Review of the superconducting properties of MgB₂

This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since the early 1950s but only recently discovered to be superconductive at a remarkably high critical temperature T_c = 40 K for a binary compound. What makes MgB₂ so special? Its high T_c, simple crystal structure, large coherence lengths, high critical current densities and fields, and transparency of grain boundaries to current promise that MgB₂ will be a good material for both large-scale applications and electronic devices. During the last seven months, MgB₂ has been fabricated in various forms: bulk, single crystals, thin films, tapes and wires. The largest critical current densities, greater than 10 MA cm⁻², and critical fields, 40 T, are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is yet to be resolved as a wide range of values have been reported, γ = 1.2–9. Also, there is no consensus on the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB₂ represents a new class of superconductors, which is the tip of an iceberg awaiting to be discovered. To date MgB₂ holds the record for the highest T_c among simple binary compounds. However, the discovery of superconductivity in MgB₂ revived the interest in non-oxides and initiated a search for superconductivity in related materialss; several compounds have since been announced to be superconductive: TaB₂, BeB_2.75, C–S composites, and the elemental B under pressure.