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# Spin-orbit-controlled metal–insulator transition in Sr2IrO4

## Abstract

In the context of correlated insulators, where electron–electron interactions (U) drive the localization of charge carriers, the metal–insulator transition is described as either bandwidth- or filling-controlled1. Motivated by the challenge of the insulating phase in Sr2IrO4, a new class of correlated insulators has been proposed, in which spin–orbit coupling (SOC) is believed to renormalize the bandwidth of the half-filled jeff = 1/2 doublet, allowing a modest U to induce a charge-localized phase2,3. Although this framework has been tacitly assumed, a thorough characterization of the ground state has been elusive4,5. Furthermore, direct evidence for the role of SOC in stabilizing the insulating state has not been established, because previous attempts at revealing the role of SOC6,7 have been hindered by concurrently occurring changes to the filling8,9,10. We overcome this challenge by employing multiple substituents that introduce well-defined changes to the signatures of SOC and carrier concentration in the electronic structure, as well as a new methodology that allows us to monitor SOC directly. Specifically, we study Sr2Ir1−xTxO4 (T = Ru, Rh) by angle-resolved photoemission spectroscopy, combined with ab initio and supercell tight-binding calculations. This allows us to distinguish relativistic and filling effects, thereby establishing conclusively the central role of SOC in stabilizing the insulating state of Sr2IrO4. Most importantly, we estimate the critical value for SOC in this system to be λc = 0.42 ± 0.01 eV, and provide the first demonstration of a spin–orbit-controlled metal–insulator transition.

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## Data availability

The data represented in Figs. 2 and 3 are available as source data in Supplementary Data 2 and 3. All other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

We thank A. Nocera, M. Franz and G.A. Sawatzky for critical reading of the manuscript and useful discussions. This research was undertaken thanks in part to funding from the Max Planck-UBC-UTokyo Centre for Quantum Materials and the Canada First Research Excellence Fund, Quantum Materials and Future Technologies Program. The work at UBC was supported by the Killam, Alfred P. Sloan and Natural Sciences and Engineering Research Council of Canada’s (NSERC’s) Steacie Memorial Fellowships (A.D.), the Alexander von Humboldt Fellowship (A.D.), the Canada Research Chairs Program (A.D.), NSERC, Canada Foundation for Innovation (CFI) and the CIFAR Quantum Materials Program. E.R. acknowledges support from the Swiss National Science Foundation (SNSF, grant no. P300P2_164649). B.J.K. was supported by IBS - R014-A2.

## Author information

Authors

### Contributions

B.Z. and A.D. conceived the experiment. B.Z., E.R. and M.M. collected the experimental data. N.X., M.S. and J.D.D. provided experimental support. G.C., S.C., K.U., J.B., H.T. and B.J.K. grew the single crystals. B.Z. and R.P.D. performed data analysis. B.Z. performed simulations, with input from R.P.D., I.S.E. and A.D. B.Z., R.P.D. and A.D. wrote the manuscript, with input from all authors. I.S.E. and A.D. supervised the project. A.D. was responsible for overall project direction, planning and management.

### Corresponding author

Correspondence to A. Damascelli.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer Review Information Nature Physics thanks Fahad Mahmood and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–4, discussion and references.

## Source data

### Source Data Fig. 2

Source data for Fig. 2 panels.

### Source Data Fig. 3

Source data for Fig. 3 panels.

## Rights and permissions

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Zwartsenberg, B., Day, R.P., Razzoli, E. et al. Spin-orbit-controlled metal–insulator transition in Sr2IrO4. Nat. Phys. 16, 290–294 (2020). https://doi.org/10.1038/s41567-019-0750-y

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• DOI: https://doi.org/10.1038/s41567-019-0750-y

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