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 T_c^onset 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, T_c^onset 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 coeffcient 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.