Abstract

Despite having been synthesized as far back as the 1950s, the superconducting properties of magnesium diboride (MgB₂) were only discovered at the beginning of 2001. Akimitsu and his group were attempting to make a chemical analogue of CaB₆ by replacing Ca with Mg (Cava 2001). They intended to use MgB₂, a compound commercially available, as starting material and, during a routine characterization of MgB₂ properties before using it in the synthesis, they discovered that it had a superconducting transition T _c at 39 K (Nagamatsu et al., 2001). Although high-T _c superconductivity in cuprates had already been studied for 15 years, this discovery aroused huge surprise because the highest T _c in a simple intermetallic compound was 23 K (Nb₃Ge) and because the theoretical limit for conventional BCS superconductors, whose properties are mediated by the electron–phonon interaction, was thought to be 30 K. Moreover, from a practical point of view, MgB₂'s high critical temperature in principle allowed working with liquid hydrogen or cryocoolers at an operational temperature above 20 K, thus obviating the expense and availability issues of liquid helium.