Imaging phase transitions in quantum materials

ongoing

Strongly interacting electron systems lead to a wealth of competing phases, phase transitions, and quantum critical points. When probed globally, the inherent inhomogeneities, disorder, localization, and mixture with other phases can be a stumbling block in detecting and controlling the various electronic states. Armed with a suitable local probe, however, spatial inhomogeneities turn from a concealing factor into the key to unveil new exotic electronic phases. Our unique tool, the scanning SQUID, is the most suitable probe, as it provides both extremely high magnetic sensitivity - capable of detecting trace amounts of conductivity, superconductivity and magnetism - with a high spatial resolution. We will integrate our state-of-the-art sensor with a set of tuning knobs, to enable simultaneous manipulation and imaging of quantum phase transitions. Our key goal is to provide clear-cut evidence for elusive many-body states that are in the blind spot of global measurements. We will detect hidden phases, such as traces of superconducting islands in an insulator, puddles of strongly correlated electrons at the onset of metallicity, and protected states in topological phases. The spatial distribution of states and disorder-related inhomogeneities will serve as the main tool in our quest. We will elucidate the correlations between emergent states that show non-trivial coexistence, such as magnetism and superconductivity, conductivity in a ferroelectric medium and itinerant ferromagnetism. We will provide clues about the mechanisms that drive fundamental transitions, such as the metal-insulator and the superconductor-insulator transitions. We will track phases and fluctuations near quantum criticality, and use the local information to bridge the gap between the microscopic behavior and the thermodynamic limit, where critical phenomena emerge. We aim to explore fundamental questions like the universality of transitions and assist the development of quantum materials.