Thin films differ from bulk single crystals primarily in two aspects: defect density and strain state. Due to the nature of the growth process, superconducting thin films generally contain many more structural defects than single crystals. These include threading dislocations, twin or anti-phase boundaries, and precipitates. While such defects are detrimental in some contexts, in superconducting films they often act as strong pinning centers, significantly enhancing the critical current density. Another important difference is strain. Epitaxial films are typically strained due to lattice mismatch with the substrate. This in-plane strain can persist through a certain film thickness—and sometimes even beyond—resulting in compensating strain along the out-of-plane direction, often along the c-axis. This strain state can lead to noticeable shifts in the critical temperature Tc, particularly in iron based superconductors. To decouple the effects of defect density and strain, and to facilitate a more accurate comparison between thin films and bulk single crystals, we have developed multiple techniques to obtain substrate-free Fe(Se,Te) thin films. These include adhesive cleavage at or slightly above the film–substrate interface, as well as the use of a watersoluble sacrificial buffer layer. In this presentation, I will examine the structural and superconducting properties of the films prepared by these two methods and discuss their correspondence to the behavior observed in bulk single crystals. References: 1) L. Masuda et al., submitted in Supercond. Sci. Technol.
Masuda et al. (Wed,) studied this question.