Pressure evolution of Eu valence in non-centrosymmetric EuTGe3 (T=Co, Rh) studied by high resolution x-ray absorption spectroscopy

Recently, heavy fermion superconductors have been discovered among the compounds without inversion symmetry in their crystal structure, and attracting considerable attentions due to the possible coexistence of spin singlet and triplet Cooper pairing channels initiated by antisymmetric spin-orbit coupling and strong correlations [1]. The superconductivity in such systems tends to develop under pressure. Indeed, an emergence of superconductivity has been reported among a series of antiferromagnet Ce$TX_3$ ($T$: transition metal, $X$=Si, Ge) [2]. Non-centrosymmetric ternaries have also been discovered among Eu-compounds, and a possible emergence of pressure induced superconductivity has been investigated [3-5]. Non-centrosymmetric Eu$T$Ge$_3$ ($T$=Co, Rh) possesses the BaNiSn$_3$-type structure. The Eu valence in both EuCoGe$_3$ and EuRhGe$_3$ is expected to be Eu$^{2+}$ at ambient pressure, and antiferromagnetic ordering emerges at $T_{\rm N}\sim$15.4 and $\sim$11.3 K, respectively [5]. The magnetic moments order along the $c$-axis in EuCoGe$_3$, while perpendicular to the $c$-axis in EuRhGe$_3$ [5]. We have performed high resolution x-ray absorption spectroscopy (XAS) on EuCoGe$_3$ and EuRhGe$_3$ as a function of pressure. The Eu $L_3$ XAS spectra of both EuCoGe$_3$ and EuRhGe$_3$ show a dominant Eu$^{2+}$ peak at ambient pressure. With increasing pressure, the Eu valence increases linearly up to 25 GPa, and exhibits a gradual increase at higher pressure in EuRhGe$_3$. Around 40 GPa, the Eu valence reaches $\sim$2.4 [6]. While in EuCoGe$_3$, the Eu valence shows only a slight change by pressure. Even at 50 GPa, the Eu valence only reaches $\sim$2.3. We also measured the Ge $K$ and Co $K$ XAS spectra as a function pressure, though no remarkable changes have been observed. The results indicate a stable magnetic phase against pressure in EuCoGe$_3$. Our study reveals the pressure evolution of Eu$T$Ge$_3$ and the different pressure response between 3$d$ and 4$d$ transition metal substitution.