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Evidence for orbital order and its relation to superconductivity in FeSe0.4Te0.6

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Science Advances  16 Oct 2015:
Vol. 1, no. 9, e1500206
DOI: 10.1126/sciadv.1500206
  • Fig. 1 Composition analysis from the topography and spatial map of the superconducting gap.

    (A) STM topography of FeSe0.4Te0.6 taken with bias voltage V = 80 mV and tunneling current I = 2.0 nA at 2.1 K. The inset is a close-up of the atomically resolved chalcogenide layer. (B) Schematic of the atomic structure of Fe(Se,Te), oriented as in the topography (not to scale). (C) Fourier transform of the topographic image shown in (A). Bragg peaks associated with tellurium/selenium (qCh) atoms are visible. (D) Histogram of the height of Se/Te atoms in (A); the composition is obtained from two Gaussians fitted to the histogram. The result (37 ± 4% Se and 63 ± 4% Te) closely matches the composition determined by EDX (see also fig. S2 and note S1). (E) Spatial map of the size of the superconducting gap in an area of 50 × 50 nm2, taken at T = 2.1 K. (F) Histogram of the gap size for the map in (E).

  • Fig. 2 Symmetry-breaking QPI in FeSe0.4Te0.6.

    (A to C) Spectroscopic maps of the differential conductance g(x, V) taken at T = 2.1 K (50 × 50 nm2, V = 40 mV, I = 0.3 nA, Vmod = 600 μV). (D to F) Processed Fourier transform images Embedded Image obtained from (A) to (C) (see note S2 for details of data processing).

  • Fig. 3 Analysis of QPI.

    (A and B) Line cuts in horizontal and vertical directions from the center in Fig. 2 (D to F). A clear anisotropy is seen at negative bias voltages, where the horizontal cut along the Fe-Fe direction shows strong scattering at |q| = 0.12π/aFe-Fe, where aFe-Fe is the atomic distance between two Fe atoms. (C and D) Line cuts as in (A) and (B) obtained from a map measured at T = 16 K > Tc (see also fig. S4). (E) Magnitude |q| of the dominant scattering vector due to the symmetry-breaking state as a function of energy. The symmetry-breaking excitations persist above Tc.

  • Fig. 4 Relation between symmetry-breaking excitations and superconductivity.

    (A) Autocorrelation of a gap map (38 × 38 nm2). (B) Line cuts (blue and green symbols) that are extracted along horizontal and vertical directions in (A) are fitted by an exponential decay function and yield decay lengths ξx = 1.38 nm and ξy = 3.0 nm. (C) Correlation plot between a low-pass filtered conductance map taken at V = −4.5 mV (see also fig. S5) and the local gap size in Fig. 1E. A clear anticorrelation (correlation coefficient ~−0.5) is found.

  • Fig. 5 Comparison with the tight-binding model.

    (A) Fermi surface of FeSe0.4Te0.6 as obtained from a tight-binding calculation in the normal state. (B) JDOS calculation corresponding to (A). (C) Fermi surface with orbital splitting (Δ) between dxz and dyz orbitals of 8 mV included. (D) Corresponding JDOS calculation. The dominant scattering vector near q = 0 is due to interband scattering between the α2 and α3 bands. The inset shows the magnified view of the central part. (E) Orbital splitting versus the dominant scattering vector q. The dominant scattering occurs on the hole pockets around the Γ point; a renormalization of 10 has been accounted for [which would be appropriate for the α3 band (33)].

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/9/e1500206/DC1

    Fig. S1. Magnetic susceptibility.

    Note S1. Composition analysis from STM data.

    Fig. S2. Composition analysis.

    Note S2. QPI data processing.

    Fig. S3. Unprocessed Fourier transforms and autocorrelation.

    Fig. S4. Spectroscopic maps and QPI at 16 K.

    Note S3. Correlation with symmetry-breaking states.

    Fig. S5. Correlation with symmetry-breaking states.

    Note S4. Tight-binding calculation.

    Fig. S6. Details of the tight-binding model.

    Note S5. JDOS calculation including orbital character.

    Fig. S7. JDOS calculation including orbital character.

    References (55, 56)

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Magnetic susceptibility.
    • Note S1. Composition analysis from STM data.
    • Fig. S2. Composition analysis.
    • Note S2. QPI data processing.
    • Fig. S3. Unprocessed Fourier transforms and autocorrelation.
    • Fig. S4. Spectroscopic maps and QPI at 16 K.
    • Note S3. Correlation with symmetry-breaking states.
    • Fig. S5. Correlation with symmetry-breaking states.
    • Note S4. Tight-binding calculation.
    • Fig. S6. Details of the tight-binding model.
    • Note S5. JDOS calculation including orbital character.
    • Fig. S7. JDOS calculation including orbital character.
    • References (55, 56)

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