where Fk(t) is the Fourier transform of wave number k for the external driving field. We assume that the external driving
field is turned on adiabatically at the initial time t = 0, and that the boson system and heat reservoir in the external static
field are in the thermal equilibrium state at the initial time t = 0, i.e., |ρT(0)i = |ρTEi. Then, using the TCLE method in
the lowest Born approximation for the boson–reservoir interaction, the admittance χaka
Ď
k
(ω) of the boson system takes the
following expressions