Abstract
A class of simple, translationally invariant, nearest-neighbor, isotropic XY models that possess novel topological defects will be presented.1,2 As the first example, I shall present a model that exhibits, in addition to integer vortices, half-integer-vortex, and ‘‘string’’ excitations. The interaction between the half-integer vortices is a superposition of the usual logarithmic Coulomb potential and a linear potential mediated by the strings. Due to these new excitations the system possesses a rich phase diagram. As the second example, I shall present a model that supports commensurate and incommensurate vortices. Due to a novel ground-state degeneracy, the ‘‘charges’’ of the incommensurate vortices are fixed by their cost in zero-point entropy and are independent of the parameters in the Hamiltonian. Again, as a result of these new defects the model exhibits a very rich phase diagram.
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