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

The mismatch in lattice or thermal expansion coefficients between iron-based superconducting
thin films and their substrates significantly impacts the superconducting transition temperature,
Tc. 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 FeSe0.5Te0.5 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 FeSe0.5Te0.5 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
FeSe0.5Te0.5 single crystals. Additionally, a Tc of 13.6 K was observed for both relaxed films,
closely matching the Tc of the corresponding bulk sample. Although the composition and Tc of
the films are nearly identical to those of the single crystal, the upper critical field and
irreversibility field are significantly different. ... mehrThis suggests that bulk single crystals, commonly
used as strain-free references, may not be an ideal comparison with thin films.