Enhanced coherence and decoupled surface states in topological insulators through structural disorder.

To harness the true potential of topological insulators as quantum materials for information processing, it is imperative to maximise topological surface state conduction, while simulateneously improving their quantum coherence. However, these goals have turned out to be contradictory. Surface dominated transport in topological insulators has been achieved primarily through compensation doping of bulk carriers that introduces tremendous electronic disorder and drastically deteriorates electronic coherence. In this work, we use structural disorder instead of electronic disorder to manipulate electrical properties of thin films of topological insulator Bi$_2$Se$_3$. We achieve decoupled surface state transport in our samples and observe significantly suppressed carrier dephasing rates in the coupled surface state regime. As the film thickness is decreased, the dephasing rate evolves from a linear to a super-linear temperature dependence. While the former is consistent with Nyquist electron-electron interactions, the latter leads to significantly enhanced coherence at low temperatures and is indicative of energy exchange due to frictional drag between the two surface states. Our work opens up the way to harness topological surface states, without being afflicted by the deleterious effects of compensation doping.