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Abstract

Transition-metal oxides (TMOs) are a widely studied class of materials with fascinating electronic properties and a great potential for applications. Sr₂IrO₄ is such a TMO, with a partially filled 5d t₂g shell. Given the reduced Coulomb interactions in these extended 5d orbitals, the insulating state in Sr₂IrO₄ is quite unexpected. To explain this state, it has been proposed that SOC entangles the t₂g states into a filled jeff = 3/2 state and a half-filled jeff = 1/2 state, in which a smaller Coulomb interaction can open a gap. This new scheme extends filling and bandwidth, the canonical control parameters for metal-insulator transitions, to the relativistic domain. Naturally the question arises whether in this case, SOC can in fact drive such a transition. In order to address this question, we have studied the behaviour of Sr₂IrO₄ when substituting Ir for Ru or Rh. Both of these elements change the electronic structure and drive the system into a metallic state. A careful analysis of filling, bandwidth, and SOC, demonstrates that only SOC can satisfactorily explain the transition. This establishes the importance of SOC in the description of metal-insulator transitions and stabilizing the insulating state in Sr₂IrO₄. It has furthermore been proposed that the jeff = 1/2 model in Sr₂IrO₄ is an analogue to the superconducting cuprates, realizing a two-dimensional pseudo-spin 1/2 model. We test this directly by measuring the spin-orbital entanglement using circularly polarized spin-ARPES. Our results indicate that there is a drastic change in the spin-orbital entanglement throughout the Brillouin zone, implying that Sr₂IrO₄ can not simply be described as a pseudo-spin 1/2 insulator, casting doubt on direct comparisons to the cuprate superconductors. We thus find that the insulating ground state in Sr₂IrO₄ is mediated by SOC, however, SOC is not strong enough to fully disentangle the jeff = 1/2 state, requiring that Sr₂IrO₄ is described as a multi-orbital relativistic Mott insulator.