Relation of local Cu2+ spins, crystal microstructure, and charge transport in the CuO2 plane in Bi2Sr2CuOy

Single-phase Bi2Sr2CuOy samples with a Sr deficiency were synthesized. Resistivity measurements have shown that a decrease in Sr concentration causes the 2201 phase to change from a metal to an insulator. Changes in the structural modulation of the 2201 phase, which accompany the metal-to-insulator transition, were examined. The experimental data indicate that the modulation periodicity reduces with a decrease in Sr concentration. The Raman-scattering analysis showed that the structural distortion of the 2201 phase is enhanced with a decrease in the modulation periodicity. In addition, the behavior of localized Cu2+ spins at the metal-insulator transition and its relation to the structural distortion were studied by means of electron paramagnetic resonance. The experimental data reveal that the fraction of localized Cu2+ spins increased appreciably with the occurrence of the metal-insulator transition. The relationship between the localized Cu2+ spins associated with the structural distortion and charge transport in the CuO2 layer is discussed, based on these experimental results.