High-Pressure Transport Properties of the Superconducting Spin-Ladder System Sr_14-xCa_xCu₂₄O_41+δ

We report the electrical resistivity, the Hall coefficient, R_H, and the Magnetoresistance, MR, at pressures up to 2.0 GPa, of the spin-ladder compound Sr_14-xCa_xCu₂₄O_41+δ, on both a polycrystal (x = 13.6) and single crystals (x = 11 and 12), in which superconductivity has been observed only in a range of both Ca content (∼ 11 < x < 14) and pressure (∼ 1.5 < P <∼ 8 GPa). The pressure dependence of the R_H, with positive sign, indicates that carrier density increases with increasing pressure and localization of hole carriers at low temperature is substantially suppressed by applying pressure. We have also found that a positive MR, which violates Kohler's rule, seems to appear below the temperature at which a spin gap begins to open and strongly depends on the temperature as well as both on Ca content and applied pressure, suggesting that this MR is closely related to the mobile holes in the spin singlet state.