Direct observation of the Ce<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>4</mml:mn><mml:mi>f</mml:mi></mml:math>states in the Kondo semiconductor CeRhAs and related compounds:  A high-resolution resonant photoemission study

Abstract

Ce $4f$ derived states at the Fermi level ${(E}_{F})$ of the isostructural single crystals CeRhAs, CeRhSb, and CePtSn were observed directly by means of high-resolution $(\ensuremath{\Delta}E=18--20\mathrm{meV}),$ low-temperature (10--12 K) photoemission spectroscopy with a photon energy of $h\ensuremath{\nu}=126\mathrm{eV}.$ The Ce $4f$ spectrum for the Kondo semiconductor CeRhAs exhibited no peak structure near ${E}_{F},$ and its spectral intensity decreases monotonically above the binding energy $\ensuremath{\sim}90\mathrm{meV},$ thereby forming a large gap structure. The spectrum of the semimetal CeRhSb is enhanced above $\ensuremath{\sim}120\mathrm{meV},$ but decreases steeply above $\ensuremath{\sim}13\mathrm{meV},$ which indicates the existence of a narrow pseudogap at ${E}_{F}.$ A clear crystal field excitation at $\ensuremath{\sim}27\mathrm{meV},$ and a weak Kondo resonance at ${E}_{F},$ were found in the metal CePtSn.