Temperature dependence of iron local magnetic moment in phase-separated superconducting chalcogenide

We have studied local magnetic moment and electronic phase separation in superconducting ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ by x-ray emission and absorption spectroscopy. Detailed temperature-dependent measurements at the Fe K-edge have revealed coexisting electronic phases and their correlation with the transport properties. By cooling down, the local magnetic moment of Fe shows a sharp drop across the superconducting transition temperature ($T_c$) and the coexisting phases exchange spectral weights with the low-spin state, gaining intensity at the expense of the higher-spin state. After annealing the sample across the iron-vacancy order temperature, the system does not recover the initial state and the spectral weight anomaly at $T_c$ as well as superconductivity disappear. The results clearly underline that the coexistence of the low-spin and high-spin phases and the transitions between them provide unusual magnetic fluctuations and have a fundamental role in the superconducting mechanism of the electronically inhomogeneous ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ system.

Figure 1

(a) Fe $K\beta$ XES measured on ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ at several temperatures. The spectra are normalized to the integrated area of the XES lines. Inset: A zoom over the main $K{\beta }_{1,3}$ emission peak. (b)–(g) Fe $K\beta$ emission spectra of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ (blue) at several temperatures are shown compared with the one measured on FeSe at RT (red). The difference spectra (green) are also shown (multiplied by four).

Figure 2

Integrated absolute difference (IAD) calculated with respect to the FeSe as a function of temperature (see text). The thick continuous line (blue) is a guide to the eyes. The size of the symbols represents maximum uncertainty, estimated by analyzing different XES scans at each temperature.

Figure 3

(a) Partial fluorescence yield (PFY) XAS spectra measured at the Fe K-edge of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ at various temperatures. (b) A zoom over the prepeak region with spectral differences across the transition temperature (multiplied by five) with respect to a spectrum measured below the transition. (c) Deconvoluted absorption preedge peak in two components (P1 and P2) after subtracting a constant arctangent background from the normalized absorption (the latter are also shown).

Figure 4

Relative weight of the P1 [Fig. 3] as a function of temperature. The thick continuous line (blue) is a guide to the eyes. The size of the symbols represents maximum uncertainty, estimated by Gaussian fits of different spectra at each temperature.

Figure 5

(a) ${\mathrm{Fe}}^{2+}$ HS domains gradually grow with cooling from 520 to 300 K. (b) Filamentary ${\mathrm{Fe}}^{2+}$ LS domain is established with cooling from 300 to 180 K. (c) The interface region between the ${\mathrm{Fe}}^{2+}$ HS and ${\mathrm{Fe}}^{2+}$ LS domains becomes magnetic with cooling from 180 to 32 K. (d) The magnetic interface state turns into the superconducting state with ${\mathrm{Fe}}^{2+}$ LS.