  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.
