The Interacting Impurity Josephson Junction: Variational Wavefunctions and Slave Boson Mean Field Theory

We investigate the Josephson coupling between two superconductors mediated through an infinite U Anderson impurity, adapting a variational wavefunction approach which has proved successful for the Kondo model. Unlike the Kondo problem, however, a crossing of singlet and doublet state energies may be produced by varying the ratio of Kondo energy to superconducting gap, in agreement with recent work of Clerk and Ambegaokar. We construct the phase diagram for the junction and discuss properties of different phases. In addition, we find the singlet and doublet state energies within a slave boson mean field approach. We find the slave boson mean field treatment is unable to account for the level crossing.Comment: 5 pages; 4 encapsulated PostScript figures; submitted to Phys. Rev.

Collapse of the critical state in superconducting niobium

Giant abrupt changes in the magnetic flux distribution in niobium foils were studied by using magneto-optical visualization, thermal and magnetic measurements. Uniform flux jumps and sometimes almost total catastrophic collapse of the critical state are reported. Results are discussed in terms of thermomagnetic instability mechanism with different development scenarios.Comment: arXiv.org produced artifacts in color images (three versions were attempts to make better images). Download clean PDF and watch video-figures at: "http://cmp.ameslab.gov/supermaglab/video/Nb.html

The Effect of the Northeast Dairy Compact on Producers and Consumers, with Implications of Compact Contagion

Balagtas andSumnermodel and measure the effects of the Northeast Dairy Compact on prices, quantities, and producer and consumer welfare, underscoring the distribution of these effects across regions and among producer and buyers. Using 1999 as a base year, simulations show that the Compact raised the farm price of milk in the Northeast by $0.45/cwt., lowered the farm price of milk in the rest of the country by$0.02/cwt., and transferred income from producers outside the Compact region and buyers in the Compact region to producers in the Compact region. Non-Compact producer losses exceeded Compact producer gains. Similar results are found for a scenario of Compact contagion, extension of the Compact to include additional states. In both cases, the Compact changed the distribution of the costs and benefits of price discrimination as practiced by milk marketing orders. The implication is that the regional distribution of the Compact's welfare effects raises again the question of the organization of a government-sponsored milk marketing plan such as the federal milk marketing order system.Health and Safety, Regulatory Reform

Inverse transition in the two dimensional dipolar frustrated ferromagnet

We show that the mean field phase diagram of the dipolar frustrated ferromagnet in an external field presents an inverse transition in the field-temperature plane. The presence of this type of transition has recently been observed experimentally in ultrathin films of Fe/Cu(001). We study a coarse-grained model Hamiltonian in two dimensions. The model supports stripe and bubble equilibrium phases, as well as the paramagnetic phase. At variance with common expectations, already in a single mode approximation, the model shows a sequence of paramagnetic-bubbles-stripes-paramagnetic phase transitions upon lowering the temperature at fixed external field. Going beyond the single mode approximation leads to the shrinking of the bubbles phase, which is restricted to a small region near the zero field critical temperature. Monte Carlo simulations results with a Heisenberg model are consistent with the mean field results.Comment: 8 pages, 6 figure