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Researchers have discovered quantum oscillations of electrical resistivity in an insulator for the first time. The new discovery, made in the mixed valence Kondo insulator ytterbium dodecaboride, is unexpected since these oscillations are usually only seen in metals. The result will help shed more light on the electronic properties of these unusual materials, which are important for fundamental studies in condensed-matter physics.

The orbital motions of conduction electrons on the Fermi surface in metals are quantized in magnetic fields and show up as quantum oscillations in the electrical resistivity of the metal. This so-called Landau quantization is not usually seen in insulators, however. A team of researchers led by Lu Li of the University of Michigan in the US and Yuji Matsuda of the University of Kyoto in Japan has now found that it exists in crystals of ytterbium dodecaboride (YbB12). The bulk of this rare-earth intermetallic compound is an insulator but its surface conducts electricity very efficiently. The quantum oscillations observed by Li and Matsuda’s teams, surprisingly, come from the insulating bulk.

The result is in line with the discovery of bulk quantum oscillations in a bulk insulator in 2015 made by Suchitra Sebastian’s team at the University of Cambridge in the UK.

Li and colleagues obtained their result thanks to measurements using the strongest DC magnetic fields available (of 45 Tesla) at the National High Magnetic Laboratory in Tallahassee, Florida. They passed an electrical current through their sample while applying extremely high magnetic fields and measured the electric voltage decrease in it. This technique, which they performed at different temperatures, allowed them to determine the resistivity of the sample.

“We found that the resistivity of YbB12, which is of a much larger magnitude than the resistivity of metals, exhibits distinct quantum oscillations,” explains Li. “These unconventional oscillations arise from the insulating bulk, even though the temperature dependence of the oscillation amplitudes follows the expected and conventional Fermi liquid theory of metals (the “Lifshitz-Kosevich formula”).

“This result confirms that Kondo insulators have a dual nature – they are both electrical insulators and itinerant metals,” he tells Physics World. “This duality is a surprising consequence of the strong correlations between the electrons in the material.”

The team, which includes researchers from the Université Chretienne Bilingue du Congo, the Japan Synchrotron Radiation Research Institute, the Clarendon Laboratory at the University of Oxford and Ibaraki University, says that it is now repeating its measurements in pulsed magnetic fields. “These studies should help us pin down the angular dependence of the quantum oscillations and allow us the map of the geometry of Fermi surfaces in the insulating state,” reveals Li.

The present research is detailed in Science 10.1126/science.aap9607.

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