Out-of-plane conductivity in Bi2Sr2CuOy crystals

We present extensive measurements of anisotropic resistivity on Bi2Sr2CuOy crystals grown from melts with different Bi/Sr ratios and doped with Pb. We find that the temperature variation of the c-axis resistivity rho(c)(T) is correlated with the in-plane resistivity rho(ab). Depending on the starting compositions, the normal-state in-plane resistivity rho(ab) can either show localized conduction at low temperature or be metallic (d rho(ab)/dT > 0) in the whole temperature range. Correspondingly, a change of the T dependence of rho(c) from nonmetallic (d rho(c)/dT < 0) in the whole measured temperature range (4.2-300 K) to a mixed (d rho(c)/dT < 0 at low T but d rho(c)/dT > 0 at high T) conduction is observed. In accompany, the magnitude of rho(c) at low T decreases by about two orders. We have quantified the trend of the rho(c)(T) and examined some current proposals concerning the out-of-plane transport.