Spontaneous Interconfiguration Fluctuations in the Tm Monochalcogenides

Abstract

Spontaneous interconfiguration fluctuations have been detected in TmTe and TmSe by x-ray photoemission. The resulting instantaneous picture of the Tm ions fluctuating between the divalent and trivalent state within the same environment allows a direct measurement of the magnitude of the Coulomb correlation energy, ${U}_{\mathrm{eff}}$. Spontaneous interconfiguration fluctuations are shown also to affect the binding energies of core levels within the same rare-earth ion, resulting in a chemical shift of 3.5 \ifmmode\pm\else\textpm\fi{} 0.1 eV for the Tm ($5p$) core levels.