36,190 research outputs found
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 0.02/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
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