Steady state behaviors as a function of the middle bath temperature T_m within the nonequilibrium NIBA at strong coupling (α_m = 4): (a) transition rates Gu±(u=l,m,r) in Eqs. (13a)–(13c), and (b) average energy quanta 〈ω〉_u,± (u = l,r) in Eqs. (14a) and (14b); (c) heat current J_m,NIBA and its main components in Eqs. (20a) and(20b), and (d) comparison of the approximate amplification factor β_r,NIBA with β_r. (e) and (f) Schematic illustrations of flow components Jm,NIBA(a) and Jm,NIBA(b) . The other parameters are the same as those in Fig. 5.