CM In the literature: contested

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

In plain words

Some insulators built from heavy-fermion elements, such as YbB12 and SmB6, show metal-like signatures, heat conduction by mobile carriers and quantum oscillations, while not conducting electricity; this would require particles that carry heat but no charge.

Precise statement

Determine whether a finite residual thermal conductivity $\kappa_{xx}/T$ at $T \to 0$ and bulk quantum oscillations in insulating Kondo insulators (YbB12, SmB6) 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.

What would settle it

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.

Status in the literature

Sato et al. (Nature Physics 2019) reported finite $\kappa/T$ at $T \to 0$ in YbB12; for SmB6, Hartstein et al. (2018) attributed field-dependent heat transport to neutral fermions and Boulanger et al. (2018) to phonons scattered by magnetic impurities.