The non-equilibrium thermo-field dynamics proposed by Arimitsu and Umezawa are
generalized to the case of a non-bilinear unperturbed Hamiltonian which includes not only
a bilinear part but also a non-bilinear part with momentum mixing. The forms of the quasiparticle
operators for a semi-free boson field are derived. The form of the two-point Green’s
function for the semi-free boson field is evaluated. A form of the admittance for a boson
system interacting with its heat reservoir, which includes effects of the initial correlation
and memory, is derived using the TCLE method formulated in terms of generalized nonequilibrium
thermo-field dynamics. The expressions of the zeroth-order, first-order and
second-order parts of the admittance in powers of the boson–boson interaction, are
derived.