We introduce the concept of multisymplectic formalism, familiar in covariant field theory, for the study of integrable defects in 1 + 1 classical field theory. The main idea is the coexistence of two Poisson brackets, one for each spacetime coordinate. The Poisson bracket corresponding to the time coordinate is the usual one describing the time evolution of the system. Taking the nonlinear Schrödinger (NLS) equation as an example, we introduce the new bracket associated to the space coordinate. We show that, in the absence of any defect, the two brackets yield completely equivalent Hamiltonian descriptions of the model. However, in the presence of a defect described by a frozen Bäcklund transformation, the advantage of using the new bracket becomes evident. It allows us to reinterpret the defect conditions as canonical transformations. As a consequence, we are also able to implement the method of the classical r matrix and to prove Liouville integrability of the system with such a defect. The use of the new Poisson bracket completely bypasses all the known problems associated with the presence of a defect in the discussion of Liouville integrability. A by-product of the approach is the reinterpretation of the defect Lagrangian used in the Lagrangian description of integrable defects as the generating function of the canonical transformation representing the defect conditions.
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B. McCoy and J. Perk, Two-spin correlation functions of an Ising model with continuous exponents, Phys. Rev. Lett.44 (1980) 840.CrossRefADSGoogle Scholar
N.R. Jungwirth et al., A single-molecule approach to ZnO defect studies: single photons and single defects, arXiv:1402.1773.
F. Nabarro, Theory of crystaline dislocation, Clarendon Press, Oxford U.K. (1967).Google Scholar
S. Chandrasekhar and G. Ranganath, The structure and energetics of defects in liquid crystals, Adv. Phys.35 (1986) 507.CrossRefADSGoogle Scholar
T. Lubensky et al., Topological defects and interactions in Nematic emulsions, cond-mat/9707133.
D. Vollhardt and C. Wolfe, The phases of Helium 3, Taylor & Francis (1990).Google Scholar