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

Fabrication of epitaxial FeSeTe thin films using pulsed laser deposition (PLD) enables improving their superconducting transition temperature (T ) by more than ~40% than their bulk T . Intriguingly, T enhancement in FeSeTe thin films has been observed on various substrates and with different Se content, x. To date, various mechanisms for T enhancement have been reported, but they remain controversial in universally explaining the T improvement in the FeSeTe films. In this report, we demonstrate that the controversies over the mechanism of T enhancement are due to the abnormal changes in the chalcogen ratio (Se:Te) during the film growth and that the previously reported T enhancement in FeSeTe thin films is caused by a remarkable increase of Se content. Although our FeSeTe thin films were fabricated via PLD using a FeSeTe target, the precisely measured composition indicates a Se-rich FeSeTe (0.6 < x < 0.8) as ascertained through accurate compositional analysis by both wavelength dispersive spectroscopy (WDS) and Rutherford backscattering spectrometry (RBS). We suggest that the origin of the abnormal composition change is the difference in the thermodynamic properties of ternary FeSeTe, based on first principle calculations.