Kondo breakdown in the topological Kondo insulator SmB6 studied by point-contact Andreev reflection spectroscopy

We studied the topological features in Kondo insulator $\mathrm{Sm}{\mathrm{B}}_{6}$ by point-contact Andreev reflection spectroscopy with a Nb probe tip. Below the superconducting transition of Nb, the spectra exhibited a narrow dip at around zero bias superposed on a broad asymmetric background caused by the Kondo resonance. The contact-size dependence of the spectra revealed that the width of the resonance is suppressed near the surface. The spin polarization in the surface state was $\ensuremath{\sim}0.56$, which is smaller than that in Bi-based topological insulators. These indicate that the Kondo breakdown plays a crucial role for the topological features, not only in the bulk state near the surface but also in the surface state on $\mathrm{Sm}{\mathrm{B}}_{6}$.