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The direct observation of the magnetic flux motion near individual twin boundaries in YBa2Cu307y sho~s that twin boundaries are not channels for easy flux penetration but planar pinning barriers near which vortices are concentrated, giving rise to guided vortex motion. PACS numbers: 74.60.Ge, 74.72.Bk Twin boundaries (TBs) are an important component of the defect structure in the YBa2Cu307b high-T, materials. Theoretical considerations have predicted both an increasing [1,2] and a decreasing [3] superconducting order

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