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(\nu, T) = \sigma_{DC}(T) + A(T)\nu^{s}, where s \approx 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. 
T. Vuletic, B. Korin-Hamzic, S. Tomic, B. Gorshunov, P. Haas, M.   Dressel, J. Akimitsu, T. Sasaki, T. Nagata