Enhanced Superconductivity in FeSe Thin Films on LiF Substrates

FeSe films have attracted extensive research interest in iron-based superconductors owing to their highly tunable superconducting properties. Here, we fabricated a series of FeSe films with various thicknesses on LiF substrates using pulsed-laser deposition. The onset superconducting transition temperature (Formula presented) (Formula presented) reaches ∼22 K for ∼20–50 nm-thick films, which is nearly three times that of bulk FeSe and much higher than values previously reported for FeSe films on fluoride substrates. Additionally, (Formula presented) (Formula presented) increases anomalously as the film thickness decreases. Transport measurements reveal no clear signatures of interfacial electron doping from the LiF substrate. Structural characterizations indicate an out-of-plane c-axis expansion accompanied by an in-plane lattice contraction in FeSe/LiF films. The large thermal expansion coefficient of LiF implies that the enhanced T_c originates from strong in-plane compressive strain induced during post-growth cooling. This study provides an interpretation of the varying superconducting properties in FeSe films and offers valuable insights into further enhancing T_c through heterostructure engineering.