High resolution Compton profiles have been measured in the single crystal of ${\mathrm{CeRu}}_2{\mathrm{Si}}_2$ above and below the Kondo temperature to elucidate the change of the $\mathrm{Ce}\mathrm{\text{−}}4f$ electron from localized to itinerant states. Two-dimensional electron occupation number densities projected on the first Brillouin zone, which are obtained after a series of analyses, clearly specify the difference between itinerant and localized states. The contribution of $\mathrm{Ce}\mathrm{\text{−}}4f$ electrons to the electronic structure is discussed by contrast with a band calculation.

• Received 22 September 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.136401