Real-Space Mapping of Emergent Correlated Electronic States With Scanning SQUID Microscopy

Scanning superconducting quantum interference device (SQUID) microscopy provides ultrahigh magnetic field sensitivity and has emerged as a powerful real-space probe for correlated quantum phenomena in condensed matter. By reducing the superconducting loop to submicron regime, scanning SQUID microscopy achieves magnetic field sensitivities on the order of ≈10 nT Hz^-1/2 while simultaneously enabling spatial resolution below 50 nm, allowing direct visualization of local magnetic textures and current distributions. Unlike the conventional SQUID measurements that can only probe spatially averaged magnetic responses of bulk samples, the scanning-probe technique enables the detection of tiny, spatially inhomogeneous magnetic signals associated with emergent electronic states. This review focuses on recent progress in applying scanning SQUID microscopy to topological materials, magnetic systems, and unconventional superconductors, with an emphasis on the underlying mechanisms that can be uniquely revealed by real-space magnetic imaging based on scanning SQUIDs. We discuss how scanning SQUID measurements have revealed phenomena that are inaccessible to bulk probes, including chiral edge currents in topological insulators, coexistence/competition between superconductivity and ferromagnetism, and anomalous flux quantization in multiband superconductors. Finally, we outline the future directions and challenges of scanning SQUID, with particular focuses on understanding correlated quantum phenomena and guiding the design of quantum information devices.