thin films as a platform for exploring exotic superconducting states*

Iron-based superconductor FeSe_xTe_1-x (FST) has been intensively studied due to its very unique electronic states driven by strong spin-orbit coupling and electronic correlation. For example, it is considered to be one of the promising candidates for a topological superconductor [1]. Moreover, it has been pointed out that FST has the strongest electronic correlation among the iron-based superconductor family, leading to realization of exotic superconducting states such as FFLO and BCS-BES crossover [2]. 

In this study, we demonstrate successful fabrication of FST thin films on CdTe(100) substrates covering the entire composition range of 0 ≤ x ≤ 1 by using molecular beam epitaxy (MBE) technique. We observed the FST thin films show superconductivity in the entire composition range, taking an optimal T_c ~ 12 K at around x = 0.1. The observed phase diagram shows stark contrast to that of bulk crystals, suggesting that substrate plays an important role for modifying the electronic structure. To get insight into the pairing mechanism, we also performed electrical transport measurements with a 60 T pulse magnet to systematically investigate upper critical fields H_c2 in various compositions of x. Remarkably, we observed strong suppression of orbital pair-breaking effect in Te-rich FST thin films irrespective the applied field direction, suggesting strong and nearly isotropic coupling of the Cooper pairs. The result suggests strong renormalization of electrons, which prefers exotic superconducting states such as FFLO and BCS-BEC crossover. Our work thus would provide materials platform to further investigate exotic superconducting states that arise from the strong spin-orbit coupling and the electronic correlation.

[1] N. Hao and J. Hu, National Science Review 6, 213 (2018).

[2] T. Shibauchi et al., Journal of the Physical Society of Japan 89, 102002 (2020).