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

We studied the topological features in Kondo insulator SmB6 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 similar to 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 SmB6.