Departmental Oral Examination (Thesis Title: "Spin-Orbit Coupling in Iridates")

Event Date:
2019-08-29T09:30:00
2019-08-29T11:30:00
Event Location:
AMPEL 488
Speaker:
BEREND ZWARTSENBERG
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Intended Audience:
Public
Event Information:

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 spin-orbit coupling (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 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.

Add to Calendar 2019-08-29T09:30:00 2019-08-29T11:30:00 Departmental Oral Examination (Thesis Title: "Spin-Orbit Coupling in Iridates") Event Information: 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 spin-orbit coupling (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 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. Event Location: AMPEL 488