Metal-insulator transition in the S = 1 2 spin ladder system (Sr_0.4Ca_0.6)₁₄Cu_24-xCo_xO_41-δ

We have found that the (Sr_0.4Ca_0.6)₁₄Cu_24-xCo_xO_41-δ system shows a metal-insulator transition at Co content x between 4.0 and 4.5 after high-pressure O₂ annealing. For the as-sintered samples the T dependence of the electrical resistivities is semionductting over the whole x range and the associated activation energies systematically increase with increasing x. For the high-pressure O₂ annealed samples the T dependence of the electrical resistivities is semiconducing, similar to the as-sintered ones at x below 3.8, whereas the electrical resistivities scarcely depend on the temperature variation with two types of small activation energies, ∼ 59 meV (200-100 K) and ∼ 2 meV (100-4.2 K) at x = 4.8. The behaviors of the electrical resistivities are characterized by the one-dimensional variable-range hopping scheme at a low-temperature region. These facts can be interpreted: the hole carriers beyond the capacity of the 1D CuO₂ chain for trapping holes are released into the ladder by Co doping x above 2.9 under high-pressure O₂ annealing (650°C, 8 MPa, 20 h).