Novel quantum state and magnetic excitations in a spin-orbit driven 4d antiferromagnet

An intricate interplay between spin-orbit interaction and electronic correlations can lead to collective quantum phenomena with exotic excitations in 4d and 5d transition metals. According to recent theoretical work, the d4 electronic configuration is quite interesting as super-exchange opposes to spin-orbit coupling, yielding a phase transition from nonmagnetic atomic singlets to novel and unexplored magnetic phases. A promising candidate to test this physics is La2MgRuO6, a double perovskite with an ordered B-site where the Ru4+ is in the 4d4 configuration. Our preliminary characterizations suggests that this novel material could be at the border between those phases. The magnetic susceptibility and specific heat show neither any signature of magnetic ordering nor spin-freezing down to 50 mK despite a strong antiferromagnetic interaction between Ru4+ (4d4) moments as indicated by a large Curie-Weiss temperature of -104 K. In addition, the power law behavior of low-T magnetic specific heat suggests a gapless excitation spectrum. We plan to perform inelastic neutron scattering on MERLIN and OSIRIS time-of-flight spectrometers to shed insights into the correlated quantum state and associated magnetic excitations over a broad energy scale owing to the subtle interplay between emergent degrees of freedom in this novel Ru-based spin-orbit-driven quantum magnet.