Physicists led by Lu Li at the University of Michigan have discovered quantum oscillations originating from the bulk of an insulating material, challenging long-held assumptions in physics. The finding, published in Physical Review Letters, suggests a new duality where materials can behave as both metals and insulators. This bizarre phenomenon was observed using powerful magnets at the National Magnetic Field Laboratory.
Lu Li, a professor of physics at the University of Michigan, and an international team of scientists have uncovered quantum oscillations in the heat capacity of the Kondo insulator ytterbium boride (YbB12). These oscillations, typically seen in metals where electrons act like tiny springs vibrating in magnetic fields, were previously debated in insulators: whether they arise from the surface or the bulk interior.
The researchers conducted experiments at the National Magnetic Field Laboratory, utilizing magnets up to 35 Tesla—about 35 times stronger than a hospital MRI. Their results, detailed in Physical Review Letters (2025; 135 (15)), confirm the oscillations are intrinsic to the material's bulk, not just a surface effect.
"I would love to claim that there's a great application, but my work keeps pushing that dream further away," Li said. "But what we've found is still really bizarre and exciting."
The discovery points to a "new duality" in physics, akin to the wave-particle duality of light and matter discovered over a century ago. Li explained, "Effectively, we're showing that this naive picture where we envisioned a surface with good conduction that's feasible to use in electronics is completely wrong. It's the whole compound that behaves like a metal even though it's an insulator."
The collaboration involved over a dozen scientists from six institutions in the United States and Japan, including 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 pursued the answer to a fundamental question about the carrier origin in this exotic insulator: Is it from the bulk or the surface, intrinsic or extrinsic?" said Chen. "We are excited to provide clear evidence that it is bulk and intrinsic."
Zhu added, "Confirming that the oscillations are bulk and intrinsic is exciting. We don't yet know what kind of neutral particles are responsible for the observation. We hope our findings motivate further experiments and theoretical work."
Li admitted, "I wish I knew what to do with that, but at this stage we have no idea. What we have right now is experimental evidence of a remarkable phenomenon, we've recorded it and, hopefully, at some point, we'll realize how to use it."
The project was supported by the U.S. National Science Foundation, U.S. Department of Energy, Institute for Complex Adaptive Matter, Gordon and Betty Moore Foundation, Japan Society for the Promotion of Science, and Japan Science and Technology Agency.