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Various orientations of YBa2Cu307 x grains in polycrystalline films prepared on (OO1) MgO substrates by in situ laser deposition were determined using electron diffraction. Eight different types of in-plane orientations have been observed. These orientations agree wen with the prediction of a simplified theory of near coincidence site lattice between YBa2Cu307 _ x and MgO. The YBa2Cu307 x grains were found to have a high probability of forming low angle or low I boundaries among themselves. These grain boundaries are of low energy and should exhibit a high connectivity of Cu-O-Cu chains. Therefore, YBa2Cu307 x thin films on MgO can attain a Tc of~ 90 K and a Je of~ 106 A/cmZ at 77 K.

Single-crystalline MgO is a popular substrate for the deposition of YBa2Cu, 07 x thin films. Due to the large mismatch of the lattice constants, so far only polycrystalline YBa2Cu307 x thin films have been obtained on MgO …