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.

The discovery of gravitation by Isaac Newton in the 17th century is often referred to as the beginning of physics as we know it. More than two centuries later, the value of the gravitational constant, “the big G,” serves as the fundamental basis for understanding large-scale cosmic structures, while providing the essential theoretical foundation for general relativity. Yet, despite centuries of experimentation, the global dataset for G remains frustratingly inconsistent. Compared to the strength of the electromagnetic interaction, known to a relative uncertainty of 0.2 parts-per-billion, G is only known to 22 parts-per-million. This staggering five-order-of-magnitude gap highlights the unique stubbornness of gravity. Two primary factors contribute to this: (1) unlike its dimensionless electromagnetic counterpart, G is a dimensioned quantity tied to our macroscopic units, and (2) the gravitational force is extremely weak compared to other fundamental forces and impossible to shield. In this talk, I will discuss the recent NIST measurement that we undertook to better understand the global dataset. I will attempt to answer three central questions: Why is G so difficult to pin down? How did we measure it? And what now for the future of the gravitational constant?

No colloquium; Fall Break.