Ising Model with Adjustable Boundary Conditions: Exact Results for Finite Lattice Mass Gaps

Abstract
Using the spinor approach, we calculate exactly the complete spectrum of the transfer matrix for the finite-size, planar Ising model with adjustable boundary conditions. The mass gap is found to vanish exponentially or algebraically, depending on the boundary conditions used. These results are compared with recent numerical simulations, and with random-walk and capillary-wave arguments.