Collapse of the Cooper pair phase coherence length at a superconductor-to-insulator transition

The superconductor-to-insulator transition (SIT) in quasi-2D thin films now serves as a canonical quantum phase transition (QPT). However, variation in its features between different material systems continues to raise perplexing questions about universality and universality classes in QPTs. A key to understanding the different manifestations of SITs has been through experimental probes of microscopic transport mechanisms. In this talk, I will present transport experiments on two morphologies of amorphous Bi quench-condensed films patterned with a nanohoneycomb array of holes, where the holes provide an embedded probe of local phase coherence. Films with smooth, nano-scale undulations in thickness exhibit a bosonic insulating phase that includes a giant magnetoresistance peak. In contrast, films of uniform thickness exhibit a fermionic insulating phase. The distinct phases are supported by the presence or absence of magneto-resistance oscillations that occur when Cooper pairs are phase coherent over a scale similar to the spacing of the nanohoneycomb array. These data provide a microscopic view of locally phase coherent Cooper pairs, confirm the existence of the bose insulator, and distinguish two classes of disorder-tuned SITs.