Samarium hexaboride

Early studies

It was first studied by soviet scientists in the early 1960s. Further studies were then undertaken at Bell Laboratories. By 1968 their researchers had noted changes in the electronic configuration at different temperatures. At temperatures above 50 K its properties are typical of a Kondo metal, with metallic electrical conductivity characterized by strong electron scattering, whereas at low temperatures, it behaves as a non-magnetic insulator with a narrow band gap of about 4–14 meV. The cooling-induced metal-insulator transition in SmB6 is accompanied by a sharp increase in thermal conductivity, peaking at about 15 K. The reason for this increase is that electrons do not contribute to thermal conductivity at low temperatures, which is instead dominated by phonons. The decrease in electron concentration reduced the rate of electron-phonon scattering.

Twenty first century research

By the twenty first century condensed matter physicists grew more interested in with claims that it may be a topological insulator. Other researchers found no evidence of topological surface states.

The increasing electrical resistance with a reduction in temperature indicates that the material behaves as an insulator; however, recent measurements reveal a Fermi surface (an abstract boundary of electrons in momentum space) characteristic of a good metal, indicating a more exotic dual metal-insulating ground state. The electrical resistivity at temperatures below 4K displays a distinct plateau, which is thought to be the coexistence of an insulating state (bulk) and a conducting state (surface). At temperatures approaching absolute zero, the quantum oscillations of the material grow as the temperature declines, a behavior that contradicts both the Fermi analysis and the rules that govern conventional metals. While it has been argued that quantum oscillations on samples grown from aluminium flux may arise from aluminum inclusions, such an explanation is excluded for samples grown by the image furnace method rather than by the flux growth method.

References

References

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11. (2018). "Samarium hexaboride is a trivial surface conductor". Nature Communications.
12. (2015). "Unconventional Fermi surface in an insulating state". Science.
13. Natalie Wolchover. (2 July 2015). "Paradoxical Crystal Baffles Physicists". Quanta Magazine.
14. (2013). "Large, high quality single-crystals of the new Topological Kondo Insulator, SmB6". Nature.
15. Borghino, Dario. (July 7, 2015). "Puzzling material acts as conductor and insulator at the same time".
16. (2014-12-05). "Two-dimensional Fermi surfaces in Kondo insulator SmB6". Science.
17. (2019). "Quantum oscillations in flux-grown SmB6 with embedded aluminum". Physical Review Letters.