Investigating Surfaces and Interfaces in Thin-Film Topological Materials and Heterostructures Using Spin-Momentum Locking

Nontechnical Description Fundamentally, the electrical, magnetic, thermal, and optical properties of materials are all connected and originate in the behavior of electrons. In some special materials, the structure of the material places constraints on how the electrons can move and interact with each other. When electrons have to satisfy multiple constraints at the same time, their behavior can become severely limited, and the surfaces – where those constraints change – can behave very differently from the bulk. This is the basic idea behind topological materials, and it creates opportunities for a range of future quantum technologies. Most studies of topological materials rely on techniques that only measure the exposed surface, leaving the behavior inside the material and at buried interfaces largely unclear. In this project, we will develop new ways to probe these hidden electronic states using spin-momentum locking – a signature of topology. These measurements will be combined with electrical transport measurements and structural measurements using neutron reflectometry and magnetic resonance. The goal is to determine where these states form, how they change when materials with different constraints are grown in layered structures, and if we can use them as indicators of the underlying topology. Understanding these buried interfaces is an important step toward building quantum materials and devices that use spin, charge, and topology in controlled ways. Technical Description This project investigates surface and buried-interface states in thin-film topological materials and heterostructures. In these systems, spin, charge, and momentum are coupled by the electronic band structure, producing experimental signatures that can be used to identify topological states even when they are not accessible to conventional surface-sensitive probes. This is particularly important in multilayers, where the relevant states may be displaced, broadened, or reconstructed by band bending, charge transfer, or proximity coupling to adjacent materials. We will combine depth-sensitive neutron measurements, magnetic resonance methods, and electrical transport to probe these buried electronic states and determine their spatial extent, spin ordering, and response to nearby materials. The measurements will be applied to thin films and selected heterostructures with metallic, magnetic and superconducting layers, and adjacent topological films. The goal is to establish practical experimental probes of buried topology and use them to determine how topological protection evolves at functional interfaces. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.