Spectral Evidence for Pseudogap Formation in Kondo Insulators CeRhAs and CeRhSb

High-resolution photoemission spectroscopy has been performed on CeRhAs and CeRhSb to study the temperature-induced evolution of the “Kondo-insulator” gap. We have observed that a pseudogap opens at $E_F$ at low temperatures while it is gradually filled with increasing temperature. The size of the pseudogap is well scaled with the Kondo temperature $\left(T_K\right)$ of each compound while the temperature evolution is dominated by another characteristic temperature $T_{\mathrm{coh}}$ $(<T_K)$. The results are qualitatively consistent with the theoretical prediction from the Anderson lattice model.