All-metal Superconductivity at 40K

In the early days of superconductivity research most of the samples were metals or combinations of metals, and the mechanism for producing the super-current state was described by the BCS theory, named for John Bardeen, Leon Cooper, and Robert Schrieffer. In this theory electrons pair up and eventually fall into a single quantum state in which the moving electron pairs are immune from electrical resistance---the hallmark of superconductivity---courtesy of a wavelike flexing of the crystal of atoms. An equivalent way of expressing this idea is to say that electrons pair up by exchanging phonons.

And then the Woodstock of Physics happened in 1987; this was the physics meeting at which a series of ceramic compounds (e.g., yttrium-barium-copper oxide) were revealed to superconduct up to 90 K and higher. Suddenly oxide superconductors were the rage, the BCS theory was felt to be inadequate to describing the new "high-temperature" materials, and study of the intermetallics (i.e., using only metallic elements) languished by comparison. Now this might change.

At a meeting on January 10, in Sendai Japan, Jun Akimitsu of the Aoyama-Gakuin University reported that they had observed superconductivity in a magnesium-boron compound at 39 K, a transition temperature almost twice the size for any previous intermetallic material. More recently a group at the Ames Lab at Iowa State (Paul Canfield, 515-294-6270, canfield@ameslab.gov) has taken a MgB2 sample and scrutinized it using different isotopes of B. Not only are the samples superconducting, at 40.2 K when using boron with an atomic mass of 10 and at 39.2 K for a boron mass of 11, but the dependence on isotope shows that even at this temperature, the BCS mechanism (which stipulates how the transition temperature should change with the mass of the isotope) is at work. (Bud'ko et al., Physical Review Letters, 26 February 2001; text at Physics News Select)