The kinetics of homogeneous melting beyond the limit of superheating

Molecular dynamics simulation is used to study the time-scales involved in the homogeneous melting of a superheated crystal. The interaction model used is an embedded-atom model for Fe developed in previous work, and the melting process is simulated in the microcanonical $(N, V, E)$ ensemble. We study periodically repeated systems containing from 96 to 7776 atoms, and the initial system is always the perfect crystal without free surfaces or other defects. For each chosen total energy $E$ and number of atoms $N$, we perform several hundred statistically independent simulations, with each simulation lasting for between 500 ps and 10 ns, in order to gather statistics for the waiting time $\tau_{\rm w}$ before melting occurs. We find that the probability distribution of $\tau_{\rm w}$ is roughly exponential, and that the mean value $<\tau_{\rm w} >$ depends strongly on the excess of the initial steady temperature of the crystal above the superheating limit identified by other researchers. The mean $$ also depends strongly on system size in a way that we have quantified. For very small systems of $\sim 100$ atoms, we observe a persistent alternation between the solid and liquid states, and we explain why this happens. Our results allow us to draw conclusions about the reliability of the recently proposed Z method for determining the melting properties of simulated materials, and to suggest ways of correcting for the errors of the method.Comment: 19 pages, 8 figure

A comparison between detailed and configuration-averaged collisional-radiative codes applied to non-local thermal equilibrium plasma

A collisional-radiative model describing nonlocal-thermodynamic-equilibrium plasmas is developed. It is based on the HULLAC (Hebrew University Lawrence Livermore Atomic Code) suite for the transitions rates, in the zero-temperature radiation field hypothesis. Two variants of the model are presented: the first one is configuration averaged, while the second one is a detailed level version. Comparisons are made between them in the case of a carbon plasma; they show that the configuration-averaged code gives correct results for an electronic temperature Te=10 eV (or higher) but fails at lower temperatures such as Te=1 eV. The validity of the configuration-averaged approximation is discussed: the intuitive criterion requiring that the average configuration-energy dispersion must be less than the electron thermal energy turns out to be a necessary but far from sufficient condition. Another condition based on the resolution of a modified rate-equation system is proposed. Its efficiency is emphasized in the case of low-temperature plasmas. Finally, it is shown that near-threshold autoionization cascade processes may induce a severe failure of the configuration-average formalism.Comment: 9

DNA adducts in peripheral blood lymphocytes from aluminum production plant workers determined by 32P-postlabeling and enzyme-linked immunosorbent assay

32P-Postlabeling analysis and enzyme-linked immunosorbent assay (ELISA) have been used to detect DNA adducts in peripheral blood lymphocytes from primary aluminum production plant workers who were exposed occupationally to a mixture of polycyclic aromatic hydrocarbons (PAHs). Preliminary results reported here are from a comparative study being performed in two aluminum plants. The levels of aromatic DNA adducts have been determined by the 32P-postlabeling assay in samples collected on two occasions, 1 year apart. PAH-DNA adduct levels have also been determined by competitive ELISA in the second set of DNA samples. The results show the necessity of follow-up biomonitoring studies to detect possible alterations in biological effect induced by changing exposures. The comparison of the results obtained by 32P-postlabeling and ELISA may lead to a better understanding of the power and weaknesses of the two methods applied in these studies

Investigating the roles of phonological and semantic memory in sentence recall

The mechanisms underlying short-term sentence recall have been the subject of recent investigations. While both semantic and phonological information have been found to play a role in sentence recall, this has been established using different paradigms in previous research. As a result, it is not clear whether the contributions of semantic and phonological information are equivalent subject only to specific experimental conditions. The present study used a common paradigm with equally plausible lures to systematically compare the roles of semantic and phonological information in short-term sentence recall. The lure intrusion paradigm was used with three different lures that were all equally plausible in the context of the sentence: semantic, onset, and rhyme. Further, no contextual cues were provided in the sentence to bias the participant. The findings indicate that there were significantly more semantic and onset intrusions compared to rhyme intrusions. This is interpreted in light of models incorporating lexical information during sentence production

Quantum Hall resistance standards from graphene grown by chemical vapor deposition on silicon carbide

Replacing GaAs by graphene to realize more practical quantum Hall resistance standards (QHRS), accurate to within $10^{-9}$ in relative value, but operating at lower magnetic fields than 10 T, is an ongoing goal in metrology. To date, the required accuracy has been reported, only few times, in graphene grown on SiC by sublimation of Si, under higher magnetic fields. Here, we report on a device made of graphene grown by chemical vapour deposition on SiC which demonstrates such accuracies of the Hall resistance from 10 T up to 19 T at 1.4 K. This is explained by a quantum Hall effect with low dissipation, resulting from strongly localized bulk states at the magnetic length scale, over a wide magnetic field range. Our results show that graphene-based QHRS can replace their GaAs counterparts by operating in as-convenient cryomagnetic conditions, but over an extended magnetic field range. They rely on a promising hybrid and scalable growth method and a fabrication process achieving low-electron density devices.Comment: 12 pages, 8 figure

Évaluation de la capacité de rétention des métaux dissous d'un marais artificiel en utilisant le périphyton et le gastéropode Helisoma trivolvis

Le but de cette étude est de déterminer si le périphyton et le gastéropode pulmoné Helisoma trivolvis peuvent être utilisés pour évaluer la capacité de rétention des métaux dissous d'un marais construit. Cette étude a été menée dans un marais de la région d'Ottawa-Carleton (Ontario, Canada), qui a été construit en 1995 afin d'améliorer la qualité des eaux de ruissellement provenant d'un bassin versant à usage résidentiel et agricole. Au cours du mois de septembre 1999, des échantillons d'eau ont été prélevés, des escargots (H. trivolvis) ont été recoltés et un substrat artificiel a été utilisé pour faire croître du périphyton, à l'entrée et à la sortie d'eau du marais. En moyenne, les calculs de balance de masse indiquaient une rétention des formes dissoutes du Cu, Mn, Ni et Zn dans le marais. Cependant, les tendances observées pour les concentrations de métaux dissous et pour les concentrations dans les organismes différaient pour certains métaux. Les concentrations de Cd et Ni dans les tissus de H. trivolvis et le périphyton étaient significativement plus élevées à l'entrée qu'à la sortie. Cependant, les concentrations de Cr et Al dans les organismes n'étaient pas significativement différentes entre l'entrée et la sortie alors que celle de Mn était significativement plus élevée à la sortie du marais. Pour tous les métaux sauf le Cd et le Zn, les concentrations dans le périphyton étaient en moyenne plus elevées que celles dans les escargots. Le périphyton peut donc fournir une mesure plus conservatrice de la contamination du milieu pas les métaux. Cette étude montre l'importance de considérer, non-seulement les mesures chimiques, mais aussi les mesures biologiques, dans l'évaluation de l'efficacité d'un ouvrage de contrôle de pollution.Urban and agricultural development has had a significant impact on the water quality of rivers and lakes around the world. In the last few decades, constructed wetlands have been designed as wastewater treatment systems to prevent water quality deterioration in natural receiving waters. Constructed wetlands are built because they are considered sinks for many pollutants thereby protecting the water quality of downstream ecosystems. The treatment performance of these wetlands is generally assessed using mass balance calculations. However, the retention of metals by constructed wetlands is highly variable and the factors involved are still poorly understood. If wetlands are sinks for metals, the metal content of organisms should be lower downstream than upstream. In this context, organisms can be useful to assess the retention or transformation of metals by wetlands.The objective of this study was to determine whether periphyton and the gastropod Helisoma trivolvis could be used to evaluate the retention of dissolved metals in a constructed wetland. H. trivolvis is a freshwater pulmonate snail widespread in ponds across North America. It feeds mostly on periphyton and is more or less sedentary. Snails have been used as biomonitors because several species are metal tolerant. However, compared to snails, periphytic microorganisms may track more closely dissolved metal concentrations as they take up metals principally from the water column.This study was conducted at the Monahan Pond in Kanata, Ontario (Canada). This wetland was built in 1995 to treat run-off from an agricultural and residential watershed. Water chemistry samples, snails and periphyton grown on artificial substrata were collected at both the inlet and the outlet of the wetland in the fall of 1999. Tissue samples were digested with concentrated nitric acid and metal analyses were done by ICP-MS (Inductively Coupled Plasma Mass Spectrometry). During the experiment several chemical parameters differed between the inlet and the outlet. Alkalinity was significantly higher at the inflow and all major cation concentrations were higher at the inflow. The temperature was on average 3 ºC higher at the outlet. Mass balance calculations showed that the wetland was a sink for most dissolved metals. Snails and periphyton tissue metal concentrations were higher at the inflow than at the outlet for Cd and Ni. However, no significant differences were observed between inflow and outlet tissue concentrations for Cr and Al, whereas Mn was actually significantly higher at the outlet. As a result, the metal content of the organisms did not consistently reflect the dissolved metal concentrations in water. For all metals except Cd and Zn, periphyton concentrations were on average higher than snail metal concentrations. Periphyton analyses can provide a more conservative measure of metal contamination and, when artificial substrates are used, correspond to defined and known periods of exposure.This study demonstrates that constructed wetlands may lead to increased metal content of downstream organisms even if these wetlands appear to be overall sinks for dissolved metals based on mass balance calculations. It also shows the need to consider not only metal concentrations, but also biological data when assessing the performance of pollution control facilities