Enhanced superconducting transition temperature in FeSe _0.5Te_0.5 thin films

We report magnetoresistive and structural measurements of superconducting FeSe_0.5Te_0.5 epitaxial thin films grown by pulsed laser deposition. Enhanced onset superconducting transition temperature (∼17 K) is observed in some of these films. Structural analysis by x-ray diffraction and high resolution transmission electron microscopy reveal that these films generally have significantly shorter out-of-plane lattice constant c than the bulk value, suggesting that the out-of-plane changes have a dominating impact on the superconducting transition in iron-based superconductors. Our data also indicate that the upper critical field H_c2 (0) of those films may reach as high as 50 T.