Improvement of superconducting properties by high mixing entropy at blocking layers in BiS2-based superconductor REO0.5F0.5BiS2

To investigate the interlayer interaction in the recently synthesized high-entropy-alloy-type (HEA-type) REO0.5F0.5BiS2 superconductors (RE: rare earth), we have systematically synthesized two sets of samples with close lattice parameters (close to those of PrO0.5F0.5BiS2 or CeO0.5F0.5BiS2) but having different mixing entropy (Delta S-mix) for the RE site. The crystal structure was investigated using synchrotron X-ray diffraction and Rietveld refinement. For the examined samples having different Delta S-mix, the increase in Delta S-mix does not largely affect the bond lengths and the bond angle of the BiS2 conducting layer but clearly suppresses the in-plane disorder at the in-plane S1 site, which is the parameter essential for the emergence of bulk superconductivity in the REO0.5F0.5BiS2 system. Bulk nature of superconductivity is improved by the increase in Delta S-mix for the present samples. The results of this work clearly show that the increase in mixing entropy at the blocking layer can positively affect the emergence of bulk superconductivity by modifying the local structure of the conducting layer. This is the evidence of the interaction between the high entropy states of the blocking layers and the physical properties of the conducting layers.