Quantum Otto Refrigerator Based On Two-qubit Isotropic Heisenberg Model

Heat engines and refrigerators are crucial tools in post-industrial age and their miniaturization is a hot topic.Ever since Scovil and Schulz-Du Bois who first proposed the concept of quantum heat engine.More and more attentions have been attracted in this field.This thesis focuses on a four-level refrigerator.The effects of physical parameter in the working substance on the performance of the refrigerator are discussed.This thesis contains three chapter.In chapter 1,the background and the development for quantum thermodynamics are introduced and the differences between classical models and quantum models are analyzed.In chapter 2,basic concepts and theorems about the quantum mechanics and thermodynamics,which are necessary in the following studies,are presented.Chapter 3 in the main body of this thesis where a quantum Otto refrigerator based on spin-1/2 isotropic two-qubit Heisenberg model is established.At first,two types of different adiabatical schemes are considered,which are(1)the external magnetic field B varies but the coupling constant J unchanges.(2)the coupling constant J changes but the external magnetic field B does not,respectively.The conditions under which the refrigerator can be operated are discovered and the effects of physical parameters on the performance of the refrigerator are studied.Then,the input work and the refrigeration coefficient as functions of temperature of the hot heat bath are analyzed for afferent coupling constant J and external magnetic field B,respectively.Finally,a special adiabatic process,in which the external magnetic field B and coupling constant J are changed in the same ratio,i.e.(?)kholds during the adiabatic process,are studied.The differences are compared between these three types of refrigerators schemes.Finally,conclusion and outlook are presented.