Research ArticleCONDENSED MATTER PHYSICS

Superconductivity with twofold symmetry in Bi2Te3/FeTe0.55Se0.45 heterostructures

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Science Advances  08 Jun 2018:
Vol. 4, no. 6, eaat1084
DOI: 10.1126/sciadv.aat1084
  • Fig. 1 STM/STS characterization of Bi2Te3/FeTe0.55Se0.45 heterostructure.

    (A and B) Atomically resolved topography (with bias voltage Vbias = 10 mV and tunneling current It = 200 pA) of the (A) FeTe0.55Se0.45 substrate and (B) 2-QL Bi2Te3 thin film. The measured lattice constants of the top Te/Se surface on FeTe0.55Se0.45 with square lattice and Te surface on Bi2Te3 with hexagonal lattice are 3.80 and 4.38 Å, respectively. Using a deposition rate of about 0.5 QL/min, we obtain the Bi2Te3 thin film with very good connectivity. The thickness changes from place to place. (C) Topographic image (Vbias = 2 V, It = 10 pA, 150 × 150 nm2) of Bi2Te3 film with different thicknesses. The height difference for each step along the dashed yellow arrowed line is about 1.0 nm, which equals a thickness of 1 QL. (D) Typical differential conductance spectra (set-point bias voltage Vset = 0.45 V, Iset = 300 pA) measured on FeTe0.55Se0.45 substrate and Bi2Te3 thin films with different thicknesses. The black arrows show the positions of the typical kink features probably from the Dirac point of the Bi2Te3 films of different thicknesses. (E) Tunneling spectra with SC feature (Vset = 10 mV, Iset = 50 pA) measured on FeTe0.55Se0.45 substrate and 1- to 4-QL Bi2Te3/FeTe0.55Se0.45. The short horizontal bars with the same color mark the zero differential conductance for each curve. The typical feature of the spectra on the FeTe0.55Se0.45 substrate is that one or two pairs of coherence peaks can be observed with peak energies varying from 1.1 to 2.1 meV. The tunneling spectrum measured on 1-QL Bi2Te3 also has the trace of coherent peaks of FeTe0.55Se0.45. This feature is absent on the 2-QL or thicker Bi2Te3 films. a.u., arbitrary units.

  • Fig. 2 Twofold SC gap resolved by QPI measurements.

    (A to J) The FT-QPI images derived from Fourier transformation to the QPI images at different energies with real-space area of 84 × 84 nm2 (Vset = 10 mV, Iset = 50 pA). The inset in (F) shows the topography associated with the QPI measurements. (K to O) Control experimental FT-QPI results on another 2-QL Bi2Te3/FeTe0.55Se0.45 heterostructure. The FT-QPI at 4.0 mV presented in (O) shows very clearly the feature of sixfold symmetry. (P) The averaged FT-QPI intensity per pixel in the q space of (G) and (H) within the band bounded by the two parallel dashed hexagons in (A) with increment of every 5°. The initial angle of φ is defined from one Γ-K direction, as marked by a red arrow in (A). (Q) Schematic top view of the Te atom layer of Bi2Te3 surface. (R) Schematic 2D hexagonal Fermi surface and the resultant angular dependence of the anisotropic SC gap. (S) Schematic image of SC gap structures with an oval Fermi surface; the color gives a qualitative distribution of the SC gap.

  • Fig. 3 Vortex image and vortex core states.

    (A) Zero-bias differential conductance map of a single vortex. The inset shows the topography measured near the vortex, and the vortex is elongated along the a axis. (B) Spectroscopic image of the vortex lattice consisted of elongated vortices measured at zero bias. This elongated vortex shape is very different from that in the bare FeTe0.55Se0.45 because of distinct values of Fermi energies. (C and D) Tunneling spectra measured along a axis and perpendicular to a axis directions, respectively. The curves marked by the red arrow represent the spectrum measured at the center of the vortex core. It should be noted that the dark dashed lines in (A) only denote the directions of the spectra measurements but do not represent the spatial distance of the measurements. The spectra shown in (C) and (D) are taken along much longer lines than those shown in (A), and the real spatial distance for measuring spectra is given by the vertical dashed arrowed line on the left-hand side of each panel. All data in this figure are taken at T = 0.4 K and B = 0.7 T with Vset = 10 mV and Iset = 50 pA.

  • Fig. 4 Energy evolution of vortex image.

    (A) Topography of the view in an area (Vbias = 1 V, It = 10 pA, 140 × 140 nm2). (B to H) Vortex image measured with different bias voltages (Vset = 10 mV, Iset = 50 pA) at 0.4 K and 0.7 T. Twofold symmetry of the vortex images appears at bias below the gap maximum around 2 meV (B to F). When the measuring voltage is beyond 2 meV (G and H), the vortex images become roughly isotropic dark discs, and the twofold symmetry is absent. This can be understood by the relative reduction of DOS within a certain region in the center compared with that outside that region.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/6/eaat1084/DC1

    fig. S1. Atomically resolved topographies and Fourier transform images of Bi2Te3 thin films with thicknesses of 1 and 2 QLs.

    fig. S2. Series of tunneling spectra measured on FeTe0.55Se0.45 and Bi2Te3 thin films.

    fig. S3. Real-space QPI patterns at series of energies on the 2-QL Bi2Te3/FeTe0.55Se0.45 heterostructure.

    fig. S4. Control experimental QPI results on the 2-QL Bi2Te3/FeTe0.55Se0.45 heterostructure.

    fig. S5. Simulation of FT-QPI pattern from a 2D hexagonal Fermi surface.

    fig. S6. Typical tunneling spectra and theoretical fitting results.

    fig. S7. Vortex image on FeTe0.55Se0.45 substrate without Bi2Te3 film.

    fig. S8. The spatial evolution of differential conductance across the elongated vortex.

    fig. S9. The control experiments of the elongated vortices.

    table S1. SC gap functions and fitting parameters used in the fitting to the tunneling spectra measured on Bi2Te3 with different thicknesses at 0.4 K.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Atomically resolved topographies and Fourier transform images of Bi2Te3 thin films with thicknesses of 1 and 2 QLs.
    • fig. S2. Series of tunneling spectra measured on FeTe0.55Se0.45 and Bi2Te3 thin films.
    • fig. S3. Real-space QPI patterns at series of energies on the 2-QL Bi2Te3/FeTe0.55Se0.45 heterostructure.
    • fig. S4. Control experimental QPI results on the 2-QL Bi2Te3/FeTe0.55Se0.45 heterostructure.
    • fig. S5. Simulation of FT-QPI pattern from a 2D hexagonal Fermi surface.
    • fig. S6. Typical tunneling spectra and theoretical fitting results.
    • fig. S7. Vortex image on FeTe0.55Se0.45 substrate without Bi2Te3 film.
    • fig. S8. The spatial evolution of differential conductance across the elongated vortex.
    • fig. S9. The control experiments of the elongated vortices.
    • table S1. SC gap functions and fitting parameters used in the fitting to the tunneling spectra measured on Bi2Te3 with different thicknesses at 0.4 K.

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