Review of the superconducting properties of MgB₂

TLDR

MgB2, a binary compound discovered to superconduct at 40 K, has sparked renewed interest in non‑oxide superconductors and raised the question of whether it represents a new class of high‑Tc materials. The review seeks to identify the unique features that make MgB2 special as a superconductor. MgB2 has been fabricated in bulk, single crystals, thin films, tapes, and wires. MgB2 exhibits a record‑setting 40 K Tc for a binary compound, exceptionally high critical current densities (>10 MA cm⁻²) and fields (≈40 T) in thin films, large coherence lengths, and grain‑boundary transparency, yet its anisotropy (γ = 1.2–9) and gap structure remain unresolved.

Abstract

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 Tc = 40 K for a binary compound. What makes MgB2 so special? Its high Tc, 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 Tc 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, BeB2.75, C–S composites, and the elemental B under pressure.