means a compound of magnesium and boron that conducts electricity with zero resistance below about 39 kelvin, the highest critical temperature of any simple metallic superconductor.

from chemists knew the compound since the 1950s as a dull industrial ceramic, but nobody bothered testing it for superconductivity until a japanese team led by jun akimitsu measured it in 2001 and found it superconducting far above expectations, embarrassing decades of overlooked shelf-chemistry.