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Samarium
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====Samarium hexaboride==== {{Main|Samarium hexaboride}} Samarium hexaboride is a typical intermediate-valence compound where samarium is present both as Sm<sup>2+</sup> and Sm<sup>3+</sup> ions in a 3:7 ratio.<ref name="smb6b">{{cite journal|last1=Nickerson|first1=J.|last2=White|first2=R.|last3=Lee|first3=K.|last4=Bachmann|first4=R.|last5=Geballe|first5=T.|last6=Hull|first6=G.|title=Physical Properties of SmB<sub>6</sub> |journal=Physical Review B|volume=3|page=2030|date=1971|doi=10.1103/PhysRevB.3.2030|issue=6|bibcode=1971PhRvB...3.2030N }}</ref> It belongs to a class of [[Kondo insulator]]s; at temperatures above 50 K, its properties are typical of a Kondo metal, with metallic electrical conductivity characterized by strong electron scattering, whereas at lower temperatures, it behaves as a non-magnetic insulator with a narrow [[band gap]] of about 4β14 [[electronvolt|meV]].<ref>{{cite journal |doi=10.1103/PhysRevB.52.R14308 |pmid=9980746 |last1=Nyhus |date=1995 |first1=P. |pages=14308β14311 |volume=52|last2=Cooper|journal=Physical Review B|first2=S.|last3=Fisk|first3=Z.|author4-link=John Sarrao |last4=Sarrao |first4=J. |title=Light scattering from gap excitations and bound states in SmB<sub>6</sub> |issue=20|bibcode=1995PhRvB..5214308N }}</ref> The cooling-induced metal-insulator transition in SmB<sub>6</sub> is accompanied by a sharp increase in the [[thermal conductivity]], peaking at about 15 K. The reason for this increase is that electrons themselves do not contribute to the thermal conductivity at low temperatures, which is dominated by [[phonon]]s, but the decrease in electron concentration reduces the rate of electron-phonon scattering.<ref>{{cite journal |last1=Sera |first1=M. |last2=Kobayashi |first2=S. |last3=Hiroi |first3=M.|last4=Kobayashi|first4=N.|last5=Kunii|first5=S.|title=Thermal conductivity of RB<sub>6</sub> (R=Ce, Pr, Nd, Sm, Gd) single crystals |journal=Physical Review B |volume=54 |date=1996 |doi=10.1103/PhysRevB.54.R5207 |pmid=9986570|issue=8 |pages=R5207βR5210|bibcode=1996PhRvB..54.5207S }}</ref>
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