We studied the thermal annealing effects on the micro structure of the grain boundary of a 36.8° and a 24° symmetric  tilt SrTiO3 bicrystal. Scanning tunneling microscopy and atomic force microscopy were used for the observation of the vacuum exposed boundary structures. Annealing the bicrystalline substrates at temperatures as low as 780°C led to the formation of grooves at their boundaries. This provides direct evidence that the thickness depression of YBa2Cu3O7-δ films at the bicrystal boundaries originates from the underlying grooved substrates. A simple structural model has been proposed to explain the 1/φ dependence of the normalized critical currents flowing cross the grain boundaries. PACS nos: 61.16.Ch, 61.72.Mm, 68.35.-p, 68.35.Ct, 68.35.
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