Evolution of the resistivity anisotropy in Bi_{2}Sr_{2-x}La_{x}CuO_{6+δ} single crystals for a wide range of hole doping

Abstract

To elucidate how the temperature dependence of the resistivity anisotropy of the cuprate superconductors changes with hole doping, both the in-plane and the out-of-plane resistivities (ρ_{ab} and ρ_{c}) are measured in a series of high-quality Bi_{2}Sr_{2-x}La_{x}CuO_{6+δ} (BSLCO) single crystals for a wide range of x (x = 0.23 - 1.02), which corresponds to the hole doping per Cu, p, of 0.03 - 0.18. The anisotropy ratio, ρ_{c}/ρ_{ab}, shows a systematic increase with decreasing p at moderate temperatures, except for the most underdoped composition where the localization effect enhances ρ_{ab} and thus lowers ρ_{c}/ρ_{ab}. The exact p dependence of ρ_{c}/ρ_{ab} at a fixed temperature is found to be quite peculiar, which is discussed to be due to the effect of the pseudogap that causes ρ_{c}/ρ_{ab} to be increasingly more enhanced as p is reduced. The pseudogap also causes a rapid growth of ρ_{c}/ρ_{ab} with decreasing temperature, and, as a result, the ρ_{c}/ρ_{ab} value almost reaches 10^6 in underdoped samples just above T_c. Furthermore, it is found that the temperature dependence of ρ_{c} of underdoped samples show two distinct temperature regions in the pseudogap phase, which suggests that the divergence of ρ_{c} below the pseudogap temperature is governed by two different mechanisms.