Direct Visualization of Electron Spatial Order in Quantum Materials Unveiled

The mystery of quantum phenomena inside materials—such as superconductivity, where electric current flows without energy loss—lies in when electrons move together and when they break apart. KAIST researchers have succeeded in directly observing the moments when electrons form and dissolve ordered patterns. Research teams led by Professors Yongsoo Yang, SungBin Lee, Heejun Yang, and Yeongkwan Kim of the Department of Physics, in an international collaboration with Stanford University, have become the first in the world to spatially visualize the formation and disappearance of charge density waves (CDWs) inside quantum materials. The research is published in Physical Review Letters. Superconductivity is a state in which electrical current flows with 100% efficiency and no energy loss, occurring only in certain materials at extremely low temperatures. While electrons, which carry negative charge, normally repel each other, electrons in a superconducting state are known to pair up and move together in an unusual way. This property is already utilized in technologies such as MRI scanners and maglev trains. Such strongly correlated quantum states, formed by tightly entangled charges, also serve as the foundation for next-generation quantum technologies, including quantum computers. Challenges in observing quantum patterns To apply superconductivity and other ultralow-temperature quantum phenomena to quantum computing and related technologies, it is essential to precisely control electrons inside materials. However, the spatial patterns of charge density waves formed by electrons at cryogenic temperatures have remained largely hidden, as their formation and disappearance are extremely difficult to observe directly. The research team employed a special electron microscope cooled with liquid helium, along with four-dimensional scanning transmission electron microscopy (4D-STEM), to observe changes in electronic patterns in real time. This study is analogous to filming the growth of ice crystals as water freezes using an ultra-high-magnification camera. The difference is that instead of water, the researchers observed electrons arranging themselves at temperatures as low as approximately –253 °C, and instead of a camera, they used an electron microscope capable of resolving features as small as one hundred-thousandth the width of a human hair. Key findings and implications The results showed that electronic patterns do not appear uniformly throughout the material. In some regions, clear stripe-like patterns were visible, while in adjacent areas, no such patterns appeared at all—resembling a lake that does not freeze all at once, but instead contains a mixture of ice and liquid water. The team further revealed that this phenomenon is closely linked to extremely subtle internal strain within the material. Tiny pressures or distortions—imperceptible to the naked eye—can disrupt the formation of electronic patterns. Conversely, in certain regions, the electronic patterns persisted even as the temperature increased, instead of disappearing readily. This behavior suggests the existence of isolated "islands" of quantum order that remain stable at higher temperatures—an observation that is difficult to explain using existing theoretical frameworks. Another major achievement of the study was the world's first quantitative determination of how far electrons forming a charge density wave influence one another spatially. This goes beyond simply identifying whether a pattern exists or not, providing a new analytical framework for understanding how electronic order is connected and maintained within quantum materials. Charge density waves and superconductivity are known to sometimes compete with each other and sometimes cooperate. The findings of this study therefore connect naturally to research on high-temperature superconductors. By understanding the conditions under which electronic patterns remain stable, researchers can open new pathways toward designing materials in which superconducting currents flow more efficiently. Professor Yongsoo Yang, who led the study, said, "Until now, we had to rely on theory or indirect measurements to study subtle changes in electronic order and quantum states at ultralow temperatures. This work allows us to directly 'see' these phenomena for the first time. "By uncovering hidden order in quantum materials, this research provides a crucial breakthrough that will accelerate the development of materials for future quantum technologies."