November 9, 2021 (HP147). 1. Introduction  2021 is the 20^th anniversary of the discovery of MgB₂. So, the 20^th anniversary symposium has been planned in many places. After the discovery of MgB₂, I had the ambition to search for a new superconductor with a higher-Tc, and had little desire to investigate the physical properties of MgB₂ itself or to apply it. At the early stage of its application, I heard the news that Jc and Hc of MgB₂ seems to be not so high and I was pessimistic with respect its application.  After the 20 years has passed, the application of MgB₂ has steadily been developed, even just looking at the table of contents of the recent symposium.  I am surprised at the improvement of its quality and its wonderful application to various fields. Twenty years seem to be short, but I am amazed at its progress. Here, it's a little personal impression, but I would like to look back on 20 years ago.

2. Prehistory of MgB₂ discovery

As is well known, superconductivity was discovered in 1907 by Kamerlingh Onnes in mercury (Hg). After that, the new superconductors have been discovered  by various peoples, and during that time, B. Matthias carried out the most vigorous and systematic investigation from single element to two-element system. Figure 1 shows the well-known superconducting transition temperature (Tc) vs. its year of discovery. Looking at this figure, Tc vs. its year of discovery is almost straight and rising slowly until the discovery of high temperature superconductor by Bednorz and Müller in 1986. The main materials of these were discovered by Matthias and his group. I don't guarantee the truth, but according to the story I heard, one of the students told to Matthias, "Professor, if this straight line is extended, room temperature superconductor will be realized in about ✕✕ years." However, Matthias replied, "No, these are the materials I found, so if I die, Tc will not be raised." In fact, it is true that after the discovery of Nb₃Ge in around 1970, Tc was not improved until 1986.

Read Full Text by Jun Akimitsu (Research Institute for Interdisciplinary Science, Okayama University, Okayama, Japan)

Contributed by Kazuhiko Hayashi, SNF Co-editor, Large Scale