Probing the density of states in EuFe_2-xRu_xAs₂

Electron spin resonance of Eu²⁺ (4f⁷, S = 7/2) in europium-based iron pnictides successfully probes the local density of states of the conduction electrons. Starting from the mother compound EuFe₂As₂, the usual metallic phase is characterized by the linear increase of the linewidth on increasing temperature (Korringa slope b = 8 Oe K^-1) due to the Korringa relaxation via the conduction electrons, while this Relaxation contribution is switched off in the spin-density wave phase (T < T_SDW), where the linewidth is mainly determined by the crystal-electric field of the ligands [2]. Thus, we observe the same phenomenology like in insulators, despite of the high conductivity. Substitution of ruthenium for iron gradually suppresses the SDW phase up to x = 0.5 and reduces the Korringa slope down to about b = 0.3 Oe K^-1 for x = 2. This indicates a continuously decreasing conduction-electron density of states at the Fermi energy on increasing Ru Substitution in a good agreement with band-structure calculations.