MgB 2 Crystallites Much Less Anisotropic than Cuprate HTS
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RESEARCH/RESEARCHERS
Superconducting Mechanism of MgB2 Consistent with BCS Theory Researchers R. Osborn and colleagues at Argonne National Laboratory have confirmed that the superconducting mechanism of MgB2, a newly discovered noncopper-oxide superconductor with critical transition temperature (Tc) of 39 K, can be explained by the Bardeen–Cooper– Schrieffer (BCS) theory. As reported in the June 18 issue of Physical Review Letters, the researchers performed inelastic neutronscattering measurement of the phonon density of states on an isotopically enriched polycrystalline sample of Mg11B2, using time-of-flight neutron spectroscopy. They found that boron vibration frequencies extend up to energies of 100 meV. The acoustic phonons extend in energy to 36 meV, and there are highly dispersive optic branches peaking at 54, 78, 89, and 97 meV. The frequencies of the main peaks are well represented by the Born–von Kármán force-constant model, and from which an electron-phonon coupling of ~0.9 was estimated from the Allen-Dynes equation. The measured values of the Tc and isotope effect are consistent with a conventional phonon mechanism for superconductivity with moderately strong electron-phonon coupling. SHIMING WU
MgB2 Crystallites Much Less Anisotropic than Cuprate HTS Relatively high values of critical current density (Jc ) and the absence of a grain-boundary weak-link problem have been observed for MgB 2, making it a potentially useful material for superconductive devices. However, the layered lattice structure of MgB 2 has made researchers wonder whether this compound is anisotropic like all the high Tc cuprates. In an article appearing in the June 25 issue of Physical Review Letters, direct measurements of upper critical field anisotropy and mass anisotropy ratio were made by O.F. de Lima and colleagues at UNICAMP in Brazil. The MgB2 sample was weakly sintered from a stoichiometric mixture of pure boron and pure magnesium powders. It was then crushed and milled to crystallites. Small amounts of the crystallites were c-axis aligned on paper. The real component of ac susceptibility was measured with an external magnetic field H applied parallel and perpendicular to the sample c-axis. The upper critical field H c 2, above which the superconductivity is destroyed, was calculated from the data taken in both orientations. The ratio between the upper critical field when H is applied parallel to the ab plane, and when it is along MRS BULLETIN/JULY 2001
the c direction, was evaluated at different temperatures, producing a ratio of about 1.7. This ratio, in turn, implies an anisotropy of the coherence length of 1.7 and a mass anisotropy ratio of 0.3. Compared to the mass anisotropy ratio values found for the high Tc compounds Y-Ba-Cu-O (0.04) and Bi-Sr-Ca-Cu-O (10-4), both of which are known to be highly anisotropic materials, MgB 2 is mildly anisotropic. In practical applications, the high Tc materials have to be textured to a maximum degree of c-axis alignment in order to obtain optimum Jc. While the texture might not be
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