Superconducting properties of mechanically exfoliated macro strain-free Fe(Se,Te) films

Abstract The mismatch in lattice or thermal expansion coefficients between iron-based superconducting thin films and their substrates significantly impacts the superconducting transition temperature, T c . Bulk single crystals, often used as strain-free references, are not ideal for comparison due to the non-equilibrium growth conditions of thin films. Free-standing thin films, grown under identical conditions and separated from the substrate, provide a more accurate basis for comparison. Here, we have successfully fabricated strain-free FeSe 0.5 Te 0.5 thin films using two methods: i) peeling the superconducting layer with a razor blade, and ii) applying epoxy resin to a quartz substrate, placing an FeSe 0.5 Te 0.5 thin film on top, securing it, and subsequently cleaving the film. The c -axis length of both relaxed films was nearly identical to that of bulk FeSe 0.5 Te 0.5 single crystals. Additionally, a T c of 13.6 K was observed for both relaxed films, closely matching the T c of the corresponding bulk sample. Although the composition and T c of the films are nearly identical to those of the single crystal, the upper critical field and irreversibility field are significantly different. This suggests that bulk single crystals, commonly used as strain-free references, may not be an ideal comparison with thin films.