4,713 research outputs found
Dynamics of clade diversification on the morphological hypercube
Understanding the relationship between taxonomic and morphological changes is
important in identifying the reasons for accelerated morphological
diversification early in the history of animal phyla. Here, a simple general
model describing the joint dynamics of taxonomic diversity and morphological
disparity is presented and applied to the data on the diversification of
blastozoans. I show that the observed patterns of deceleration in clade
diversification can be explicable in terms of the geometric structure of the
morphospace and the effects of extinction and speciation on morphological
disparity without invoking major declines in the size of morphological
transitions or taxonomic turnover rates. The model allows testing of hypotheses
about patterns of diversification and estimation of rates of morphological
evolution. In the case of blastozoans, I find no evidence that major changes in
evolutionary rates and mechanisms are responsible for the deceleration of
morphological diversification seen during the period of this clade's expansion.
At the same time, there is evidence for a moderate decline in overall rates of
morphological diversification concordant with a major change (from positive to
negative values) in the clade's growth rate.Comment: 8 pages, Latex, 2 postscript figures, submitted to Proc.R.Soc.Lond.
Neutron diffraction in a model itinerant metal near a quantum critical point
Neutron diffraction measurements on single crystals of Cr1-xVx (x=0, 0.02,
0.037) show that the ordering moment and the Neel temperature are continuously
suppressed as x approaches 0.037, a proposed Quantum Critical Point (QCP). The
wave vector Q of the spin density wave (SDW) becomes more incommensurate as x
increases in accordance with the two band model. At xc=0.037 we have found
temperature dependent, resolution limited elastic scattering at 4
incommensurate wave vectors Q=(1+/-delta_1,2, 0, 0)*2pi/a, which correspond to
2 SDWs with Neel temperatures of 19 K and 300 K. Our neutron diffraction
measurements indicate that the electronic structure of Cr is robust, and that
tuning Cr to its QCP results not in the suppression of antiferromagnetism, but
instead enables new spin ordering due to novel nesting of the Fermi surface of
Cr.Comment: Submitted as a part of proceedings of LT25 (Amsterdam 2008
Pengukuran Tingkat Kebisingan di Jalan Raya Pekanbaru-bangkinang Sebelum di Bangun Bebas Hambatan (Highway)
Measurement of the noise level in the roadway of Pekanbaru-Bangkinang before building of Highway has been done. The sources of noise originated from the sound of vehicles that passed through the roadway. The level of noise has been measured by using a Sound Level Meter (SLM). The measurement of noise level carried out in two locations, first location was on roadway of Pekanbaru-Bangkinang KM 21 and the second location was at round aboard of Jalan Garuda Sakti and Jalan PekanbaruBangkinang. The noise level at first location was measured as a function of time from 07.00 until 18.00 pm during one week. In this research, the number of vehicles that passed through the first location have been counted every hour. Furthermore, the noise level was measured as a function of distance from the noise source using two SLMs, namely SLM type MS 6701 and SL 4112. The results showed that the noise level at the first location occurred on Saturday and Sunday at 16.00 to 18.00 with the highest value of 79.09 dBA and 78.28 dBA respectively, while the lowest noise value occurred on Friday at 12.00 to 13.00 pm with the value of 67.97 dBA. The highest noise level at the second location occurred at 17.00 to 18.00 with a value of 81.60 dBA. The value of noise level at the second location was higher compared to that of the first location. This was due to more number of vehicles that passed through that location compared to that of the first location. The profile of noise level occurred on Saturday was compared to that of the theoretical calculations. The result showed that both results had a similar characteristics. Moreover, the results of this research showed that the level of noise as a function of distance from the noise source was decrease with increasing the distance. The decrease of the noise level was caused by the absorption of the noise by the surrounding parameters such as air,vegetation and buildings nearby
Comment on "Self-Purification in Semiconductor Nanocrystals"
In a recent Letter [PRL 96, 226802 (2006)], Dalpian and Chelikowsky claimed
that formation energies of Mn impurities in CdSe nanocrystals increase as the
size of the nanocrystal decreases, and argued that this size dependence leads
to "self-purification" of small nanocrystals. They presented
density-functional-theory (DFT) calculations showing a strong size dependence
for Mn impurity formation energies, and proposed a general explanation. In this
Comment we show that several different DFT codes, pseudopotentials, and
exchange-correlation functionals give a markedly different result: We find no
such size dependence. More generally, we argue that formation energies are not
relevant to substitutional doping in most colloidally grown nanocrystals.Comment: 1 page, 1 figur
Overdamped Stress Relaxation in Buckled Rods
We present a comprehensive theoretical analysis of the stress relaxation in a
multiply but weakly buckled incompressible rod in a viscous solvent. In the
bulk two interesting regimes of generic self--similar intermediate asymptotics
are distinguished, which give rise to two classes of approximate and exact
power--law solutions, respectively. For the case of open boundary conditions
the corresponding non--trivial boundary--layer scenarios are derived by a
multiple--scale perturbation (``adiabatic'') method. Our results compare well
with -- and provide the theoretical explanation for -- previous results from
numerical simulations, and they suggest new directions for further fruitful
numerical and experimental investigations.Comment: 20 pages, 12 figure
Micro-optic lens for data storage
A new type of microlens for data storage applications that has improved off-axis performance is described. The lens consists of a micro Fresnel pattern on a curved substrate. The radius of the substrate is equal to the focal length of the lens. If the pattern and substrate are thin, the combination satisfies the Abbe sine condition. Therefore, the lens is free of coma. We analyze a 0.5 numerical aperture, 0.50 mm focal length lens in detail. A 0.16 numerical aperture lens was fabricated holographically, and results are presented
Important role of alkali atoms in A4C60
We show that hopping via the alkali atoms plays an important role for the t1u
band of A4C60 (A=K, Rb), in strong contrast to A3C60. Thus the t1u band is
broadened by more than 40 % by the presence of the alkali atoms. The difference
between A4C60 and A3C60 is in particular due to the less symmetric location of
the alkali atoms in A4C60.Comment: 5 pages, revtex, 2 figures, submitted to Phys. Rev. B more
information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
M-grid: Using Ubiquitous Web Technologies to create a Computational Grid
There are many potential users and uses for grid computing. However, the concept of sharing computing resources excites security concerns and, whilst being powerful and flexible, at least for novices, existing systems are complex to install and use. Together these represent a significant barrier to potential users who are interested to see what grid computing can do. This paper describes m-grid, a system for building a computational grid which can accept tasks from any user with access to a web browser and distribute them to almost any machine with access to the internet and manages to do this without the installation of additional software or interfering with existing security arrangements
Stiff Polymers, Foams and Fiber Networks
We study the elasticity of fibrous materials composed of generalized stiff
polymers. It is shown that in contrast to cellular foam-like structures affine
strain fields are generically unstable. Instead, a subtle interplay between the
architecture of the network and the elastic properties of its building blocks
leads to intriguing mechanical properties with intermediate asymptotic scaling
regimes. We present exhaustive numerical studies based on a finite element
method complemented by scaling arguments.Comment: 4 pages, 5 figure
- …