1,785 research outputs found
Freezing in random graph ferromagnets
Using T=0 Monte Carlo and simulated annealing simulation, we study the energy
relaxation of ferromagnetic Ising and Potts models on random graphs. In
addition to the expected exponential decay to a zero energy ground state, a
range of connectivities for which there is power law relaxation and freezing to
a metastable state is found. For some connectivities this freezing persists
even using simulated annealing to find the ground state. The freezing is caused
by dynamic frustration in the graphs, and is a feature of the local
search-nature of the Monte Carlo dynamics used. The implications of the
freezing on agent-based complex systems models are briefly considered.Comment: Published version: 1 reference deleted, 1 word added. 4 pages, 5
figure
Improvement of Indoor Air Quality by MDF panels containing walnut shells
High levels of Volatile Organic Compounds (VOCs) and extremes of Relative Humidity (RH) commonly reduce indoor air quality with associated negative effects on human health and wellbeing. Interior materials are known to be one of the main contributors to poor indoor air quality. Notwithstanding, they can also act as a sink for airborne pollutants and excess moisture through adsorption. In this paper, we evaluate the ability of Medium Density Fibreboard (MDF) modified with walnut shell to regulate RH, toluene, limonene, dodecane and formaldehyde. The physicochemical properties, including molecular size/shape, vapour pressure, polarity and boiling point of VOCs allowed them to represent a range of pollutants. Adsorption and desorption behaviour of MDF containing up to 15% walnut shell was evaluated in 2-L environmental chambers under dynamic conditions at 23 °C and 50% RH. The porous microstructure of the MDF and walnut shell and their chemical composition were analysed using SEM, XRD and FTIR. Compared to a control panel, walnut shell additions showed an improved ability to remove VOCs and formaldehyde from the indoor air and buffer humidity. Of particular significance was the irreversible sink effect of formaldehyde and dodecane. This was attributed to the porous surface of walnut shell increasing the specific surface area of the panel and thus its adsorption capacity. The improved capacity of buffer humidity also increased the ability to adsorb water soluble VOCs such as formaldehyde. This research provides for the first time significant evidence that walnut shell modified MDF can improve indoor air quality
Static Cylindrical Matter Shells
Static cylindrical shells composed of massive particles arising from matching
of two different Levi-Civita space-times are studied for the shell satisfying
either isotropic or anisotropic equation of state. We find that these solutions
satisfy the energy conditions for certain ranges of the parameters.Comment: 9 pages, 3 figures, Latex; Final version, To appear in General
Relativity and Gravitatio
Glauber dynamics in a single-chain magnet: From theory to real systems
The Glauber dynamics is studied in a single-chain magnet. As predicted, a
single relaxation mode of the magnetization is found. Above 2.7 K, the
thermally activated relaxation time is mainly governed by the effect of
magnetic correlations and the energy barrier experienced by each magnetic unit.
This result is in perfect agreement with independent thermodynamical
measurements. Below 2.7 K, a crossover towards a relaxation regime is observed
that is interpreted as the manifestation of finite-size effects. The
temperature dependences of the relaxation time and of the magnetic
susceptibility reveal the importance of the boundary conditions.Comment: Submitted to PRL 10 May 2003. Submitted to PRB 12 December 2003;
published 15 April 200
The low energy limit of the non-commutative Wess-Zumino model
The non-commutative Wess-Zumino model is used as a prototype for studying the
low energy behaviour of a renormalizable non-commutative field theory. We start
by deriving the potential mediating the fermion-fermion and boson-boson
interactions in the non-relativistic regime. The quantum counterparts of these
potentials are afflicted by irdering ambiguities but we show that there exists
an ordering prescription which makes them hermitean. For space/space
noncommutativity it turns out that Majorana fermions may be pictured as rods
oriented perpendicularly to the direction of motion showing a lack of
localituy, while bosons remain insensitive to the effects of noncommutativity.
For time/space noncommutativity bosopns and fermions can be regarded as rods
oriented along the direction of motion. For both cases of noncommutativity the
scattering state described scattered waves, with at least one wave having
negative time delay signalizing the underlying nonlocality. The superfield
formulation of the model is used to compute the corresponding effective action
in the one- and two-loop approximations. In the case of time/space
noncommutativity, unitarity is violated in the relativistic regime. However,
this does not preclude the existence of the unitary low energy limit.Comment: 14 pages, 2 figures, minor correction
Decomposition and nutrient release of leguminous plants in coffee agroforestry systems.
Leguminous plants used as green manure are an important nutrient source for coffee plantations, especially for soils with low nutrient levels. Field experiments were conducted in the Zona da Mata of Minas Gerais State, Brazil to evaluate the decomposition and nutrient release rates of four leguminous species used as green manures (Arachis pintoi, Calopogonium mucunoides, Stizolobium aterrimum and
Stylosanthes guianensis) in a coffee agroforestry system under two different climate conditions. The initial N contents in plant residues varied from 25.7 to 37.0 g kg-1 and P from 2.4 to 3.0 g kg-1. The lignin/N, lignin/polyphenol and(lignin+polyphenol)/N ratios were low in all residues studied. Mass loss rates were highest in the first 15 days, when 25 % of the residues were decomposed. From 15 to 30 days, the decomposition rate decreased on both farms. On the farm in Pedra Dourada (PD), the decomposition constant k increased in the order C. mucunoides < S. aterrimum < S. guianensis < A. pintoi. On the farm in Araponga (ARA), there was no difference in the decomposition rate among leguminous plants. The N release rates varied from 0.0036 to 0.0096 d-1. Around 32 % of the total N content in the plant material was released in the first 15 days. In ARA, the N concentration in the S. aterrimum residues was always significantly higher than in the other residues. At the end of 360 days, the N released was 78 % in ARA and 89 % in PD of the initial content. Phosphorus was the most rapidly released nutrient (k values from 0.0165 to 0.0394 d-1). Residue decomposition and nutrient release did not correlate with initial residue chemistry and biochemistry, but differences in climatic conditions between the two study sites modified the decomposition rate constants
Measurement of single electron emission in two-phase xenon
We present the first measurements of the electroluminescence response to the
emission of single electrons in a two-phase noble gas detector. Single
ionization electrons generated in liquid xenon are detected in a thin gas layer
during the 31-day background run of the ZEPLIN-II experiment, a two-phase xenon
detector for WIMP dark matter searches. Both the pressure dependence and
magnitude of the single-electron response are in agreement with previous
measurements of electroluminescence yield in xenon. We discuss different
photoionization processes as possible cause for the sample of single electrons
studied in this work. This observation may have implications for the design and
operation of future large-scale two-phase systems.Comment: 11 pages, 6 figure
Green manure in coffee systems in the region of Zona da Mata, Minas Gerais: characteristics and kinetics of carbon and nitrogen mineralization.
The use of green manure may contribute to reduce soil erosion and increase the soil organic matter content and N availability in coffee plantations in the Zona da Mata, State of Minas Gerais, in Southeastern Brazil. The potential of four legumes (A. pintoi, C. mucunoides, S. aterrimum and S. guianensis)to produce above-ground
biomass, accumulate nutrients and mineralize N was studied in two coffee plantations of subsistence farmers under different climate conditions. The biomass production of C. mucunoides was influenced by the shade of the coffee plantation.C. mucunoides tended to mineralize more N than the other legumes due to the low polyphenol content and polyphenol/N ratio. In the first year, the crop establishment of A. pintoi in the area took longer than of the other legumes, resulting in lower biomass production and N2 fixation. In the long term, cellulose was the main
factor controlling N mineralization. The biochemical characteristics, nutrient accumulation and biomass production of the legumes were greatly influenced by
the altitude and position of the area relative to the sun
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