期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：28
页码：E3645-E3650
DOI：10.1073/pnas.1502330112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceThe surface states of topological insulators (TIs) and magnetically doped TIs exhibit considerable inhomogeneities at the nanoscale. Methods are needed to probe the degree of heterogeneity as a function of depth in nanoscale layers. We present a method that can directly visualize TIs in a depth-resolved manner and report on their electronic and magnetic properties. For example, in epitaxial thin films we demonstrate an increase in the density of states, a weakening of the ferromagnetic order when approaching the TI edges, as detected by measurements of the electron-nuclear hyperfine interaction, the effective s-d exchange integral, and local moment density. Depth profiling is expected to help uncover exotic physics of pure and ferromagnetic TIs and TI heterostructures. Considerable evidence suggests that variations in the properties of topological insulators (TIs) at the nanoscale and at interfaces can strongly affect the physics of topological materials. Therefore, a detailed understanding of surface states and interface coupling is crucial to the search for and applications of new topological phases of matter. Currently, no methods can provide depth profiling near surfaces or at interfaces of topologically inequivalent materials. Such a method could advance the study of interactions. Herein, we present a noninvasive depth-profiling technique based on {beta}-detected NMR ({beta}-NMR) spectroscopy of radioactive 8Li+ ions that can provide "one-dimensional imaging" in films of fixed thickness and generates nanoscale views of the electronic wavefunctions and magnetic order at topological surfaces and interfaces. By mapping the 8Li nuclear resonance near the surface and 10-nm deep into the bulk of pure and Cr-doped bismuth antimony telluride films, we provide signatures related to the TI properties and their topological nontrivial characteristics that affect the electron-nuclear hyperfine field, the metallic shift, and magnetic order. These nanoscale variations in {beta}-NMR parameters reflect the unconventional properties of the topological materials under study, and understanding the role of heterogeneities is expected to lead to the discovery of novel phenomena involving quantum materials.
