When the boson–reservoir interaction is given by (2.39), which includes not only the non-adiabatic part but also the
adiabatic part with momentum mixing, the collision operator C
(2)
takes the expression (2.45), which includes not only the
bilinear part but also the non-bilinear part with momentum mixing. Therefore, the unperturbed Hamiltonian Hˆ
S0 + iC
(2)
in
the non-equilibrium thermo-field dynamics takes the non-bilinear form which includes not only the bilinear part but also the
non-bilinear part with momentum mixing, where the non-bilinear part comes from the adiabatic part of the boson–reservoir
interaction (2.39). When Ψk [=φ
zz
k
(0)] given by (2.82) is real, the quasi-particle operators for the semi-free boson field take
the forms (2.80) and (2.81), where Φ
0
k
is non-negative for non-negative k and Ψk is non-negative in general. Then, the twopoint
Green’s function for the semi-free boson field is given by (3.14). We see that the quasi-particle and the two-point
Green’s function for the semi-free boson field depend on the non-bilinear interaction with momentum mixing.
The admittance for the boson system takes the form (4.5), which has been obtained using the TCLE method formulated
in terms of generalized non-equilibrium thermo-field dynamics, and includes effects of the initial correlation and memory
for the boson system and heat reservoir. The zeroth-order, first-order and second-order parts of the admittance (4.5) in
powers of the boson–boson interaction are, respectively, given by (4.11), (4.18) and (4.31) and depend on the non-bilinear
interaction with momentum mixing. The higher-order parts can be calculated using the Feynman diagram method.
The results that the quasi-particle and the two-point Green’s function for the semi-free boson field depend on the nonbilinear
interaction, are expected to give some novel and interesting non-equilibrium phenomena. Recently, we have studied
the linear response of a ferromagnetic spin system interacting with a phonon reservoir in the spin-wave approximation using
the TCLE method [24]. In the forthcoming paper, we will discuss the transverse magnetic susceptibility for a ferromagnetic
spin system in the resonance region, by assuming the spin–phonon interaction similar to (2.39).