Studying everyday materials and creating substances with new properties leads to the development of products that make society more convenient. The field of study that examines the properties of interest and explores new substances is called "condensed matter physics," and research has been flourishing in recent years with the introduction of the concept of topology. RIKEN ECL Research Unit Leader Yukako Fujishiro at the RIKEN Center for Emergent Matter Science, aims to develop materials for the next generation through research on electronic properties that utilize the geometric characteristics of topology.
Aspiring to be a researcher before elementary school: "Making life better" rather than romance
The materials we handle daily are made up of vast numbers of atoms and molecules. The properties of each material are largely determined by the movement of electrons within the atoms and molecules. For example, metals such as iron, gold, and silver have many electrons that can move freely inside them, resulting in high electrical and thermal conductivity. On the other hand, rubber and glass have no freely moving electrons, making them insulators that do not conduct electricity. Additionally, materials that conduct or do not conduct electricity depending on conditions are called semiconductors.
Semiconductors are essential for the smartphones and computers that we use in our daily lives. There are countless other products brought to society through condensed matter physics research, including LEDs, DVDs, liquid crystal panels, and solar panels.
RIKEN ECL Research Unit Leader Yukako Fujishiro is also taking on the challenge of developing materials with new functions. In particular, she has achieved remarkable results in cutting-edge research examining the behavior and properties of electrons in solids using topology theory. Fujishiro says that, influenced by her parents, who were computer science researchers, she wanted to become a researcher even before entering elementary school. "To my younger self, science had an image of dreams and romance. However, when I learned in university lectures that there were people trying to make life better using science, I decided to major in condensed matter physics," she recalls.
"Skyrmions" of swirling magnetism: Expected application for information bits and more
Groups of electrons in materials sometimes exhibit behavior that cannot be understood with existing electromagnetic knowledge. One such structure is called a "skyrmion." Electrons have a property called "spin," which is deeply related to the magnetic properties of materials. In most materials, the spin directions are random, so they have no magnetic force, but in some materials such as iron, the spins align in one direction, giving them magnetic force. In skyrmions, thousands of electrons in a material come together, with their spin directions forming a vortex. When skyrmions form, properties appear from a new phase that is neither a magnetic phase nor a non-magnetic phase, such as the generation of a magnetic field felt only by electrons moving in the material.
Skyrmions were originally a particle model proposed in nuclear physics but later attracted attention in the field of condensed matter physics, and the existence of skyrmions in materials was confirmed in 2009. Furthermore, the 2016 Nobel Prize in Physics was awarded to three physicists who elucidated the mechanism of phase transitions through topology theory, providing additional momentum. Research on topological materials is now advancing worldwide.
In these materials, topological magnetic structures, including skyrmions, are known to couple with electrons and generate a virtual effective magnetic field called an "emergent magnetic field" in solids, dramatically affecting electron motion. Therefore, elucidating this mechanism and controlling electron movement has become an important challenge that forms the foundation for various electronic devices, and is expected to lead to next-generation storage media, energy-efficient devices, and ultra-miniaturization of conventional devices.
Student days spent fascinated by phase transitions: Discovery of the world's highest-density spin structure
Fujishiro is working to elucidate unknown electronic properties that occur when these topological magnetic structures undergo phase transitions, utilizing extreme conditions such as ultrahigh pressure and strong magnetic fields. One of Fujishiro's research achievements is the successful synthesis of the solid solution MnSi1-xGex from the topological material "manganese silicon (MnSi)," which has a skyrmion structure, and the topological material "manganese germanium (MnGe)," which has an emergent magnetic monopole structure with a hedgehog-like spin structure (Figure 1).
One day, Fujishiro, who says she has been captivated by the appeal of phase transitions since her student days, wondered what structure an intermediate solid solution between MnSi and MnGe would take. In other words, what kind of topological magnetic structure would be created in a material mixing manganese, germanium, and silicon?
Fujishiro created numerous samples with gradually changing ratios of germanium and silicon and repeated experiments. Since they don't mix well under normal pressure, she applied ultrahigh pressure of about 60,000 atmospheres and heated them to several hundred degrees (Figure 2). She discovered that new topological magnetic structures were forming in the materials created in this way. Her discovery led to clarification of the process by which skyrmion structures are converted to emergent magnetic monopole structures.
"This structure was, at the time, the world's highest-density spin structure, capable of creating a larger magnetic field in the material. Being able to create something new with my own abilities for the first time gave me great confidence," Fujishiro says, emphatically.
Achieving results by breaking hard-to-break structures: Opening the path to devices that efficiently utilize waste heat
Once topological structures form, many of them exist stably. Therefore, attempting to break that state requires significant energy, similar to how thermal energy is needed for solid ice to change into liquid water. Fujishiro investigated what phenomena would occur when applying a strong magnetic field to the topological material MnGe to break its spin structure. During this process, she found that giant magnetic fluctuations occurred, and the thermoelectric efficiency-the efficiency of converting heat to electricity-became one order of magnitude larger than in ordinary metal compounds. "When I'm told something is hard to break, I can't help wanting to break it somehow, and these results came from that," Fujishiro laughs.
We consume various forms of energy, including electrical energy, in our daily lives. However, not all electrical energy can be converted into work, and just as electrical appliances heat up during use, some is dissipated as thermal energy. The huge thermoelectric effect in topological magnetic structures discovered by Fujishiro represents an entirely new principle of high-efficiency thermoelectric conversion, with the potential for application to devices that use waste heat and other sources more efficiently.
Based on her achievements so far, she won the Grand Prize of the Fourth Marie Sklodowska Curie Award in 2025 (Figure 3). "I was surprised when I heard the news of the award, thinking 'Me, of all people?' I want to continue my research while expanding my network worldwide, and I hope this will be an opportunity for many people to learn about condensed matter physics," she says with joy.
Photo courtesy of the Embassy of the Republic of Poland in Japan.
Exploring properties under ultrahigh pressure: New materials unconstrained by common sense
Currently, Fujishiro is taking on the challenge of creating materials with new properties in extreme environments, including ultrahigh pressure conditions (Figure 4). She is particularly interested in superconducting materials. Superconductivity is a phenomenon where electrical resistance becomes zero, its history beginning with its discovery in mercury in 1911, and research has been advancing. Nevertheless, much about the properties of materials under ultrahigh pressure environments remains unknown. From Fujishiro's research alone, phase transitions involving state changes are known to occur. In the future, she plans to explore not only the topological spin structures she has worked on so far, but also new superconducting materials.
Previous materials research has centered on the 1 atmosphere state in which we normally live, but there are limits to searching for new materials only in the normal pressure world. "Those of us living on Earth take the 1 atmosphere environment for granted, but when it comes to materials, different environments may allow them to demonstrate their true capabilities. I want to search for interesting new materials without being bound by common sense," she says, describing her outlook.
Fujishiro also speaks of her interest in activities to convey the joy of science to children. "When I ask fellow researchers what inspired them to become researchers, many talk about when they were in the first or second grade of elementary school. By interacting with children of that age, I want to help them become interested in science and create an environment where they can casually aspire to become researchers," she says, showing enthusiasm for outreach activities. It will be exciting to see how Fujishiro, who is pioneering new condensed matter physics from topology, will soar.
(Article: Yoshitaka Arafune, Photography: Erika Shimamoto)
Condensed matter physics and topology
Topology as a field of mathematics is a discipline that focuses on properties preserved even when a shape or space is continuously deformed, just as the property of having one hole remains unchanged even when a mug is deformed into a donut shape. When continuous deformation can reach one form from the other, as with a mug and a donut, they are said to "have the same topology." When deformation cannot reach one form from the other, as with a sphere without holes and a donut, they are said to "have different topologies."
For a long time, condensed matter physics focused on the characteristics of electronic band structures and spin structures of individual materials. However, topology focuses on the "twisting" of band structures and spin structures created by electrons in materials. By introducing the new concept of "topological invariants"-a new quantity that characterizes twisting-it became clear that phenomena unexplainable by conventional theories could be explained and predicted. Today, it is attracting attention as an indispensable concept for understanding the properties of nanoscale magnetic vortex structures and new groups of materials that do not fit within the frameworks of insulators, conductors, and semiconductors.
