Probing Vortex Unbinding via Dipole Fluctuations

We develop a numerical method for detecting a vortex unbinding transition in a two-dimensional system by measuring large scale fluctuations in the total vortex dipole moment ${\vec P}$ of the system. These are characterized by a quantity $\cal F$ which measures the number of configurations in a simulation for which the either $P_x$ or $P_y$ is half the system size. It is shown that $\cal F$ tends to a non-vanishing constant for large system sizes in the unbound phase, and vanishes in the bound phase. The method is applied to the XY model both in the absence and presence of a magnetic field. In the latter case, the system size dependence of $\cal F$ suggests that there exist three distinct phases, one unbound vortex phase, a logarithmically bound phase, and a linearly bound phase.Comment: 6 pages, 2 figure

Frustrated two-dimensional Josephson junction array near incommensurability

To study the properties of frustrated two-dimensional Josephson junction arrays near incommensurability, we examine the current-voltage characteristics of a square proximity-coupled Josephson junction array at a sequence of frustrations f=3/8, 8/21, 0.382 $(\approx (3-\sqrt{5})/2)$, 2/5, and 5/12. Detailed scaling analyses of the current-voltage characteristics reveal approximately universal scaling behaviors for f=3/8, 8/21, 0.382, and 2/5. The approximately universal scaling behaviors and high superconducting transition temperatures indicate that both the nature of the superconducting transition and the vortex configuration near the transition at the high-order rational frustrations f=3/8, 8/21, and 0.382 are similar to those at the nearby simple frustration f=2/5. This finding suggests that the behaviors of Josephson junction arrays in the wide range of frustrations might be understood from those of a few simple rational frustrations.Comment: RevTex4, 4 pages, 4 eps figures, to appear in Phys. Rev.

Trans-oceanic population genetic structure of humpback whales in the north and south pacific

We examined genetic diversity of humpback whales in the North and adjacent South Pacific Oceans to investigate the history and dynamics that resulted in their current population structure and for which trans-oceanic gene flow is a phenomenon of great importance. Analysis of mitochondrial DNA variation suggests that humpback whale populations are subjected to contraction and expansion cycles associated with glaciations. Contrast between nuclear and mitochondrial genetic diversities show that expansion phases may be related to regional differentiation dependent upon sex-biased dispersal. To explain trans-oceanic gene flow from sex-biased dispersal, we analysed the species' wintering habits in the Mexican Pacific as described from the sex composition and temporal profile of social groupings. In consideration of the energetic burden for reproduction of female humpback whales and the resultant pre-copulatory competition among males, trans-oceanic gene flow may be explained by changes in winter distribution driven by male dispersal dynamics and gametic exchange across high productivity areas close to the equatorial coast of the American Pacific, as well as by the influence of long-term climatic change in forming trans-equatorial corridors for female interchange. Because of the sensitivity of humpback whale reproduction and dispersal to environment perturbations, our results raise concerns about the effects of climate change on the phylogeographic structure and thereby the evolution and long-term conservation of this species