Physical review. B, Condensed matter
Quantum studies of the two-dimensional axial next-nearest-neighbor Ising model: A BCS-type theory
Santiago D’Elía
Abstract
The Hamiltonian limit of the two-dimensional axial next-nearest-neighbor Ising model is mapped into a cutoff interacting field fermion theory. The single-particle spectrum is defined by a set of integral equations with a singular kernel at the ferromagnetic critical line (FCL). The interaction operator V${\mathrm{^}}_{\mathrm{in}}$ is calculated using single-particle states with definite momentum and parity. There are various interaction processes associated with V${\mathrm{^}}_{\mathrm{in}}$. One of these is, at the FCL, an attractive pairing force V${\mathrm{^}}_{\mathit{p}}$. Allowance for V${\mathrm{^}}_{\mathit{p}}$ leads to the formation of a condensate of Cooper pairs, each pair having opposite momenta and parities. This is shown by using Green's functions.