J. R. GonçalvesJ. PoulterJ. A. BlackmanUniversity of ReadingMahidol University2018-07-042018-07-041997-05-07Journal of Physics A: Mathematical and General. Vol.30, No.9 (1997), 2947-2962030544702-s2.0-0040092366https://repository.li.mahidol.ac.th/handle/20.500.14594/18009The Villain model and the triangular antiferromagnet are the two canonical fully frustrated two-dimensional Ising systems. Exact results are presented for the entropy changes that occur when bond and site defects are introduced into these systems. The results for single defects are obtained first. Pairs of site defects are then discussed. The change in entropy is evaluated to the leading term in an expansion in powers of R -1 where R is the separation of the defects. The entropy shift is ∼ R -2 in both models except for special values of R in the Villain model where it is ∼ R -4.Mahidol UniversityMathematicsPhysics and AstronomyArticleSCOPUS10.1088/0305-4470/30/9/010