Strongly correlated 4f-electron system has been a platform of various anomalous phenomena, such as valence fluctuation, unconventional superconductivity, heavy fermion behavior and spin/charge ordering [1]. The ground state property of these compounds is characterized by conflicting Kondo effect and Rudermann-Kittel-Kasuya-Yoshida (RKKY) interaction. Both interactions originate in the interplay of localized f electron and itinerant conduction electrons, though the former quenches the magnetic moments, while the latter leads to magnetic ordering in the ground state. The competition between Kondo effect and RKKY interactions in Ce and Yb-compounds are often discussed using a Doniach phase diagram [2], while Eu-compounds exhibit very different phase diagrams from them. Most of the reported Eu-compounds favor Eu²⁺ (4f ⁷, J=7/2) valence state with an antiferromagnetic ground state. However, the energy difference between Eu²⁺ and non-magnetic Eu³⁺ (4f ⁶, J=0) valence state is not so large and is reachable by applying external pressure or chemical substitution, that is accompanied by a first order phase transition. Due to the different ionic size between Eu²⁺ and Eu³⁺, the change of the Eu valence state is often assigned to the Kondo volume collapse effect.

Eu-based ternary germanide EuTGe₃ (T= Co, Ni, Rh, Ir) possesses BaNiSn₃-type crystal structure without inversion symmetry. The existence of the polycrystalline EuTGe₃ has been known for decades, though a high quality single crystal becomes available only recently and its unique magnetic property is discovered [3]. The magnetic susceptibility measurement and Mössbauer spectroscopy report the presence of magnetic Eu²⁺ ions in all the compounds and localized Eu 4f moments order antiferromagnetically at similar temperature [3-6]. The magnetic moments order antiferromagnetically along the c-axis at T_N=15.4, 13.5 and 12.3 K for EuCoGe₃, EuNiGe₃ and EuIrGe₃, respectively. Conversely in EuRhGe₃, the magnetic moments order perpendicular to the c-axis at ∼12 K. Despite the variation of transition metal substitution and the change in the unit cell volume, the Eu ions seem to have robust Eu²⁺ valence state with T_N being barely affected. In order to study the effect of transition metal substitution on the electronic structure, a bulk sensitive hard x-ray photoelectron spectroscopy (HAXPES) was performed at GALAXIES beamline of SOLEIL synchrotron. In the seminar talk, I will present a detailed HAXPES result of EuTGe₃.

[1] C. M. Varma, Rev. Mod. Phys. 48, 219 (1976).

[2] S. Doniach, Physica B 91, 231 (1977).

[3] O. Bednarchuk, et al., J. Alloys Comp. 622, 432-439 (2015): A. Phys. Pol. A 127, 418 (2015).

[4] A. Maurya, et al., J. Phys. : Condens. Matter 26, 216001 (2014).

[5] A. Maurya, et al., J. Mag. Mag. Mat. 401, 823-831 (2016).

[6] M. Kakihana, et al., J. Alloys Comp. 694, 439-451 (2017).