3,388 research outputs found
On the problem of an electron scattering in an arbitrary one-dimensional potential field
Recurrent representations for an electron transmission and reflection
amplitudes for a one-dimensional chain are obtained. The linear differential
equations for scattering amplitudes of an arbitrary potential are found.Comment: 6 pages, no figs. To be submitted to Phys. Lett.
Autonomous agile teams: Challenges and future directions for research
According to the principles articulated in the agile manifesto, motivated and
empowered software developers relying on technical excellence and simple
designs, create business value by delivering working software to users at
regular short intervals. These principles have spawned many practices. At the
core of these practices is the idea of autonomous, self-managing, or
self-organizing teams whose members work at a pace that sustains their
creativity and productivity. This article summarizes the main challenges faced
when implementing autonomous teams and the topics and research questions that
future research should address
Evolution of virulence: triggering host inflammation allows invading pathogens to exclude competitors.
Virulence is generally considered to benefit parasites by enhancing resource-transfer from host to pathogen. Here, we offer an alternative framework where virulent immune-provoking behaviours and enhanced immune resistance are joint tactics of invading pathogens to eliminate resident competitors (transferring resources from resident to invading pathogen). The pathogen wins by creating a novel immunological challenge to which it is already adapted. We analyse a general ecological model of 'proactive invasion' where invaders not adapted to a local environment can succeed by changing it to one where they are better adapted than residents. However, the two-trait nature of the 'proactive' strategy (provocation of, and adaptation to environmental change) presents an evolutionary conundrum, as neither trait alone is favoured in a homogenous host population. We show that this conundrum can be resolved by allowing for host heterogeneity. We relate our model to emerging empirical findings on immunological mediation of parasite competition
Proof for an upper bound in fixed-node Monte Carlo for lattice fermions
We justify a recently proposed prescription for performing Green Function
Monte Carlo calculations on systems of lattice fermions, by which one is able
to avoid the sign problem. We generalize the prescription such that it can also
be used for problems with hopping terms of different signs. We prove that the
effective Hamiltonian, used in this method, leads to an upper bound for the
ground-state energy of the real Hamiltonian, and we illustrate the
effectiveness of the method on small systems.Comment: 14 pages in revtex v3.0, no figure
Interannual variability of Alexandrium fundyense abundance and shellfish toxicity in the Gulf of Maine
Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B. V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 52 (2005): 2843-2855, doi:10.1016/j.dsr2.2005.06.020.Six years of oceanographic surveys of Alexandrium fundyense concentrations in the Gulf of Maine are combined with shellfish toxicity records from coastal monitoring stations to assess covariations of these quantities on seasonal to interannual time scales. Annual mean gulf-wide cell abundance varies by less than one order of magnitude during the time interval examined (1993-2002). Fluctuations in gulf-wide annual mean cell abundance and shellfish toxicity are not related in a consistent manner. This suggests that interannual variations in toxicity may be regulated by transport and delivery of offshore cell populations, rather than the absolute abundance of the source populations themselves.We gratefully acknowledge the support of the US ECOHAB Program, sponsored by NOAA, NSF, EPA, NASA, and ONR
Superconductivity in the two dimensional Hubbard Model.
Quasiparticle bands of the two-dimensional Hubbard model are calculated using
the Roth two-pole approximation to the one particle Green's function. Excellent
agreement is obtained with recent Monte Carlo calculations, including an
anomalous volume of the Fermi surface near half-filling, which can possibly be
explained in terms of a breakdown of Fermi liquid theory. The calculated bands
are very flat around the (pi,0) points of the Brillouin zone in agreement with
photoemission measurements of cuprate superconductors. With doping there is a
shift in spectral weight from the upper band to the lower band. The Roth method
is extended to deal with superconductivity within a four-pole approximation
allowing electron-hole mixing. It is shown that triplet p-wave pairing never
occurs. Singlet d_{x^2-y^2}-wave pairing is strongly favoured and optimal
doping occurs when the van Hove singularity, corresponding to the flat band
part, lies at the Fermi level. Nearest neighbour antiferromagnetic correlations
play an important role in flattening the bands near the Fermi level and in
favouring superconductivity. However the mechanism for superconductivity is a
local one, in contrast to spin fluctuation exchange models. For reasonable
values of the hopping parameter the transition temperature T_c is in the range
10-100K. The optimum doping delta_c lies between 0.14 and 0.25, depending on
the ratio U/t. The gap equation has a BCS-like form and (2*Delta_{max})/(kT_c)
~ 4.Comment: REVTeX, 35 pages, including 19 PostScript figures numbered 1a to 11.
Uses epsf.sty (included). Everything in uuencoded gz-compressed .tar file,
(self-unpacking, see header). Submitted to Phys. Rev. B (24-2-95
Gyroid cuticular structures in butterfly wing scales: biological photonic crystals
We present a systematic study of the cuticular structure in the butterfly wing scales of some papilionids (Parides sesostris and Teinopalpus imperialis) and lycaenids (Callophrys rubi, Cyanophrys remus, Mitoura gryneus and Callophrys dumetorum). Using published scanning and transmission electron microscopy (TEM) images, analytical modelling and computer-generated TEM micrographs, we find that the three-dimensional cuticular structures can be modelled by gyroid structures with various filling fractions and lattice parameters. We give a brief discussion of the formation of cubic gyroid membranes from the smooth endoplasmic reticulum in the scale's cell, which dry and harden to leave the cuticular structure behind when the cell dies. The scales of C. rubi are a potentially attractive biotemplate for producing three-dimensional optical photonic crystals since for these scales the cuticle-filling fraction is nearly optimal for obtaining the largest photonic band gap in a gyroid structure
The fine-tuning cost of the likelihood in SUSY models
In SUSY models, the fine tuning of the electroweak (EW) scale with respect to
their parameters gamma_i={m_0, m_{1/2}, mu_0, A_0, B_0,...} and the maximal
likelihood L to fit the experimental data are usually regarded as two different
problems. We show that, if one regards the EW minimum conditions as constraints
that fix the EW scale, this commonly held view is not correct and that the
likelihood contains all the information about fine-tuning. In this case we show
that the corrected likelihood is equal to the ratio L/Delta of the usual
likelihood L and the traditional fine tuning measure Delta of the EW scale. A
similar result is obtained for the integrated likelihood over the set
{gamma_i}, that can be written as a surface integral of the ratio L/Delta, with
the surface in gamma_i space determined by the EW minimum constraints. As a
result, a large likelihood actually demands a large ratio L/Delta or
equivalently, a small chi^2_{new}=chi^2_{old}+2*ln(Delta). This shows the
fine-tuning cost to the likelihood (chi^2_{new}) of the EW scale stability
enforced by SUSY, that is ignored in data fits. A good
chi^2_{new}/d.o.f.\approx 1 thus demands SUSY models have a fine tuning amount
Delta<<exp(d.o.f./2), which provides a model-independent criterion for
acceptable fine-tuning. If this criterion is not met, one can thus rule out
SUSY models without a further chi^2/d.o.f. analysis. Numerical methods to fit
the data can easily be adapted to account for this effect.Comment: 10 pages (v3: small comment added
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