Research Background and Challenges
Electron microscopy, with its atomic-level resolution, has become an indispensable tool for materials observation in contemporary science. However, using electron microscopy to measure the effective electron mass and Fermi velocity presents a novel challenge. Dr. Chu Ming-Wen, Director of the Condensed Matter Center and Co-Director of the Atomic-Level Science Research Center for Novel Materials, as well as the leader of the Electron Microscopy Key Technology Platform at National Taiwan University, developed a method combining transmission electron microscopy with electron energy loss spectroscopy (EELS) to delve into key issues in quantum materials. One such issue is whether the charge-density-wave ordering, a common phenomenon in quantum materials, increases the effective electron mass and slows down the Fermi velocity due to electron correlations, thus affecting related applications.
Breakthrough Discovery
Dr. Chu's team developed the internationally rare momentum-resolved electron energy loss spectroscopy (q-EELS) technique. They used an electron beam to excite charge-density waves in the semimetal CuTe, inducing a dynamic wave oscillation known as plasmon. This allowed CuTe to exhibit both static charge-density wave ordering and dynamic plasmon oscillations. According to plasmon theory developed in the 1950s, dynamic plasmon fluctuations are influenced by the kinetic energy changes induced by static charge-density waves, and this change is a function of the effective electron mass and Fermi velocity. By measuring the energy change of the plasmon, the team could directly measure these key quantum material properties. Despite the fact that the theoretical framework for measuring effective mass and Fermi velocity had been neglected for nearly 70 years and experimental validation was extremely difficult, Dr. Chu’s team successfully demonstrated that plasmon can be used to capture electron effective mass and Fermi velocity. Furthermore, they discovered that the static charge-density wave in CuTe actually makes the Dirac-like relativistic fermions in the material have lighter effective mass and faster Fermi velocity, which is more favorable for quantum applications. This counterintuitive discovery was verified through first-principles electronic structure and spectral calculations conducted by Professor Hung-Chung Hsueh's team at Tamkang University.
Technical Implementation and Collaboration
This research not only uncovered a theory that had been overlooked for nearly 70 years but also provided new insights into charge-density wave ordering, becoming a driving force for the functionality of quantum materials. The momentum-resolved electron energy loss spectroscopy results were achieved with the guidance of Professor Chun-Wei Chen from National Taiwan University’s Department of Materials Science, as well as the collaborative efforts of PhD student Yi-Da Wang from the International PhD Program in Molecular Science and Technology, and Dr. Chou Da-Lei, a postdoctoral researcher at the Condensed Matter Center. Despite using an electron microscopy and energy loss spectroscopy device that was nearly 21 years old, with energy resolution far below current standards, the results garnered widespread international attention, leading to invitations from laboratories in the United States, Canada, and Japan to conduct high-energy-resolution electron energy loss spectroscopy experiments and share experimental techniques for collaborative research.
Significance and Outlook
Dr. Chu sincerely hopes that Taiwan will enter the high-energy-resolution and high-momentum-resolution electron energy loss spectroscopy field as soon as possible. This will not only serve as an important tool for quantum materials research but also be used to probe the effective electron mass and Fermi velocity in small semiconductor devices. Fundamentally, this technology can stay in Taiwan, contributing to scientific breakthroughs and strengthening Taiwan’s technological prowess. Special thanks go to Professor Chin-Shan Lue and PhD student Chia-Nung Kuo from the Department of Physics at National Cheng Kung University for providing high-quality CuTe single crystals, which laid a solid foundation for the success of this research, from crystal growth to advanced spectral measurements and microscopic theoretical verification. This research was published in Nature Communications on October 29, 2024, and the journal editors summarized the findings as follows: "Charge density waves usually cause electrons to become heavier and slow down. Here, the authors find the opposite is true in CuTe and study the phenomenon using advanced q-EELS."
研究團隊。
Link to the paper: https://www.nature.com/articles/s41467-024-53653-z
Source: https://www.ntu.edu.tw/spotlight/2024/2334_20241211.html