Control of structural transition in FeSe1−xTex thin films by changing substrate materials

Abstract

Iron chalcogenide superconductors FeSe1−xTex 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 FeSe1−xTex 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 FeSe1−xTex films clearly shows that T* decreases with increasing x, and that Tc 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 Tc in iron chalcogenides.

Comparative Review on Thin Film Growth of Iron-Based Superconductors

“…Successful suppression of phase separation in [132]. Suppression of phase separation is also possible with LAO substrates; in this case the optimal composition is x = 0.4 [133]. These results demonstrate that the optimal composition, x opt , of FeSe 1−x Te x is in the phase separation region.…”

“…In this figure, the horizontal axis is the nominal Te content of the polycrystalline target. We strongly believe that Te contents of the grown films are nearly identical to the nominal values of the targets, because the c-axis lengths of the films exhibit almost linear dependence on x including two end member materials, FeSe and FeTe [131,133]. Therefore, in Figure 3b, the horizontal axis is able to be regarded as the Te content of the resultant film.…”

“…These results confirmed that the suppression of T c in films with x < x opt cannot be explained by the local structural parameters. Therefore, we believe that the sudden change in T c at x opt in Figure 3b is probably related to the structural/nematic transition since the Te content where T s disappears coincides x opt [133], and that the orthorhombic distortion and/or nematic order may not be favorable for the superconductivity in iron chalcogenides. At x opt , which is closely related to the structural/nematic transition, big changes are observed in some physical properties other than T c : the upper critical field, B c2 [209], the superconducting transition width [213], the ratio of B c2 along the ab-plane and the c-axis in the orbital limit, γ ≡ B //ab c2,0K /B //c c2,0K [213], the temperature dependence of Hall coefficient, R H (T) [209], and the optical conductivity [214].…”

“…T s is considered to be a temperature where a temperature derivative of resistivity takes its local minimum (maximum) for FeSe single crystals with large values of RRR [197] (samples with small values of RRR [206,207]). We can define T s at x = 0-0.1 for films on CaF 2 and x = 0-0.3 for films on LAO [133]. At the other compositions, there are no peak-nor dip-structure in a temperature derivative of resistivity, which shows that structural/nematic transition does not occur.…”

“…", and "AF" are abbreviations for orthorhombic, tetragonal, monoclinic phases, and bicollinear antiferromagnetic-ordered state, respectively. Here, for discussing the relation between local structural parameters and superconductivity, we focus on the peak intensity of out-of-plane XRD patterns [131,133], especially the intensity ratio of the 003 reflections to the 002 reflections, r = I 003 /I 002 , where I 002 and I 003 are intensities of 002 and 003 reflections, respectively. The structural analysis of FeSe 1−x Te x bulk samples at room temperatures revealed that the space group and the atomic coordinates other than the z-coordinate of the Chalcogen (Se or Te) site did not change with changing Te contents [210].…”