Role of chalcogen vapor annealing in inducing bulk superconductivity in ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$

Recent investigations have shown that ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ can be made superconducting by annealing it in Se and O vapors. The current lore is that these chalcogen vapors induce superconductivity by removing the magnetic excess Fe atoms. To investigate this phenomenon, we performed a combination of magnetic susceptibility, specific heat, and transport measurements together with scanning tunneling microscopy and spectroscopy and density functional theory calculations on ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ treated with Te vapor. We conclude that the main role of the Te vapor is to quench the magnetic moments of the excess Fe atoms by forming ${\mathrm{FeTe}}_m$ ($m\ge 1$) complexes. We show that the remaining ${\mathrm{FeTe}}_m$ complexes are still damaging to the superconductivity and therefore that their removal potentially could further improve superconductive properties in these compounds.

Figure 1

(a) Picture of an as-grown ${\mathrm{Fe}}_{1.05}{\mathrm{Fe}}_{0.73}{\mathrm{Se}}_{0.27}$ sample. (b) Transport measurements of as-grown ${\mathrm{Fe}}_{1.05}{\mathrm{Fe}}_{0.75}{\mathrm{Se}}_{0.25}$ sample (red curve) and annealed ${\mathrm{Fe}}_{1.05}{\mathrm{Fe}}_{0.75}{\mathrm{Se}}_{0.25}$ sample under Te vapor (black curve). (c) Magnetic susceptibility measurements of as-grown ${\mathrm{Fe}}_{1.05}{\mathrm{Fe}}_{0.75}{\mathrm{Se}}_{0.25}$ sample (black curve) and annealed ${\mathrm{Fe}}_{1.05}{\mathrm{Fe}}_{0.75}{\mathrm{Se}}_{0.25}$ sample under Te vapor (red curve) (d) Zero-field-cooled (zfc) and field-cooled (fc) magnetic susceptibility measurements of as-grown ${\mathrm{Fe}}_{1.04}{\mathrm{Fe}}_{0.75}{\mathrm{Se}}_{0.25}$ sample (black curves) and annealed ${\mathrm{Fe}}_{1.04}{\mathrm{Fe}}_{0.75}{\mathrm{Se}}_{0.25}$ sample under Te vapor (bottom red curves) at low temperatures.

Figure 2

(a) Scanning tunneling microscopy topographic image of the annealed ${\mathrm{Fe}}_{1.05}{\mathrm{Te}}_{0.75}{\mathrm{Se}}_{0.25}$. The image was taken with a bias of 50 mV and a tunneling current of 100 pA at about 4.3 K. There are larger white blobs that are marked with arrows, and relatively smaller bright spots that are marked with open white circles (b) Typical $dI/dV$ spectra taken on the pristine area (red curve) and on the top of an impurity (black curve). The two spectra (1 and 2) correspond to the locations marked by spots 1 and 2 in (a). The spectra have been smoothed for better illustration.

Figure 3

(a) Zoomed-in image for the region marked with the square in Fig. 2 reveals an individual impurity (bright spot) as well as surface lattice of the annealed ${\mathrm{Fe}}_{1.05}{\mathrm{Te}}_{0.75}{\mathrm{Se}}_{0.25}$. The dashed line marks where the height profile in b is extracted. The image was taken with a bias of 50 mV and a tunneling current of 150 pA at about 4.3 K. An image flattening process has been applied for better illustration. (b) Height profile along the horizontal dashed line across the bright spot in (a), shows the height of impurity is about 1.0–1.5 Å. The height profile was extracted before the image flattening process was applied to (a). (c) Relaxed atomic positions obtained from first principles calculations for a single Fe interstitial atom on an FeTe surface. (d) Relaxed atomic positions obtained from first-principles calculations for a single FeTe complex on an FeTe surface.

Figure 4

(a) Gap mapping for the same area shown in Fig. 3. This gap mapping is obtained by fitting superconducting gap as described by the references cited in the main text. (Some data points in this map, were corrected manually to 0 value for the reason of the failure of correct fitting by the computer program). (b) The $dI/dV$ spectra measured near and away from the impurity along the arrow mark in Fig. 3, showing the suppression of superconducting gap around the impurity. More interestingly, around the impurity, we also observed that impurity-induced states (indicated by green and pink arrows) appear in the $dI/dV$ spectra with the suppression of the superconducting gap, when tip approaching to the impurity. The top $dI/dV$ curve was taken near the impurity, at the location marked with the white dot in a, which is set as the reference spot (0 Å). The distance marked for each curve, indicates the linear distance away from the reference spot toward the left side. The setup conditions for the spectra are $V=50$ mV, $I_{\mathit{t}}=150$ pA, $V_{\mathrm{mod}}=0.5$ mV. The spectra have been smoothed for better illustration.