Coexistence of different electronic phases in the K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ superconductor: A bulk-sensitive hard x-ray spectroscopy study

We have studied electronic and magnetic properties of the K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ superconductor by x-ray absorption and emission spectroscopy. Detailed temperature-dependent measurements along with a direct comparison with the binary FeSe system have revealed coexisting electronic phases: a majority phase with a high-spin ${}^{3+}$Fe state and a minority phase with an intermediate-spin ${}^{2+}$Fe state. The effect of high-temperature annealing suggests that the compressed phase with a lower-spin ${}^{2+}$Fe state is directly related to the high-$T_c$ superconductivity in the title system. The results clearly underline the glassy nature of superconductivity in the electronically inhomogeneous K${}_{0.8}$Fe${}_{1.6}$Se${}_2$, similar to the superconductivity in granular phases.

Figure 1

(a) PFY spectra measured at the Fe K edge of K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ at RT, 10 K, and RT after cooling, compared with the PFY spectrum of binary FeSe. The inset shows a close-up of the prepeak region. (b) The prepeak of FeSe is shown deconvoluted in two components representing the low (dot-dashed line) and high (solid line) spin ${}^{2+}$Fe states. (c)–(e) The prepeak of K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ is deconvoluted in two Gaussian features, A and B. (f) Temperature cycle of the relative intensities of the two features, ($I_B-I_A)/(I_B+I_A$), while cooling down (I) followed by warming up to RT again (II). The inset represents theoretical simulation of the preedge region for tetrahedral high-spin ${}^{2+}$Fe (solid line) and ${}^{3+}$Fe (dashed line) complexes.[19]

Figure 2

(a) Fe K$\beta$ emission spectra of K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ at different temperatures, compared with the spectra measured on FeSe and SmFeAsO. The spectra are normalized to the integrated area. The inset shows a close-up of the main K${\beta }_{1,3}$ emission line. (b) and (c) Resonant x-ray emission maps at RT and 10 K, respectively, measured at the K$\beta$ emission line with the incoming energy around the absorption prepeak. The arrows indicate the two features resonating at $E_A$ and $E_B$. (d) IDA as a function of temperature. Open squares represent the data while cooling down the sample (I), and the solid circles represent warming up from 10 K (II). The scale on the right-hand side shows the local magnetic moment $\mu$ determined following Gretarsson et al.[21] (e) Energy position of the K${\beta }_{1,3}$ main line as a function of temperature. The uncertainties are of the order of the symbols size.

Figure 3

(a) PFY spectrum at the Fe K edge of K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ measured at 10 K after high-temperature annealing (triangles), compared with the PFY spectrum on the as-grown sample (squares). The inset shows a close-up of the prepeak region. (b) and (c) The prepeaks before and after annealing are shown deconvoluted in two Gaussian components. (d) Comparison of the Fe K$\beta$ emission spectra collected at 10 K on the sample as-grown (squares) and after the high-temperature annealing (triangles).