c-Axis Transport and Resistivity Anisotropy of Lightly- to Moderately-Doped La_{2-x}Sr_{x}CuO_{4} Single Crystals: Implications on the Charge Transport Mechanism

Abstract

Both the in-plane and the out-of-plane resistivities (ρ_{ab} and ρ_{c}) are measured in high-quality La_{2-x}Sr_{x}CuO_{4} (LSCO) single crystals in the lightly- to moderately-doped region, x = 0.01 to 0.10, and the resistivity anisotropy is determined. In all the samples studied, the anisotropy ratio ρ_{c}/ρ_{ab} quickly increases with decreasing temperature, although in non-superconducting samples the strong localization effect causes ρ_{c}/ρ_{ab} to decrease at low temperatures. Most notably, it is found that ρ_{c}/ρ_{ab} at moderate temperatures (100 - 300 K) is almost completely independent of doping in the non-superconducting regime (x = 0.01 to 0.05); this indicates that the same charge confinement mechanism that renormalizes the c-axis hopping rate is at work down to x = 0.01. It is discussed that this striking x-independence of ρ_{c}/ρ_{ab} is consistent with the idea that holes form a self-organized network of hole-rich regions, which also explains the unusually metallic in-plane transport of the holes in the lightly-doped region. Furthermore, the data for x > 0.05 suggest that the emergence of the superconductivity is related to an increase in the c-axis coupling.