Samarium hexaboride (SmB₆) is an intermediate-valence compound where

samarium Samarium is a chemical element with symbol Sm and atomic number 62. It is a moderately hard silvery metal that slowly oxidizes in air. Being a typical member of the lanthanide series, samarium usually has the oxidation state +3. Compounds of samar ...

is present both as Sm²⁺ and Sm³⁺

ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

at the ratio 3:7. It belongs to a class of

Kondo insulator In solid-state physics, Kondo insulators (also referred as Kondo semiconductors and heavy fermion semiconductors) are understood as materials with strongly correlated electrons, that open up a narrow band gap (in the order of 10 meV) at low tem ...

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

electron scattering Electron scattering occurs when electrons are deviated from their original trajectory. This is due to the electrostatic forces within matter interaction or, if an external magnetic field is present, the electron may be deflected by the Lorentz fo ...

, whereas at low temperatures, it behaves as a non-magnetic insulator with a narrow

band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...

of about 4–14 meV. The cooling-induced metal-insulator transition in SmB₆ is accompanied by a sharp increase in

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

, 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

phonon In physics, a phonon is a collective excitation in a periodic, Elasticity (physics), elastic arrangement of atoms or molecules in condensed matter physics, condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phon ...

s. The decrease in electron concentration reduced the rate of electron-phonon scattering. Some research claims that it may be a

topological insulator A topological insulator is a material whose interior behaves as an electrical insulator while its surface behaves as an electrical conductor, meaning that electrons can only move along the surface of the material. A topological insulator is an ...

. 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 In condensed matter physics, the Fermi surface is the surface in reciprocal space which separates occupied from unoccupied electron states at zero temperature. The shape of the Fermi surface is derived from the periodicity and symmetry of the cryst ...

(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 Absolute zero is the lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as zero kelvin. The fundamental particles of nature have minimum vibration ...

, 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

{{Samarium compounds Samarium compounds Borides Correlated electrons

See also

References