Squeezed-field path-integral description of BCS superconductors

We develop a squeezed-field path-integral representation for BCS superconductors utilizing a generalized completeness relation of squeezed-fermionic coherent states. We derive a Grassmann path integral of fermionic quasiparticles that explicitly includes the collective degrees of freedom of the order-parameter dynamics governed by the classical Anderson pseudospin model. Based on this method, we analyze the spectral function of the single-particle excitations, and show that the squeezed-field path integral for the BCS Hamiltonian describes the dispersion relation and the mass gap of the Higgs amplitude mode of BCS superconductors, as well as the quasiparticle and quasihole excitation branches described by the BCS mean-field approximation.