As-grown Growth of MgB2 Thin Films and Fabrication of Josephson Junctions

In the fall of 2001, a group of researchers at Aoyama Gakuin University published a report in Nature on a new superconducting material, MgB2 (magnesium boride), which has a critical temperature of 39 K [1], the finding of which sent shock waves around the world. The discovery of superconductivity in Hg traces back to 1911. Since then, various metals and metal compounds have been confirmed to show superconductivity. However, no improvement in the critical temperature had been reported since 23 K for Nb3Ge. The discovery of a series of high temperature superconductors that started in the La-related materials in 1986 increased the critical temperature dramatically. It is still well remembered how superconductors with critical temperatures above the 77 K liquid nitrogen temperature raised a superconductivity fever not only in the superconductor research community but also in the general public. Compared to high temperature superconductors, MgB2 has a low critical temperature. Nevertheless, this critical temperature is the highest among the metal compound superconductors. It is a common substance that appears in material catalogues but its superconductivity had been overlooked. While various borides are known to show superconductivity, it seems MgB2 alone was accidentally omitted from these investigations. The group at Aoyama Gakuin University studied the appearance of superconductivity in the 3-element system of Mg, B, and Ti, and finally discovered superconductivity in the 2-element system of Mg and B, without Ti. Figure 1 shows the crystalline structure of the compound. The crystalline structure is a simple hexagonal system, and the separate Mg and B layers repeat in the caxis direction. The NICT are presently conducting studies on high frequency superconductivity applications. To apply the superior techniques established for superconductor detectors using conventional Nb superconductors to MgB2, a number of problems must be solved. The first is thin film fabrication. Without high quality thin film fabrication techniques, no success