Tuning the electronic and magnetic states of Ca2RuO4 with proton evolution

With strong correlations between lattice, spin, and charge degrees of freedom, the layered ruthenate Ca₂RuO₄ has attracted considerable interest over the past few decades due to its metal-insulator transition, antiferromagnetic-to-ferromagnetic transition, metamagnetic transition, and orbital ordering. Much effort has been devoted to manipulating its crystalline structure through epitaxial strain, chemical substitution, and pressure to clarify the underlying many-body physics and related quantum critical phenomena. Here we report a comprehensive proton intercalation study of Ca₂RuO₄ thin films and investigate their magneto-transport properties arising from structural deformations and carrier doping. It reveals a rich phase diagram with distinct electronic and magnetic ground states. Specifically, with increasing gate voltage during ionic liquid gating, the film first evolves from an insulating state into a metallic state and then gradually turns towards an exotic Mott insulator. Furthermore, we observed an emergent metamagnetic transition from a canted antiferromagnetic to a nearly ferromagnetic state, a characteristic feature conventionally triggered by external magnetic field, but here with electron doping. Our first-principles calculations reveal that these unexpected features could be attributed to the proton evolution-induced synergistic structural distortion and electron doping during ionic liquid gating. Our findings highlight the important role of both lattice and charge degrees of freedom in the intriguing electronic states of Ca₂RuO₄ and provide an effective approach to uncover different properties in quantum materials.