Variable-range hopping conductivity in the copper-oxygen chains of La_3Sr_3Ca_8Cu_24O_41

We show that the spin chain/ladder compound La_3Sr_3Ca_8Cu_24O_41 is an insulator with hopping transport along the chains. In the temperature range 35 - 280 K, DC conductivity sigma_DC(T) follows Mott's law of variable-range hopping conduction; the frequency dependence has the form sigma(ν, T) = σ_DC(T) + A(T)ν^s, where s ≈ 1. The conduction mechanism changes from variable-range hopping to nearest-neighbor hopping around T_c =300 K. The chain array thus behaves like a one-dimensional disordered system. Disorder is due to random structural distortions of chains induced by irregular coordination of the La/Sr/Ca ions.