NEW HIGH [T.sub.c] BORON SUPERCONDUCTOR.

The surprise discovery very recently of superconductivity at 40 K in the binary [MgB.sub.2] system has triggered enormous interest in the structural, electronic, and superconducting properties of simple systems in general and this class of compounds in particular. From the experimental viewpoint, it is important to determine whether [MgB.sub.2] is an isolated, special exception, or if it is representative of a broad class of new superconducting materials. The emerging picture of this system is quite interesting. The crystal structure is layered, similar to intercalated graphite, and the band structure shows that it is a good metal due to the boron orbitals at the Fermi surface, while the Mg does not contribute appreciably to the conductivity. The hole-type conduction band is reminiscent of the high [T.sub.c] cuprates, but, in contrast to the cuprates, the normal-state conductivity is three-dimensional in nature instead of being highly anisotropic, thus eliminating the "weak-link" problem that has plagued wide spread commercialization of the cuprates. The conduction electron density and normal-state conductivity are also one to two orders-of-magnitude higher than either the Nb-based alloys or Bi-based cuprates used in present day wires, contradicting the conventional wisdom that good superconductors are poor conductors because of the strong electron-phonon interaction and at the same time providing encouragement that higher [T.sub.c] materials will be found in this class.

From a fundamental point of view, the central question is whether the high [T.sub.c] in this new system can be understood within the framework of a conventional electron-phonon mechanism, or a more exotic mechanism is responsible for the superconducting pairing. To answer this question, scientists from the Nist Center for Neutron Research, Princeton University, the University of Maryland, and the University of Pennsylvania have carried out temperature-dependent neutron measurements of the crystal structure and phonon density of states, and have compared these results with detailed first-principles calculations of the lattice dynamics and electronic band structure for [MgB.sub.2]. Excellent agreement is found between theory and experiment, demonstrating that the calculations are able to capture the essential physics of this class of materials. The numerical results demonstrate that the in-plane boron phonons are strongly coupled to the conduction electrons, providing the large electron-phonon interaction in th is system. This coupling gives rise not only to strong anharmonicity for these phonon modes, but to a large non-linear electron-phonon coupling that explains the high [T.sub.c] in [MgB.sub.2].

The many interesting properties described above, combined with the low cost, lightweight, and easy fabrication of wires and thin films, makes this new material quite attractive for many applications. Additional information can be obtained at http://webster.ncnr.nist.gov/staff/taner/mgb2/.