The square lattice Ising model under the Brascamp-Kunz boundary conditions is a well-known exactly solvable lattice model. The exact solution of this system has been derived within the framework of Pfaffian-type method. In this paper, we provide a derivation for the solution by the Schultz-Mattis-Lieb method in the transfer matrix formalism. We set special interactions on the boundaries and take certain limit of these interactions, so that the system under the Brascamp-Kunz boundary conditions is transformed into another system under the toroidal boundary conditions. The Schultz-Mattis-Lieb method is applied to the mapping system and the partition function is exactly solved in the fermionic representation. The Fisher zeros are analytically calculated and the physical critical point is identified. We also discuss the difference between the transfer matrix approaches to the Brascamp-Kunz and to the toroidal boundary conditions. Our work introduces a member to the family of transfer-matrix-based studies for Ising model under various boundary conditions.
A family of interacting bosonic models is studied. A picture is suggested where each such conformal field theory is tensored with a conjugate one to give a free bosonic theory. It is shown that the lattice of the bosonic theory is determined in each case, and how the spectrum and correlation functions can be calculated. It is explained how flat directions can arise in such theories, allowing for an exact solution for a family of parameters. A large class of examples is discussed in detail. It is shown that for any rational value of the central charge c ≥ 3 there is an infinite number of distinct rational N = 2 theories which can be contracted by this method.