Research ArticleMATERIALS SCIENCE

Superconductivity across Lifshitz transition and anomalous insulating state in surface K–dosed (Li0.8Fe0.2OH)FeSe

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Science Advances  14 Jul 2017:
Vol. 3, no. 7, e1603238
DOI: 10.1126/sciadv.1603238
  • Fig. 1 Topographic image, tunneling, and ARPES spectra of as-cleaved (Li0.8Fe0.2OH)FeSe.

    (A) Topographic image of as-cleaved, FeSe-terminated surface (Vb = 100 mV and I = 50 pA); inset shows the surface lattice. (B) Low-energy dI/dV spectrum of as-cleaved FeSe surface, which displays double superconducting gaps of size Δ1 = 15 meV and Δ2 = 9 meV. a.u., arbitrary units. (C) Larger energy scale dI/dV spectrum. Arrows indicate the onset of the α and β bands (see text). Horizontal bar indicates the range of the δ band. (D) ARPES measurement of as-cleaved (Li0.8Fe0.2OH)FeSe. Solid curves track the dispersion of the β and δ bands, whereas the α band above EF is sketched with red dashed curve.

  • Fig. 2 Topographic images of the FeSe surface with a different Kc.

    (A) Kc = 0.008 ML. (B) Kc = 0.048 ML. (C) Kc = 0.098 ML. (D) Kc = 0.124 ML. (E) Kc = 0.226 ML. (F) Kc = 0.306 ML. Typical imaging parameters are Vb = 0.5 V and I = 50 pA. The red and blue arrows in (D) indicate the orientation of two different rotational domains. The white dashed arrow marks the position where the STS in Fig. 5E is taken.

  • Fig. 3 Evolution of dI/dV spectra taken on the FeSe surface with various Kc as labeled.

    (A) Typical dI/dV spectra taken within large energy range (±200 meV). Red and blue dashed lines track the onsets of the α and β bands. The zero positions of the spectra at Kc = 0.306, 0.264, and 0.226 ML are marked by short horizontal bars. (B) Typical dI/dV spectra taken near EF (±27 meV). Two blue dashed lines track the superconducting coherence peaks at negative bias. The curves at Kc ≤ 0.075 ML are normalized by the dI/dV value at Vb = −27 mV, and the curves at Kc > 0.075 ML are normalized by the value at Vb = 27 mV. EF (Vb = 0) is indicated by gray dashed lines. At Kc = 0.111, 0.124, and 0.129 ML, the gap edge positions (defined as Δ3) are marked by short dashed lines.

  • Fig. 4 Doping dependence of the energy band position and the DOS near EF.

    (A) The doping dependence of the band bottom (top) energy of the α (β) band. At Kc = 0.080 ML, the α band begins to cross EF. (B) Integrated dI/dV values within the bias range of ±8 meV as a function of Kc, which reflects the DOS near EF.

  • Fig. 5 QPI measurement of the α band and the spatial and temperature dependence of its gap.

    (A) Topographic image of the mapping area of size 100 × 100 nm2 (Kc = 0.124 ML). (B) Typical dI/dV map taken at Vb = 10 mV. The set point for dI/dV map is as follows: Vb = 50 mV, I = 150 pA, and ΔV = 3 mV. (C) FFT image of (B). (D) Intensity plot of the FFT linecuts through q = (0, 0); dashed curve is the parabolic fit. Note that the small gap is not observable here because of the large modulation (ΔV). (E) A dI/dV linecut taken along the dashed arrow in Fig. 2D, showing a spatially uniform gap. Bars indicate the coherence peaks. (F) Temperature dependence of the gap taken on a different sample with Kc ~ 0.12 ML.

  • Fig. 6 ARPES measurement of the band structure of surface K–dosed (Li0.8Fe0.2OH)FeSe.

    (A) ARPES intensity along cut #1 shown in (C), as a function of Kc and deposition time (t). Red dashed line in the third panel (Kc ~ 0.12 ML) represents the band dispersion of α that derived from QPI (Fig. 5D). (B) ARPES intensity along cut #2 shown in (C), as a function of Kc and t. Dashed lines track the dispersion of the δ band. (C) Sketch of the Brillouin zone of (Li0.8Fe0.2OH)FeSe. (D) Evolution of the MDC along cut #1 upon K dosing, integrated over ±14 meV at EF (curves are shifted vertically for clarity). The intensity at Γ increases up to Kc ~ 0.12 ML. The decreased intensity at Kc ~ 0.27 ML could be due to approaching to the insulating phase (consistent with Fig. 4B). (E) Evolution of the EDC taken around k = 0 (Γ point) upon K dosing (Kc = 0 to 0.12 ML). The increased intensity between −0.04 and 0 eV is consistent with the emergence of an electron pocket. (F) Symmetrized EDC showing the evolution of the superconducting gap on the δ band, as a function of Kc. The momenta of individual spectra are indicated by the arrows in (B).

  • Fig. 7 Summarized phase diagram of surface K–dosed (Li0.8Fe0.2OH)FeSe.

    The insets in regimes I and II sketch the Fermi surface before and after the Lifshitz transition. The red, blue, and green dots represent the value of Δ1, Δ2, and Δ3, respectively. Gray circles represent the ARPES measured gap size on the δ band (gray dashed line traces its variation). ARPES measurement suggests that Δ1 and Δ2 would not suddenly disappear when entering regime II, as illustrated by the short black dashed lines. SC, superconducting.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/7/e1603238/DC1

    fig. S1. Resistivity, dc magnetic susceptibility measurement, and optical microscopy image of (Li0.8Fe0.2)OHFeSe single crystal.

    fig. S2. Topographic image and STS taken on the as-cleaved Li0.8Fe0.2OH surface.

    fig. S3. Spatial distribution of the superconducting gap on the as-cleaved FeSe surface.

    fig. S4. Additional topographic images of the FeSe surface after K dosing.

    fig. S5. Unnormalized dI/dV spectra at the Kc near Lifshitz transition.

    fig. S6. dI/dV maps and corresponding FFTs taken in an area of 100 × 100 nm2 of the FeSe-terminated surface at Kc = 0.124 ML.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Resistivity, dc magnetic susceptibility measurement, and optical microscopy image of (Li0.8Fe0.2)OHFeSe single crystal.
    • fig. S2. Topographic image and STS taken on the as-cleaved Li0.8Fe0.2OH surface.
    • fig. S3. Spatial distribution of the superconducting gap on the as-cleaved FeSe surface.
    • fig. S4. Additional topographic images of the FeSe surface after K dosing.
    • fig. S5. Unnormalized dI/dV spectra at the Kc near Lifshitz transition.
    • fig. S6. dI/dV maps and corresponding FFTs taken in an area of 100 × 100 nm2 of the FeSe-terminated surface at Kc = 0.124 ML.

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