Imaging Nematic Transitions in Iron-Pnictide Superconductors with a Quantum Gas*

The SQCRAMscope is a recently realized Scanning Quantum CRyogenic Atom Microscope that utilizes an atomic Bose-Einstein condensate to measure magnetic fields emanating from solid-state samples. Here, we combine the SQCRAMscope with an in situ microscope that measures optical birefringence near the surface of a sample to study iron-pnictide superconductors, where the relationship between electronic and structural symmetry-breaking resulting in a nematic phase is under debate. We conduct simultaneous and spatially resolved measurements of both bulk and surface manifestations of nematicity via transport and structural deformation channels, respectively. By performing the first local measurement of emergent resistivity anisotropy in iron pnictides, we observe a spatially inhomogeneous increase in the temperature at which optical birefringence appears near the surface over that at which anisotropic local transport appears within the bulk. This is consistent with the existence of a higher-temperature surface nematic transition, albeit one that emerges inhomogeneously. More broadly, these measurements demonstrate the SQCRAMscope's ability to reveal important insights into the physics of complex quantum materials.