Samarium hexaboride (SmB6) is an intermediate-valence compound wheresamarium is present both as Sm2+ and Sm3+ions at the ratio 3:7.[2] It is aKondo insulator having a metallicsurface state.[3]
It was first studied by soviet scientists in the early 1960s.[4] Further studies were then undertaken atBell Laboratories.[4] By 1968 their researchers had noted changes in the electronic configuration at different temperatures.[5] 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 narrowband gap of about 4–14 meV.[6] The cooling-inducedmetal-insulator transition in SmB6 is accompanied by a sharp increase inthermal 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 byphonons. The decrease in electron concentration reduced the rate of electron-phonon scattering.[7]
By the twenty first centurycondensed matter physicists grew more interested inSmB6 with claims that it may be atopological insulator.[8][9][10] Other researchers found no evidence of topological surface states.[11]
The increasing electrical resistance with a reduction in temperature indicates that the material behaves as an insulator; however, recent measurements reveal aFermi surface (an abstract boundary of electrons in momentum space) characteristic of a good metal, indicating a more exotic dual metal-insulating ground state.[12][13] The electrical resistivity at temperatures below 4K displays a distinct plateau,[14] which is thought to be the coexistence of an insulating state (bulk) and a conducting state (surface). At temperatures approachingabsolute 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.[12][15][13] While it has been argued that quantum oscillations on samples grown from aluminium flux[16] may arise from aluminum inclusions,[17] such an explanation is excluded for samples grown by the image furnace method[12][14] rather than by the flux growth method.[16][17]
