Bulk superconductivity in FeTe${}_{1-x}$Se${}_x$ via physicochemical pumping of excess iron

The iron-based superconductor FeTe${}_{1-x}$Se${}_x$ has attracted considerable attention as a candidate topological superconductor owing to a unique combination of topological surface states and bulk high-temperature superconductivity. The superconducting properties of as-grown single crystals, however, are highly variable and synthesis dependent due to excess interstitial iron impurities incorporated during growth. Here we report a novel physicochemical process for pumping this interstitial iron out of the FeTe${}_{1-x}$Se${}_x$ matrix and achieving bulk superconductivity. Our method should have significant value for the synthesis of high-quality single crystals of FeTe${}_{1-x}$Se${}_x$ with large superconducting volume fractions.