Superconductivity in the crystallogenide LaFeSiO_1−δ with squeezed FeSi layers

Pnictogens and chalcogens are both viable anions for promoting Fe-based superconductivity, and intense research activity in the related families has established a systematic correlation between the Fe-anion height and the superconducting critical temperature Tc, with an optimum Fe-anion height of ~1.38 Å. Here, we report the discovery of superconductivity in the compound LaFeSiO1−δ that incorporates a crystallogen element, Si, and challenges the above picture: considering the strongly squeezed Fe–Si height of 0.94 Å, the superconducting transition at Tc ~ 10 K is unusually high. In the normal state, the resistivity displays non-Fermi-liquid behavior while NMR experiments evidence weak antiferromagnetic fluctuations. According to first-principles calculations, the Fermi surface of this material is dominated by hole pockets without nesting properties, which explains the strongly suppressed tendency toward magnetic order and suggests that the emergence of superconductivity materializes in a distinct set-up, as compared to the standard s- and d-wave electron-pocket-based situations. These properties and its simple-to-implement synthesis make LaFeSiO1−δ a particularly promising platform to study the interplay between structure, electron correlations, and superconductivity.