Study of Bulk and Surface Conduction in Alloys of Samarium Hexaboride

Recent conjecture of topologically-protected surface state in samarium hexaboride (SmB$_{\mathrm{6}})$ and the verification of robust surface conduction below 4 K have led to a large effort to understand this material. Transport properties of SmB$_{\mathrm{6}}$ can be altered by introducing either vacancies or substitutional atoms. Previous studies indicate the material would still exhibit an activated behavior as well as a robust low temperature resistance plateau in the dilute-doped samples. In some cases, the resistance plateau value is a few orders of magnitude lower than that of pure SmB$_{\mathrm{6}}$. This is very difficult to explain with the topological insulator framework, because the increased disorder would normally result in a lower mobility, and therefore a lower conductivity. In order to resolve this issue, we have studied samples with vacancies and samples with La substitution using a double-sided Corbino geometry, which allowed us to extract temperature dependent surface and bulk conductivities. For a sample with 25{\%} vacancy in the Sm site, we found the bulk to be truly insulating and the surface conductivity to be similar to that of pure SmB$_{\mathrm{6}}$, which is inconsistent with previous reports.