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

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 perturbed 1 – 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 Sr 2 IrO 4 (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. Spin-polarized tunnelling data show that the breakdown of antiferromagnetic order and the collapse of the spectral gap are not correlated in Sr 2 IrO 4 . This indicates that short-range magnetic correlations are not behind the emergence of the pseudogap.