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Suppression of phase separation and giant enhancement of superconducting transition temperature in FeSe1−xTex thin films

Significance To clarify the mechanism of superconductivity in iron-based superconductors, it is crucial to investigate FeSe1−xTex, which has the simplest crystal structure among them. There is, however, a serious obstacle to the understanding of its superconductivity; phase separation occurs in the region of 0.1≤x≤0.4, and thus a whole phase diagram has not been available. Here we report the successful fabrication of FeSe1−xTex films with 0≤x≤1. This is, to our knowledge, the first demonstration of the suppression of the phase separation. What is more notable is that a giant enhancement of Tc is observed in the “phase-separation region.” The complete phase diagram that we present provides a previously unidentified perspective for the mechanism of superconductivity of this material. We demonstrate the successful fabrication on CaF2 substrates of FeSe1−xTex films with 0≤x≤1, including the region of 0.1≤x≤0.4, which is well known to be the “phase-separation region,” via pulsed laser deposition that is a thermodynamically nonequilibrium method. In the resulting films, we observe a giant enhancement of the superconducting transition temperature, Tc, in the region of 0.1≤x≤0.4: The maximum value reaches 23 K, which is ∼1.5 times as large as the values reported for bulk samples of FeSe1−xTex. We present a complete phase diagram of FeSe1−xTex films. Surprisingly, a sudden suppression of Tc is observed at 0.1<x<0.2, whereas Tc increases with decreasing x for 0.2≤x<1. Namely, there is a clear difference between superconductivity realized in x=0−0.1 and in x≥0.2. To obtain a film of FeSe1−xTex with high Tc, the controls of the Te content x and the in-plane lattice strain are found to be key factors.