Topological defects in the fully frustrated XY model and in3He-A films

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Abstract
The ground states of both the fully frustrated XY model in two dimensions and 3He-A films have Z2*U(1) symmetries. Both line defects (domain walls) and point defects (vortices) can disrupt ordering in these systems at finite temperatures. The presence of line defects induces a breakdown on vortex charge quantisation in both of these systems. In the fully frustrated XY model on a square lattice, integral quantisation of charge is replaced by quarter-integral quantisation; in 3He-A, quantisation of charge is completely destroyed. It follows that in both of these systems phase ordering cannot coexist with Z2 disorder. The fully frustrated XY model is relevant to Josephson junction arrays in a transverse magnetic field, at a value of the field where half of a magnetic flux quantum threads each cell of the array.