Mechanism for Magnetic Flux Generation in Grain Boundaries of YBCO
  The spontaneously generated magnetic flux observed by Mannhart et. al. (1996) in 45 degree grain boundaries in YBCO was explained by them in terms of a d-wave superconducting order parameter and grain boundary faceting. This article argues that twin boundaries which contact the grain boundary also play an important role. 
M. B. Walker