Superconductivity in the crystallogenide LaFeSiO_1−δ with squeezed FeSi layers

Iron-based superconductors (IBSCs) are a class of unconventional superconductors featuring a square planar lattice of Fe atoms coordinated by pnictogen or chalcogen elements (X). The superconducting critical temperature (*T*_c) correlates with the Fe-X height (*h*_Fe−X), with an optimum around 1.38 Å. Recent research has extended IBSCs to include crystallogens (group 14 elements) like Ge and Si, replacing pnictogens/chalcogens. This is surprising as crystallogens were thought to favor ferromagnetism, detrimental to superconductivity. This work focuses on LaFeSiO_1−δ, a new iron-crystallogen superconductor with an exceptionally low Fe-Si height of 0.94 Å, yet exhibiting superconductivity below *T*_c ~ 10 K. This challenges the existing understanding of the relationship between structure and superconductivity in IBSCs.

Existing literature establishes a strong correlation between the Fe-anion height and the superconducting critical temperature in iron-based superconductors. An optimal Fe-anion height of approximately 1.38 Å is associated with maximum *T*_c values. Recent studies have explored the possibility of extending this family of superconductors by incorporating crystallogen elements such as Ge and Si. However, the use of crystallogens has been previously discussed as potentially detrimental to superconductivity due to a predicted preference for a ferromagnetic ground state rather than the antiferromagnetic ground state usually associated with the parent compounds of IBSCs. This paper expands upon this prior work by investigating a new material, LaFeSiO_1−δ, and its superconducting properties.

Polycrystalline LaFeSiO_1−δ samples were synthesized from a non-superconducting LaFeSi precursor through oxygenation at 330 °C in an Ar/O₂ atmosphere. Energy-dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM) confirmed the elemental composition and crystal structure. Neutron powder diffraction (NPD) was employed to refine the structural parameters, revealing the presence of oxygen in the La₄ tetrahedron and the unusually low Fe-Si height. The superconducting properties were characterized via electrical resistivity measurements on a small grain and a cold-pressed pellet, magnetization measurements (detecting a type-II superconductor hysteresis loop), and estimations of the upper and lower critical fields. Normal-state properties were investigated using ²⁹Si nuclear magnetic resonance (NMR) to probe spin fluctuations and electronic structure, and the latter was also explored by density functional theory (DFT) calculations. Further techniques employed include X-ray powder diffraction and thermogravimetric analysis (TGA).

LaFeSiO_1−δ exhibits superconductivity with an onset *T*_c ≈ 10 K, despite having a significantly compressed Fe-Si layer (*h*_Fe−Si = 0.94 Å). Electrical resistivity measurements show a complete superconducting transition in small grains, confirming the bulk nature of superconductivity. Magnetization measurements confirm the superconducting nature of the material and provide estimates for the critical fields, *H*_c1 and *H*_c2. The normal-state resistivity follows a *T*^1.4 dependence, characteristic of non-Fermi-liquid behavior. ²⁹Si NMR reveals weak antiferromagnetic fluctuations, and the spin-lattice relaxation rate 1/T₁ shows an enhancement at low temperatures, indicative of spin fluctuation enhancement but weaker than in many other Fe-based pnictides. DFT calculations show a Fermi surface dominated by hole pockets without significant nesting properties, explaining the suppressed tendency towards magnetic order. The calculated structure shows excellent agreement with the experimental findings. This reduced tendency toward magnetism is directly linked to the structure of LaFeSiO_1−δ. The moderate superconducting volume fraction observed is possibly linked to chemical inhomogeneity in oxygen content. The observed pseudogap-like behavior below 20 K warrants further investigation.

The discovery of superconductivity in LaFeSiO_1−δ with its unusually low Fe-Si height challenges the established correlation between Fe-anion height and *T*_c in Fe-based superconductors. The weak antiferromagnetic fluctuations and the Fermi surface dominated by non-nesting hole pockets suggest a superconductivity mechanism distinct from the conventional *s*-wave and *d*-wave scenarios. The non-Fermi-liquid behavior in the normal state hints at the importance of electronic correlations in this material. The findings raise the question of whether superconductivity in this compound arises from a different pairing mechanism or if the standard correlation exists but with a different optimum due to the unique crystal structure and Fermi surface topology. The simple synthesis and rich electronic properties of LaFeSiO_1−δ make it an exciting candidate for future studies.

This study demonstrates superconductivity in LaFeSiO_1−δ, a crystallogenide with an unexpectedly low Fe-Si height, challenging existing understandings of Fe-based superconductivity. The material exhibits weak antiferromagnetic fluctuations, non-Fermi-liquid behavior, and a Fermi surface dominated by non-nesting hole pockets. Further investigations are needed to fully elucidate the mechanism of superconductivity in LaFeSiO_1−δ and to explore the potential for tuning *T*_c by controlling the oxygen content.

The relatively low superconducting volume fraction and broad transition observed in resistivity measurements might be attributed to chemical inhomogeneity due to the oxygenation process. Further work is necessary to fully optimize the synthesis and achieve a higher degree of homogeneity. The NMR measurements were performed in a high magnetic field (15 T), which may suppress some superconducting effects. The origin of the pseudogap-like behavior below 20 K requires further investigation.