Classical XY model (Wikipedia)
Z Gulacsi, M Gulacsi, Theory of phase transitions in two-dimensional systems, AP 47, 1 (1998)
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Spin ice (Wikipedia)
H Kawamura, Universality of phase transitions of frustrated antiferromagnets, JPC 10, 4707 (1998)
du Croo de Jongh M S L, Denteneer P J H, Spin stiffness in the frustrated Heisenberg antiferromagnet, PRB 55, 2713 (1997)