LAUSR

Information processing devices operating in the quantum mechanical regime strongly rely on the quantum coherence of charge carriers. Studies of electronic dephasing in conventionalmetallic and semiconductor systems have not only paved the way toward high coherence quantum electronics, but also led to fundamental new insights in condensed matter physics. In this paper, we perform a spatially resolved study of electronic dephasing in three-dimensional topological insulators by exploiting an edge-versus surface-contacted measurement scheme. Unlike conventional two-dimensional systems that are characterized by a single dephasing mechanism, we find that dephasing in our samples evolves from a variable-range-hopping-type mechanism on the sample surface to a Nyquist-type electron-electron interaction mechanism in the subsurface layers. This is confirmed independently by the temperature and chemical potential dependence of the dephasing length, and gate-dependent suppression/enhancement of the weak antilocalization effect. Our devices are fabricated using bulk insulating topological insulator BiSbTe1.25Se1.75 capped with hexagonal-boron nitride in an inert environment, ruling out any extrinsic effects and confirming the topological surface state origin of our results. Our work addresses the idea of spatially resolved electronic dephasing and coherent transport in perhaps the most important topological insulator discovered so far. Our edge-versus-surface scheme may be applied to dephasing studies in a wide class of 2D materials.