Research ArticleCONDENSED MATTER PHYSICS

Tuning across the BCS-BEC crossover in the multiband superconductor Fe1+ySexTe1−x: An angle-resolved photoemission study

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Science Advances  21 Apr 2017:
Vol. 3, no. 4, e1602372
DOI: 10.1126/sciadv.1602372
  • Fig. 1 SC state ARPES spectra.

    (A to C) ARPES spectra for three Fe1+ySexTe1−x samples in order of decreasing y (excess Fe) from left to right. The spectra are normalized using the intensity from high-order photons, and a constant background is removed. The spectra are sharpened by adding a small part of their second derivative to the original data. The green dashed line is the best fit to the data using a simple parabolic dispersion. (D to F) Spectral functions, calculated using the model and parameters described in the text, to describe the BCS-BEC crossover seen in the data in the top panels.

  • Fig. 2 Schematic of electronic structure and transport data for Fe1+ySexTe1−x samples with different amounts of excess Fe y.

    In all panels, the red and blue curves correspond to the small and large y samples, respectively. (A) Schematic band structure and the effect of y on various bands (see text for details). (B) Resistivity (R) as a function of temperature (T). The small y sample has an SC Tc = 14 K, whereas the large y sample has Tc = 12 K. (C) Hall resistance RH of the same two samples. RH of the small y sample changes sign at around 40 K, whereas that for the large y sample is always positive.

  • Fig. 3 ARPES data showing the effect of y on the band structure.

    (A and B) ARPES spectra around the M point for two samples with small and large amounts of excess Fe, respectively. A shallow electron pocket can be seen, whose occupied bandwidth decreases with excess Fe. (C and D) The green lines are best fits using a simple parabolic model to the MDCs. The red lines represent the α2 dispersion for the same samples. (E and F) ARPES spectra of the same two samples around the Γ point using vertically polarized 22-eV light. In this polarization, α1 and α3 can be seen. The blue curves represent the dispersion of α3, and the green lines represent that of α1.

  • Fig. 4 Coherence peak dispersion in the SC state.

    (A to C) EDCs of the same three samples shown in Fig. 1 (A to C), measured at 1 K using horizontally polarized 22-eV photons. The blue line represents the EDC at the Γ point, and the black lines are the EDC at kF. a.u., arbitrary units. (D to F) The blue dots represent the dispersion of the EDC coherence peaks extracted from (A) to (C), respectively. The red dots represent MDC peak positions for binding energies between −5 and −50 meV. The red lines are fits to the latter using a simple parabolic model.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/4/e1602372/DC1

    Comparison between superconducting and normal state ARPES data

    Modeling the spectral function

    fig. S1. ARPES spectra above and below Tc.

    fig. S2. Bogoliubov dispersion from BCS to BEC.

    References (38, 39)

  • Supplementary Materials

    This PDF file includes:

    • Comparison between superconducting and normal state ARPES data
    • Modeling the spectral function
    • fig. S1. ARPES spectra above and below Tc.
    • fig. S2. Bogoliubov dispersion from BCS to BEC.
    • References (38, 39)

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