Influence of Dislocations in Thomson's Problem

We investigate Thomson's problem of charges on a sphere as an example of a system with complex interactions. Assuming certain symmetries we can work with a larger number of charges than before. We found that, when the number of charges is large enough, the lowest energy states are not those with the highest symmetry. As predicted previously by Dodgson and Moore, the complex patterns in these states involve dislocation defects which screen the strains of the twelve disclinations required to satisfy Euler's theorem.Comment: 9 pages, 4 figures in gif format. Original PS files can be obtained in http://fermi.fcu.um.es/thomso

Re: Practice of breastfeeding and factors that affect breastfeeding in Hong Kong [2]

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Non-integer flux quanta for a spherical superconductor

A thin film superconductor shaped into a spherical shell at whose center lies the end of long thin solenoid in which there is an integer flux $N\Phi_0$ has been previously extensively studied numerically as a model of a two-dimensional superconductor. The emergent flux from the solenoid produces a radial B-field at the superconducting shell and $N$ vortices in the superconducting film. We study here the effects of including a second solenoid (carrying a flux $f$) which is inserted inside the first solenoid but passing right across the sphere. This Aharonov-Bohm (AB) flux does not have to be quantized to make the order parameter single valued. The Ginzburg-Landau (GL) free energy is minimized at fixed $N$ as a function of $f$ and it is found that the minimum is usually achieved when the AB flux $f$ is half a flux quantum, but depending on $N$ the minimum may be at $f=0$ or values which are not obvious rational fractions.Comment: 6 pages, RevTeX, 5 figures include

Becoming a role model: The breastfeeding trajectory of Hong Kong women breastfeeding longer than 6 months

While a substantial proportion of breastfeeding women stop early in the postpartum period, some women are able to breastfeed for longer periods. The aim of this research was to explore the experience of breastfeeding with a subsample of Hong Kong women who have breastfed for longer than 6 months. Participants (n=17) were recruited from a larger infant-feeding study (n=360) conducted in tertiary-care hospitals in Hong Kong. In-depth qualitative interviews were conducted and content analysis was used to analyse the data. Data analysis revealed four themes that encompassed the women's experiences: (1) making the decision, (2) maintaining family harmony, (3) overcoming barriers, and (4) sustaining lactation. Antenatally, participants anticipated that breastfeeding would be very 'difficult' and described how the practice did not fit with the image of a professional woman in Hong Kong. Despite family opposition, frequently from their mother-in-law, and lack of societal acceptance, difficulties were overcome by what the Chinese people call hung-sum or determination. This study highlights unique cultural and social findings affecting breastfeeding women in Hong Kong which may be useful to health-care providers working with Chinese women locally and internationally. © 2004 Elsevier Ltd. All rights reserved.postprin

First Order Premelting Transition of Vortex Lattices

Vortex lattices in the high temperature superconductors undergo a first order phase transition which has thus far been regarded as melting from a solid to a liquid. We point out an alternative possibility of a two step process in which there is a first order transition from an ordinary vortex lattice to a soft vortex solid followed by another first order melting transition from the soft vortex solid to a vortex liquid. We focus on the first step. This premelting transition is induced by vacancy and interstitial vortex lines. We obtain good agreement with the experimental transition temperature versus field, latent heat, and magnetization jumps for YBCO and BSCCO.Comment: revised version replaces 9705092, 5 pages, Latex, 2 postscript figures, defect line wandering is included, 2 step melting is propose

Phase Transitions in Isolated Vortex Chains

In very anisotropic layered superconductors (e.g. Bi$_2$Sr$_2$CaCu$_2$O$_x$) a tilted magnetic field can penetrate as two co-existing lattices of vortices parallel and perpendicular to the layers. At low out-of-plane fields the perpendicular vortices form a set of isolated vortex chains, which have recently been observed in detail with scanning Hall-probe measurements. We present calculations that show a very delicate stability of this isolated-chain state. As the vortex density increases along the chain there is a first-order transition to a buckled chain, and then the chain will expel vortices in a continuous transition to a composite-chain state. At low densities there is an instability towards clustering, due to a long-range attraction between the vortices on the chain, and at very low densities it becomes energetically favorable to form a tilted chain, which may explain the sudden disappearance of vortices along the chains seen in recent experiments.Comment: 9 pages, 10 figure

Five rules for managing large, complex projects

Large-scale, long-term projects are notoriously difficult to manage. But recent research on megaprojects — defined as projects costing more than $1 billion — reveals five lessons that can help executives manage any big, complex project more effectively