Evolution of Superconducting Properties in Fe1.1Se0.8Te0.2 Films Before and After Structure Avalanche.

By preparing a series of high-quality Fe1.1Se0.8Te0.2 films on the CaF2 substrate via pulsed laser deposition, we reveal the evolution of the structure as well as the superconductivity with the film thickness. We have found that there exists a threshold thickness above which the critical temperature Tc reaches its optimal value of 23.18 K with large activation energy, promising for high-field technological applications. Most importantly, the thick films have been found in a metastable state due to the fragile balance between the increased strain energy and the large compressive stress. Once the balance is broken by an external perturbation, a unique structure avalanche happens with a large part of the film exfoliated from the substrate and curves out. The exfoliated part of the film remains a single phase, with its lattice parameter and Tc recovering the bulk values. Our results clearly demonstrate the close relation between the compressive stress of the film/substrate interface and the high critical temperature observed in FeSeTe films. Moreover, this also provides an efficient way to fabricate free-standing single-phase FeSeTe crystals in the phase-separation regime.