Intercalant-Driven Superconductivity in YbC6_{6} and CaC6_{6}

Recently deiscovered superconductivity in YbC$_6$ and CaC$_6$ at temperatures substantially higher than previously known for intercalated graphites, raised several new questions: (1) Is the mechanism considerably different from the previously known intercalated graphites? (2) If superconductivity is conventional, what are the relevant phonons? (3) Given extreme similarity between YbC$_6$ and CaCa$_6$, why their critical temperatures are so different? We address these questions on the basis of first-principles calculations and conclude that coupling with intercalant phonons is likely to be the main force for superconductivity in YbC$_6$ and CaC$_6$, but not in alkaline-intercalated compounds, and explain the difference in $T_c$ by the ``isotope effect'' due to the difference in Yb and Ca atomic masses.Comment: 4 pages, embedded postscript figire

Theory of Magnetic Properties and Spin-Wave Dispersion for Ferromagnetic (Ga,Mn)As

We present a microscopic theory of the long-wavelength magnetic properties of the ferromagnetic diluted magnetic semiconductor (Ga,Mn)As. Details of the host semiconductor band structure, described by a six-band Kohn-Luttinger Hamiltonian, are taken into account. We relate our quantum-mechanical calculation to the classical micromagnetic energy functional and determine anisotropy energies and exchange constants. We find that the exchange constant is substantially enhanced compared to the case of a parabolic heavy-hole-band model.Comment: 9 pages, 4 figure

Resolution of two-dimensional Currents in Superconductors from a two-dimensional magnetic field measurement by the method of regularization

The problem of reconstructing a two-dimensional (2D) current distribution in a superconductor from a 2D magnetic field measurement is recognized as a first-kind integral equation and resolved using the method of Regularization. Regularization directly addresses the inherent instability of this inversion problem for non-exact (noisy) data. Performance of the technique is evaluated for different current distributions and for data with varying amounts of added noise. Comparisons are made to other methods, and the present method is demonstrated to achieve a better regularizing (noise filtering) effect while also employing the generalized-cross validation (GCV) method to choose the optimal regularization parameter from the data, without detailed knowledge of the true (and generally unknown) solution. It is also shown that clean, noiseless data is an ineffective test of an inversion algorithm.Comment: To appear in the Physical Review B. Some text/figure additions and modification

On the role of pre-existing, unhealed cracks on the bending strain response of Ag-clad (Bi,Pb)2Sr2Ca2Cu3Ox tapes

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Suppression of magnetic granularity by transport current in (Bi,Pb)2Sr2Ca2Cu3Ox tapes

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Modeling the current-voltage characteristics of silver-sheathed Bi-Sr-Ca-Cu-O tapes

This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder