891 research outputs found
Emergence of buffel grass (Cenchrus ciliaris) from seed after flooding
The emergence of 7 buffel grass Cenchrus ciliaris cultivars after simulated flooding for 0, 10, 20, 30 or 40 days was investigated in pot trials. Seed of known germination percentage was sown in a medium-heavy alluvial clay soil, which was immediately flooded
Flooding tolerance of Panicum coloratum
Five Panicum coloratum cultivars and one of Panicum maximum were flooded in pots. Flooding damage was assessed by visually rating the induced chlorosis, counting live tillers per pot and measuring dry weight of the plants
Effect of flooding on the regeneration of six tropical grasses after defoliation.
The effect of flooding immediately and 15 days after defoliation on the survival and growth of Panicum coloratum, Panicum maximum, Urochloa mosambicensis and three Cenchrus ciliaris cultivars was studied in a pot experiment at Mackay, Queensland
Stability Walls in Heterotic Theories
We study the sub-structure of the heterotic Kahler moduli space due to the
presence of non-Abelian internal gauge fields from the perspective of the
four-dimensional effective theory. Internal gauge fields can be supersymmetric
in some regions of the Kahler moduli space but break supersymmetry in others.
In the context of the four-dimensional theory, we investigate what happens when
the Kahler moduli are changed from the supersymmetric to the non-supersymmetric
region. Our results provide a low-energy description of supersymmetry breaking
by internal gauge fields as well as a physical picture for the mathematical
notion of bundle stability. Specifically, we find that at the transition
between the two regions an additional anomalous U(1) symmetry appears under
which some of the states in the low-energy theory acquire charges. We compute
the associated D-term contribution to the four-dimensional potential which
contains a Kahler-moduli dependent Fayet-Iliopoulos term and contributions from
the charged states. We show that this D-term correctly reproduces the expected
physics. Several mathematical conclusions concerning vector bundle stability
are drawn from our arguments. We also discuss possible physical applications of
our results to heterotic model building and moduli stabilization.Comment: 37 pages, 4 figure
Spectral Degeneracies in the Totally Asymmetric Exclusion Process
We study the spectrum of the Markov matrix of the totally asymmetric
exclusion process (TASEP) on a one-dimensional periodic lattice at ARBITRARY
filling. Although the system does not possess obvious symmetries except
translation invariance, the spectrum presents many multiplets with degeneracies
of high order. This behaviour is explained by a hidden symmetry property of the
Bethe Ansatz. Combinatorial formulae for the orders of degeneracy and the
corresponding number of multiplets are derived and compared with numerical
results obtained from exact diagonalisation of small size systems. This
unexpected structure of the TASEP spectrum suggests the existence of an
underlying large invariance group.
Keywords: ASEP, Markov matrix, Bethe Ansatz, Symmetries.Comment: 19 pages, 1 figur
Operator-Based Truncation Scheme Based on the Many-Body Fermion Density Matrix
In [S. A. Cheong and C. L. Henley, cond-mat/0206196 (2002)], we found that
the many-particle eigenvalues and eigenstates of the many-body density matrix
of a block of sites cut out from an infinite chain of
noninteracting spinless fermions can all be constructed out of the one-particle
eigenvalues and one-particle eigenstates respectively. In this paper we
developed a statistical-mechanical analogy between the density matrix
eigenstates and the many-body states of a system of noninteracting fermions.
Each density matrix eigenstate corresponds to a particular set of occupation of
single-particle pseudo-energy levels, and the density matrix eigenstate with
the largest weight, having the structure of a Fermi sea ground state,
unambiguously defines a pseudo-Fermi level. We then outlined the main ideas
behind an operator-based truncation of the density matrix eigenstates, where
single-particle pseudo-energy levels far away from the pseudo-Fermi level are
removed as degrees of freedom. We report numerical evidence for scaling
behaviours in the single-particle pseudo-energy spectrum for different block
sizes and different filling fractions \nbar. With the aid of these
scaling relations, which tells us that the block size plays the role of an
inverse temperature in the statistical-mechanical description of the density
matrix eigenstates and eigenvalues, we looked into the performance of our
operator-based truncation scheme in minimizing the discarded density matrix
weight and the error in calculating the dispersion relation for elementary
excitations. This performance was compared against that of the traditional
density matrix-based truncation scheme, as well as against a operator-based
plane wave truncation scheme, and found to be very satisfactory.Comment: 22 pages in RevTeX4 format, 22 figures. Uses amsmath, amssymb,
graphicx and mathrsfs package
Quest for a Nuclear Georeactor
Knowledge about the interior of our planet is mainly based on the
interpretation of seismic data from earthquakes and nuclear explosions, and of
composition of meteorites. Additional observations have led to a wide range of
hypotheses on the heat flow from the interior to the crust, the abundance of
certain noble gases in gasses vented from volcanoes and the possibility of a
nuclear georeactor at the centre of the Earth. This paper focuses on a proposal
for an underground laboratory to further develop antineutrinos as a tool to map
the distribution of radiogenic heat sources, such as the natural radionuclides
and the hypothetical nuclear georeactor.Comment: Invited talk presented at the International Symposium on Radiation
Physics, Cape Town, 2003. Manuscript is submitted to Radiation Physics and
Chemistr
One Dimensional Chain with Long Range Hopping
The one-dimensional (1D) tight binding model with random nearest neighbor
hopping is known to have a singularity of the density of states and of the
localization length at the band center. We study numerically the effects of
random long range (power-law) hopping with an ensemble averaged magnitude
\expectation{|t_{ij}|} \propto |i-j|^{-\sigma} in the 1D chain, while
maintaining the particle-hole symmetry present in the nearest neighbor model.
We find, in agreement with results of position space renormalization group
techniques applied to the random XY spin chain with power-law interactions,
that there is a change of behavior when the power-law exponent becomes
smaller than 2
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