Seeing the wholeDr. Chung’s research field is condensed matter physics. This field is not familiar to many people but has a long history, as well as broad reach,tracing back to the origin of the field in the 19th century when British scientist Humphry Davy and Michael Faraday distinguished between conductor and insulator. In the beginning of the 20th century, the state of a solid was interpreted from a quantum mechanical perspective. This research stream, coined solid state physics, had many achievements. As the field developed, U.S. physicist Philip Warren Anderson began to use the term “condensed matter physics”. In 1978, the American Physical Society renamed their Solid State Physics Division to the Condensed Matter Physics Division. As implied by the term“condensed”, condensed matter physics deals with the macroscopic condition of a material consisting of a number of particles such as heat, elasticity, electricity, magnetics, and optics.

In particular, the core subject is crystalline materials which are multi-particle systems, easily interpreted by quantum mechanics. Thanks to this, achievements of condensed matter physics are often applied directly to the development of semiconductors and new materials. To facilitate understanding of condensed matter physics, let’s think about thermodynamics.Thermodynamics isn’t concerned with velocities of each individual particle even though equations representing the temperature contain the average velocity of all particles. Similarly, condensed matter physics is concerned with phenomena involving the entire system rather than with each constituent particle. This is the field of ‘emergent phenomenon’ research.

Condensed matter physics approaches physical phenomena from macroscopic perspective. It can also be thought of as the study of phase. Condensed matter physics seeks to interpret observed phenomena. When an unexpected phenomenon is seen in the system consisting of particles, condensed matter physics interprets the phenomenon as a single entity rather than dissecting it into the behavior of each particle. This is similar to the methodology of thermodynamics.”

Dr. Chung said that, “Phenomena in condensed matter physics came from the interaction of already known elementary particles, citing superconductivity as an example.” Since the discovery of superconductivity by Heike Kamerlingh Onnes, scientists have struggled to find the power that enabled superconductivity. Superconductivity looked like a qualitatively new phenomenon that could not be explained by previous theories. John Bardeen, John Robert Schrieffer, and Leon Cooper, however, succeeded in explaining superconductivity simply by citing the electrostatic attraction between already known particles. The BSC theory, named after its creators, shows several limitations but has been the firm orthodoxy for superconductivity.

The physics community accepted the BSC theory quickly, but it was not easily acceptable to all people. Some were uncomfortable with the fact that a known interaction can lead to unexpected results in a many body system. When Niels Bohr,who was a founder of quantum physics, heard about the BSC theory in a lecture by Schrieffer,he said “it cannot be true”.

Condensed matter physics is one of the actively studied areas recently.This picture took in 1995 isa 3-dimensional snapshot of rubidium atoms acting as a single atom due to the Bose-Einstein condensate at extremely low temperatures.

This does not mean, however, that conventional particle physics takes the opposite side of condensed matter physics. Their relationship can be considered complementary. As stated, while particle physics separates particles one by one,condensed matter physics deals with phenomena caused by the interaction between particles. Usually, condensed matter physics deals with phenomena that cannot occur at the ground state of particle physics. For this reason, condensed matter physics is thought to cover those areas that cannot be easily studied by particle physics.

"Suppose there is no interaction between particles. Physics at the undergraduate level can explain almost all physical phenomena. If there is no interaction between particles, the qualities of retention and fermion should show the natures of super-fluid and metal, respectively." The presence of the interactions on the systemic scale, however, makes the world a more complex place. Condensed matter physics is the study of the effect of this interaction.

Following a traceDr. Chung’s main research topic is the Majorana fermion. The Majorana fermion is known as “the particle that becomes anti-particle of their own.”This concept was introduced by Italian physicist Ettore Majorana in 1937 when he established the theory of the neutrino. Because the Majorana fermion is the key to the study of dark matter,physicists have struggled to directly identify it.What in the Majorana fermion study attracted Dr. Chung to the study of condensed matter physics?

“One of the interesting things about the Majorana fermion is that quantized units look like they are being fractionalized. One well known example of this is the ‘Fractional quantum Hall’ effect. This effect is the phenomenon where the unit charge appears to be one-third of the electron charge. Unless particles do not fractionalize, it is difficult to explain this phenomenon using the laws of particle physics."

If we look at the system as a whole, however, we can avoid this difficulty.” The reason why Dr. Chung joined the Center for Correlated Electron Systems is that his main interest is to analyze phenomena occurring in the entire entity thus finding out what is happening with constituent parts.
“The BCS theory is very useful in explaining what seemed impossible. For example, the nature of electron can be reconciled with superconductivity through pairing of electrons. Similarly, rather than finding the Majorana fermion itself, we will look for evidence of the particles. This approach is often more efficient than the direct attempt to find the particles themselves. We expect that the metal oxides studied at the Center for Correlated Electron Systems will give us the information we seek. Recently, many particle physicists often gain perspective from condensed matter physics. I think my work reflects this trend."

Culture differences in approach“I have been asked a lot about the differences between the domestic and international research environments. I think there is difference in the approach rather than the level of achievement.Let me give you an example. Assuming research institutions are assigned to study a sphere and a donut. Most domestic researchers at the institutions will first check if there is a hole in the center of both the sphere and the donut. On the contrary, most researchers at overseas institutions study the subjects based on basic theories, such as a topological differences. The findings of domestic and international groups are the same but the starting points are different. This is the difference between checking the entire part before applying theory and checking each result before analyzing specific differences. I think that harmony between the approaches would be ideal.”

In the area of technology, the condensed matter physics is the ‘hot’ area. This is because we can quantify new physical properties of materials and develop application methods. Naturally, though condensed matter physics is part of basic science,cooperation with engineering has been very active. “When I studied at Stanford University, I found that the cooperation between the Department of Physics and the Department of Material Science and Engineering was extremely active. I think that interdisciplinary researches are prevalent domestically and that focusing on the basic science leads to the latest technologies. My impression was, at Stanford University, the basic science played the leading role. This feature can be expressed in giving priority to the theories and principles of the entire part. I think that domestic research mainly focuses on individual technologies, consequently, specific results. I think this needs to be complemented. If we focus on only individual performance, we may miss the overall perspective. If we think that innovative research is ‘the research that destroys the existing frame and opens up new areas’, the role of basic science needs to be highlighted more than ever.”

What’s the exchange between researchers seen by Dr. Chung? He emphasizes that it is important to create conditions where researchers in various fields interact freely. For active collaboration and exchange, an environment that encourages such effort needs to be created first.“The most frequent issue in communicating with researchers in other areas is the issue of expression. A concept or term regarded as‘A’ in one area was often regarded as ‘B’ in other areas. When meeting with people who were studying different research topics, it was sometimes hard to communicate due to a lack of understanding of the others research area."

We often envy the overseas researchers who have a congenial exchange on a coffee break or debate topics by writing on a blackboard. For free communication, I think that the number of people who can act as intermediaries needs to be increased. As, “intermediary,” I mean a person who can not only understand the various research activities, but also convey that thought to other researchers.