In a remarkable advancement in the field of physics, Li, a professor at the University of Michigan, has collaborated with an international team of scientists to uncover a puzzling phenomenon known as quantum oscillations. This discovery, recently published in the prestigious Physical Review Letters, has captivated the attention of the scientific community. Although Li expresses a desire for immediate applications of the findings, he acknowledges that the research continues to push those dreams further out of reach. "What we've found is still really bizarre and exciting," he remarked.
The research, supported by the U.S. National Science Foundation and the U.S. Department of Energy, centers around the effect of quantum oscillations, which typically occurs in metals. In these materials, electrons behave akin to tiny springs, vibrating in response to external magnetic fields. By adjusting the strength of these magnetic fields, scientists can influence the oscillation frequency of these electron springs. Interestingly, recent studies have revealed that similar quantum oscillations can also occur in insulators—materials traditionally known for their inability to conduct electricity or heat. This groundbreaking revelation has sparked intense debate among researchers regarding whether these oscillations arise from the surface of the materials or from their internal structure, known as the bulk.
If it is determined that the oscillations originate from the surface, the implications for future technologies could be significant. Topological insulators, for example, are materials that can conduct electricity on their surfaces while remaining insulating internally. These materials are already under investigation for their potential applications in next-generation electronic, optical, and quantum devices. To delve deeper into this mystery, Li and his collaborators utilized the National Magnetic Field Laboratory, home to the world's most powerful magnets. Their experiments provided compelling evidence that the oscillations do not merely occur at the surface; rather, they are intrinsic properties derived from the bulk of the material itself. "I wish I knew what to do with that, but at this stage, we have no idea," Li admitted. "What we have right now is experimental evidence of a remarkable phenomenon, and hopefully, we'll eventually discover how to harness it."
The study was a collaborative effort involving over a dozen scientists from six institutions across the United States and Japan. Notable contributors included research fellow Kuan-Wen Chen and graduate students Yuan Zhu, Guoxin Zheng, Dechen Zhang, Aaron Chan, and Kaila Jenkins from the University of Michigan. For years, scientists have sought answers to fundamental questions regarding the origin of carriers in this exotic insulator: Are they derived from the bulk or the surface, and are they intrinsic or extrinsic? Chen expressed excitement at providing clear evidence that the oscillations are indeed bulk and intrinsic.
Li characterizes this finding as part of a new duality in physics. The traditional duality, recognized over a century ago, revealed that light and matter can function as both waves and particles. This pivotal discovery transformed the landscape of physics, leading to innovations such as solar cells and electron microscopes. The new duality, as described by Li, pertains to materials that exhibit behaviors characteristic of both conductors and insulators. His team investigated this phenomenon using a compound known as ytterbium boride (YbB12) within an extraordinarily powerful magnetic field of 35 Tesla—approximately 35 times stronger than that of a typical hospital MRI machine. "We're demonstrating that the simplistic notion of a surface with good conduction suitable for electronics is fundamentally flawed," Li elaborated. "It's the entire compound that behaves like a metal, despite being classified as an insulator."
While this metal-like behavior is only observable under extreme magnetic conditions, the findings raise critical questions about material behavior at the quantum level. "Confirming that the oscillations are bulk and intrinsic is thrilling," noted Zhu. "However, we still do not know what kind of neutral particles are responsible for this observation. We hope our findings will inspire further experimental and theoretical investigations." This project received additional support from esteemed organizations including the Institute for Complex Adaptive Matter, the Gordon and Betty Moore Foundation, the Japan Society for the Promotion of Science, and the Japan Science and Technology Agency.