Research ArticleCONDENSED MATTER PHYSICS

Superconducting pairing of topological surface states in bismuth selenide films on niobium

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Science Advances  27 Apr 2018:
Vol. 4, no. 4, eaar7214
DOI: 10.1126/sciadv.aar7214
  • Fig. 1 Cleavage-based sample preparation and characterization.

    (A) Photo and schematic diagram of assembled Bi2Se3(0001)/Nb sample structure before cleavage. (B) Same sample structure after cleavage exposing a “fresh” surface of the Bi2Se3 film with a predetermined thickness of four QLs. (C) ARPES maps for samples with 4, 7, and 10 QLs of Bi2Se3 films. The spectra were recorded at photon energies of 18, 35, and 35 eV, respectively. (D) An x-ray photoelectron spectroscopy spectrum taken from a cleaved-off Al2O3 substrate reveals that only a trace amount of Bi is left on the substrate after cleavage, thus confirming that the cleavage occurs at the Bi2Se3/Al2O3 interface. (E) Atomic force microscopy (AFM) image of the Bi2Se3 film surface after cleavage. The step structure corresponds well to that of the Al2O3 substrate. a.u., arbitrary units.

  • Fig. 2 ARPES maps revealing proximity-induced superconductivity.

    (A) ARPES map taken from a sample with a 10-QL Bi2Se3 film at T = 10 K using 6.994-eV photons. It shows a Dirac cone from the TSSs within the bulk band gap. (B) Detailed ARPES maps for energies close to the Fermi level for the 10-QL film sample at T = 10 and 1.5 K, respectively. (C) Corresponding symmetrized ARPES maps reveal coherence peaks and superconducting gaps throughout momentum space at 1.5 K. (D) EDCs at three selected momenta corresponding to a BS, +TSS, and −TSS demonstrate proximity-induced superconductivity in all of these states. (E to H) Similar results for a sample with a four-QL Bi2Se3 film.

  • Fig. 3 Symmetrized ARPES EDCs as a function of temperature and Bi2Se3 film thickness.

    Results for a bare Nb sample as well as for BS, +TSS, and −TSS at Bi2Se3 film thicknesses of 4, 5, 7, and 10 QLs, respectively. The height of the coherence peaks and the size of the superconducting gap increase with decreasing temperature below TCNb and completely vanish for temperatures above TCNb; both superconducting features become less pronounced for increasing Bi2Se3 film thicknesses at a given temperature below TCNb.

  • Fig. 4 Extracted superconducting gaps for the BSs and TSSs.

    (A) Superconducting gaps for the BS and the TSS as a function of temperature deduced from fitting the symmetrized EDCs with a Dynes function. Results for different film thicknesses and a bare Nb sample are shown. The curves are fit using Eq. 1; the fits yield the zero-temperature gap Δ(0) for each case. (B) Δ(0) for a BS and 〈TSS〉 as a function of film thickness. The curve is an exponential fit.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/4/eaar7214/DC1

    section S1. Characterization of as-grown Bi2Se3 films

    section S2. Characterization of cleaved Bi2Se3/Nb samples

    section S3. Stability and accuracy for measuring the Fermi level EF and the gap

    section S4. Comparison to a model calculation

    section S5. ARPES measurements of the superconducting proximity effect at the One-Cube Beamline at BESSY II

    fig. S1. Characterization of as-grown Bi2Se3 films.

    fig. S2. ARPES maps of a seven-QL Bi2Se3/Nb sample.

    fig. S3. Surface morphology of cleaved Bi2Se3/Nb samples.

    fig. S4. Superconducting gap of Nb as a function of detector angular channels.

    fig. S5. Superconducting gaps as a function of film thickness.

    fig. S6. ARPES results obtained at the One-Cube Beamline at BESSY II.

    References (2527)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Characterization of as-grown Bi2Se3 films
    • section S2. Characterization of cleaved Bi2Se3/Nb samples
    • section S3. Stability and accuracy for measuring the Fermi level EF and the gap
    • section S4. Comparison to a model calculation
    • section S5. ARPES measurements of the superconducting proximity effect at the One-Cube Beamline at BESSY II
    • fig. S1. Characterization of as-grown Bi2Se3 films.
    • fig. S2. ARPES maps of a seven-QL Bi2Se3/Nb sample.
    • fig. S3. Surface morphology of cleaved Bi2Se3/Nb samples.
    • fig. S4. Superconducting gap of Nb as a function of detector angular channels.
    • fig. S5. Superconducting gaps as a function of film thickness.
    • fig. S6. ARPES results obtained at the One-Cube Beamline at BESSY II.
    • References (25–27)

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