A Guide to Designing Strongly Correlated Electron Materials

Masanori Matoba (Professor, Department of Applied Physics and Physico-Informatics)

In any world, you'll find "roguish older men" like those featured in the magazine *LEON*. In the world of electrons, too, there exists a class of electrons that are wild and tough, like Panzetta Girolamo. When many of these "roguish older men" are present, their individual personalities clash, leading to numerous "squabbles" that far exceed public expectations. The more attractive this is, the more it draws admiring gazes. After all, it was the earnest older gentlemen who supported Japan's high-growth period. However, in the near future when Moore's Law reaches its limit (around 2020, when transistors shrink to the atomic level), it may be the collective of "roguish older men" that saves the day. I've written with a bit of a swagger up to this point, but from here on, let's discuss the "stage for the collective performance of these roguish older men" in a more academic manner.

Since electrons are negatively charged particles, a Coulomb repulsion force acts between them. However, in solids such as ordinary metals and semiconductors, this Coulomb repulsion is relatively small, and the charge carriers (valence electrons or their vacancies, known as holes) can be described as spatially extended waves. In conventional semiconductor electronics, various functions are realized by controlling the electric current carried by these mutually independent waves (individual electrons or holes). A representative material is Si (silicon), which has supported the high growth of our society as a standard-bearer for semiconductor electronics.

On the other hand, there is a movement to apply electron groups where the Coulomb repulsion is too large to be ignored (strongly correlated electron systems) to electronics. This is called strongly correlated electronics, and it aims to realize functions far beyond the predictions of conventional theory by using systems where all electrons move in a strongly correlated manner due to mutual interactions (strongly correlated electrons) as base materials (for example, transition metal oxides). Superconductivity in copper-oxide superconductor materials and ferromagnetism in heavy fermion system materials are also examples of such strongly correlated electron systems. In a situation where there is approximately one electron per site (half-filled), if the Coulomb repulsion is too large, the electrons can hardly move (they become localized at each site). However, by skillfully creating holes (electron vacancies), a "bad metal"-like state emerges, where many electrons can move in a strongly correlated manner. The phenomenon of high-temperature superconductivity, which far exceeds the predictions of conventional theory, also emerges under these conditions. The environment within such a "bad metal" can be considered an emergent environment for electrons—one where unexpected new properties appear. In other words, a "bad metal" provides a stage where electrons can perform in an emergent fashion.

Next, I would like to discuss the material design of these "roguish metals" or "bad metals." To put it stylishly, how do we design a stage where correlated electrons can perform emergently? Examples of such stages are known to be the CuO2 planar units (Cu: copper, O: oxygen) in copper-oxide high-temperature superconductors and the ThCr2Si2-type units (Th: thorium, Cr: chromium, Si: silicon) representative of heavy fermion system materials. Therefore, our task is to design and search for materials that possess these units. From the perspective of developing materials that integrate these nano-functional units, we have been studying the materials shown in the figure.

We have been conducting a project to explore the functions and elucidate the factors controlling the physical properties of transition metal oxysulfides, materials in which Cu2S2 units (Cu: copper, S: sulfur) and MO2 planar units (M=Mn: manganese, Co: cobalt, Zn: zinc) are naturally stacked. Although we were able to develop a novel layered magnetic material with a giant thermoelectric power [1], we were unfortunately unable to discover the function of high-temperature superconductivity. However, Dr. Yoichi Kamihara (a member of the first graduating class of the Department of Applied Physics and Physico-Informatics), who participated in this research project for six years from April 1999 to March 2005 (from his fourth undergraduate year to his third doctoral year) and earned his PhD, went on to become a postdoctoral researcher in Professor Hosono's lab at Tokyo Tech. In early 2008, he discovered a new iron-based high-temperature superconductor [2], a layered compound of Fe2As2 units (Fe: iron, As: arsenic) and LaO units (La: lanthanum, O: oxygen). With this, he demonstrated a divinely inspired (and magnificent) stage design where correlated electrons could perform emergently in a "bad metal."

When I caught a glimpse of Dr. Yoichi Kamihara's historic achievement of masterfully manipulating electrons in an iron-based material to induce high-temperature superconductivity, I felt that strongly correlated electron materials must hold the potential for many more unexpected physical phenomena. I realized that we do research precisely because we don't know what will happen. I felt that I must devote myself to research with a mind as clear as a cloudless sky and a spirit as bright and sunny [3], just like the founder of the Juku, Yukichi Fukuzawa. Is it possible to apply these "roguish metals" or "bad metals" to near-future electronics?! I believe that when the time comes that we can freely manipulate "electron correlation"—one of the most difficult problems in physics—just as we do with conventional semiconductor electronics, a great new culture of a new era, "strongly correlated electronics," will be born, based on creativity and imagination.

[1] Y. Kamihara, M. Matoba, T. Kyomen, and M. Itoh, J. Appl. Phys. 91, 8864 (2002).

[2] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am. Chem. Soc. 130, 3296 (2008).

[3] Yukichi Fukuzawa and Masafumi Tomita, "Shin-tei Fuku-ō Jiden" (Iwanami Bunko, 1978).