Do Kondo insulators host charge-neutral fermions with a Fermi surface?

Determine whether a finite residual thermal conductivity $\kappa_{xx}/T$ at $T \to 0$ and bulk quantum oscillations in insulating Kondo insulators ($\mathrm{YbB_{12}}$, $\mathrm{SmB_6}$) arise from charge-neutral fermions with a Fermi surface (Majorana or composite-exciton Fermi seas) or from phonons, magnetic impurities and metallic inclusions. An answer is field-dependent $\kappa_{xx}/T$ and specific heat $\gamma$ on crystals with controlled impurity content, compared with a theory that predicts their magnitudes.

Thermal conductivity and specific heat at millikelvin temperatures on sets of crystals with independently measured impurity and inclusion densities, showing whether the residual terms survive as extrinsic content goes to zero.