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 (AF) Mott insulator remains difficult due to inherent phase separation as the Mott state is perturbed. Measuring magnetic and electronic properties at the atomic length scales would provide crucial insight, but this is yet to be experimentally achieved. Here we use spectroscopic-imaging spin-polarized scanning tunneling microscopy (SP-STM) to visualize periodic spin-resolved modulations originating from the AF order in a relativistic Mott insulator Sr2IrO4, and study these as a function of doping. We find that near insulator-to-metal transition (IMT), the long-range AF order melts into a fragmented state with short-range AF correlations. Crucially, we discover that the short-range AF order is locally uncorrelated with the observed spectral gap magnitude. This strongly suggests that short range AF correlations are unlikely to be the culprit behind inhomogeneous gap closing and the emergence of pseudogap regions near IMT. Our work establishes SP-STM as a powerful tool for revealing atomic-scale magnetic information in complex oxides.A Mott insulator, characterized by localization of electrons due to strong electron-electron interactions (1), is typically accompanied by magnetic ordering (2-4). The antiferromagnetic (AF)

Imaging antiferromagnetic domain fluctuations and the effect of atomic scale disorder in a doped spin-orbit Mott insulator

“…4, C and D), spectral gapmaps look virtually identical (Fig. 4, G and H), thus confirming that short-range AF order is not the cause behind the spatially inhomogeneous gap closing (16). While our work points toward La 2 substitutions in the middle SrO layer having the strongest correlation with the regions where the AF order is absent, it remains to be seen what is the physical effect driving this behavior.…”

“…Inset in (F) is a 2D intensity cross-correlation histogram of | M 1 ( r , 5 K)| and | M 2 ( r , 5 K)|. ( G and H ) Maps of the spectral gap Δ( r ) [section S3 and ( 16 , 34 , 35 )] at 9 and 5 K, respectively, both obtained over the same area as | M ( r , T )| maps. STM setup condition: M ( r ) maps: V sample = 600 mV, I set = 100 pA, ±4 T, 5 K (A), 7 K (B), 9 K (C), and 5 K (D); Δ( r ) maps: V sample = 300 mV, I set = 200 pA, V exc = 6 mV, 0 T, 9 K (G) and 5 K (H).…”

“…Last, we note that, in the single-layer iridate Sr 2 IrO 4 , spins are oriented in-plane, which can lead to AF domains with spins rotated by 90° in-plane ( 16 ). This can, in principle, lead to some AF domains that are undetectable by an STM tip, if tip polarization is exactly orthogonal to the spin orientation of the sample [see more details in supplementary information of ( 16 )]. This is not the case in Ir-327 because spins point out-of-plane and no orthogonally oriented domains should exist.…”

“…Over the past decade, Ruddlesden-Popper iridates (12)(13)(14)(15) have emerged as an intriguing family of two-dimensional (2D) AF spinorbit Mott insulators, where the AF Mott ground state can be tuned by different parameters, for example, temperature and chemical composition. The experiments thus far revealed a remarkable nanoscale phase separation in iridates, reflected in the granular fabric of both their electronic and magnetic properties (4,5,16). While, in many correlated oxides, the competition between different phases provides a natural tendency toward phase separation (5,17,18), chemical disorder that is often necessary to induce a new phenomenon can also play a substantial role (5,19,20).…”

“…While, in many correlated oxides, the competition between different phases provides a natural tendency toward phase separation (5,17,18), chemical disorder that is often necessary to induce a new phenomenon can also play a substantial role (5,19,20). Our recent spin-polarized scanning tunneling microscopy (SP-STM) experiments revealed the emergence of AF puddles near insulator-to-metal transition in the Ruddlesden-Popper iridate Sr 2 IrO 4 (16). However, many questions remain regarding the formation of AF clusters in this family of oxides.…”