardeen et al. and the cluster-underweight model of Jing, Mo, & B\"orner are considered in addition to the pure dark matter models. Based on a large set of high resolution simulations, we have measured the redshift power spectrum for the three tracers from the linear to the nonlinear regime. We investigate the validity of the relation - guessed from linear theory - in the nonlinear regime PS(k,μ)=PR(k)[1+βμ2]2D(k,μ,σ12(k)), P^S(k,\mu)=P^R(k)[1+\beta\mu^2]^2D(k,\mu,\sigma_{12}(k)), where PR(k)P^R(k) is the real space power spectrum, and β\beta equals Ω00.6/bl\Omega_0^{0.6}/b_l. The damping function DD which should generally depend on kk, μ\mu, and σ12(k)\sigma_{12}(k), is found to be a function of only one variable kμσ12(k)k\mu\sigma_{12}(k). This scaling behavior extends into the nonlinear regime, while DD can be accurately expressed as a Lorentz function - well known from linear theory - for values D>0.1D > 0.1. The difference between σ12(k)\sigma_{12}(k) and the pairwise velocity dispersion defined by the 3-D peculiar velocity of the simulations (taking r=1/kr=1/k) is about 15%. Therefore σ12(k)\sigma_{12}(k) is a good indicator of the pairwise velocity dispersion. The exact functional form of DD depends on the cosmological model and on the bias scheme. We have given an accurate fitting formula for the functional form of DD for the models studied.Comment: accepted for publication in ApJ;24 pages with 7 figures include
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