Low-temperature thermal transport in the Kondo insulator SmB$_6$

The striking observation of quantum oscillations in the Kondo insulator SmB$_6$ suggests that there may be chargeless fermionic excitations at low temperature in the bulk of this material [1]. One way to detect such putative excitations is through their ability to carry entropy, which a measurement of thermal transport should in principle detect as a non-zero residual linear term in the $T=0$ limit, {\it i.e.} $\kappa_0/T > 0$. Here we report low-temperature measurements of the thermal conductivity $\kappa$ in SmB$_6$, down to 50~mK, performed on various single crystals in magnetic fields up to 15~T. By extrapolating, we obtain $\kappa_0/T$ at each field. We observe no residual linear term at any field, {\it i.e.} $\kappa_0/T = 0$ at all $H$, in agreement with a previous study [2]. In other words, we do not detect mobile fermionic excitations. However, unlike in the prior study [2], we observe a large enhancement of $\kappa(T)$ with increasing field. We discuss possible interpretations of this field dependence. \\ \\ $[1]$ B.~S.~Tan~{\it et al.}, Science~{\bf 349},~287~(2015). \\ $[2]$ Y.~Xu~{\it et al.}, Physical Review Letters~{\bf 116},~246403~(2016).