Out-of-plane and in-plane conduction in insulating and strongly underdoped cuprates

We report measurements of out-of-plane (rho(c)) and in-plane (rho(ab)) normal-state resistivities of single crystals of insulating PrBa2Cu3O7-delta and strongly underdoped oxygen deficient YBa2Cu3O6.41 using a flux transformer method. In the superconducting specimens, the onset of superconductivity was suppressed by a magnetic field of 9 T. We have found that the anisotropy rho(c)/rho(ab) of these samples increases monotonically at low temperatures with no signs of saturation. The temperature dependence of rho(c)/rho(ab) for YBa2Cu3O6.41 is well described by rho(c)/rho(ab) = a + bT(-2/3), but over a smaller temperature range than for insulating PrBa2Cu3O7-delta. Both the absence of saturation of rho(c)/rho(ab) and its T-2/3 dependence indicate two-dimensional conduction. This means that the average in-plane hopping distance of the localized charge carriers increases with decreasing T according to Mott's (R) over bar similar to T-1/3 law, while the elementary step in the c-direction remains T independent, equal to the spacing between the bilayers.