Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires
Johannes Gooth,*,†,‡ Robert Zierold,† Philip Sergelius,† Bacel Hamdou,† Javier Garcia,§ Christine Damm,§ Bernd Rellinghaus,§ Håkan Jan Pettersson,∥,⊥ Anna Pertsova,# Carlo Canali,# Mattias Borg,‡ and Kornelius Nielsch*,†,§ †
Institute of Nanostructure and Solid State Physics, Universität Hamburg, Jungiusstrasse 11 B, 20355 Hamburg, Germany IBM Research-Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland § Institute for Metallic Materials, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany ∥ Division of Solid State Physics and NanoLund, Lund University, Box 118, 22100 Lund, Sweden ⊥ Center for Applied Mathematics and Physics, Halmstad University, Box 823, 30118 Halmstad, Sweden # Department of Physics and Electrical Engineering, Linnaeus University, 39182 Kalmar, Sweden ‡
S Supporting Information *
ABSTRACT: Locally induced, magnetic order on the surface of a topological insulator nanowire could enable room-temperature topological quantum devices. Here we report on the realization of selective magnetic control over topological surface states on a single facet of a rectangular Bi2Te3 nanowire via a magnetic insulating Fe3O4 substrate. Low-temperature magnetotransport studies provide evidence for local time-reversal symmetry breaking and for enhanced gapping of the interfacial 1D energy spectrum by perpendicular magnetic-field components, leaving the remaining nanowire facets unaffected. Our results open up great opportunities for development of dissipationless electronics and spintronics. KEYWORDS: topological insulator, nanowire, magnetism, surface, 1D confinement
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conical Dirac band structure of the surface states splits into discrete 1D subbands that are robust against disorder (Figure 1a). While these azimuthal 1D modes have already been used as an Aharonov−Bohm (AB) interferometer pierced by magnetic flux quanta,29 they are highly fragile against external uniform perpendicular magnetic fields. Within this context, local magnetic manipulation, e.g., via selective magnetic doping or proximity to a magnetic substrate should locally split up the 1D bands,1 which, combined with Fermi level tuning, would confine electron transport to the remaining facets of the nanowire (Figure 1b). Realization of such magnetic control of the electronic structure and corresponding transport properties would allow for the development of functional spin information processing devices that are not limited by spin relaxation processes and spin-injection efficiencies. Furthermore, reverse magnetization of opposite NW surfaces and an altered surface magnetization along the NW axis are expected to host chiral edge modes as well as chiral bound states, carrying spins and charges without losses. Hence, breaking TRS in TI NWs is very interesting both from a fundamental physics point of view as
elective magnetic manipulation of topological insulator (TI) nanowires (NWs) has recently been proposed as a versatile tool to realize a variety of exotic topological quantum devices.1 Successfully developed, families of dissipation-less electronic and spintronic components are foreseen including low-power magnetoresistive switches with ultrahigh on−off ratios. The most distinct predicted transport features of such systems are 100% spin-polarized currents, highly tunable spin-polarized quantum dots, a quantized topological magnetoelectric response, and the half-integer quantum anomalous Hall (QAH) effect. Inducing magnetic order on the surface of TIs breaks time reversal symmetry (TRS) and introduces axion electrodynamics2 that is generally reflected by a gapped Dirac spectrum.3−6 In fact, an effective way to break TRS in 3D TIs is via magnetic doping7−15 or proximity induced coupling at a TI/ magnetic insulator interface.11,16−19 Both strategies have led to the discovery of various quantum phenomena, such as hedgehog-like spin textures,20,21 the integer QAH effect,22−25 and the giant magnetooptical Kerr effect.26 However, the predicted physics of TI NWs significantly deviates from their 3D counterparts, providing an additional parameter space to utilize. A key difference is their unique 1D surface band structure (Figure 1a,b)27−29 due to the confined periodic boundary conditions around the NW’s perimeter. The 2D © 2016 American Chemical Society
Received: May 28, 2016 Accepted: June 28, 2016 Published: June 28, 2016 7180
DOI: 10.1021/acsnano.6b03537 ACS Nano 2016, 10, 7180−7188
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Figure 1. Microscopic and magnetic device characterization. (a) Scheme of the 1D mode transport on the surface of a TI NW. Generally, the Dirac spectrum of the surface is gapped, due to the finite cross section of the NW. This gap is of the order of 1 meV for the NWs considered here. (b) Magnetic interactions in TIs, typically facilitated by magnetic doping or coupling to a magnetic substrate on a single facet of NW locally opens the gap of the order of tens of meV, due to broken TRS. Electrostatic interaction with the substrate can lead to an additional potential shift. The two shifted sets of 1D sub-bands are mainly localized on opposite NW facets. If the Fermi level EF lies within the magnetically induced gap of the Dirac states, transport will occur through the remaining facets of the NW, which correlates with the results of the magnetotransport measurements. (c) False-colored SEM image of a typical measurement device. The Bi2Te3 NW (red) is placed on a Fe3O4 film (green) between two contact electrodes (gray). The inset shows a close-up of a representative NW with 130 nm width and 45 nm height. (d) Transmission electron micrograph of a Bi2Te3 NW/Fe3O4 contact interface. (e,f) Selected area electron diffraction pattern of a representative Bi2Te3 NW, revealing single-crystallinity and a (110) growth direction as well as the crystallinity of the Fe4O3 film, respectively. (g,h) Magnetization isotherms at different temperatures for in-plane and out-of-plane magnetic field, respectively, of the 50 nm thick Fe3O4 thin film on a Si substrate. (i) In-plane (open triangles) and out-of-plane (open circles) coercive fields BC on the left axis and normalized magnetization Mnorm (green) of the Fe4O3 film on the right axis as a function of T.
RESULTS AND DISCUSSION The experiments were performed on individual singlecrystalline rectangular Bi2Te3 NWs grown by the vapor− liquid−solid method,31 which have been previously shown to exhibit TI surface states with up to 70% surface contribution to the total electrical transport.32,33 Delicate control of the NW growth in a three-zone furnace enables us to inherently adjust the Fermi level EF close the Dirac point (only tens of meV above), making the NWs sensitive to magnetic-field-induced gapping of the surface states. Two-terminal NW devices have been fabricated (Figure 1c) to measure their electrical resistance R as a function of the temperature T and the magnetic field B, using a low-frequency (130 K) to GΩ in the low-temperature limit (
