title: Imaging Dirac-mass disorder from magnetic dopant atoms in the ferromagnetic topological insulator Crx(Bi0.1Sb0.9)2-xTe3.
abstract: 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 Δ(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 Δ(r) not inconsistent with predictions for surface ferromagnetism mediated by those states. Moreover, despite the intense Dirac-mass disorder, the anticipated relationship [Formula: see text] is confirmed throughout and exhibits an electron-dopant interaction energy J* = 145 meV·nm(2). 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.
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973;
authors.forename: Inhee
authors.initials: I
authors.lastname: Lee
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973;
authors.forename: Chung Koo
authors.initials: CK
authors.lastname: Kim
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973; Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Korea; Center for Correlated Electron Systems, Institute for Basic Science, Seoul 151-742, Korea;
authors.forename: Jinho
authors.initials: J
authors.lastname: Lee
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973; Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027;
authors.forename: Simon J L
authors.initials: SJ
authors.lastname: Billinge
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973; Materials Science and Engineering Department and.
authors.forename: Ruidan
authors.initials: R
authors.lastname: Zhong
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973; Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794;
authors.forename: John A
authors.initials: JA
authors.lastname: Schneeloch
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973; School of Chemical Engineering and Environment, North University of China, Shanxi 030051, China;
authors.forename: Tiansheng
authors.initials: T
authors.lastname: Liu
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973;
authors.forename: Tonica
authors.initials: T
authors.lastname: Valla
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973;
authors.forename: John M
authors.initials: JM
authors.lastname: Tranquada
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973;
authors.forename: Genda
authors.initials: G
authors.lastname: Gu
authors.affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973; Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, NY 14853; School of Physics and Astronomy, University of St. Andrews, Fife KY16 9SS, Scotland; and Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, NY 14853 jcseamusdavis@gmail.com.
authors.forename: J C Séamus
authors.initials: JC
authors.lastname: Davis
keywords: Dirac-mass disorder; Dirac-mass gapmap; ferromagnetic topological insulator; magnetic dopant atoms