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

Fabrication of epitaxial FeSe_xTe_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_xTe_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_xTe_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_0.5Te_0.5 thin films is caused by a remarkable increase of Se content. Although our FeSe_xTe_1-x thin films were fabricated via PLD using a Fe_0.94Se_0.45Te_0.55 target, the precisely measured composition indicates a Se-rich FeSe_xTe_1-x (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 FeSe_xTe_1-x, based on first principle calculations.