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# Atomic-scale fragmentation and collapse of antiferromagnetic order in a doped Mott insulator

## Abstract

Disentangling the relationship between the insulating state with a charge gap and the magnetic order in an antiferromagnetic Mott insulator remains difficult due to inherent phase separation as the Mott state is perturbed1,2,3,4,5,6,7. Measuring magnetic and electronic properties at atomic length scales would provide crucial insight, but this is yet to be experimentally achieved. Here, we use spin-polarized scanning tunnelling microscopy (SP-STM) to visualize the periodic spin-resolved modulations originating from the antiferromagnetic order in a relativistic Mott insulator Sr2IrO4 (refs. 8,9), and how they change as a function of doping. We find that near the insulator-to-metal transition (IMT), the long-range antiferromagnetic order melts into a fragmented state with short-range correlations. Crucially, we discover that the short-range antiferromagnetic order is locally uncorrelated with the observed spectral gap magnitude. This suggests that static short-range antiferromagnetic correlations are unlikely to be the cause of the inhomogeneous closing of the spectral gap and the emergence of pseudogap regions near the IMT. Our work establishes SP-STM as a powerful tool for revealing atomic-scale magnetic information in complex oxides.

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## Data availability

The data represented in Figs. 1f,g, 2g,h and 4e,f are available as Supplementary information files. All other data that support the plots within this paper and the other findings of this study are available from the corresponding author on reasonable request.

## Code availability

The computer code used for data analysis is available on request from the corresponding author.

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## Acknowledgements

We thank J. Hoffman, P. Lee and V. Madhavan for valuable discussions, and G. Gu for supplying the FeTe single crystals for characterizing spin-polarized STM tips. The spin-polarized STM measurements were supported by the US Department of Energy Early Career Award DE-SC0020130. I.Z. also acknowledges the support from the Army Research Office grant number W911NF-17-1-0399 (H.Z.) and the National Science Foundation grant number NSF-DMR-1654041 (A.U.) for developing the spin-polarized STM capability. Z.W. acknowledges the support from the US Department of Energy, Basic Energy Sciences grant no. DE-FG02-99ER45747. S.M. and J.M. would like to acknowledge the Office of Naval Research grant N00014-16-1-2657, the National Science Foundation grant DMR-1700137 and a grant from the John Templeton Foundation. S.D.W. acknowledges the support from the National Science Foundation award no. DMR-1905801 (S.D.W.), and additional funding support from the Army Research Office award W911NF-16-1-0361 (Z.P.).

## Author information

Authors

### Contributions

STM experiments were carried out by H.Z. H.Z., S.M. and A.U. were responsible for fabrication and initial characterization of spin-polarized STM tips. Iridate single crystals were grown by Z.P. and X.C., supervised by S.D.W. H.Z. analysed the STM data with guidance from I.Z. Z.W. provided theoretical input on the interpretation of the STM data. I.Z., S.D.W., Z.W., H.Z. and J.M. wrote the manuscript with input from all the authors. I.Z. supervised the project.

### Corresponding author

Correspondence to Ilija Zeljkovic.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Physics thanks Milan Allan 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. 1–13 and refs. 1–16.

## Source data

### Source Data Fig. 1

Data plotted in Fig. 1f,g.

### Source Data Fig. 2

Data plotted in Fig. 2g,h.

### Source Data Fig. 4

Data plotted in Fig. 4e,f.

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Reprints and Permissions

Zhao, H., Manna, S., Porter, Z. et al. Atomic-scale fragmentation and collapse of antiferromagnetic order in a doped Mott insulator. Nat. Phys. 15, 1267–1272 (2019). https://doi.org/10.1038/s41567-019-0671-9

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• DOI: https://doi.org/10.1038/s41567-019-0671-9

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