Research ArticleCONDENSED MATTER PHYSICS

Experimental observation of dual magnetic states in topological insulators

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Science Advances  08 Feb 2019:
Vol. 5, no. 2, eaav2088
DOI: 10.1126/sciadv.aav2088
  • Fig. 1 Experimental configuration.

    (A) Conceptual illustration of the Dirac fermion states of the Bi2Se3 topological insulator. In the magnetically doped Bi2Se3 (right), a bandgap is present between the upper and lower Dirac cones. (B) Sample configuration of the global-, surf-, and mid-doped Bi2Se3. Each cuboid represents one Bi2Se3 quintuple layer (QL).

  • Fig. 2 The global-doped Bi2Se3.

    (A to G) Typical total XAS and XMCD and their deconvoluted spectra of the global-doped Bi2Se3 at 2 to 80 K, respectively. The (percentage) XMCD intensity at the Cr L3 edge decreases with the increasing temperature within the measured range. The best fitting was obtained by a linear superposition of dsurf3.70 and dbulk2.79, with ~1:3 for the total XAS and ~1:2 for the XMCD. (H) Schematic diagram of the experimental set up and the measurement processes. Circularly polarized x-rays were used in normal incidence with respect to the sample plane and parallel to the applied magnetic field. Samples were cooled down to 2 K without magnetic field, and data were collected in the warm-up cycle.

  • Fig. 3 The modulation-doped Bi2Se3.

    Typical total XAS and XMCD and their deconvoluted spectra of the (A) surf-doped and (B) mid-doped Bi2Se3 at 3 to 80 K, respectively. For the surf-doped Bi2Se3, the best fitting was obtained by a linear superposition of dsurf3.70 and dbulk2.79, with ~4:5 for the total XAS and ~2:1 for the XMCD. No appreciable Cr dsurf3.70 but only Cr dbulk2.79 was obtained from the mid-doped Bi2Se3.

  • Fig. 4 M-T relationships.

    (Top)The XAS/XMCD-derived mspin and morb of the dsurf3.70 and dbulk2.79 versus temperature (T) at 3 to 80 K of the modulation-doped Bi2Se3 thin films. The dashed lines are the best fit within the mean-field approximation. (Bottom) Schematic illustration of the three-step transition: Both the surface and bulk are magnetically ordered below Tc (phase I); between Tc and Tc′, the surface retains magnetization while the bulk does not anymore (phase II); eventually, beyond Tc′, both the surface and bulk lose their magnetic orders (phase III).

  • Table 1 Summary of the XMCD-derived ms, Tc, and γ of the modulation-doped Bi2Se3 thin films.
    SampleCr dopantsmsB/atom)Tc (K)γ
    Surf-dopeddsurf3.702.49 ± 0.2546. 4 ± 2.50.40 ± 0.14
    dbulk2.791.30 ± 0.1031.0 ± 3.20.29 ± 0.17
    Mid-dopeddbulk2.791.34 ± 0.1031.3 ± 2.90.58 ± 0.10

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/2/eaav2088/DC1

    Section S1. Sample preparation

    Section S2. XAS/XMCD measurement

    Section S3. Multiplet calculations

    Section S4. Sum-rules analysis

    Fig. S1. Schematic diagram of the experimental setup for XAS and XMCD measurement.

    Fig. S2. Deconvolution of the mixed Cr valences.

    Fig. S3. The sum-rules analysis.

    Table S1. Summary of the XMCD-derived mspin for the global-, surf-, and mid-doped Cr-doped Bi2Se3, respectively, at 3 K.

    References (3743)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Sample preparation
    • Section S2. XAS/XMCD measurement
    • Section S3. Multiplet calculations
    • Section S4. Sum-rules analysis
    • Fig. S1. Schematic diagram of the experimental setup for XAS and XMCD measurement.
    • Fig. S2. Deconvolution of the mixed Cr valences.
    • Fig. S3. The sum-rules analysis.
    • Table S1. Summary of the XMCD-derived mspin for the global-, surf-, and mid-doped Cr-doped Bi2Se3, respectively, at 3 K.
    • References (3743)

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