We have found that the (Sro.4Ca0.6)14Cu24_xCOxO41_ 8 system shows a metal-insulator transition at Co content x between 4.0 and 4.5 after high-pressure 0 2 annealing. For the as-sintered samples the T dependence of the electrical resistivities is semiconducting over the whole x range and the associated activation energies systematically increase with increasing x. For the high-pressure O 2 annealed samples the T dependence of the electrical resistivities is semiconducting, 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 carders beyond the capacity of the 1D CuO 2 chain for trapping holes are released into the ladder by Co doping x above 2.9 under high-pressure 0 2 annealing (650°C, 8 MPa, 20 h).
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suggested that Sr n_ 1GUn+ 102n described by the antiferromagnetic intra-ladder coupling J and a weak and frustrated inter-ladder coupling J' will be a spin liquid state and have a gap in the spin-excitation spectra (spin gap) for n = 3, 7, 11 .... compound [2]. They also concluded that when the holes are lightly doped, the spin gap would remain and superconductivity should occur. Actually, Azuma et al. and Ishida et al. showed evidence for the existence of a spin gap for SrECU406 (n = 3 phase) from the magnetic measurement [7] and 63Cu NMR spectrum [8]. Unfortunately, however, hole-carrier doping into this system has not been successfully done. Siegrist et al. and McCarron et al. reported the syntheses and structure refinements of (Sr,Ca)14-Cu24041_ 8 [9,10]. From their structure refinements, this phase is composed of the layer containing 1D