Tripling the critical temperature of KFe₂As₂ by carrier switch

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Superconductivity of high critical temperature (Tc) superconductors is usually realized through chemical dopant or application of pressure in a similar way to induce charge carriers of either electrons or holes into their parent compounds. For chemical doping, superconductivity behaves asymmetrically with the maximum Tc often higher for optimal hole-doping than that of optimal electron-doping on the same parent compound. However, whether electron carriers could be in favour of higher Tc than holes in such high-Tc superconductors is unknown but attractive. Here we show that the application of pressure can drive KFe₂As₂ from hole- to electron-superconductivity after passing the previously reported V-shape or oscillation regime. The maximum Tc in the electron-dominated region is tripled to the initial value of 3.5 K or the average in the low-pressure hole-dominated region. The structural transition takes place from the tetragonal to collapsed tetragonal phase when the carrier characteristic is changed upon compression. Our results unambiguously offer a new route to further improve superconductivity with huge Tc enhancement for a compound through carrier switch. The strong electronic correlations in KFe₂As₂ are suggested to account for the unexpected enhancement of superconductivity in the collapsed tetragonal phase.