An insulator with conducting electrons?

Samarium hexaboride is a curious material. For starters, it’s a Kondo insulator. Jun Kondo proposed in 1963 that the decrease in conductivity observed in some metals at low temperature is due to interactions between fixed, localized spins—magnetic impurities—and delocalized, roaming conduction electrons. In SmB₆, a good conductor at room temperature, it is the localized f-shell electrons that interact with the conduction electrons through the Kondo effect, and below 50 K that coupling turns the material into an insulator. At still lower temperatures, below about 4 K, SmB₆ is also increasingly suspected of being a topological insulator: Though insulating in bulk, it appears to support two-dimensional conducting surface states that have zero effective mass, have fixed spin orientation, and are protected by symmetry from scattering. (For more on topological insulators, see the article by Xiao-Liang Qi and Shou-Cheng Zhang, Physics Today, January 2010, page 33 .) Now Suchitra Sebastian (Cambridge University) and her colleagues report additional unusual behavior in SmB₆: quantized oscillations in the material’s magnetization as a function of magnetic field (see figure). The oscillations are a bulk feature—they don’t correspond to the protected surface states. Yet the bulk material is insulating, while the quantized oscillations are a hallmark of conduction electrons, a so-called Fermi liquid. Indeed, the oscillations in SmB₆ strongly resemble those seen in metallic hexaborides that incorporate lanthanum, cerium, and praseodymium instead of samarium. But as the temperature cools below 2 K, the quantum oscillations grow dramatically and deviate markedly from conventional metal behavior. The SmB₆ results rule out impurities and other easy explanations, leaving a puzzle for experimentalists and theorists to solve. (B. S. Tan et al., Science 349, 287, 2015, doi:10.1126/science.aaa7974 .)