What is SmB6: Samarium hexaboride

Samarium hexaboride (SmB₆) is an intermediate-valence compound where samarium is present both as Sm²⁺ and Sm³⁺ ions at the ratio 3:7. It belongs to a class of Kondo insulators.

At temperatures above 50 K its properties are typical of a Kondo metal, with metallic electrical conductivity characterized by strongelectron 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.

New research seems to show that it may be a topological insulator.

Its electrical resistance indicates that the material behaves as an insulator; however, its Fermi surface (an abstract boundary used to reliably predict the properties of materials) contradicts this, indicating that the material actually behaves as a good metal. 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.

Researchers have identified  material (SmB6: Samarium hexaboride) that behaves as a conductor and an insulator at the same time, challenging current understanding of how materials behave, and pointing to a new type of insulating state.

(Image: PhD student Maria Kiourlappou holding a piece of SmB6)

The material, a much-studied compound called samarium hexaboride or SmB6, is an insulator at very low temperatures, meaning it resists the flow of electricity. Its resistance implies that electrons (the building blocks of electric currents) cannot move through the crystal more than an atom’s width in any direction. And yet, Sebastian and her collaborators observed electrons traversing orbits millions of atoms in diameter inside the crystal in response to a magnetic field — a mobility that is only expected in materials that conduct electricity. Calling to mind the famous wave-particle duality of quantum mechanics, the new evidence suggests SmB6 might be neither a textbook metal nor an insulator, Sebastian said, but “something more complicated that we don’t know how to imagine.”