Control of structural transition in FeSe_1-xTe_x thin films by changing substrate materials

Iron chalcogenide superconductors FeSe_1-xTe_x are important materials for investigating the relation be-tween the superconductivity and the orbital and/or electronic nematic order, because the end member material FeSe exhibits a structural transition without a magnetic phase transition. However, the phase separation occurs in the region of 0.1 ≤ x ≤ 0.4 for bulk samples, and it prevents the complete understanding of this system. Here, we report the successful fabrication of epitaxial thin films of FeSe_1-xTe_x with 0 ≤ x ≤ 0.7, which includes the phase-separation region, on LaAlO₃ substrates via pulsed laser deposition. In the temperature dependences of differential resistivity for these films with 0 ≤ x ≤ 0.3, the dip- or peak- anomalies, which are well-known to be originated from the structural transition in FeSebulk samples, are observed at the characteristic temperatures, T*. The doping-temperature (x-T) phase diagram of FeSe_1-xTe_x films clearly shows that T* decreases with increasing x, and that T_c suddenly changes at a certain Te content where T* disappears, which turns out to be commonly observed for both films on LaAlO₃ and CaF₂. These indicate the importance of controlling the structural transition to achieve high T_c in iron chalcogenides.