| The main topic of this thesis is the local cooling effect in nonequilibrium quantum impurity systems,especially the local cooling effect in strongly correlated systems,which is the Kondo cooling predicted in this thesis.Before the study of open quantum impurity systems,first of all,it is necessary to select a suitable theoretical method for dealing with quantum systems.In an open quantum system,the dissipation between the system and the environment and the strong electron-electron interactions have extremely important effect on the dynamic and thermodynamic properties of the system.The hierarchical equations of motion approach has great advantages in dealing with dissipation and strong correlation effects.Therefore,the investigation in this thesis is based on the hierarchical equations of motion approach.As we all know,after passing an electric current to an electronic component,it will generate heat locally on the component,and the increase of the temperature of the electronic component will reduce its performance,or even cause its damage.Therefore,it is of great significance to study the local cooling effect.It is precisely because of this,researchers have made great efforts in the study on local cooling effects.Theoretically they have discovered various cooling mechanisms in nanojunctions,and later experimentally they directly observed Peltier cooling in molecule junctions.Nonetheless,the strength of cooling in nanojunctions is not yet sufficient for practical applications.Besides,the effect of strong correlation effects and quantum resonances on Peltier cooling has remained a topic barely touched on.To shed light on the effect of strong correlation effects and quantum resonances on Peltier cooling in nanojunctions,and to further enhance Peltier cooling,the thermoelectric effects in quantum impurity systems were investigated by means of theoretical analysis and numerical simulations.An important criterion of cooling in an impurity system is the lower local temperature than the background temperature of the impurity.Therefore,before studying Peltier cooling,it is necessary to find a suitable proposal to measure the local temperature of the quantum impurity systems.A few years ago,our research group further improved the minimal-perturbation condition protocol for measuring the local temperature of quantum impurity systems.This protocol does not require the measurement of heat current and is therefore experimentally operational.Based on this protocol,the correspondence relationship between the nonequilibrium system and a reference equilibrium system is established,thereby revealing the thermodynamic meaning of the measured local temperature from the physical nature.However,this protocol is only suitable for the measurement of the local temperature in single-impurity systems,not for that in multi-impurity systems.It was found that there is no real thermodynamically local cooling in nonequilibrium singleimpurity quantum systems.Therefore,it is urgent to develop a protocol suitable for measuring the local temperature in multi-impurity systems.Based on the previous work of our research group,recently important progress in the study of methods for measuring the local temperature has been made in this thesis,especially the proposal of the local minimal-perturbation condition protocol.The new protocol can be applied not only to the measurement of the local temperature in a single impurity system,but also to that in a multi-impurity system,and the measured local temperature can satisfy the correspondence relationship to the greatest extent.Using this new protocol,the conditions for local cooling in quantum impurity systems were found and the Kondo cooling phenomenon was predicted.This thesis is organized as follows:In the first chapter,the open quantum system is briefly introduced,then the development history of the research method of quantum impurity systems is reviewed,then the hierarchical equation of motion(HEOM)and the calculation examples in simulation program based on HEOM,the Hierarchical Equations of Motion for QUantum Impurity with a Correlated Kernel(HEOM-QUICK),are introduced in detail,and finally the Kondo effect and thermoelectric effect are introduced.Some basic concepts,physical phenomena and research methods introduced in this chapter are the basis of theoretical investigation in the following chapters.In Chapter 2,the local temperature in nonequilibrium single-impurity systems is investigated.First the uniqueness of the local temperature measured based on the minimal-perturbation condition is revealed.It is then found that there are some abrupt changes in the local temperature measured using the minimal-perturbation condition near the Fermi level.Finally,from both theoretical analysis and numerical simulations,it shows that such sudden changes of the local temperature originates from the strong quantum resonances near the Fermi level.In Chapter 3,the local minimal-perturbation condition protocol is proposed.First it is verified that this new protocol is equivalent to the minimal-perturbation condition protocol in a single impurity system.Then,the local minimal-perturbation condition protocol is applied to measure the local temperature in two-impurity systems,and the determined local temperature can satisfiy the correspondence relationship well,which proves that new protocol is suitable for the measurement of the local temperature in two-impurity systems.From the theoretical analysis,the local minimal-perturbation condition protocol is applicable to any impurity system,including single-impurity and multi-impurity systems.Therefore,there is no problem at all for measuring the local temperature in other impurity systems by means of the local minimal-perturbation condition protocol.Finally,using the local minimal-perturbation condition protocol,the local temperature in four-impurity chain systems is studied,and it is found that quantum resonances will cause a strong oscillation of the local temperature on the impurity chain.In Chapter 4,the Peltier effect in noninteracting serial double quantum dots under a bias voltage is studied.Firstly,the variation of the local temperature of the systems with interdot and dot-lead coupling is calculated by the local minimal-perturbation condition protocol,and it is found that the local temperature of the two quantum dots is different from each other.Then,the variation of the corresponding Peltier heat and Joule heat of the systems is represented.Finally,it is verified that the variation of the Peltier and Joule heats leads to the change in local temperature.In Chapter 5,an unconventional Peltier effect in quantum impurity systems,i.e.,the Kondo cooling phenomenon,is predicted.First the relationship between heat current and thermopower is uncovered,according to which it can be concluded that a large thermopower in absolute value is the premise and necessary condition for Peltier cooling.Then,based on the fact that the orbital Kondo effect will lead to a large thermopower(in absolute value),a serial double impurity system with a strong orbital Kondo effect is designed.Next,the existence of Peltier cooling in impurities is verified by two criteria of an inverse heat current and a lower local temperature than the background temperature.Due to the key role of the orbital Kondo effect in such local cooling,the Peltier cooling phenomenon caused by the orbital Kondo effect is named as Kondo cooling.Finally,Peltier cooling under a wide range of extrinsic conditions are investigated and it is found that Kondo cooling is the only effective means to achieve cooling at already low temperatures.The discovery of Kondo cooling provides a new path towards nanorefrigeration.Finally,in the sixth chapter,the investigation in the above chapters of this thesis is summarized,and the directions that can be further studied in the future are discussed around the topics such as the local temperature and Kondo cooling in open quantum impurity systems. |
