The road to magnesium diboride thin films, Josephson junctions and SQUIDs

The remarkably high critical temperature at which magnesium diboride (MgB₂) undergoes transition to the superconducting state, T_c ≈ 40 K, has aroused great interest and has encouraged many groups to explore the properties and application potential of this novel superconductor. For many electronic applications and further basic studies, the availability of superconducting thin films is of great importance. Several groups have succeeded in fabricating superconducting MgB₂ films. An overview of the deposition techniques for MgB₂ thin film growth will be given, with a special focus on the in situ two-step process.

Although, meanwhile, many problems to obtain suitable films have been solved, such as oxygen impurities and magnesium volatility, the question of how single-phase epitaxial films can be grown still remains. The possibility of growing single-crystalline epitaxial films will be discussed from the deposition conditions' point of view as well as substrate choice. Necessary conditions are discussed and possible routes are reviewed.

The applicability of MgB₂ in superconducting electronic devices depends on the possibility of making well-controlled, i.e., reproducible and stable, Josephson junctions. The first attempts to make MgB₂–MgO–MgB₂ ramp-type junctions and SQUIDs from MgB₂ nanobridges are discussed.