The Gibbs-Thomson formula at small island sizes - corrections for high vapour densities

In this paper we report simulation studies of equilibrium features, namely circular islands on model surfaces, using Monte-Carlo methods. In particular, we are interested in studying the relationship between the density of vapour around a curved island and its curvature-the Gibbs-Thomson formula. Numerical simulations of a lattice gas model, performed for various sizes of islands, don't fit very well to the Gibbs-Thomson formula. We show how corrections to this form arise at high vapour densities, wherein a knowledge of the exact equation of state (as opposed to the ideal gas approximation) is necessary to predict this relationship. Exploiting a mapping of the lattice gas to the Ising model one can compute the corrections to the Gibbs-Thomson formula using high field series expansions. We also investigate finite size effects on the stability of the islands both theoretically and through simulations. Finally the simulations are used to study the microscopic origins of the Gibbs-Thomson formula. A heuristic argument is suggested in which it is partially attributed to geometric constraints on the island edge.Comment: 27 pages including 7 figures, tarred, gzipped and uuencoded. Prepared using revtex and espf.sty. To appear in Phys. Rev.

Renormalization group approach to multiscale modelling in materials science

Dendritic growth, and the formation of material microstructure in general, necessarily involves a wide range of length scales from the atomic up to sample dimensions. The phase field approach of Langer, enhanced by optimal asymptotic methods and adaptive mesh refinement, copes with this range of scales, and provides an effective way to move phase boundaries. However, it fails to preserve memory of the underlying crystallographic anisotropy, and thus is ill-suited for problems involving defects or elasticity. The phase field crystal (PFC) equation-- a conserving analogue of the Hohenberg-Swift equation --is a phase field equation with periodic solutions that represent the atomic density. It can natively model elasticity, the formation of solid phases, and accurately reproduces the nonequilibrium dynamics of phase transitions in real materials. However, the PFC models matter at the atomic scale, rendering it unsuitable for coping with the range of length scales in problems of serious interest. Here, we show that a computationally-efficient multiscale approach to the PFC can be developed systematically by using the renormalization group or equivalent techniques to derive appropriate coarse-grained coupled phase and amplitude equations, which are suitable for solution by adaptive mesh refinement algorithms

A dopaminergic switch for fear to safety transitions

Overcoming aversive emotional memories requires neural systems that detect when fear responses are no longer appropriate. The midbrain ventral tegmental area (VTA) dopamine system has been implicated in reward and more broadly in signalling when a better than expected outcome has occurred. This suggests that it may be important in guiding fear to safety transitions. We report that when an expected aversive outcome does not occur, activity in midbrain dopamine neurons is necessary to extinguish behavioral fear responses and engage molecular signalling events in extinction learning circuits. Furthermore, a specific dopamine projection to the nucleus accumbens medial shell is partially responsible for this effect. By contrast, a separate dopamine projection to the medial prefrontal cortex opposes extinction learning. This demonstrates a novel function for the canonical VTA-dopamine reward system and reveals opposing behavioural roles for different dopamine neuron projections in fear extinction learning

Epidemiological study of road traffic accident cases from Western Nepal

Background: Road Traffic Accident (RTA) is one among the top five causes of morbidity and mortality in South-East Asian countries. [1] Its socioeconomic repercussions are a matter of great concern. Efficient addressing of the issue requires quality information on different causative factors. Research Question: What are different epidemiological determinants of RTA in western Nepal? Objective: To examine the factors associated with RTA. Study Design: Prospective observational. Setting: Study was performed in a tertiary healthcare delivery institute in western Nepal. Participants: 360 victims of RTA who reported to Manipal Teaching hospital in one year. Study Variables: Demographic, human, vehicular, environmental and time factors. Statistical analysis: Percentages, linear and logarithmic trend and Chi-square. Results: Most of the victims i.e. 147 (40.83%) were young (15 to 30 years); from low i.e. 114 (31.66%) and mid i.e. 198 (55%) income families and were passengers i.e. 153 (42.50%) and pedestrians i.e. 105 (29.16%). Sever accidents leading to fatal outcome were associated with personal problems (P< 0.01, χ2 - 8.03), recent or on-day conflicts (P< 0.001, χ2 - 18.88) and some evidence of alcohol consumptions (P< 0.001, χ2 - 30.25). Increased prevalence of RTA was also noticed at beginning i.e. 198 (55%) and end i.e. 69 (19.16%) of journey; in rainy and cloudy conditions (269 i.e. 74.72%) and in evening hours (3 to 7 p.m. 159 i.e. 44.16%). Out of 246 vehicles involved; 162 ( 65.85%) were old and ill maintained. The contributions of old vehicle to fatal injuries were 33 (50%). Head injury was found in 156 (43.33 %) cases and its associated case fatality rate was 90.90%. In spite of a good percentage receiving first aid i.e. 213 (59.16%) after RTA; there was a notable delay (174 i.e. 48.33% admitted after 6 h) in shifting the cases to the hospitals. The estimated total days lost due to hospital stay was 4620 with an average of 12.83 days per each case. Conclusion: Most of the factors responsible for RTA and its fatal consequences are preventable. A comprehensive multipronged approach can mitigate most of them

Photoacoustic and X-ray photoelectron spectroscopic studies in reduced lead zirconate titanate ceramics

Lead zirconate titanate (PZT) is known to become a semiconductor after reduction in a hydrogen atmosphere. An attempt is made here to characterize the reduced, as well as the unreduced, PZT system with the help of photoacoustic (PA) spectroscopy and X-ray photoelectron spectroscopy (XPS) and to obtain useful information regarding its electronic behaviour. The band gap of the unreduced PZT is estimated to be 3.56 eV from PA spectra which is observed to remain unaffected after doping with lanthanum, niobium and strontium. XPS results indicate the presence of metallic lead in PZT after hydrogen reduction whereas other elements remain unaffected. This becomes responsible for the change in its conductivity and also for the increased absorption in the visible range which is reflected in the PA spectra. The electronic structure based on PA spectra is also presented for the reduced and unreduced PZT systems