Local-structure insight into the improved superconducting properties of Pb-substituted La(O, F)BiS₂: a photoelectron holography study

Pb-substituted La(O, F)BiS₂ (Pb-LaOFBiS₂) exhibits improved superconducting properties and a resistivity anomaly around 100 K that is attributed to a structural transition. We have performed temperature(T)-dependent photoelectron holography (PEH) to study dopant incorporation sites and the local structure change across the anomaly. The PEH study of Pb-LaOFBiS₂ provided evidence for the dominant incorporation sites of Pb and F: Pb atoms are incorporated into the Bi sites and F atoms are incorporated into the O site. No remarkable difference in the local structures around Pb and Bi atoms was observed. Across the temperature of the resistivity anomaly (T^*), photoelectron holograms of Bi 4f changed. Comparisons of holograms with those of non-substituted LaOFBiS₂ sample, as well as simulated holograms, suggested that (1), above T^*, the tetragonal structure of Pb-LaOFBiS₂ is different from the tetragonal structure of LaOFBiS₂ and (2), below T^*, the tetragonal structure still remains in Pb-LaOFBiS₂. We discuss a possible origin of the difference in the structure above T^* and the implication of the result below T^*, which are necessary ingredients to understand the physical properties of Pb-LaOFBiS₂.