Special coupled quantum Otto cycles

We study a special quantum Otto heat engine, in which the external magnetic field B and the coupling constant J are changed by the same ratios in the quantum adiabatic processes, with a composed quantum system as working substance. We find that the positive work condition and efficiency for the composed system are the same as in the case where a single spin is considered as working substance. The effects of coupling on the work output and the work relations between the total system and local subsystems are studied. We find that, under proper conditions, the local subsystems can serve as a refrigerator while the total system is a heat engine. These results can be explained and can be divided into two categories by the concept of local effective temperature. Our results can be generalized to a general coupled system.