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Supplementary Information
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Materials and methods
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Single crystals of KFS were grown by the following method. Precursor FeSe was synthesized using Se powder (Alfa 99.999$\%$) and Fe powder (99.998$\%$) in a 1:1 molar ratio. The reagents were weighed and placed in a 2mL alumina crucible. The quartz tube was sealed after being flushed with argon and evacuated. The sealed quartz tube was placed in a furnace and heated from room temperature to 1050$^\circ$C in 12 hours. The furnace remained at 1050$^\circ$C for 12 hours, then was shut off and cooled to room temperature. Single crystals of KFS were obtained by a self-flux method with mixtures of K (Alfa, 99.95$\%$), and FeSe in molar ratios of 1:3 respectively. Potassium has a low boiling point so a small amount of additional K needs to be incorporated into the growth during synthesis. The reagents were weighed and placed in a 2mL alumina crucible, which 