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Modulation of the dynamic resistance versus the magnetic field of YBa2Cu307~single crystal shows two groups of peak structures with nearly uniform magnetic field spacings. The spacings suggest two kinds of Josephson junctions: one formed by the Cu02-Y-Cu02 atomic planes within each unit cell and the other formed by the Cu02 bilayers between different unit cells. The intra-unit-cell junctions appear first as the temperature is lowered from T, . The existence of CuO&-Y-Cu02 Josephson junctions implies two-dimensional superconductivity in the Cu02 atomic planes.

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