Superconductivity in the crystallogenide LaFeSiO$_{1-\delta}$ 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 systematic correlation between the Fe-anion height and the superconducting critical temperature $T_c$, with an optimum Fe-anion height of $\sim$ 1.38 \r{A}. Here, we report the discovery of superconductivity in a novel compound LaFeSiO$_{1-\delta}$ that incorporates a crystallogen element, Si, and challenges the above picture: considering the strongly squeezed Fe-Si height of 0.94 \r{A}, the superconducting transition at $T_{c}$ = 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 towards magnetic order and suggests that the emergence of superconductivity materializes in a distinct set-up, as compared to the standard $s_\pm$- and $d$-wave electron-pocket-based situations. These properties and its simple-to-implement synthesis make LaFeSiO$_{1-\delta}$ a particularly promising platform to study the interplay between structure, electron correlations and superconductivity.