Persistent Mott state and vanishing Néel state in single-crystal Ca₂Ru_1-xRh_xO₄ (0≤ x ≤0.33)

The 4d-based Ca₂RuO₄ is a spin-orbit coupled Mott insulator with a first-order metal-insulator transition concurrent with a violent structural transition at T_MI = 357 K, followed by a well-separated antiferromagnetic state at T_N = 110 K stabilized by further structural distortions. Here we report the results of our recent study on Rh⁴⁺ (4d⁵) substituted Ca₂RuO₄ or Ca₂Ru_1-xRh_xO₄ single crystals. Rh substituting weakens the structural distortions and effectively removes the first-order metal-insulator transition at T_MI while unexpectedly retaining the Mott state. The magnetic properties, on the other hand, changes dramatically throughout the same Rh substitution range. Consequently, T_N drops linearly with increasing x, from 110 K for x = 0 down to 54 K for x = 0.33, accompanied by an elongated c-axis lattice parameter. The unparallel behavior of the transport and magnetic states indicates a disassociation between them in this highly distorted ruthenate.