Different evolution of the intrinsic gap in strongly correlated SmB6 in contrast to YbB12

Dependence of the spectral functions near the Fermi level on temperature and rare-earth atom doping was studied in detail for strongly correlated alloys Sm _1− _x Eu _x B _6 and Yb _1− _x Lu _x B _12 by photoelectron spectroscopy at ∼8000 eV as well as at 7 and 8.4 eV. It was found that the 4f lattice coherence and intrinsic gap are robust for Sm _1− _x Eu _x B _6 at least up to the Eu substitution of x = 0.15 while both collapse by Lu substitution already at x = 0.125 for Yb _1− _x Lu _x B _12 . As for the temperature dependence of the spectral shapes near the Fermi level at low temperatures, rather contrasting results were observed between YbB _12 and SmB _6 . Although the gap shape does not change below 15 K for YbB _12 with the characteristic temperature T * of 80 K, the spectral shape of SmB _6 with a T * of 140 K shows that the peak beyond the gap is further increased below 15 K. The temperature dependence of the spectra near the intrinsic gap is clearly different between SmB _6 and YbB _12 , although both materials have so far been categorized in the same kind of strongly correlated semiconductor. The possibility of the surface contribution is discussed for SmB _6 .