3,352 research outputs found
Analytical approximations for the orientation distribution of small dipolar particles in steady shear flows
Analytic approximations are obtained to solutions of the steady Fokker-Planck equation describing the probability density functions for the orientation of dipolar particles in a steady, low-Reynolds-number shear flow and a uniform external field. Exact computer algebra is used to solve the equation in terms of a truncated spherical harmonic expansion. It is demonstrated that very low orders of approximation are required for spheres but that spheroids introduce resolution problems in certain flow regimes. Moments of the orientation probability density function are derived and applications to swimming cells in bioconvection are discussed. A separate asymptotic expansion is performed for the case in which spherical particles are in a flow with high vorticity, and the results are compared with the truncated spherical harmonic expansion. Agreement between the two methods is excellent
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Production of high molecular weight organic compounds on the surfaces of amorphous iron silicate catalysts: Implications for organic synthesis in the solar nebula
The high molecular weight organic products of Fischer-Tropsch/Haber-Bosch syntheses on the surfaces of Fe-silicate catalysts have been studied by GCMS
On the stability of the primordial closed string gas
We recast the study of a closed string gas in a toroidal container in the
physical situation in which the single string density of states is independent
of the volume because energy density is very high. This includes the gas for
the well known Brandenberger-Vafa cosmological scenario. We describe the gas in
the grandcanonical and microcanonical ensembles. In the microcanonical
description, we find a result that clearly confronts the Brandenberger-Vafa
calculation to get the specific heat of the system. The important point is that
we use the same approach to the problem but a different regularization. By the
way, we show that, in the complex temperature formalism, at the Hagedorn
singularity, the analytic structure obtained from the so-called
F-representation of the free energy coincides with the one computed using the
S-representation.Comment: 20 pages and 1 figure. The final version that appeared in JHE
Field-induced thermal metal-to-insulator transition in underdoped LSCO
The transport of heat and charge in cuprates was measured in undoped and
heavily-underdoped single crystal La_{2-x}Sr_xCuO_{4+delta} (LSCO). In
underdoped LSCO, the thermal conductivity is found to decrease with increasing
magnetic field in the T --> 0 limit, in striking contrast to the increase
observed in all superconductors, including cuprates at higher doping. The
suppression of superconductivity with magnetic field shows that a novel thermal
metal-to-insulator transition occurs upon going from the superconducting state
to the field-induced normal state.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
The vegetation history of South Australia
Published online: 11 April 2018South Australia today is one of the most arid regions on Earth, with a vegetation that is well adapted to either a strongly developed winter rainfall pattern with associated hot, dry summers (mostly near the south coast), or, across the rest of the State, to highly intermittent rainfall and otherwise extremely hot and dry conditions. Despite being a very stable piece of land with a deep geological history, South Australia, as an integral part of Australia, has had a highly variable history in terms of its global positioning and its climate, so that even within the past 65 million years (since the catastrophic event that signalled the end of the Cretaceous), the position of South Australia has changed dramatically, from very close to the South Pole, through to its current position in midsouthern latitudes. During that time the climate has changed to such an extent that the vegetation has reduced by declining from highly diverse, very complex, broad-leafed rainforest, through to today’s scleromorphic forests and shrublands and various other forms of desert vegetation. The transition between these extremes has not been a smooth one, and especially in more recent times there has been significant controversy over the impact on the vegetation coincident with the arrival of Homo sapiens and the demise of the remarkable megafauna.R.S. Hill, M.A. Tarran, K.E. Hill & Y.K. Bee
Characterization of the S = 9 excited state in Fe8Br8 by Electron Paramagnetic Resonance
High Frequency electron paramagnetic resonance has been used to observe the
magnetic dipole, M = 1, transitions in the excited
state of the single molecule magnet FeBr. A Boltzmann analysis of the
measured intensities locates it at 24 2 K above the ground
state, while the line positions yield its magnetic parameters D = -0.27 K, E =
0.05 K, and B = -1.3 10 K. D is thus smaller by 8%
and E larger by 7% than for . The anisotropy barrier for is
estimated as 22 K,which is 25% smaller than that for (29 K). These
data also help assign the spin exchange constants(J's) and thus provide a basis
for improved electronic structure calculations of FeBr.Comment: 7 pages, Figs included in text, submitted to PR
Macroinvertebrate taxonomic and functional trait compositions within lotic habitats affected by river restoration practices
The widespread degradation of lotic ecosystems has prompted extensive river restoration efforts globally, but many studies have reported modest ecological responses to rehabilitation practices. The functional properties of biotic communities are rarely examined within post-project appraisals, which would provide more ecological information underpinning ecosystem responses to restoration practices and potentially pinpoint project limitations. This study examines macroinvertebrate community responses to three projects which aimed to physically restore channel morphologies. Taxonomic and functional trait compositions supported by widely occurring lotic habitats (biotopes) were examined across paired restored and non-restored (control) reaches. The multivariate location (average community composition) of taxonomic and functional trait compositions differed marginally between control and restored reaches. However, changes in the amount of multivariate dispersion were more robust and indicated greater ecological heterogeneity within restored reaches, particularly when considering functional trait compositions. Organic biotopes (macrophyte stands and macroalgae) occurred widely across all study sites and supported a high alpha (within-habitat) taxonomic diversity compared to mineralogical biotopes (sand and gravel patches), which were characteristic of restored reaches. However, mineralogical biotopes possessed a higher beta (between-habitat) functional diversity, although this was less pronounced for taxonomic compositions. This study demonstrates that examining the functional and structural properties of taxa across distinct biotopes can provide a greater understanding of biotic responses to river restoration works. Such information could be used to better understand the ecological implications of rehabilitation practices and guide more effective management strategies
Multiband superconductivity in NbSe_2 from heat transport
The thermal conductivity of the layered s-wave superconductor NbSe_2 was
measured down to T_c/100 throughout the vortex state. With increasing field, we
identify two regimes: one with localized states at fields very near H_c1 and
one with highly delocalized quasiparticle excitations at higher fields. The two
associated length scales are most naturally explained as multi-band
superconductivity, with distinct small and large superconducting gaps on
different sheets of the Fermi surface.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
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