NMR Study in the Iron-Selenide ${\mathrm{Rb}}_{0.74}{\mathrm{Fe}}_{1.6}{\mathrm{Se}}_2$: Determination of the Superconducting Phase as Iron Vacancy-Free ${\mathrm{Rb}}_{0.3}{\mathrm{Fe}}_2{\mathrm{Se}}_2$

${}^{77}\mathrm{Se}$ and ${}^{87}\mathrm{Rb}$ nuclear magnetic resonance (NMR) experiments on ${\mathrm{Rb}}_{0.74}{\mathrm{Fe}}_{1.6}{\mathrm{Se}}_2$ reveal clearly distinct spectra originating from a majority antiferromagnetic (AF) and a minority metallic-superconducting (SC) phase. The very narrow NMR line of the SC phase evidences the absence of Fe vacancies and any trace of AF order. The Rb content of the SC phase is deduced from intensity measurements identifying ${\mathrm{Rb}}_{0.3(1)}{\mathrm{Fe}}_2{\mathrm{Se}}_2$ as the actual composition of the SC fraction. The resulting estimate of $0.15\mathrm{electrons}/\mathrm{Fe}$ brings this class of superconductors 245 family closer to the other Fe-based superconductor families.

Figure 1

${}^{87}\mathrm{Rb}$ NMR spectra for $H\Vert c$ at various temperatures display a narrow-superconducting (SC) and broad-antiferromagnetic (AF) contribution which can be separated by contrast measurements as shown at the bottom of the figure. Curves are vertically shifted with respect to each other.

Figure 2

(a) ${}^{77}\mathrm{Se}$ and ${}^{87}\mathrm{Rb}$ NMR room temperature spectra for the SC phase only (obtained by contrast experiments for Rb) for $H\Vert c$. (b) Corresponding full width at half maximum. (c) Temperature dependence of the SC Knight shift of ${}^{87}\mathrm{Rb}$ and ${}^{77}\mathrm{Se}$. (d) Temperature dependence of the ${}^{87}\mathrm{Rb}$ relaxation rate $\frac{1}{T_1}$ for the SC (red) and AF (green) phases. In all figures, the hatched region indicates where superconductivity occurs.

Figure 3

(a) Proposition for an alternated layer arrangement of the AF ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.6}{\mathrm{Se}}_2$ (green) and SC ${\mathrm{Rb}}_{0.3}{\mathrm{Fe}}_2{\mathrm{Se}}_2$ (red) phases from 20. (b), (c), and (d) the Rb NMR, $\mu \mathrm{SR}$ from [8] and Mössbauer from 9 data all show separated contributions from the SC (red) and AF (green) phases.

Figure 4

${}^{87}\mathrm{Rb}$ quadrupolar satellite (black) is compared to EFG point-charge simulations in ${\mathrm{Rb}}_x^{+}{\mathrm{Fe}}_{2-y}^{2+}{\mathrm{Se}}_2^{2-}$ assuming randomly distributed Rb sites for $x=0.3$ (red) or Fe vacancies either randomly distributed (green, blue) or ordered in a $\sqrt{5}\times \sqrt{5}$ pattern (magenta).