Superconducting properties of the strain-free Fe(Se,Te) film

The mismatch in lattice or thermal expansion coefficients between iron-based superconducting thin films and their substrates significantly affects the superconducting transition temperature (Tc) 1. Bulk single crystals, generally used as strain-free references, are not ideal for comparison due to the non-equilibrium growth conditions of thin films. Freestanding thin films, grown under identical conditions and separated from the substrate, are better for accurate comparisons. This study used Fe (Se, Te), the simplest iron-based superconductor, to fabricate freestanding thin films via three methods. The films were prepared by pulsed laser deposition with a KrF excimer laser. Initially, a CeO₂ buffer was deposited on an SrTiO₃ (STO) (001) substrate. Freestanding thin films were fabricated through three methods: (1) peeling the superconducting layer with a razor blade (Film B), (2) applying epoxy resin to a quartz substrate, placing Fe (Se, Te) thin films on top, and holding for 10 hours to ensure sufficient solidification, followed by cleaving the film, and (3) using water-soluble Sr₃Al₂O₆ layers on STO and LaAlO₃ (LAO) (001) substrates, followed by immersion in deionized water (Film D and E). Figure 1 (a) shows X-ray diffraction (XRD) patterns of as-grown thin films, Film B, and Film C. The weak 00l peak intensity in Films B and C is due to reduced Fe (Se, Te) volume fractions after detachment or cleavage. Figure 1 (b) highlights the 004 peak shift to higher angles in Films B and C compared to as-grown films. The onset Tc decreased from about 17 K (as-grown) to 14 K for Films B and C, comparable to bulk crystals 2. Figure 1 (c) shows the XRD patterns of Films D and E, with all layers oriented along 001. Before immersion, we confirmed zero resistance at approximately 4 K and 6 K for Films D and E, respectively.