The New Magnesium Boride Superconductors

The new magnesium superconductors were the subject of a mammoth session at the APS meeting. Even bigger (in terms of papers delivered---79 altogether) than the1987 "Woodstock of Physics" meeting where ceramic superconductors came to prominence, this "Woodstock West" session showcased results from a dozen countries performed within weeks or days of the January announcement of superconductivity in MgB₂.

The heart of superconductivity is the formation and flow of pairs of charges, and each time a new material is found to sustain resistanceless currents the nature of the pairing has to be explored all over again. Typical diagnostic tests include (1) measuring the energy (the "energy gap") that holds electron pairs together in the superconducting state; (2) measuring the density of phonon states in the material (phonons are the particle equivalent of the subtle lattice vibrations that, according to the BCS theory, hold the electron pairs together; in these tests a beam of neutrons are sent into the sample, where they excite vibrations equivalent to striking a bell and listening for the characteristic tones; and (3) seeing what happens when atoms in the material are replaced by different isotopes (boron and magnesium in this case) or even atoms of different elements.

Besides these plentiful yeoman measurements, what discoveries came to light at the Seattle session? Robert Cava of Princeton announced superconductivity in a MgCNi₃ sample at a temperature of 8 K. Cava claimed that this represents the first metallic perovskite superconductor. Jun Akimitsu of Aoyama Gakuin University in Tokyo, whose discovery set off the MgB research frenzy in the first place, reported superconductivity at 35 K in a Mg-B-Be compound. Beryllium, unfortunately, is a toxic material, and increasing the Be fraction in the recipe actually depressed the superconductivity transition temperature.

APS site on the Seattle MgB₂ session