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 Sm1-xEuxB6 and Yb1-xLuxB12 by photoelectron spectroscopy at similar to 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 Sm1-xEuxB6 at least up to the Eu substitution of x = 0.15 while both collapse by Lu substitution already at x = 0.125 for Yb1-xLuxB12. As for the temperature dependence of the spectral shapes near the Fermi level at low temperatures, rather contrasting results were observed between YbB12 and SmB6. Although the gap shape does not change below 15 K for YbB12 with the characteristic temperature T* of 80 K, the spectral shape of SmB6 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 SmB6 and YbB12, 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 SmB6.