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：5
页码：1316-1321
DOI：10.1073/pnas.1424322112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceSurface states of topological insulators (TIs) should exhibit extraordinary electronic phenomena when a Dirac-mass gap' is opened in their spectrum, typically by creating a ferromagnetic state. However, our direct visualization of the Dirac-mass gap [IMG]f1.gif" ALT="Formula" BORDER="0"> in a ferromagnetic TI reveals its intense disorder at the nanoscale. This is correlated with the density of magnetic dopant atoms [IMG]f2.gif" ALT="Formula" BORDER="0">, such that [IMG]f3.gif" ALT="Formula" BORDER="0"> as anticipated for surface-state-mediated ferromagnetism. Consequent new perspectives on ferromagnetic TI physics include that the quantum anomalous Hall effect occurs in this environment of extreme Dirac-mass disorder and that paths of associated chiral edge states must be tortuous. To achieve all the exotic physics expected of ferromagnetic TIs, greatly improved control of dopant-induced Dirac-mass gap disorder is therefore required. To achieve and use the most exotic electronic phenomena predicted for the surface states of 3D topological insulators (TIs), it is necessary to open a "Dirac-mass gap" in their spectrum by breaking time-reversal symmetry. Use of magnetic dopant atoms to generate a ferromagnetic state is the most widely applied approach. However, it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale or, conversely, whether the ferromagnetic interactions between dopant atoms are influenced by the topological surface states. Here we image the locations of the magnetic (Cr) dopant atoms in the ferromagnetic TI Cr0.08(Bi0.1Sb0.9)1.92Te3. Simultaneous visualization of the Dirac-mass gap {Delta}(r) reveals its intense disorder, which we demonstrate is directly related to fluctuations in n(r), the Cr atom areal density in the termination layer. We find the relationship of surface-state Fermi wavevectors to the anisotropic structure of {Delta}(r) not inconsistent with predictions for surface ferromagnetism mediated by those states. Moreover, despite the intense Dirac-mass disorder, the anticipated relationship [IMG]f3.gif" ALT="Formula" BORDER="0"> is confirmed throughout and exhibits an electron-dopant interaction energy J* = 145 meV{middle dot}nm2. These observations reveal how magnetic dopant atoms actually generate the TI mass gap locally and that, to achieve the novel physics expected of time-reversal symmetry breaking TI materials, control of the resulting Dirac-mass gap disorder will be essential.
