Different evolution of the intrinsic gap in strongly correlated SmB sub(6) in contrast to YbB sub(12)

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 sub(1-x)Eu sub(x)B sub(6) and Yb sub(1-x)Lu sub(x)B sub(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 sub(1-x)Eu sub(x)B sub(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 sub(1-x)Lu sub(x)B sub(12). As for the temperature dependence of the spectral shapes near the Fermi level at low temperatures, rather contrasting results were observed between YbB sub(12) and SmB sub(6). Although the gap shape does not change below 15K for YbB sub(12) with the characteristic temperature T* of 80 K, the spectral shape of SmB sub(6) with a T* of 140K 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 sub(6) and YbB sub(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 sub(6).