The electrical resistance R of SmB6 as a function of temperature T and pressure P has been measured in the range 1 K<∼T<∼300 K and 0< 220 kbar. The behavior of R(T) changes continuously from that of a narrow gap semiconductor to that of a metal in the range of 0< 70 kbar. The dependence of R on T and P can be analyzed phenomenologically within the context of a thermal activation model with an activation energy that decreases linearly with pressure from 33 K at zero pressure to zero at 70 kbar. The data resemble those of SmS and SmSe under pressure and suggest a general behavior of R(T,P) for intermediate-valence Sm compounds. © 1983 The American Physical Society.

We report the results of optical reflectivity measurements on MBe13(M=U,Ce,Th) from 0.1 to 30 eV at T=300 K. The dielectric function ()=1+i2() is determined from a Kramers-Kronig analysis of the data. The intraband and interband contributions to () were separated via a low-energy Drude analysis. The following free-carrier plasma frequencies were obtained: p(UBe13)=5.9 eV, p(CeBe13)=5.3 eV, and p(ThBe13)=4.4 eV. Estimated values for the optical mass of the free carriers fall in the range 1