Effect of Ca substitution and pressure on the transport and magnetic properties of ${\mathrm{Sr}}_{14}$${\mathrm{Cu}}_{24}$${\mathrm{O}}_{41}$ with doped two-leg Cu-O ladders

Anisotropic electrical resistivity and magnetic susceptibility are measured on single crystals of ${\mathrm{Sr}}_{14\mathrm{-}\mathrm{x}}$ ${\mathrm{A}}_{\mathrm{x}}$ ${\mathrm{Cu}}_{24}$ ${\mathrm{O}}_{41}$ (A = Ca, Y). This material contains two-leg antiferromagnetic spin ladders that are progressively doped with hole carriers upon Ca substitution and thus determine the charge-transport properties. Application of hydrostatic pressure to this system has an effect similar to Ca substitution, increasing the hole density in the ladders. The resistivity shows remarkable anisotropy, indicative of a one-dimensional electronic state.