2 Moreoever, the 39-K T c comes straight off the rack. Tailoring MgB 2 's material properties could send T c higher. Akimitsu published his discovery in the 1 March issue of Nature , but the first MgB 2 preprint appeared on the Los Alamos server on 31 January, just three weeks after the Sendai symposium. In that early paper, Paul Canfield's group at Iowa State University explained how to make pure samples of MgB 2 and measured the compound's isotope effect. Lighter isotopes lead to higher phonon frequencies, which, in turn, lead to higher transition temperatures.
T c cuprates, whose superconductivity depends on subtle, phonon-free coupling between electrons.
By finding a large isotope effect, Canfield and company confirmed the key role of electron-phonon coupling in MgB 2 . A host of subsequent measurements by other groups have further elucidated MgB 2 's normal and superconducting states. Just above T c , its resistivity is about 75 mW cm, which is lower than many simple metals. And although most experiments point to MgB 2 being a conventional s -wave BCS superconductor, some see possible evidence of unconventional behavior. T c cuprates in the mid-1980s, researchers looked to BCS theory to guide them toward higher T c .