[No. 62] Yoichi Kamihara

As a researcher at the Japan Science and Technology Agency (JST), I am engaged in the search for superconductors.

Superconductivity is a phenomenon where the electrical resistance of a metal drops to zero below a certain temperature (Tc). Since my time at Keio University, I have consistently pursued research to "make and measure" unknown materials, which led to my involvement in the discovery of iron-based high-temperature superconductors. In this article, I will introduce this discovery along with some memories from my student days.

During my undergraduate years, I was a member of the Palette Club, a general arts group, where I painted with oils. I found it thrilling when unexpected colors and shapes emerged as I moved my brush across the canvas with a clear mind. On a different note, in the Department of Applied Physics and Physico-Informatics, I was able to explore a wide range of academic fields, from control engineering to the phenomenology of phase transitions in solids. To current students, I recommend taking as many classes as possible and not wasting your tuition fees; a motivation of simply "being aware" is sufficient. Even if your understanding is shallow, broadening your knowledge will definitely be useful when you encounter "unexpected results."

In the Department of Applied Physics and Physico-Informatics, under the guidance of Professor Masanori Matoba, I conducted my graduation research on the synthesis and functional exploration of crystals called layered oxysulfides (meaning oxide + sulfide). This was my first time conducting "make and measure" research. Although sulfur (S) is an element that tends to bond with oxygen (O) and become a cation, in the case of oxysulfides, S anions and O anions coexist within the same unit cell. These crystals are one of a mysterious class of materials called mixed-anion compounds. Believing that this structure held hidden functions (electronic properties), I continued my research and advanced to the doctoral program. However, I was unable to demonstrate these functions during my time as a student. Nevertheless, my experience in "making and measuring" mixed-anion compounds became the catalyst for my search for superconductors.

A conference presentation in Seattle during my student days

When I obtained my degree in 2005, the research on the aforementioned mixed-anion compounds was at the world's forefront, led by the group of Professor Hideo Hosono (Applied Ceramics Laboratory, Tokyo Institute of Technology). Wanting to master that technology, I applied for a postdoctoral position and was accepted thanks to my "make and measure" experience. In the Hosono group at that time, the functional exploration of mixed-anion compounds called oxypnictides [meaning oxide + pnictide (phosphorus, arsenic, antimony, etc.)] was a "hot" topic. As shown in figure (a), this material has a layered structure consisting of lanthanoid (Ln) oxide (LnO) layers and transition metal [e.g., iron (Fe)] pnictide (FePn) layers. The initially envisioned theme was to demonstrate ferromagnetic semiconductors using oxypnictides.

Quickly establishing priority is crucial in research. To that end, we divided the themes among several people, and I happened to be assigned the compound with the composition LaFePO. After making LaFePO and examining its magnetic properties, I found that it was non-magnetic, like copper or aluminum, despite containing iron, a typical magnetic element. This was an unexpected result. Out of curiosity, I measured the electrical resistance of this compound down to 4 K and obtained an even more unexpected result (= a discovery): the resistance dropped to zero. At the time, I was a novice in superconductivity research, so whether to continue the research was a delicate issue. However, because the crystal structure of LaFePO was similar to that of the copper-oxide high-temperature superconductors I had learned about in classes as a student, this material looked like a gold mine to me.

When I discussed this with my supervisor, I received approval to continue the research. Fortunately, in a material where phosphorus (P) was replaced with arsenic (As), the Tc rose to 26 K. Furthermore, spurred by this discovery, groups around the world began searching for related compounds, and materials with a Tc exceeding 50 K were found. The crystal structures shown in the figure are all of superconductors discovered in 2008. Because they exhibit a high Tc second only to copper-oxide high-temperature superconductors, they are recognized as iron-based high-temperature superconductors.

This discovery was selected as one of the top ten scientific breakthroughs of 2008 by the American journal *Science*.

Not sparing the effort to "make and measure," even for a theme where it was uncertain whether any novel results would be obtained, yielded an unexpected result: an iron superconductor. Furthermore, I believe that scientific progress—the discovery of iron-based high-temperature superconductors—was achieved by mobilizing all available knowledge in response to the obtained results and appropriately changing the research theme.