Research ArticleSUPERCONDUCTIVITY

Time-reversal symmetry-breaking superconductivity in epitaxial bismuth/nickel bilayers

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Science Advances  31 Mar 2017:
Vol. 3, no. 3, e1602579
DOI: 10.1126/sciadv.1602579
  • Fig. 1 Structure and Kerr signal of a Bi (25 nm)/Ni (2 nm) sample.

    (A) Left: Side view of the sample structure with a transmission electron microscopy image. Right: Schematic diagram of Kerr rotation measurement. (B) Kerr angle (purple) and resistance (green) measured on the Bi side for ZF cooldown, showing the onset of θK at Tc = 4.1 K. The pink dashed line is a guide to the eye with the form of 1 − (T/Tc)2. (C) Kerr signal measured on the Ni side at 1600-Oe applied perpendicular magnetic field (red) or at ZF (blue). In either case, no change of the Kerr signal was observed across Tc = 4.1 K.

  • Fig. 2 Chirality training and domain size estimation of a Bi (20 nm)/Ni (2 nm) sample.

    (A) ZF warm-up data after cooling down in +190-Oe magnetic field. (B) ZF warm-up data after cooling in −70-Oe magnetic field. Pink dashed lines in (A) and (B) are guides to the eye with the form of 1 − (T/Tc)2. (C) Kerr effect θK measured during ZF warm-up, after cooling down in ZF. Inset: Random (2D) chiral domains under the optical spot. (D) SD σ (θK) between experiments that contain two random contributions: σ0 due to chiral domains and σapp from the apparatus.

  • Fig. 3 Shifted Fermi surface and pairing symmetry of Bi electrons.

    (A) The Fermi circle of the electron surface states in Bi. The original dashed blue Fermi circle centered at Embedded Image is shifted by the vector w to the solid blue circle (shaded area) centered at Embedded Image because of the in-plane magnetization M produced by Ni. The electron momentum p is measured from Embedded Image and characterized by the azimuthal angle φp. The green arrows show spin polarization locked to the momentum direction. (B) Superconducting pairing of the electrons with opposite spins and opposite momenta p and p. The TRSB condensate has the total angular momentum Jz = ±2, corresponding to the d ± id pairing, as indicated by the red double-curved arrows. A weak training magnetic field can select one of the two degenerate states.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/3/e1602579/DC1

    Experimental methods and supplementary data

    Theory of superconducting pairing in epitaxial bismuth/nickel bilayers

    fig. S1. Schematic of Sagnac interferometer.

    fig. S2. RHEED patterns of the substrate and the sample.

    fig. S3. Supplementary data of the Bi (25 nm)/Ni (2 nm) sample.

    fig. S4. Dependence of the critical temperature Tc on the thickness of the Bi and Ni layers.

    fig. S5. Kerr signal of the Bi (40 nm)/Ni (2 nm) sample.

    fig. S6. Optical signal from the MgO substrate.

    References (3141)

  • Supplementary Materials

    This PDF file includes:

    • Experimental methods and supplementary data
    • Theory of superconducting pairing in epitaxial bismuth/nickel bilayers
    • fig. S1. Schematic of Sagnac interferometer.
    • fig. S2. RHEED patterns of the substrate and the sample.
    • fig. S3. Supplementary data of the Bi (25 nm)/Ni (2 nm) sample.
    • fig. S4. Dependence of the critical temperature Tc on the thickness of the Bi and
      Ni layers.
    • fig. S5. Kerr signal of the Bi (40 nm)/Ni (2 nm) sample.
    • fig. S6. Optical signal from the MgO substrate.
    • References (31–41)

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