Influence of strain and composition on Tc in FeSe1-xTex films

FeSe1-xTex films with wide ranging strain and composition were deposited using pulsed laser deposition, and lattice parameters and critical temperature (T-c) were evaluated in the films to understand influence of strain and composition on T-c of FeSe1-xTex films. c was larger than that in bulk and a was smaller than the bulk value due to strain (substrate effect), where c and a are c-axis length (perpendicular to film surface) and a-axis length (parallel to film surface) in the c-axis oriented FeSe1-xTex films. Depending on c value, two kinds of regions were observed regarding relationship between a and T-c. In the case of small c (c < 5.9 angstrom: small Te content), lattice parameter dependence of T-c in the films was different from that in bulk with different Te content, and T-c strongly increased with decrease in a. On the other hand, in the case of large c (c > 5.9 angstrom: large Te content), lattice parameter dependence of T-c in the films was similar to that in bulk with different Te content, and in-plane film strain did not vary T-c. In first principle band calculation, ab-plane bi-axial strain varied the Fermi surface in FeSe (c < 5.9 angstrom), but such strain-induced variation of Fermi surface was smaller in FeSe0.5Te0.5 (c > 5.9 angstrom), which is consistent with the experimental results. The present results suggest that much higher T-c than the bulk values is obtained by decreasing a (compressive ab-plane strain) with keeping c < 5.9 angstrom (small Te content) using hydrostatic pressure and/or film strain. (C) 2014 AIP Publishing LLC.