Layered cobalt oxides have been naturally attracting great interest as intercalation compounds in chemistry and for their strongly-correlated electrons and triangular lattices in physics. They are also versatile materials; Li_xCoO₂ is used in lithium batteries powering portable electric devices, while Na_xCoO₂ is expected to be a thermoelectric material. Therefore, there is the potential interest in layered cobalt oxide systems, which is one of the reasons why so many studies have been done on our superconductor. The other reason is the focus of a large number of researchers on the subject of superconductivity. They were mothered by a so-called "superconductivity fever," which started in 1986. The record for T_c for cuprates was rewritten on a daily basis, and it increased from 40 K to 110 K during only two years; indeed many dreamed of the appearance of room-temperature superconductivity. However, it has not been broken since the discovery of the Hg system in 1993, and then the fever went down. The similarities and differences between the superconducting cobalt oxide and high-T_c cuprates fascinated these same researchers again. Another reason for the high citation rate is that anyone can synthesize the superconductor. All that one needs to replicate our study are an electric furnace to synthesize the parent material and a flask to modify it when superconducting through a soft-chemical route.

Yes, it does. The newly discovered superconductor will not be for practical use, frankly speaking, because of its low T_c, but useful and interesting for studies on superconductivity. It is only one superconductor among late 3d-transition metal oxides besides high-T_c cuprates; therefore, the comparative study seems very important. The superconductivity is induced in a two-dimensional CoO₂ system where Co atoms are triangularly arranged; such triangular arrangements frequently show interesting physics, for instance, spin frustration. In addition, the transformation from the non-superconducting phase to the superconducting one includes many interesting phenomena for chemists.

Since the discovery of high-T_c cuprates, many attempts have been made to find such superconductivity in similar late 3d-transition metal oxides, without success. This material is the first superconductor in the late 3d-transition metal oxides following the cuprates. It is similar to the cuprates also in their strong two-dimensionality and the spin state of S = 1/2, but different in the arrangement of the transition metal atoms; CoO₂ layers have triangular lattice, while CuO₂ layers do square geometry. These comparisons between the CoO₂ superconductor and high-T_c cuprates are expected to shed light on the origin of high-T_c superconductivity, which has not been elucidated, now more than 15 years after the initial discovery. Moreover, the soft-chemical process is a unique way to modify a common material to a superconductor. It is very surprising that only immersion in water, which introduces water between the CoO₂ layers, makes a superconductor.

In our research group named the Soft Chemistry Research Group, we are synthesizing titanium oxides, cobalt oxides, and their related compounds through various soft-chemical processings which include exfoliation into nanosheet materials, their reassembling, intercalation reactions, and ion-exchange reactions. I was trying to modify the layered structure of Na_xCoO₂, hopefully, into individual CoO₂ sheets. I did not succeed in exfoliation but could separate the adjacent CoO₂ layers at an unusually long distance. At that time, a researcher in the Superconducting Materials Center suggested to me that it would show interesting properties owing to the strong two-dimensionality and that I should measure its magnetization. It was the very next day after this discussion when I found the superconductivity.