5,532 research outputs found
The impact of space and space-related activities on a local economy. a case study of boulder, colorado. part ii- the income-product accounts
Total impact of space and space related activities on local economy of Boulder, Colorado - income-product account
Black hole solutions in 2+1 dimensions
We give circularly symmetric solutions for null fluid collapse in
2+1-dimensional Einstein gravity with a cosmological constant. The fluid
pressure and energy density are related by . The
long time limit of the solutions are black holes whose horizon structures
depend on the value of . The solution is the
Banados-Teitelboim-Zanelli black hole metric in the long time static limit,
while the solutions give other, `hairy' black hole metrics in this limit.Comment: 8 pages, RevTeX (to appear in Phys. Rev. D) References to Mann and
Ross, and Mann, Chan and Chan adde
Crystal Structure and Magnetism of the Linear-Chain Copper Oxides Sr5Pb3-xBixCuO12
The title quasi-1D copper oxides (0=< x =<0.4) were investigated by neutron
diffraction and magnetic susceptibility studies. Polyhedral CuO4 units in the
compounds were found to comprise linear-chains at inter-chain distance of
approximately 10 A. The parent chain compound (x = 0), however, shows less
anisotropic magnetic behavior above 2 K, although it is of substantially
antiferromagnetic (mu_{eff}= 1.85 mu_{B} and Theta_{W} = -46.4 K) spin-chain
system. A magnetic cusp gradually appears at about 100 K in T vs chi with the
Bi substitution. The cusp (x = 0.4) is fairly characterized by and therefore
suggests the spin gap nature at Delta/k_{B} ~ 80 K. The chain compounds hold
electrically insulating in the composition range.Comment: To be published in PR
The Origin of Primordial Dwarf Stars and Baryonic Dark Matter
I present a scenario for the production of low mass, degenerate dwarfs of
mass via the mechanism of Lenzuni, Chernoff & Salpeter (1992).
Such objects meet the mass limit requirements for halo dark matter from
microlensing surveys while circumventing the chemical evolution constraints on
normal white dwarf stars. I describe methods to observationally constrain this
scenario and suggest that such objects may originate in small clusters formed
from the thermal instability of shocked, heated gas in dark matter haloes, such
as suggested by Fall & Rees (1985) for globular clusters.Comment: TeX, 4 pages plus 2 postscript figures. To appear in Astrophysical
Journal Letter
Diffusion Enhances Chirality Selection
Diffusion effect on chirality selection in a two-dimensional
reaction-diffusion model is studied by the Monte Carlo simulation. The model
consists of achiral reactants A which turn into either of the chiral products,
R or S, in a solvent of chemically inactive vacancies V. The reaction contains
the nonlinear autocatalysis as well as recycling process, and the chiral
symmetry breaking is monitored by an enantiomeric excess .
Without dilution a strong nonlinear autocatalysis ensures chiral symmetry
breaking. By dilution, the chiral order decreases, and the racemic state
is recovered below the critical concentration . Diffusion effectively
enhances the concentration of chiral species, and decreases as the
diffusion coefficient increases. The relation between and for a
system with a finite fits rather well to an interpolation formula between
the diffusionless(D=0) and homogeneous () limits.Comment: 7 pages, 6 figure
The susceptibility and excitation spectrum of (VO)PO in ladder and dimer chain models
We present numerical results for the magnetic susceptibility of a Heisenberg
antiferromagnetic spin ladder, as a function of temperature and the spin-spin
interaction strengths and . These are contrasted with new
bulk limit results for the dimer chain. A fit to the experimental
susceptibility of the candidate spin-ladder compound vanadyl pyrophosphate,
(VO)PO, gives the parameters meV and meV. With these values we predict a singlet-triplet energy gap of
meV, and give a numerical estimate of the ladder triplet
dispersion relation . In contrast, a fit to the dimer chain model
leads to meV and meV, which predicts a gap of meV.Comment: 16 pages, 6 figures available upon request, RevTex 3.0, preprint
ORNL-CCIP-94-04 / RAL-94-02
Proportion Regulation in Globally Coupled Nonlinear Systems
As a model of proportion regulation in differentiation process of biological
system, globally coupled activator-inhibitor systems are studied. Formation and
destabilization of one and two cluster state are predicted analytically.
Numerical simulations show that the proportion of units of clusters is chosen
within a finite range and it is selected depend on the initial condition.Comment: 11 pages (revtex format) and 5 figures (PostScript)
Spin gap in the Quasi-One-Dimensional S=1/2 Antiferromagnet: Cu2(1,4-diazacycloheptane)2Cl4
Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4} contains double chains of spin 1/2
Cu^{2+} ions. We report ac susceptibility, specific heat, and inelastic neutron
scattering measurements on this material. The magnetic susceptibility,
, shows a rounded maximum at T = 8 K indicative of a low dimensional
antiferromagnet with no zero field magnetic phase transition. We compare the
data to exact diagonalization results for various one dimensional
spin Hamiltonians and find excellent agreement for a spin ladder with
intra-rung coupling meV and two mutually frustrating
inter-rung interactions: meV and meV. The
specific heat in zero field is exponentially activated with an activation
energy meV. A spin gap is also found through inelastic
neutron scattering on powder samples which identify a band of magnetic
excitations for meV. Using sum-rules we derive an
expression for the dynamic spin correlation function associated with
non-interacting propagating triplets in a spin ladder. The van-Hove
singularities of such a model are not observed in our scattering data
indicating that magnetic excitations in Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4}
are more complicated. For magnetic fields above T specific
heat data versus temperature show anomalies indicating a phase transition to an
ordered state below T = 1 K.Comment: 9 pages, 8 postscript figures, LaTeX, Submitted to PRB 8/4/97, e-mail
Comments to [email protected]
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