Quantum Thermodynamical Cycles For Coupled Systems As Working Substance

With the rapid development of quantum information, the interface between quantumphysics and thermodynamics has attracted more and more attention. Recently, quantumthermodynamical cycles for coupled systems as working substance have become an activetopic. In this paper, special quantum Brayton cycle and quantum Otto cycle are studied forone-dimensional isotropic Heisenberg model of spin-1/2systems as working substance.This thesis consists of three parts. The first part is composed of Chap.2and Chap.3. Themain aim of these two chapters is to introduce the fundamental conceptions of quantumphysics and quantum thermodynamical processes which are relative to our work.The second part is the Chap.4. In this part, two types of quantum Otto cycles areconsidered: In the first case, only the external magnetic field is varied during the quantumadiabatic processes. The results show that the directions of the heat transferred for the totalcoupled system and the local subsystems are opposite while both the total system and thelocal subsystems have positive work output. In the second case, the external magnetic fieldB and the coupling constant J are changed by the same ratios in the quantum adiabaticprocesses. We find that the positive work condition and efficiency for the composed systemare the same as in the case where a single spin is considered as working substance. Moreover,under some conditions, the local subsystems can serve as a refrigerator while the total systemis a heat engine.The third part is Chap.5, in which the quantum Brayton cycle is studied. Two pressurescan be defined due to the properties of coupled systems, and correspondingly two types ofquantum Brayton cycles for the composite system can be defined. The results show that thesubsystem experiences a quantum Brayton cycle in one quantum Brayton cycle, whereas thesubsystem’s cycle is quantum Otto cycle in another Brayton cycle. The efficiency for thecomposite system equals to the one for the subsystems in both cases, but the work done by thetotal system is usually larger than the sum of the work done by the two subsystems. Moreover,under contain conditions the subsystem can be a refrigerator, while the total system is a heatengine.