Fractional vortices in the XY model with pi bonds

We define a new set of excitations in the XY model which we call "fractiona l vortices." In the frustrated XY model containing pi bonds, we make the an satz that the ground state configurations can be characterized by pairs of oppositely charged fractional vortices. For a chain of a bonds, the ground state energy and the phase configurations calculated on the basis of this a nsatz agree well with the results from direct numerical simulations. Finall y, we discuss the possible connection of these results to some recent exper iments by Kirtley et al. [Phys. Rev. B 51, R12057 (1995)] on high-T-c super conductors where fractional flux trapping was observed along certain grain boundaries.