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Systematic evolution of electron localization in Bi2Sr2CuOy

The electronic transport properties in Bi2Sr2CuOy crystals have been investigated with the focus on the localized behaviours at low temperature. It is found that in the passage of transition from metallic to insulating states the samples first show two-dimensional weak localization behaviour, and cross over to the variable-range hopping conduction corresponding to the strong-localization regime. We analyse the experimental results and suggest that the metal-insulator transition is caused by the effects of both disorder in the lattice and electron correlations.