Using simple equations to capture “strongly entangled electrons” - School of Science, the University of Tokyo
Oct 28, 2015

Using simple equations to capture “strongly entangled electrons”

Department of Physics — Ogata Laboratory

Professor Masao Ogata
Department of Physics
Graduate School of Science, University of Tokyo


Professor Ogata graduated from Department of Physics, University of Tokyo in 1982, after which he advanced to the physics major in the Graduate School of Science. In 1986 he was appointed as an assistant professor at  Institute for Solid State Physics, University of Tokyo. He took postdoctoral positions at the Swiss Federal Institute of Technology in Zurich during 1989–91, and at Princeton University during 1991–93. He began teaching in Institute of Physics, College of Arts and Sciences, University of Tokyo in 1993, and then became an associate professor in Department of Physics in 2000. He became a professor in 2008.

Superconductivity was discovered in 1911, over 100 years ago, when it was found that the electrical resistance of mercury suddenly became zero when cooled to the ultra-low temperature of 4 K (–269 °C). In 1933, Meissner effect was discovered, in which superconductors completely expel magnetic fields. You may have seen a magnet placed above a superconducting material, hovering in midair despite the force of gravity.

However, it took a long time to develop a theory for understanding the mechanism of superconductivity. In 1957, superconductivity was finally explained according to “BCS theory” based on quantum mechanics, opening up entirely new horizons of physics. According to Professor Masao Ogata, “Superconductivity is interesting in that it is a realization of quantum mechanics, i.e., the microscopic law of physics, clearly visible in our macroscopic world.”

Superconductivity research took another leap forward in 1986. It was previously thought that superconductivity could only occur at ultra-cold temperatures very near absolute zero (–273 °C), but several materials were discovered in which superconductivity occurs at increasingly higher temperatures—first at 30 K (–243 °C), then progressively higher temperatures up to the present record of 166 K (–107 °C). Professor Ogata says that even in the strange world of quantum mechanics, high-temperature superconductivity is an extremely peculiar phenomenon.

“The mother materials that become high-temperature superconductors by chemical doping tend to be insulators that hardly conduct electricity. From the beginning, it is hard to explain even in the quantum mechanics why such insulators exist. It’s even harder to explain why they become superconductors through a simple chemical doping.”

Elucidating the mechanism of high-temperature superconductivity is one of the main themes of Professor Ogata’s research, starting with “strongly entangled electrons.”

“When you apply voltage to a metal, the electrons flow to form an electric current,” Professor Ogata says. “But in materials, the electrons strongly repel each other through the Coulomb repulsive force. In other words, the electrons don’t flow in a neat stream; they fight against each other and flow being strongly entangled.”

It is difficult to mathematically capture the strong entanglement. Attempts to understand it using simpler equations led to a focus on electrons in solids, and then eventually to high-temperature superconductivity. This relates to another of Professor Ogata’s research themes, namely, Dirac electrons in solids, representing a fusion of quantum mechanics with relativity theory.

Despite his love of entangled electrons, which are always complex, Professor Ogata carries with him a free, easy-going atmosphere. In his essay The Path to Becoming a Theoretical Physicist (available from the laboratory’s website), he writes that a physicist’s day begins with coffee. When Professor Ogata gets stuck in his work, he has more coffee and wanders the corridors to clear his mind. When that still isn’t enough, he heads off to banter with his students.

His students report that when he’s in his office, he’s usually humming or staring at a spot on the ceiling. He’s able to focus on his research even with people coming and going, and he interacts with students at his own pace. His seminars tend to be a barrage of questions, and he loves the chaos of debate—it is from chaos that new ideas are born, he believes. The laboratory is host to a postdoctoral researcher from Russia, so seminars are conducted in English.

Professor Ogata is comfortable working on the international stage. Based on his experience of doing research overseas, he says, “Japan is no slouch when it comes to solid-state physics. Japan is one of the best, in fact. We have many good textbooks, and the Japanese students are hard workers and good at math. What the Japanese students are not good at is showing that to others. I wish they could be a little more like some of the American students I’ve met, who will proudly brag to professors about the results they obtained. The feedback they get when they do things like that is often a huge help toward advancing their research.”

Master’s students in his group are normally sent alone to attend extended summer school overseas. “So long as you know some technical terms in the field, it doesn’t take much English to get by,” says Professor Ogata. “That way, the students understand that their competition isn’t all that fierce.” Doing physics requires a combination of brains and boldness.


aaaIn 1928 the physicist P.A.M. Dirac predicted the existence of “Dirac electrons,” a fusion of relativity and quantum theory. This is Professor Ogata’s calligraphic representation of the Dirac equations that supported that prediction.


Student Evaluations

“He loves geography, he loves trains, he loves travelogues, and he loves his family.” (Hiroyasu Matsuura, Assistant Professor)
“He gets so happy when he talks physics.” (Taro Kanao, D3)
“He always invites us out to lunch.” (Taichi Hinokihara, D1)
“He clears things up for me every time I speak with him.” (Tomonari Mizoguchi, M2)
“He seems so soft and gentle, but his questions during seminars can sting.” (Nobuyuki Okuma, M1)

― Office of Communication ―

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