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_\rm c^\rm onset reaches \sim22 K for \sim20-50 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_\rm c^\rm 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 coefficient of LiF implies that the enhanced T_\rm 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_\rm c through heterostructure engineering.