U-M Physics Professor Lu Li, Dr. Kuan-Wen Chen, Dr. Ziji Xiang and Research Teams Reveal a New State of Matter in Kondo Insulator
University of Michigan’s Professor of Physics Lu Li, along with two main contributors, Postdoctoral Fellows Dr. Kuan-Wen Chen and Dr. Ziji Xiang, conducted a recent series of experiments at the National High Magnetic Field Laboratory (National MagLab) at Los Alamos National Laboratory. Along with researchers from Kyoto University and Los Alamos, they leveraged some of the nation’s highest-powered nondestructive magnets to reveal an exotic new phase of matter at high magnetic fields. The experiments studied the unusual Kondo insulator ytterbium dodecaboride (or YbB12) and were the first results from the new 75-tesla duplex magnet housed at the National MagLab’s Pulsed Field Facility at Los Alamos.
The results of the experiment were recently published in Nature Physics. Among the co-authors from the University of Michigan Department of Physics were three recent alumni from Professor Li’s experimental research group: Dr. Lu Chen (Ph.D. ’20), winner of the 2021 ProQuest Rackham Distinguished Dissertation Award), Dr. Tomoya Asaba (Ph.D. ’18), and Dr. Colin Tinsman (Ph.D. ’19).
Metal or insulator? That is the question. This new paper reveals the dual nature of the topological Kondo insulators. They are perfect insulators like pure silicon, and their electrical resistivity diverges by more than a million times during cooling down. Yet, they show a characteristic feature of good metal—oscillations in magnetization under magnetic fields. The researchers discovered this contradiction, with a quest to observe insulators’ oscillations, not only in magnetization but also in electrical resistivity. The experiments demonstrate that the oscillatory carriers are just like electrons, following the Fermi-Dirac distributions, even in this perfect insulator. So, can the compound be both metal and insulator? Or can a fermion exist in solids even without electrical charge? They answer the question in the paper.
In Kondo insulators, there is an unusual quantum-mechanical mixing of mobile electrons and magnetic atoms, making these materials attractive as model systems for basic science, electronic devices, and possibly quantum computing. Unlike simple metals and insulators, YbB12 exhibits properties of both—its electrical resistance behaves like that of an insulator, but it also clearly shows quantum oscillations at high magnetic fields that are a fundamental metallic property.
“Quantum oscillation is a characteristic feature of metals. However, in our observation, quantum oscillations in an insulator behave like a typical metal, whereas those shown in the higher field metal behave rather differently than normal ones. This is an exciting mystery, and we hope to keep digging,” said University of Michigan Professor Lu Li.
“A plethora of theories has emerged to account for such behavior,” said John Singleton, a fellow at the MagLab’s Los Alamos campus and co-author on the paper. “Some physicists believe this is a condensed-matter incarnation of neutral Majorana fermions, entities normally explored in particle physics.”
To test these theories, the research team wanted to observe how the neutral fermions that they found in YbB12 responded to extreme conditions. The high magnetic fields in the new 75-tesla duplex magnet available at the National MagLab’s Pulsed Field Facility were used to suppress the insulating properties of YbB12 and measure quantum oscillations and various properties that are affected by the presence of the neutral fermions.
“The extra 10 tesla above our standard pulsed magnets provided by the duplex magnet enabled this new state of matter — exotic fermions gradually being drowned in a sea of normal electrons — to be tracked across a wide range of magnetic fields for the first time,” said Singleton. This confirmed that the phenomena observed were definitely associated with the neutral fermions and provided a test of the various theoretical models.
Paper: Xiang, Z., Chen, L., Chen, KW. et al. Unusual high-field metal in a Kondo insulator. Nature Physics. (2021). https://doi.org/10.1038/s41567-021-01216-0
Laura Mullane, Los Alamos National Laboratory Contact, 505.412.7733, firstname.lastname@example.org