Impact of interstitial oxygen on the electronic and magnetic structure in superconducting ${\mathrm{Fe}}_{1+y}\mathrm{Te}{\mathrm{O}}_x$ thin films

Interstitial oxygen critical to the emergence of superconductivity in ${\mathrm{Fe}}_{1+y}\mathrm{Te}{\mathrm{O}}_x$ thin films has been detected. Its location and concentration are measured by atomic-resolution electron-energy-loss spectroscopy with $x=0.09$. The density functional theory calculations show that oxygen incorporation leads to local disorder in the magnetic moments of Fe, hole doping by oxygen forming ionic bonds with Fe, and a large magnetic- and position-dependent increase or reduction in the Te-Fe-Te bond angles. An examination of bonding based on charge density further reveals covalent charge between Fe and Te, and its reduction with O doping.

Figure 1

(a) Temperature dependence of resistivity for the superconducting and nonsuperconducting films. The inset shows an onset and zero resistance transition temperature of ∼13 and 11 K, respectively, for the superconducting film. (b) Atomic-resolution HAADF-STEM image of a FeTe thin film with atomic columns labeled. (c) Electron diffraction pattern recorded from a 40-nm-diameter area centered at the film interface.

Figure 2

Line-scan EELS analysis of interstitial oxygen. (a) A typical EELS spectrum with $\mathrm{O}\text{−}K$, $\mathrm{Te}\text{−}{M}_{4,5}$ and $\mathrm{Fe}\text{−}{L}_{2,3}$ edges labeled. (b) The integrated intensity of O-$K$ is plotted by solid squares as a function of the scan path shown by the HAADF-STEM image in the inset. Error bars were estimated by assuming Poisson noise distribution for the signal and considering the standard deviation of the background intensity of the O-$K$. The corresponding smoothed curve is shown in red. The simultaneously recorded HAADF intensity along the scan path is shown below.

Figure 3

Structural models obtained from DFT calculations. A 2 × 2 × 1 supercell used for modeling is shown in (a) with Te-Fe-Te angle, $\alpha$, defined in (b). Bicollinear and collinear AFM configurations are shown in (c) and (d), respectively, with calculated Fe-O bond lengths and magnetic moments of Fe ions bonded with oxygen.

Figure 4

Difference charge density map of (a) FeTe bicollinear AFM, (b) FeTe collinear AFM, (c) FeTeO bicollinear AFM, and (d) FeTeO collinear AFM. Contour interval: $0.02\mathrm{e}/{\AA }^3$.

Figure 5

The effects of excess Fe on bonding as observed in the difference charge density maps of the (100) plane for pure FeTe (a) and ${\mathrm{Fe}}_{1.125}\mathrm{Te}$ with nonmagnetic (b), bicollinear (c), and collinear (d) AFM magnetic structure. Contour interval: $0.02\mathrm{e}/{\AA }^3$.