Influence of doping on the Hall coefficient in Sr_14−xCa_xCu₂₄O₄₁

We present Hall-effect measurements of two-leg ladder compounds Sr_14−xCa_xCu₂₄O₄₁ (0≤x≤11.5) with the aim to determine the number of carriers participating in dc transport. Distribution of holes between the ladder and chain subsystems is one of the crucial questions important for understanding the physics of these compounds. Our Hall-effect and resistivity measurements show typical semiconducting behavior for x<11.5. However, for x=11.5, the results are completely different, and the Hall coefficient and resistivity behavior are qualitatively similar to that of high-temperature copper-oxide superconductors. We have determined the effective number of carriers at room temperature and compared it to the number of holes in the ladders obtained by other experimental techniques. We propose that going from x=0 to x=11.5 less than 1 hole per f.u. is added to the ladders and is responsible for a pronounced change in resistivity with Ca doping.