Hall effect in Sr_{14 - x} Ca_x Cu₂₄ O₄₁

We present Hall effect measurements of two-leg ladder Sr_{14 - x} Ca_x Cu₂₄ O₄₁ (0 ≤ x ≤ 11.5). In these composite materials charge and spin ordering occur within chain and ladder subsystems. They have been investigated intensively during the last years because of their fascinating physical properties, especially after the discovery of superconductivity under pressure (for x > 10). They are the first superconducting copper oxides (superconductivity occurs under pressure for x > 10) without the CuO₂ planes, characteristic for high-temperature superconductors. The question of charge dynamics and hole distribution between the ladders and chains is still open. Our Hall effect results show typical semiconducting behavior for x < 10. The x = 11.5 compound shows metallic behavior and our Hall effect results match qualitatively the behavior found in high-T_c cuprates. We have determined the effective number of carriers and its dependence on composition (x) and compared it to the number of holes in the ladders obtained by other experimental techniques at room temperature.