Temperature-dependent Eu $3d\text{−}4f$ x-ray absorption and resonant photoemission study of the valence transition in $\mathrm{Eu}{\mathrm{Ni}}_2{\left({\mathrm{Si}}_{0.2}{\mathrm{Ge}}_{0.8}\right)}_2$

We study the mixed valence transition $(T_v\sim 80\mathrm{K})$ in $\mathrm{Eu}{\mathrm{Ni}}_2{\left({\mathrm{Si}}_{0.2}{\mathrm{Ge}}_{0.8}\right)}_2$ using Eu $3d\text{−}4f$ x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RESPES). The ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ main peaks show a giant resonance and the spectral features match very well with atomic multiplet calculations. The spectra show dramatic temperature $\left(T\right)$ dependent changes over large energies $(\sim 10\mathrm{eV})$ in RESPES and XAS. The observed nonintegral mean valencies of $\sim 2.35\pm 0.03(T=120\mathrm{K})$ and $\sim 2.70\pm 0.03(T=40\mathrm{K})$ indicate homogeneous mixed valence above and below $T_v$. The redistribution between ${\mathrm{Eu}}^{2+}4f^7+{\left[spd\right]}^0$ and ${\mathrm{Eu}}^{3+}4f^6+{\left[spd\right]}^1$ states is attributed to a hybridization change coupled to a Kondo-like volume collapse.

Figure 1

(Color online) Comparison of the Eu $3d\text{−}4f$ XAS experimental spectra with calculated spectra at (a) $T=120\mathrm{K}$ and (b) $40\mathrm{K}$. The spectral intensities change strongly across $T_v\sim 80\mathrm{K}$, over large energy scales. The spectra are derived from intra-atomic multiplet excitations of ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ ions by broadening the discrete energy states [(c)].

Figure 2

(Color online) The $T$-dependent Eu $3d\text{−}4f$ RESPES spectra at incident photon energies marked 0–4 in Fig. 1. The inset shows the normalization procedure using off-resonance (off-res0) spectra. The resonant features originate from the partial $4f$ DOS of ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ ions. Note the absence of $T$ dependence in the ${\mathrm{Eu}}^{2+}$ surface state at $1.7\mathrm{eV}$ BE (on-res2).

Figure 3

(Color online) Comparison of the Eu $3d\text{−}4f$ RESPES experimental spectra with atomic multiplet calculations at incident photon energies from on-res1 to on-res4, at $40\mathrm{K}$ (left panel) and $120\mathrm{K}$ (right panel).