BOND AND SITE DEFECTS IN FULLY FRUSTRATED 2-DIMENSIONAL ISING SYSTEMS

The Villain model and the triangular antiferromagnet are the two canon ical fully frustrated two-dimensional Ising systems. Exact results are presented for the entropy changes that occur when bond and site defec ts are introduced into these systems. The results for single defects a re obtained first. Pairs of site defects are then discussed. The chang e in entropy is evaluated to the leading term in an expansion in power s of R-1 where R is the separation of the defects. The entropy shift i s similar to R-2 in both models except for special values of R in the Villain model where it is similar to R-4.