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Two distinct superconducting pairing states divided by the nematic end point in FeSe1−xSx

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Science Advances  25 May 2018:
Vol. 4, no. 5, eaar6419
DOI: 10.1126/sciadv.aar6419
  • Fig. 1 Characterization of FeSe1−xSx single crystals by topographic imaging.

    (A to E) Constant-current topographic STM images of FeSe1−xSx. Feedback loop for scanning was established at the setup sample-bias voltage Vs = +20 mV and setup tunneling current Is = 100 pA. For x = 0 sample, Is = 10 pA was used. Local depressions represent sulfur atoms that replace the selenium atoms. The images were cropped from the larger images of 100 nm × 100 nm (160 nm × 160 nm for x = 0) fields of view, where all the subsequent SI-STM experiments were performed. Ortho., orthorhombic; Tetra., tetragonal. (F to J) Distributions of sulfur atoms in wider fields of view. Each red dot denotes the position of a sulfur atom. No tendency to the segregation is observed.

  • Fig. 2 In-plane QPI patterns of FeSe1−xSx at representative energies.

    (A to O) Each panel depicts the QPI pattern Lq(q, E) in q space obtained by Fourier-transforming the normalized real-space conductance map L(r, E). Tunneling spectra for each L(r, E) map were taken on a grid of 256 × 256 pixels in a 100 nm × 100 nm (160 nm × 160 nm for x = 0) field of view. Measurement conditions were Vs = +50 mV, Is = 100 pA, and the modulation amplitude for the lock-in detection Vmod = 0.85 mVrms. For the samples in the nematic phase (x < 0.17), experiments were carried out under 12-T magnetic field perpendicular to the surfaces to suppress superconductivity. In other samples, no magnetic field was applied. Strongly anisotropic QPI patterns for the x = 0 sample (A, F, and K) reflect the nematic character of the band structure. White arrow indicates the inter–electron-band QPI signal. Red and orange arrows denote the QPI signals associated with the outer hole band (see main text for details). The QPI patterns gradually become isotropic with increasing x. aFe ≡ |aFe|, bFe ≡ |bFe|, qa ≡ |qa|, and qb ≡ |qb|.

  • Fig. 3 Evolutions of QPI dispersion and band structure upon sulfur doping.

    (A to E) Energy-dependent line profiles of Fig. 2 along the qa direction. Electron-like QPI branch qe is identified. This branch becomes obscured with increasing x. (F to J) Energy-dependent line profiles of Fig. 2 along the qb direction. There are four QPI branches as labeled qhi (i = 1 − 4). To analyze the dispersions of QPI branches that cross the Fermi level (qh1 and qh2), we have determined the peak positions of the constant-energy line profile (scattering-vector distribution curve) by fitting the data with multiple Gaussian peaks. The obtained peak positions for qh1 and qh2 are plotted by orange and red open circles, respectively, on the right half of each panel. The obtained qh1(E) and qh2(E) data are fitted by quadratic functions of E shown by orange and red solid lines on the right half of each panel. On the left half of (F), intra–hole-band backscattering vectors expected from the ARPES data (15) are plotted. Orange and red lines are for the outer hole band, and green and light blue lines are for the inner hole band. Solid and dashed lines represents the backscattering q vectors at kz = 0 and kz = π/c, respectively. (K) Schematic illustration of the constant energy surface in k space (not in scale). Colors on the surface denote different orbital characters: blue, dxz; green, dyz; red, dxy. The inner hole band is not shown. (L) Evolutions of the Fermi wave vectors as functions of sulfur content x. qh1(E = 0 meV)/2 and qh2(E = 0 meV)/2 correspond to the minor axes of the elliptical cross sections of the outer hole Fermi cylinder at kz = π/c and kz = 0, respectively. Lines serve as visual guides. (M) Evolution of the Fermi velocity as a function of sulfur content x. Lines serve as visual guides. Anomalies are observed neither in the Fermi wave vector nor in the Fermi velocity at the NEP at x = 0.17.

  • Fig. 4 Evolutions of SC gap and BQPI pattern upon sulfur doping.

    (A) Averaged tunneling spectra of FeSe1−xSx showing the SC gaps. Each curve is shifted vertically by 5 nS for clarity. Spectra before the averaging were acquired on a grid of 256 × 256 pixels in a 100 nm × 100 nm (160 nm × 160 nm for x = 0) field of view. Measurement conditions were Vs = +20 mV, Is = 100 pA, and Vmod = 0.21 mVrms. The same data sets were used to obtain BQPI patterns shown in (D) to (M). (B) Energy second derivative of averaged tunneling spectra shown in (A). Dips marked by arrows correspond to the peak-like features in the original tunneling spectra. Each curve is shifted vertically by 12 nS/mV2 for clarity. (C) Evolutions of the apparent gap amplitude Embedded Image and the zero-energy spectral weight normalized by the weights at the gap-edge energies 2g(0)/(g(Embedded Image) + g(−Embedded Image)). In both quantities, there are abrupt changes at the NEP at x = 0.17. Lines serve as visual guides. (D to H) Energy-dependent line profiles of low-energy Lq(q, E)’s along the qa direction showing BQPI patterns. (I to M) Same as (D) to (H) but along the qb direction. Orange and red lines denote the fitted dispersions of qh1(E) and qh2(E) shown in Fig. 3 (F to J). In the nematic phase, there are two pairs of V-shaped dispersing BQPI branches that are smoothly connected to qh1(E) and qh2(E). These BQPI branches disappear above the NEP at x = 0.17.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/5/eaar6419/DC1

    Supplementary Text

    fig. S1. Evolution of the histogram of the apparent heights at the lattice points in FeSe1−xSx single crystals.

    fig. S2. Analysis of orthorhombic lattice distortion using the twin boundary.

    fig. S3. Extended and localized electronic modulations in the spectroscopic maps of FeSe.

    fig. S4. A complete set of the in-plane QPI patterns of FeSe1−xSx.

    References (48, 49)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • fig. S1. Evolution of the histogram of the apparent heights at the lattice points in FeSe1−xSx single crystals.
    • fig. S2. Analysis of orthorhombic lattice distortion using the twin boundary.
    • fig. S3. Extended and localized electronic modulations in the spectroscopic maps of FeSe.
    • fig. S4. A complete set of the in-plane QPI patterns of FeSe1−xSx.
    • References (48, 49)

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