In this paper a variational ground state for the sine-Gordon model is constructed based on the pairing of k, -k boson modes. This state can be realized with the help of a unitary transformation, and the variational procedure is essentially characterized by one parameter. The state is favored in a definite region of the model parameter plane where a nonzero variational parameter exists; its boundary depicts the transition line. We remark that the pairing ground state implies Bose condensation of the zero-momentum mode, and the variational parameter is related to dynamical mass generation. We also prove that these results can be recovered when one solves the Nambu-Dyson equation under the self-consistent approximation for the self-energy in which most infrared divergent diagrams are collected.

• Received 27 July 1992

DOI:https://doi.org/10.1103/PhysRevB.47.898