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The out-of-plane dielectric constant of the parent insulator of the high-temperature superconductor Bi2Sr2Dy1-xErxCu2O8 was measured and analysed from 80 to 300 K in the frequency range of 106-109 Hz. All the samples were found to show a fairly large value of 10-60, implying some kind of charge inhomogeneity in the CuO2 plane. Considering that the superconducting sample Bi2Sr2Ca0.75Pr0.25Cu2O8 also shows a similar dielectric constant, the charge inhomogeneity plays an important role in the insulator-superconductor transition.

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