Field-dependent heat transport in the Kondo insulator SmB 6 : Phonons scattered by magnetic impurities

The thermal conductivity $\kappa$ of the Kondo insulator SmB$_6$ was measured at low temperature, down to 70~mK, in magnetic fields up to 15~T, on single crystals grown using the floating-zone method. The residual linear term $\kappa_0/T$ at $T \to 0$ is found to be zero in all samples, for all magnetic fields, in agreement with a previous study. There is therefore no clear evidence of fermionic heat carriers. In contrast to prior data, we observe a large enhancement of $\kappa(T)$ with increasing field. The effect of field is anisotropic, depending on the relative orientation of field and heat current (parallel or perpendicular). However, the anisotropy goes in opposite ways in different crystals, pointing to an extrinsic, sample-dependent mechanism. We propose that heat is carried predominantly by phonons, which are scattered by magnetic impurities.