KAIST "Visualizes Birth and Disappearance of Electron Patterns... Key to Unraveling Quantum Phenomena"

The mystery behind quantum phenomena inside materials, such as superconductivity, lies in when electrons "move together" and when they "disperse." A team of Korean researchers has succeeded in directly observing the moments when electrons create and break order within a material.

KAIST announced on January 20 that a research team led by Professors Yang Yongsoo, Lee Seongbin, Yang Heejun, and Kim Yonggwan from the Department of Physics, in collaboration with Stanford University in the United States, has successfully visualized the spatial process of charge density waves (CDW) forming and disappearing inside quantum materials.

A charge density wave refers to a pattern, such as stripes or a mesh, formed when electrons in certain quantum materials are cooled to extremely low temperatures and line up at regular intervals, moving in unison.

An image visualizing an experiment measuring the spatial changes of charge density waves formed in quantum materials using a four-dimensional scanning transmission electron microscope in an ultra-low temperature environment based on liquid helium. Provided by KAIST (AI generated)

The superconducting state is a phenomenon where electric current flows with zero energy loss, but it only occurs in specific materials at very low temperatures. Electrons, which carry a negative charge, usually repel each other in ordinary conditions, but in the superconducting state, it is known that electrons move in pairs. This property has already been applied in MRI machines and magnetic levitation trains.

Such special quantum states, created by strong entanglement of charges, form the foundation of next-generation quantum technologies, including quantum computers.

To utilize ultra-low temperature quantum phenomena like superconductivity in quantum computers, it is essential to develop technologies that precisely control electrons within materials. However, because it has been difficult to directly observe how the charge density wave patterns created by electrons form and disappear in ultra-low temperature environments, these processes have remained shrouded in mystery.

To unravel the secret behind the creation and disappearance of electron patterns, the joint research team used a special electron microscope cooled with liquid helium, as well as a four-dimensional scanning transmission electron microscope (4D-STEM). This is similar in principle to filming the growth of ice crystals as water freezes using an ultra-high magnification camera.

However, instead of water, the researchers observed the arrangement of electrons at an ultra-low temperature of minus 253 degrees Celsius, using an electron microscope capable of examining details down to one-tenth the thickness of a human hair, instead of a camera.

By observing the changes in electron patterns in real time using this method, the researchers found that the electron patterns did not appear uniformly throughout the material. In some regions, clear patterns were observed, while in adjacent areas, there were none at all. This is similar to a lake not freezing all at once, but rather having a mix of ice and water.

Group photo of the KAIST research team. Photo by KAIST

The joint research team also discovered that this phenomenon is deeply connected to extremely subtle strains inside the material. The key finding is that small pressures or twists, which are difficult to detect with the naked eye, interfere with the formation of electron patterns.

Conversely, in some regions, the electron patterns persisted even as the temperature increased. This phenomenon, where "quantum order" remains isolated like small islands even at high temperatures, had previously been difficult to explain with conventional theories.

The research team also quantitatively determined how far electrons forming the charge density wave patterns influence each other. This achievement goes beyond simply observing the presence or absence of patterns and provides a new analytical framework for quantum material research by offering clues to how electronic order is connected and maintained.

Professor Yang Yongsoo stated, "Until now, we had to rely on theory or indirect measurements to study the subtle changes in electronic order and quantum states at ultra-low temperatures, but now we can directly observe them with our own eyes. This research will serve as a major breakthrough, accelerating the development of materials for future quantum technologies by revealing the hidden order of quantum materials."

Meanwhile, Hong Seokjo, Oh Jaehwan, and Park Jemin from KAIST participated as co-first authors in this research. The results were published in the international physics journal Physical Review Letters on January 6.