Characterizing the nematic phase in Ba122 iron arsenides with a SQCRAMscope

Microscopic imaging of local magnetic fields provides a window into the organizing principles of complex and technologically relevant condensed matter materials. However, a wide variety of strongly correlated and topologically nontrivial materials exhibit poorly understood phenomena outside the detection capability of state-of-the-art high-sensitivity, high-resolution scanning probe magnetometers. We have recently introduced the Scanning Quantum Cryogenic Atom Microscope (SQCRAMscope), a quantum-noise-limited scanning probe magnetometer that can operate from room-to-cryogenic temperatures. By employing a magnetically levitated atomic Bose-Einstein condensate (BEC) that can be scanned near the surface of a cryogenically cooled material, the microscope achieves unprecedented DC-field sensitivity at micron-scale resolution. Combining this unique probe with optical polarimetry, we present recent work on characterizing the nematic phase transition in unstrained Ba122 iron-based superconductors. Domain structure is visualized through spatial mapping of the current density, from which resistivity anisotropy is extracted as a proxy for the nematic order parameter. The temperature dependence of nematicity is studied and compared to optical polarimetry results from the same sample.