251,899 research outputs found
Effect of geometric and electronic structures on the finite temperature behavior of Na, Na, and Na clusters
An analysis of the evolutionary trends in the ground state geometries of
Na to Na reveals Na, an electronic closed--shell system,
shows namely an electronically driven spherical shape leading to a disordered
but compact structure. This structural change induces a strong {\it
connectivity} of short bonds among the surface atoms as well as between core
and surface atoms with inhomogeneous strength in the ground state geometry,
which affects its finite--temperature behavior. By employing {\it ab initio}
density--functional molecular dynamics, we show that this leads to two distinct
features in specific heat curve compared to that of Na: (1) The peak is
shifted by about 100 K higher in temperature. (2) The transition region becomes
much broader than Na. The inhomogeneous distribution of bond strengths
results in a broad melting transition and the strongly connected network of
short bonds leads to the highest melting temperature of 375 K reported among
the sodium clusters. Na, which has one electron less than Na,
also possesses stronger short--bond network compared with Na, resulting
in higher melting temperature (350 K) than observed in Na. Thus, we
conclude that when a cluster has nearly closed shell structure not only
geometrically but also electronically, it show a high melting temperature. Our
calculations clearly bring out the size--sensitive nature of the specific heat
curve in sodium clusters.Comment: 7 pages, 11 figure
First principles investigation of finite-temperature behavior in small sodium clusters
A systematic and detailed investigation of the finite-temperature behavior of
small sodium clusters, Na_n, in the size range of n= 8 to 50 are carried out.
The simulations are performed using density-functional molecular-dynamics with
ultrasoft pseudopotentials. A number of thermodynamic indicators such as
specific heat, caloric curve, root-mean-square bond length fluctuation,
deviation energy, etc. are calculated for each of the clusters. Size dependence
of these indicators reveals several interesting features. The smallest clusters
with n= 8 and 10, do not show any signature of melting transition. With the
increase in size, broad peak in the specific heat is developed, which
alternately for larger clusters evolves into a sharper one, indicating a
solidlike to liquidlike transition. The melting temperatures show irregular
pattern similar to experimentally observed one for larger clusters [ M. Schmidt
et al., Nature (London) 393, 238 (1998) ]. The present calculations also reveal
a remarkable size-sensitive effect in the size range of n= 40 to 55. While
Na_40 and Na_55 show well developed peaks in the specific heat curve, Na_50
cluster exhibits a rather broad peak, indicating a poorly-defined melting
transition. Such a feature has been experimentally observed for gallium and
aluminum clusters [ G. A. Breaux et al., J. Am. Chem. Soc. 126, 8628 (2004); G.
A.Breaux et al., Phys. Rev. Lett. 94, 173401 (2005) ].Comment: 8 pages, 11 figure
Corrections to the energy levels of a spin-zero particle bound in a strong field
Formulas for the corrections to the energy levels and wave functions of a
spin-zero particle bound in a strong field are derived. General case of the sum
of a Lorentz-scalar potential and zero component of a Lorentz-vector potential
is considered. The forms of the corrections differ essentially from those for
spin-1/2 particles. As an example of application of our results, we evaluated
the electric polarizability of a ground state of a spin-zero particle bound in
a strong Coulomb field.Comment: 7 pages, 1 figur
New insights into the supression of plant pathogenic fungus (Phytophthora cinnamomi) by compost leachates
Use of compost as a soil conditioner and low-grade fertiliser is gaining popularity worldwide (Epstein, 1997). Compost not only adds plant nutrients to the soil, but also improves physical properties of soil such as buffering capacity, cation exchange capacity and water holding capacity. In addition to these benefits, compost can also suppress plant diseases caused by Phytophthora cinnamomi (Hoitink et al., 1977), Pythium aphanidermatum (Mandelbaum and Hadar, 1990), Rhizoctonia solani and Sclerotium rolfoii (Gorodecki and Hadar, 1990).
Irwin et al., (1995) reported that the diseases caused by P. cinnamomi are directly responsible for considerable economic losses in many horticultural, ornamental and forestry industries throughout Australia. Phytophthora spp. continue to be the focus of attention of many researchers due to the diversity of P. cinnamomi-host interactions and their potential economic impact on a wide range of industries.
The practise of using methyl bromide and other chemicals for disinfection of soil is widespread (Trill as et al., 2002). However, the use of methyl bromide and other chemicals is phased out in the USA and Europe. The suppression of soil-borne plant fungus by composts produced from tree barks (Spencer et al., 1982) and municipal solid wastes is well documented (Trill as et al., 2002).
Composts that suppress plant disease have been extensively described and are used in greenhouse production systems (Lazarovitis et aI, 2001). However, most studies have focused on compo sting different types of materials and their effect on fungal pathogens inhibition rather than compo sting conditions that may produce suppressive composts. An objective of this study was to investigate the role of moisture, aeration and compost maturity in enhancing the inhibition effect of compost on the plant pathogen P. cinnamomi. A further objective was to generate an increased understanding of the mechanism of growth inhibition
A semantic web approach for built heritage representation
In a built heritage process, meant as a structured system of activities
aimed at the investigation, preservation, and management of architectural
heritage, any task accomplished by the several actors involved in it is deeply
influenced by the way the knowledge is represented and shared. In the current
heritage practice, knowledge representation and management have shown several
limitations due to the difficulty of dealing with large amount of extremely heterogeneous
data. On this basis, this research aims at extending semantic web
approaches and technologies to architectural heritage knowledge management in
order to provide an integrated and multidisciplinary representation of the artifact
and of the knowledge necessary to support any decision or any intervention and
management activity. To this purpose, an ontology-based system, representing
the knowledge related to the artifact and its contexts, has been developed through
the formalization of domain-specific entities and relationships between them
Computational structures for robotic computations
The computational problem of inverse kinematics and inverse dynamics of robot manipulators by taking advantage of parallelism and pipelining architectures is discussed. For the computation of inverse kinematic position solution, a maximum pipelined CORDIC architecture has been designed based on a functional decomposition of the closed-form joint equations. For the inverse dynamics computation, an efficient p-fold parallel algorithm to overcome the recurrence problem of the Newton-Euler equations of motion to achieve the time lower bound of O(log sub 2 n) has also been developed
Standardized field testing of assistant robots in a Mars-like environment
Controlled testing on standard tasks and within standard environments can provide meaningful performance comparisons between robots of heterogeneous design. But because they must perform practical tasks in unstructured, and therefore non-standard, environments, the benefits of this approach have barely begun to accrue for field robots. This work describes a desert trial of six student prototypes of astronaut-support robots using a set of standardized engineering tests developed by the US National Institute of Standards and Technology (NIST), along with three operational tests in natural Mars-like terrain. The results suggest that standards developed for emergency response robots are also applicable to the astronaut support domain, yielding useful insights into the differences in capabilities between robots and real design improvements. The exercise shows the value of combining repeatable engineering tests with task-specific application-testing in the field
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