Origin of the emergence of higher Tc than bulk in iron chalcogenide thin films

Fabrication of epitaxial FeSe x Te 1−x thin films using pulsed laser deposition (PLD) enables improving their superconducting transition temperature ( T c ) by more than ~40% than their bulk T c . Intriguingly, T c enhancement in FeSe x Te 1−x thin films has been observed on various substrates and with different Se content, x . To date, various mechanisms for T c enhancement have been reported, but they remain controversial in universally explaining the T c improvement in the FeSe x Te 1−x films. In this report, we demonstrate that the controversies over the mechanism of T c enhancement are due to the abnormal changes in the chalcogen ratio (Se:Te) during the film growth and that the previously reported T c enhancement in FeSe 0.5 Te 0.5 thin films is caused by a remarkable increase of Se content. Although our FeSe x Te 1−x thin films were fabricated via PLD using a Fe 0.94 Se 0.45 Te 0.55 target, the precisely measured composition indicates a Se-rich FeSe x Te 1−x (0.6