The electronic state of an MgB2 superconductor is reviewed, focusing on its characteristic properties such as the multi-bands and multi-gaps. The presence of two superconducting gaps causes the temperature dependence of the anisotropy in the upper critical fields. The carbon impurities can be substituted for B and act as strong scattering centers, particularly for the σ-band carriers. The s-wave symmetry of the gaps is an advantage to achieve a higher critical current.
This article reviews the developments over the past six years in the thin-film growth and junction fabrication of superconducting MgB2 materials, including future prospects for MgB2 superconducting electronics. The most serious problem in the thin-film growth of MgB2 is the high Mg vapor pressure required for phase stability. This problem has made in-situ film growth difficult. At present, however, high-quality in-situ films can be prepared either by a low-temperature route or a high-temperature route. Current best films substantially exceed bulk single crystals in size and quality. The technology to fabricate MgB2 junctions has also shown great progress, and now all-MgB2 SIS Josephson junctions are realized, which demonstrates a great potential for MgB2 in superconducting electronics.