Structure of the oxygen-annealed chalcogenide superconductor Fe${}_{1.08}$Te${}_{0.55}$Se${}_{0.45}$O${}_x$

The as-grown single crystal Fe${}_{1.08}$Te${}_{0.55}$Se${}_{0.45}$ with the tetragonal PbO-type structure is nonsuperconducting owing to the excess Fe. Superconductivity is induced after oxygen annealing with an onset and zero resistance transition temperature around 14.5 K and 11.5 K, respectively. The oxygen doping is evidenced by electron energy loss spectroscopy and accompanied by improved homogeneity in the remaining PbO-type phase, as well as an increase in the L${}_3$/L${}_2$ intensity ratio of the Fe-L${}_{2,3}$ edge, indicating an increase in Fe valence. Local phase transformation from the tetragonal PbO-type phase to the hexagonal NiAs-type phase is also observed after oxygen annealing.

Figure 1

Synchrotron powder diffraction of as-grown and oxygen-annealed samples shown in black and red, respectively.

Figure 2

Temperature dependence of resistivity for the as-grown and oxygen-annealed single crystal Fe${}_{1.08}$Te${}_{0.55}$Se${}_{0.45}$ shown as the dashed and solid curve, respectively, demonstrating the emergence of superconductivity in the nonsuperconducting crystal after oxygen annealing.

Figure 3

(a) An electron diffraction pattern of the oxygen-annealed Fe${}_{1.08}$Te${}_{0.55}$Se${}_{0.45}$ along [001] direction, demonstrating mixed phases. Reflections of the tetragonal PbO-type phase are labeled with subscript T. Two sets of diffraction patterns of the hexagonal NiAs-type phase are indexed with subscript H_a and H_b, respectively. The inset shows the enlarged pattern from the boxed region. Two DF images, formed from the reflections of (110)${}_{\mathrm{h}\_\mathrm{a}}$ and (110)${}_{\mathrm{h}\_\mathrm{b}}$, are shown in (b) and (c), respectively, which show the needle-like contrast aligning with $a$ and $b$ axis of the tetragonal phase, respectively. A combined DF image by adding (b) and (c) is shown in (d) showing the meshlike contrast.

Figure 4

(a) A HAADF image of the oxygen-annealed Fe${}_{1.08}$Te${}_{0.55}$Se${}_{0.45}$ along [001] direction at low magnification. An enlarged image of the oxygen-annealed sample is shown in (b). For comparison, a HAADF image of the as-grown Fe${}_{1.08}$Te${}_{0.55}$Se${}_{0.45}$ demonstrating spinodal decomposition is shown in (c).

Figure 5

(a) A typical EELS spectrum with O-K, Te-M${}_{4,5}$, and Fe-L${}_{2,3}$ edges labeled. (b) The background subtracted experimental data of the O-K edge are shown as the open symbols. The solid curve, obtain by smoothing the experimental data, shows three peaks (a)–(c) indicated by the arrows, respectively. The prepeak (a) indicates hybridization of Fe 3d and O 2p states. (c) Fine structure of FeO and α-Fe${}_2$O${}_3$, reproduced from Ref. 19. The prepeak (a) we observed looks like mixture of that in FeO and α-Fe${}_2$O${}_3$.

Figure 6

AFM images of as-grown and oxygen-annealed samples shown in (a) and (b), respectively.