Title: Perfect Imperfections in Quantum Matter
Abstract: Quantum materials are never perfect. Missing atoms, atomic substitutions, and other ubiquitous defects are usually considered harmful to material functionality. In this talk, however, I will discuss two positive roles played by these seemingly negative imperfections in superconductors. I will first focus on the Knoxville-born superconductor Sn/Si(111), a single atomic layer of tin on top of silicon. Here, atomic-scale lattice defects act as “quantum colliders” for Cooper pairs, generating real-space interference patterns by scattering superconducting quasiparticles. I will show that the unique “flower & star” patterns observed in Sn/Si(111) fingerprint Cooper pairs with a spontaneously acquired “handedness,” establishing the system as a rare chiral d-wave superconductor. The second part of my talk will focus on vortices, a type of topological defect that naturally appears in superconductors. I will discuss how vortices can create emergent low-dimensional topological states and consequently trap Majorana zero modes, which are fractionalized quasiparticles that hold promise for fault-tolerant quantum computation. Together, these examples illustrate how defects can serve both as microscopes for hidden quantum order and as building blocks for new topological matter.