Isovalent S and Te substitution effect on superconductivity in FeSe thin films*

We investigated chemical pressure effects from positive to negative (S and Te substitution) and in-plane strain effect from tensile to compressive on physical properties in FeSe thin films. Both S and Te substitution suppresses the structural transition temperature. The behavior of superconducting transition temperature, Tc, is completely different between S and Te substitution at the composition when the structural transition disappears. Tc increases drastically for Te substitution at the ortho.-tetra. boundary, while Tc shows monotonic decrease in S substituted samples. These results demonstrate that the relationship between the nematicity and the superconductivity is not universal in FeSe[1]. A magneto-transport study revealed a positive correlation between carrier densities and Tc in our films[2,3]. Our results suggest that the structural transition affects the electronic structure differently between Fe(Se,S) and Fe(Se,Te) and that this is the direct cause of the difference in the Tc behaviors at the ortho.-tetra. boundary[3].

[1] F. Nabeshima et al., JPSJ 87, 073704 (2018).
[2] F. Nabeshima et al., arXiv:1906.04894.
[3] F. Nabeshima et al., JJAP 57, 120314 (2018)