Magnetic quantum oscillations have been unexpectedly observed in insulators, where freely moving charge carriers are not expected to exist. A joint study by researchers from Tokyo University of Science, The University of Tokyo, and Kobe University investigated this puzzling behavior in the Kondo insulator YbB12 using ultrasound. While no oscillations were detected in the insulating state, clear signals emerged after the material became metallic, offering new insight into unusual quantum behavior in next-generation materials.
The organization and movement of electrons within a material determine whether it conducts electricity, behaves magnetically, or even becomes superconducting. Since this electronic behavior cannot be observed directly, scientists study it by observing how materials respond to external forces such as magnetic fields. One important method involves magnetic quantum oscillations (MQOs), tiny repeating changes in physical properties that occur when electrons reorganize into specific energy states under a magnetic field.
MQOs are usually observed in metals and semimetals, where electrons can move freely through the material. However, unexpectedly, they have also been found in insulators, which do not host freely moving charged quasiparticles or a Fermi surface, leaving open the question of what causes these oscillations.
Now, a research team led by Dr. Ryosuke Kurihara of the Department of Physics and Astronomy at Tokyo University of Science (TUS), Japan, in collaboration with Dr. Atsuhiko Miyata of the Institute for Solid State Physics, The University of Tokyo, Japan, Dr. Shusaku Imajo of the Department of Advanced Materials Science, The University of Tokyo and Dr. Ruo Hibino of the Department of Physics at Kobe University, Japan, has observed quantum oscillations in the field-induced metallic phase of ytterbium dodecaboride (YbB12) using ultrasonic measurements, while notably finding no such oscillations in its insulating state. Their findings, published in the journal Physical Review B on June 16, 2026, provide new insight into the puzzling behavior of this topological Kondo insulator. The paper was also selected as an Editors' Suggestion, a distinction awarded by the journal to highlight particularly significant and noteworthy research.
"Since it is known that the quantum oscillations of YbB12 are sample-dependent, we wanted to verify whether quantum oscillations are truly invisible with ultrasound by using a sample in which quantum oscillations have already been confirmed, and the data have been compiled into a paper. In our previous ultrasonic studies of YbB12, no quantum oscillations were detected. Furthermore, if quantum oscillations are not visible with ultrasound, there must be some underlying physics that explains why they are not visible, and we wanted to verify that as well," says Dr. Kurihara.
YbB12 is a Kondo insulator, a material in which strong interactions between electrons open an energy gap at low temperatures, causing it to behave as an insulator despite exhibiting unusual metallic-like electronic properties. However, previous magnetoresistance measurements, which track how a material's electrical resistance changes in a magnetic field, detected quantum oscillations while the material was still insulating. This contradicted the conventional understanding that MQOs require freely moving charged particles.
To better understand this behavior, the researchers performed bulk-sensitive ultrasonic experiments on a high-quality crystal of YbB12 that had already shown quantum oscillations in earlier magnetoresistance studies. They exposed the material to magnetic fields of up to 65 T while cooling it to temperatures as low as 485 mK, close to absolute zero.
The researchers used ultrasonic measurements to track how the material's elastic properties responded to magnetic fields, focusing on two elastic constants known as C11 and C44. In the insulating state below the field-induced insulator-to-metal transition near 45 T, the team found no clear evidence of magnetoacoustic QOs (MAQOs). Instead, they observed unusual features in the longitudinal elastic constant C11, including subtle dip-like and kink-like changes around 10 and 30 T, as well as another anomaly near 39 T. However, once YbB12 transitioned into a metallic state above approximately 45 T, clear quantum oscillations appeared in both acoustic modes, suggesting that sound waves interacted more strongly with quasiparticles in the metallic phase.
The researchers suggest that the absence of MAQOs in the insulating state may result from weak coupling between the proposed fermionic quasiparticles and acoustic phonons, the vibrations that carry sound through the material. In addition, the large cyclotron mass of these quasiparticles may suppress the oscillatory signal, making it difficult to detect using ultrasound.
By clarifying when and how these oscillations appear, the study may help researchers better understand unusual quantum states in materials and advance the search for next-generation quantum materials.
"Our findings provide further information on the puzzling behavior of the insulating state of YbB12," says Dr. Kurihara.
