Cooperative Configuration Change in EuPd2Si2

Abstract

$4f^7\rightleftarrows 4f^6+e$ charge fluctuations (inferered from ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ Mössbauer spectroscopy) in Eu${\mathrm{Pd}}_2$ ${\mathrm{Si}}_2$ show a strong nonlinear thermal dependence when compared to those observed in dilute Eu in La${\mathrm{Pd}}_2$ ${\mathrm{Si}}_2$. This nonlinear behavior is well described by extending the interconfigurational fluctuation model to include a cooperative intersite coupling of the interconfigurational fluctuation excitation energy to the $4f$ occupation number. Eu nuclear vibrational damping occurs in the region of greatest charge instability.