Kondo scaling of the pseudogap in CeOs4Sb12 and CeFe4P12

CeOs4Sb12 and CeFe4P12 are classified as Kondo semiconductors, which show coupled changes in electrical transport, thermodynamic and magnetic properties with a low-temperature semiconductor-like electrical resistivity. We have carried out core level and valence band photoemission spectroscopy on single crystal CeOs4Sb12 and CeFe4P12 to study their electronic structure and the evolution of states at the Fermi level as a function of temperature (~10?300?K). The Ce?3d core level spectra show the presence of f0, f1 and f2 final states with very different relative intensities in the two compounds. Single-impurity Anderson model calculations provide f electron counts of nf = 0.97 and 0.86 per Ce atom, suggestive of a low-?and high-TK (= single ion Kondo temperature) for CeOs4Sb12 and CeFe4P12, respectively. The high-resolution temperature-dependent near-Fermi level spectra show pseudogaps of energy ~ 50?meV and ~ 110?meV in the valence band density of states?(DOS) of CeOs4Sb12 and CeFe4P12, respectively. The temperature dependence of the DOS at the Fermi level follows the change in effective magnetic moment estimated from magnetic susceptibility for both materials, confirming the Kondo nature of the pseudogap in CeOs4Sb12 and CeFe4P12. A compilation of measured pseudogaps using photoemission and optical spectroscopy identifies the charge gaps ?C for Ce-based Kondo semiconductors and provides a direct relation with TK given by ?C ~ 2kBTK. In conjunction with the known behaviour of the spin gaps ?S ~ kBTK, the results establish the coupled energy scaling of the spin and charge gaps in Kondo semiconductors.