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Magnetism-induced massive Dirac spectra and topological defects in the surface state of Cr-doped Bi_2Se_3-bilayer topological insulators

Chen, C.-C. and Teague, M. L. and He, L. and Kou, X. and Lang, M. and Fan, W. and Woodward, N. and Wang, K.-L. and Yeh, N.-C. (2015) Magnetism-induced massive Dirac spectra and topological defects in the surface state of Cr-doped Bi_2Se_3-bilayer topological insulators. New Journal of Physics, 17 (11). Art. No. 113042. ISSN 1367-2630.

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Proximity-induced magnetic effects on the surface Dirac spectra of topological insulators are investigated by scanning tunneling spectroscopic studies of bilayer structures consisting of undoped Bi_2Se_3 thin films on top of Cr-doped Bi_2Se_3 layers. For thickness of the top Bi_2Se_3 layer equal to or smaller than 3 quintuple layers, a spatially inhomogeneous surface spectral gap Δ opens up below a characteristic temperature T_c^(2D), which is much higher than the bulk Curie temperature T_c^(3D) determined from the anomalous Hall resistance. The mean value and spatial homogeneity of the gap Δ generally increase with increasing c-axis magnetic field (H) and increasing Cr doping level (x), suggesting that the physical origin of this surface gap is associated with proximity-induced c-axis ferromagnetism. On the other hand, the temperature (T) dependence of Δ is non-monotonic, showing initial increase below T_c^(2D), which is followed by a 'dip' and then rises again, reaching maximum at T ≪ T_c^(3D). These phenomena may be attributed to proximity magnetism induced by two types of contributions with different temperature dependences: a three-dimensional contribution from the bulk magnetism that dominates at low T, and a two-dimensional contribution associated with the RKKY interactions mediated by surface Dirac fermions, which dominates at T_c^(3D) ≪ T < T_c^(2D). In addition to the observed proximity magnetism, spatially localized sharp resonant spectra are found along the boundaries of gapped and gapless regions. These spectral resonances are long-lived at H = 0, with their occurrences being most prominent near T_c^(2D) and becoming suppressed under strong c-axis magnetic fields. We attribute these phenomena to magnetic impurity-induced topological defects in the spin texture of surface Dirac fermions, with the magnetic impurities being isolated Cr impurities distributed near the interface of the bilayer system. The long-term stability of these topologically protected two-level states may find potential applications to quantum information technology.

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Yeh, N.-C.0000-0002-1826-419X
Alternate Title:Magnetism-induced massive Dirac spectra and topological defects in the surface state of Cr-doped Bi2Se3-bilayer topological insulators
Additional Information:© 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaf. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 19 June 2015. Accepted 19 October 2015. Published 17 November 2015. The work at Caltech was jointly supported by the Defense Advanced Research Projects Agency (DARPA), Focus Center Research Program: Center on Functional Engineered Nano Architectonics (FENA), and by the National Science Foundation (NSF), Physics Frontier Center: Institute for Quantum Information and Matter (IQIM). The work at UCLA was supported by DARPA/FENA. We thank Professors Gabriel Aeppli, Alexei Kitaev, Nai-Phaun Ong and Thomas Rosenbaum for useful discussions.
Group:IQIM, Institute for Quantum Information and Matter, Kavli Nanoscience Institute
Funding AgencyGrant Number
Defense Advanced Research Projects Agency (DARPA)UNSPECIFIED
Functional Engineered Nano Architectonics (FENA)UNSPECIFIED
NSF Physics Frontiers CenterUNSPECIFIED
Institute for Quantum Information and Matter (IQIM)UNSPECIFIED
Subject Keywords:topological insulators, surface state, Dirac fermions, scanning tunneling spectroscopy, proximity effect, ferromagnetism, impurity resonances
Record Number:CaltechAUTHORS:20150922-080652626
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Official Citation:C-C Chen et al 2015 New J. Phys. 17 113042
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:60404
Deposited By: Tony Diaz
Deposited On:22 Sep 2015 16:48
Last Modified:16 Nov 2017 21:31

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