Heavy fermion $\textit{d-f}$ hybrid and the SmB$_6$ low temperature phase

In this Letter we theoretically study physical properties of a model of heavy fermion $d-f$ hybrid. In the studied model two species of fermions have dispersions with different masses, one being much heavier than the other. Hybridization between the fermions at the crossing point of their dispersions doesn't open a true insulating gap leaving a heavy fermion $d-f$ hybrid at the Fermi level. As a result, our theoretical model qualitatively explains experiments on the low-temperature phase of the SmB$_6$. These are the linear in temperature specific heat, saturation of the resistance, and frequency dependence of the optical conductivity. Calculated optical conductivity shows a broadened peak at the twice the value of hybridization as well as a low-frequency tail.