where Rk and R
Ď
k
are the reservoir operators of wave number k, and hg¯ 1k and hg¯ 2kl are the coupling constants between the
boson system and heat reservoir. In the boson–reservoir interaction (2.39), the first term is non-commutative with the free
boson Hamiltonian HS0, is called the ‘non-adiabatic part’ and represents a bilinear interaction, while the second term is
commutative with HS0, is called the ‘adiabatic part’ and represents a non-bilinear interaction with momentum mixing. We
renormalize the free boson Hamiltonian HS0 and the boson–reservoir interaction HSR according to (2.17) and (2.18) as