Controlling of Localization by Elemental-substitution Effect in Layered BiCh₂-based Compounds LaO_1-xF_xBiS_2-ySe_y

We report transport properties for layered BiCh₂-based (Ch = S, Se) superconductors LaO_1-xF_xBiS_2-ySe_y (x = 0.2, 0.5, y = 0-1.05) and the observation of possible weak antilocalization (WAL). Electrical resistivity and Hall coefficients for the Se-poor samples increase with decreasing temperature. The increase becomes less pronounced with increasing Se concentration, indicating a loss of insulating behavior. Interestingly, the moderately Se-substituted samples exhibit metallic behavior in the high-temperature region and a weak increase in the resistivity in the low-temperature regions, which implies the existence of carrier localization. The heavily Se-substituted compounds show metallic behavior in the entire-temperature region. Magnetoresistance measurements indicate that WAL is realized in the heavily Se-substituted systems. The WAL behavior is weakened by the changes in F and Se concentrations. A crossover state of the WAL and weak localization (WL) emerges around the moderately F-doped and Se-free LaO_0.8F_0.2BiS₂. The change of the resistivity and Hall coefficient by the F and Se substitution clearly correlates to the difference of the magnetoconductance. We propose that the BiCh₂-based system is a good platform for studying the relationship between localization and superconductivity because those states are tunable by element substitutions with bulk single crystals.