Valence change of europium and its relation to superconductivity in EuFe$_2$As$_{1.4}$P$_{0.6}$ and compressed EuFe$_2$As$_2$

Superconductivity can be realized in Eu-containing pnictides by applying chemical (internal) and physical (external) pressure, the intrinsic physical mechanism of which attracts much attention in the studies of pnictide superconductors. Here we present the experimental evidence for the pressure-induced valence change of europium in EuFe$_{2}$As$_{1.4}$P$_{0.6}$ exposed to ambient pressure and EuFe$_{2}$As$_{2}$ to high pressure by x-ray absorption measurements on L$_{3}$-Eu edge. We found that the absorption spectrum of EuFe$_{2}$As$_{1.4}$P$_{0.6}$ showed a clear spectra weight transfer from divalent to trivalent state. Furthermore, a similar behavior of valence transition as in EuFe$_{2}$As$_{1.4}$P$_{0.6}$ was also observed in EuFe$_{2}$As$_{2}$ when pressure was applied. This is the first to report the observation of valence change in pnictide superconductors and the analysis of its influence on superconductivity in EuFe$_{2}$As$_{1.4}$P$_{0.6}$ and compressed EuFe$_{2}$As$_{2}$.