Charge Fluctuations in Europium in Metallic EuCu2Si2

Abstract

Recoilless-absorption measurements of the 21.6-keV $\gamma$ ray of ${\mathrm{Eu}}^{151}$ in Eu${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ reveal an absorption line which has a strongly temperature-dependent isomer shift. The lines fall at neither the ${\mathrm{Eu}}^{3+}$ nor the ${\mathrm{Eu}}^{2+}$ valence positions, but in between. The results are interpreted in terms of fast fluctuations ($\tau <3.5\mathrm{}\times {10}^{-11\mathrm{}}$ sec) of an electron between the localized $4f$ level and the conduction band. The difference in energy between the ${\mathrm{Eu}}^{2+}$ and the ${\mathrm{Eu}}^{3+}$ states in Eu${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ is found to be about 0.07 eV.