Large low-temperature Hall effect and resistivity in mixed-valent SmB6

The Hall coefficient $R_H$ has been measured between 2 and 300 K for small crystals of Sm${\mathrm{B}}_6$ in which the resistivity $\rho$ at 4 K is nearly ${10}^4$ times greater than its 300-K value of 290 μΩ cm. Significant differences from previous results reported for samples with a much smaller low-temperature resistivity rise have been found. It is shown that the size of the resistivity increase precludes its being ascribed to a scattering mechanism for a metallic number of carriers, and that the size of $R_H$ implies numbers of holes and electrons less than ∼5 × ${10}^{17}$/${\mathrm{cm}}^3$ at 4 K. The origin of the low-temperature residual conductivity is discussed.