X. L. Huang, L. C. Wang, X. X. Yi
We consider the quantum isobaric process and quantum Brayton cycle for a composite system as working substance. The thermodynamics relations between the total system and the subsystem are studied. We can define two pressures for the working substance of the composite system, one corresponds to the external magnetic field (named $F_x$), the other conjugates to the coupling strength (named $F_y$), and as a consequence, we can define two types of quantum Brayton cycle for the composite system. We find that the subsystem also experiences a quantum Brayton cycle in the case of cycle based on $F_x$, whereas the subsystem is a quantum Otto cycle for the cycle based on $F_y$. The efficiency for the composite system equals to the one for the subsystem in both cases. However the work done by the total system are usually larger than the sum of the two subsystems. Another interesting result is that for the cycle based on $F_y$, the subsystem can be a refrigerator while the total system is a heat engine under proper condition.
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http://arxiv.org/abs/1209.1684
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