<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>c</mml:mi></mml:math>-axis transport and resistivity anisotropy of lightly to moderately doped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mi>−</mml:mi><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>single crystals: Implications on the charge transport mechanism

Both in-plane and out-of-plane resistivities $({\ensuremath{\rho}}_{\mathrm{ab}}$ and ${\ensuremath{\rho}}_{c})$ are measured in high-quality ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ single crystals in the lightly to moderately doped region $0.01&lt;~x&lt;~0.10,$ and the resistivity anisotropy is determined. In all the samples studied, the anisotropy ratio ${\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{\mathrm{ab}}$ quickly increases with decreasing temperature, although in nonsuperconducting samples the strong localization effect causes ${\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{\mathrm{ab}}$ to decrease at low temperatures. Most notably, it is found that ${\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{\mathrm{ab}}$ at moderate temperatures $(100--300 \mathrm{K})$ is almost completely independent of doping in the nonsuperconducting regime $(0.01&lt;~x&lt;~0.05);$ 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 ${\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{\mathrm{ab}}$ 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 $x&gt;0.05$ suggest that the emergence of the superconductivity is related to an increase in the c-axis coupling.

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