Observation of the coherence transition into a collective dense Kondo state by resonant inverse photoemission spectroscopy

Abstract

By the resonant inverse photoemission spectroscopy (RIPES), we study the crossover from mixed-valent to heavy-fermion and Kondo regime in ${\mathrm{CeCoGe}}_{3\ensuremath{-}x}{\mathrm{Si}}_{x}$ $(0<~x<~3).$ We have measured the RIPE spectra as functions of composition and temperature. The strong composition dependence of $4f$ state (``${f}^{1}$ peak'') just above the Fermi level shows that the localized $4f$ electrons in the compound become itinerant due to the enhanced Kondo effect within the range $1.0<~x<~1.5.$ Temperature dependence of the ${f}^{1}$ peak indicates a transition from the paramagnetic state at room temperature to a coherent Fermi-liquid regime at low temperatures where the Kondo effect dominates. The most remarkable variation of the spectra has been observed above 200 K with increasing temperature in ${\mathrm{CeCoSi}}_{3}$ which has a very high Kondo temperature ${T}_{K}$ of about 900 K. This suggests that the change in the $4f$-electronic state of ${\mathrm{CeCoSi}}_{3}$ may be scaled by the coherence transition temperature rather than ${T}_{K}.$