Low-temperature thermal expansion ofSmB6: Evidence for a single energy scale in the thermodynamics of Kondo insulators

We have measured the thermal expansion of the Kondo insulator ${\mathrm{SmB}}_6$ from 5 to 80 K using a sensitive capacitance dilatometer. There is a pronounced negative anomaly in the linear expansion coefficient α, of electronic origin, which achieves a local minimum of -5×${10}^{\mathrm{-}6}$ ${\mathrm{K}}^{\mathrm{-}1}$ at 45 K. We interpret this anomaly as a Schottky-like contribution arising from transitions across a semiconducting energy gap ${\mathit{E}}_{\mathit{g}}$≃140 K. Although there are several energy scales in the problem, a Grüneisen analysis indicates that the thermodynamics is driven by a single energy, which we identify as the gap energy.