Research ArticleSUPERCONDUCTORS

Imaging atomic-scale effects of high-energy ion irradiation on superconductivity and vortex pinning in Fe(Se,Te)

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Science Advances  22 May 2015:
Vol. 1, no. 4, e1500033
DOI: 10.1126/sciadv.1500033
  • Fig. 1 Visualizing superconductivity in pristine Fe(Se,Te).

    (A) Large, atomically resolved constant current topograph T(r) of FeSe0.45Te0.55. The inset shows an enlargement of the atomic lattice using the same color scale. (B) Differential conduction spectra at 270 mK taken along a line just outside the FOV shown in (A): all spectra are fully gapped with clear coherence peaks (indicated by arrows). The multitude of peaks outside the gap reflects the multiband nature of the system. (C) Vortex lattice of pristine Fe(Se,Te). The lattice is predominantly hexagonal with only minor distortions because of native pinning of vortices. The inset shows the FOV average spectrum away from vortices (dashed) and a typical spectrum taken at the core of a vortex (red) at 270 mK. (D) Autocorrelation of (C) exemplifying the predominance of the hexagonal vortex structure.

  • Fig. 2 Impact on Fe(Se,Te) superconductivity of heavy ion irradiation.

    (A) High-resolution T(r) of heavy ion–irradiated FeSe0.45Te0.55. As for the pristine sample, the predominant feature is the binary Se/Te surface appearance (inset). Red and blue circles indicate the columnar and point defects that are both only observed after irradiation. Depending on the FOV, the observed damage track density during our SI-STM studies varies between 2- and 4-T effective dose. This may be because the columnar defect tracks are discontinuous or because the distribution is sufficiently heterogeneous that the FOV may not be large enough for an accurate statistical count. (B) Average differential conduction spectrum at 1.2 K of columnar defects in (A): the superconducting signature in the tunnel spectrum is completely suppressed. (C) Average differential conduction spectrum at 1.2 K of point defects in (A): the superconducting signature in the tunnel spectrum shows significant reduction. (D) F(r) as defined in the text for the same FOV as depicted in (A): note the excellent correlation between suppressed superconductivity and position of columnar and clusters of point defects, marked by red and blue dashed circles, respectively.

  • Fig. 3 Vortex halo surrounding columnar defects.

    (A) Constant current image T(r): the red and yellow rectangles mark the position of vortices shown in (B). (B) S(r) at 2 T of same FOV as in (A): the red rectangle marks a vortex that is far away from columnar defects, whereas the yellow rectangle encircles the halo of a vortex pinned to the columnar defect marked by the yellow rectangle in (A). (C) Average S(r) of all columnar defects in (A): note the vortex halo surrounding a dark core because of the strong pinning of vortices to columnar defects at low fields. (D) Average T(r) of all columnar defects in (A): the average columnar defect position agrees very well with the location of the pinning center deduced from the average vortex halo shown in (C).

  • Fig. 4 Evolution of vortex configurations with magnetic field.

    (A) Constant current image T(r) of same FOV studied in (B) to (H). (B) F(r) of same FOV as in (A). (C to H) S(r,B) for B = 0.5, 1, 2, 3, 4, and 6 T, respectively. (I) Normalized cross-correlation between S(r) and T(r) (blue), and S(r) and F(r) (red), as a function of field.

  • Fig. 5 Overview of vortex pinning sequence.

    (A) Constant current image T(r) taken on irradiated Fe(Se,Te), clearly showing the columnar defects and point defects. (B) F(r) in the same FOV as (A), illustrating the effect of the two types of defect on the superconducting tunneling signature. (C) Vortex image, S(r), taken at 3 T on the area of (A). (D) Color-coded breakdown of the interplay of irradiation-induced defects, suppression of the spectral signature of superconductivity, and vortex pinning. Note the overlap between vortices (blue) far away from columnar defects (gray) and point defects (orange). The vortex density is the area in percentage of the full FOV covered by the various features. (E) Histogram representing the relation between vortices, columnar defects, and point defects distributed in a random, mixed pinning landscape created by swift ion irradiation. Data in the histogram were obtained by analysis of the whole FOV depicted in Fig. 4.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/4/e1500033/DC1

    Fig. S1. Magnetization and critical current density.

    Fig. S2. Columnar and point defects in more detail.

    Fig. S3. Crystal structure of the columnar defects.

    Fig. S4. F(r) for pristine and irradiated Fe(Se,Te) compared.

    Fig. S5. Effect of point defects on superconductivity.

    Fig. S6. High-energy (normal-state) characteristics of damage.

    Fig. S7. Vortex halos at columnar defects.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Magnetization and critical current density.
    • Fig. S2. Columnar and point defects in more detail.
    • Fig. S3. Crystal structure of the columnar defects.
    • Fig. S4. F(r) for pristine and irradiated Fe(Se,Te) compared.
    • Fig. S5. Effect of point defects on superconductivity.
    • Fig. S6. High-energy (normal-state) characteristics of damage.
    • Fig. S7. Vortex halos at columnar defects.

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