Recent Results on the Decay of Metastable Phases

We review some aspects of current knowledge regarding the decay of metastable phases in many-particle systems. In particular we emphasize recent theoretical and computational developments and numerical results regarding homogeneous nucleation and growth in kinetic Ising and lattice-gas models. An introductory discussion of the droplet theory of homogeneous nucleation is followed by a discussion of Monte Carlo and transfer-matrix methods commonly used for numerical study of metastable decay, including some new algorithms. Next we discuss specific classes of systems. These include a brief discussion of recent progress for fluids, and more exhaustive considerations of ferromagnetic Ising models ({\it i.e.}, attractive lattice-gas models) with weak long-range interactions and with short-range interactions. Whereas weak-long-range-force (WLRF) models have infinitely long-lived metastable phases in the infinite-range limit, metastable phases in short-range-force (SRF) models eventually decay, albeit extremely slowly. Recent results on the finite-size scaling of metastable lifetimes in SRF models are reviewed, and it is pointed out that such effects may be experimentally observable.Comment: 34 pages, LaTex, 8 ps figs. on request, preprint FSU-SCRI-94-6

Scenarios of domain pattern formation in a reaction-diffusion system

We performed an extensive numerical study of a two-dimensional reaction-diffusion system of the activator-inhibitor type in which domain patterns can form. We showed that both multidomain and labyrinthine patterns may form spontaneously as a result of Turing instability. In the stable homogeneous system with the fast inhibitor one can excite both localized and extended patterns by applying a localized stimulus. Depending on the parameters and the excitation level of the system stripes, spots, wriggled stripes, or labyrinthine patterns form. The labyrinthine patterns may be both connected and disconnected. In the the stable homogeneous system with the slow inhibitor one can excite self-replicating spots, breathing patterns, autowaves and turbulence. The parameter regions in which different types of patterns are realized are explained on the basis of the asymptotic theory of instabilities for patterns with sharp interfaces developed by us in Phys. Rev. E. 53, 3101 (1996). The dynamics of the patterns observed in our simulations is very similar to that of the patterns forming in the ferrocyanide-iodate-sulfite reaction.Comment: 15 pages (REVTeX), 15 figures (postscript and gif), submitted to Phys. Rev.

Near-field optical spectroscopy and microscopy of self-assembled GaN∕AlN nanostructures

The spatial distribution and emission properties of small clusters of GaNquantum dots in an AlN matrix are studied using high-resolution electron and optical microscopy. High-resolution transmission electron microscopy reveals near vertical correlation among the GaNdots due to a sufficiently thin AlN spacer layer thickness, which allows strain induced stacking. Scanning electron and atomic force microscopy show lateral coupling due to a surface roughness of ∼50–60nm. Near-field photoluminescence in the illumination mode (both spatially and spectrally resolved) at 10K revealed emission from individual dots, which exhibits size distribution of GaNdots from localized sites in the stacked nanostructure. Strong spatial localization of the excitons is observed in GaNquantum dots formed at the tip of self-assembled hexagonal pyramid shapes with six [101¯1¯] facets

Evolution of embryonic developmental period in the marine bird families Alcidae and Spheniscidae: roles for nutrition and predation?

Background: Nutrition and predation have been considered two primary agents of selection important in theevolution of avian life history traits. The relative importance of these natural selective forces in the evolution of avianembryonic developmental period (EDP) remain poorly resolved, perhaps in part because research has tended to focuson a single, high taxonomic-level group of birds: Order Passeriformes. The marine bird families Alcidae (auks) andSpheniscidae (penguins) exhibit marked variation in EDP, as well as behavioural and ecological traits ultimately linkedto EDP. Therefore, auks and penguins provide a unique opportunity to assess the natural selective basis of variation in akey life-history trait at a low taxonomic-level. We used phylogenetic comparative methods to investigate the relativeimportance of behavioural and ecological factors related to nutrition and predation in the evolution of avian EDP.Results: Three behavioural and ecological variables related to nutrition and predation risk (i.e., clutch size, activitypattern, and nesting habits) were significant predictors of residual variation in auk and penguin EDP based on modelspredicting EDP from egg mass. Species with larger clutch sizes, diurnal activity patterns, and open nests hadsignificantly shorter EDPs. Further, EDP was found to be longer among birds which forage in distant offshore waters,relative to those that foraged in near shore waters, in line with our predictions, but not significantly so.Conclusion: Current debate has emphasized predation as the primary agent of selection driving avian life historydiversification. Our results suggest that both nutrition and predation have been important selective forces in theevolution of auk and penguin EDP, and highlight the importance of considering these questions at lower taxonomicscales. We suggest that further comparative studies on lower taxonomic-level groups will continue to constructivelyinform the debate on evolutionary determinants of avian EDP, as well as other life history parameters

Optimizing the second-order optical nonlinearities of organic molecules: asymmetric cyanines and highly polarized polyenes

e recently reported that there is an optimal combination of donor and acceptor strengths for a given molecular length and bridge structure that maximizes (beta) . For this combination, there is the correct degree of bond length alternation and asymmetry in the molecule. Our recent findings suggest that molecules that can be viewed as asymmetric cyanines with relatively small amounts of bond length alternation are nearly optimal. In this manner, we have identified molecules with nonlinearities many times that of conventional chromophores for a given length. In this paper, we will present a new computational analysis that allows the correlation of bond length alternation with hyperpolarizabilities and will present EFISH data on simple donor-acceptor polyene chromophores

Computational Sensitivity Investigation of Hydrogel Injection Characteristics for Myocardial Support

Biomaterial injection is a potential new therapy for augmenting ventricular mechanics after myocardial infarction (MI). Recent in vivo studies have demonstrated that hydrogel injections can mitigate the adverse remodeling due to MI. More importantly, the material properties of these injections influence the efficacy of the therapy. The goal of the current study is to explore the interrelated effects of injection stiffness and injection volume on diastolic ventricular wall stress and thickness. To achieve this, finite element models were constructed with different hydrogel injection volumes (150 µL and 300 µL), where the modulus was assessed over a range of 0.1 kPa to 100 kPa (based on experimental measurements). The results indicate that a larger injection volume and higher stiffness reduce diastolic myofiber stress the most, by maintaining the wall thickness during loading. Interestingly, the efficacy begins to taper after the hydrogel injection stiffness reaches a value of 50 kPa. This computational approach could be used in the future to evaluate the optimal properties of the hydrogel

The complications of ‘hiring a hubby’: gender relations and the commoditisation of home maintenance in New Zealand

This paper examines the commoditization of traditionally male domestic tasks through interviews with handymen who own franchises in the company ‘Hire a Hubby’ in New Zealand and homeowners who have paid for home repair tasks to be done. Discussions of the commoditization of traditionally female tasks in the home have revealed the emotional conflicts of paying others to care as well as the exploitative and degrading conditions that often arise when work takes place behind closed doors. By examining the working conditions and relationships involved when traditionally male tasks are paid for, this paper raises important questions about the valuing of reproductive labour and the production of gendered identities. The paper argues that while working conditions and rates of pay for ‘hubbies’ are better than those for people undertaking commoditized forms of traditionally female domestic labour, the negotiation of this work is still complex and implicated in gendered relations and identities. Working on the home was described by interviewees as an expression of care for family and a performance of the ‘right’ way to be a ‘Kiwi bloke’ and a father. Paying others to do this labour can imply a failure in a duty of care and in the performance of masculinity

Low Cost and Compact Quantum Cryptography

We present the design of a novel free-space quantum cryptography system, complete with purpose-built software, that can operate in daylight conditions. The transmitter and receiver modules are built using inexpensive off-the-shelf components. Both modules are compact allowing the generation of renewed shared secrets on demand over a short range of a few metres. An analysis of the software is shown as well as results of error rates and therefore shared secret yields at varying background light levels. As the system is designed to eventually work in short-range consumer applications, we also present a use scenario where the consumer can regularly 'top up' a store of secrets for use in a variety of one-time-pad and authentication protocols.Comment: 18 pages, 9 figures, to be published in New Journal of Physic

Convective Fingering of an Autocatalytic Reaction Front

We report experimental observations of the convection-driven fingering instability of an iodate-arsenous acid chemical reaction front. The front propagated upward in a vertical slab; the thickness of the slab was varied to control the degree of instability. We observed the onset and subsequent nonlinear evolution of the fingers, which were made visible by a {\it p}H indicator. We measured the spacing of the fingers during their initial stages and compared this to the wavelength of the fastest growing linear mode predicted by the stability analysis of Huang {\it et. al.} [{\it Phys. Rev. E}, {\bf 48}, 4378 (1993), and unpublished]. We find agreement with the thickness dependence predicted by the theory.Comment: 11 pages, RevTex with 3 eps figures. To be published in Phys Rev E, [email protected], [email protected], [email protected]