Effects of Fermion Flavor on Exciton Condensation in Double Layer Systems

We use fermionic path integral quantum Monte Carlo to study the effects of fermion flavor on the physical properties of dipolar exciton condensates in double layer systems. We find that by including spin in the system weakens the effective interlayer interaction strength, yet this has very little effect on the Kosterlitz-Thouless transition temperature. We further find that, to obtain the correct description of screening, it is necessary to account for correlation in both the interlayer and intralayer interactions. We show that while the excitonic binding cannot completely surpress screening by additional fermion flavors, their screening effectiveness is reduced leading to a much higher transition temperatures than predicted with large-N analysis.Comment: 4 pages, 3 figure

Path Integral Monte Carlo Simulations for Fermion Systems: Pairing in the Electron-Hole Plasma

We review the path integral method wherein quantum systems are mapped with Feynman's path integrals onto a classical system of "ring-polymers" and then simulated with the Monte Carlo technique. Bose or Fermi statistics correspond to possible "cross-linking" of polymers. As proposed by Feynman, superfluidity and Bose condensation result from macroscopic exchange of bosons. To map fermions onto a positive probability distribution, one must restrict the paths to lie in regions where the fermion density matrix is positive. We discuss a recent application to the two-component electron-hole plasma. At low temperature excitons and bi-excitons form. We have used nodal surfaces incorporating paired fermions and see evidence of a Bose condensation in the energy, specific heat and superfluid density. In the restricted path integral picture, pairing appears as intertwined electron-hole paths. Bose condensation occurs when these intertwined paths wind around the periodic boundaries.Comment: 14 pages, 7 figures Prepared for the 1999 International Conference on Strongly Coupled Coulomb Systems, Saint-Malo, Franc

Counties most at risk of environmental hazards are losing higher income migrants to lower risk areas, which decreases their resilience

When looking at environmental hazards, migration tends to be focused on as a result of disasters, rather than a contributor to community vulnerability. Using data from across the U.S. on hazards and county-to-county migration, J. Matthew Shumway finds that counties most at risk of experiencing severe and/or frequent environmental hazards are losing both people and income as a result of migration. He writes that counties with lower hazard levels are attracting migrants with higher incomes from those with higher hazard levels, and that this is decreasing their resilience to cope with hazards

Do the Largest Firms Grow the Fastest? The Case of U.S. Dairies

Replaced with revised version of paper 06/29/07.Livestock Production/Industries, Productivity Analysis,

What Does Initial Farm Size Imply About Growth and Diversification?

Recent consolidation in agriculture has shifted production toward fewer but larger farms, reshaping business relationships between farmers, processors, input suppliers, and local communities. We analyze growth and diversification of U.S. corn, wheat, apple, and beef farms by examining longitudinal changes in 10 size cohorts through three successive censuses. We fail to reject Gibrat’s law in apple and wheat industries and the mean reversion hypothesis in beef and corn industries. Apple and wheat farms diversify over time. The findings suggest that scale economies diminish for large farms across all four industries and scope economies dominate scale economies for large apple and wheat farms.diversification, firm growth, Gibrat’s law, longitudinal data, scale economies, scope economies, Agribusiness, Farm Management, Production Economics, Q12,

Do the Largest Firms Grow and Diversify the Fastest? The Case of U.S. Dairies

We analyze growth and diversification of U.S. dairy farms by examining changes in ten size cohorts and new entrants through three successive censuses. We reject Gibrat’s law and the mean reversion hypothesis of growth. Growth rates appear bimodal where the smallest and largest farm cohorts grow fastest. All cohorts diversify but the largest farms do not diversify as rapidly as medium-sized farms. New entrants are generally large, and they diversify more rapidly than comparably-sized incumbents. These data suggest that scale economies persist even for the largest cohort of U.S. dairy farms and scale economies dominate scope economies for large farms.: census, dairy, diversification, growth, scale, scope