Study Of The Heat Transport Through Quantum Dots System

In recent years, with the fast development of micromachining technology and nanotechnology, understanding how heat is carried, distributed, stored, and converted in nanoscale systems has garnered even more attention and has grown considerably in importance. Uncovering the laws of heat generation induced by an electric current in nanoscale devices and investigating how the heat generation may be reduced and exploited is very important for deeply understanding the electronic transport and easing the world energy crisis. Recently, much higher thermoelectrical figure of merit has been found in nanostructure-thermoelectric materials, so it may be a effective way to recycle the heat generation via thermoelectric effects. In this paper, we study the heat transport in quantum dot (QD) systems. Our purpose is to find high thermoelectrical figure of merit by design different QD structures; and on the other hand, to study the heat generation in a QD coupled to a normal metal and a superconducting lead. After brief review of the QD, the electron transport through QD systems, local heating effect and the thermoelectric effect, we present a detailed introduction the nonequilibrium Green’s function in mescoscopic transport, and give general formulas for electric current, heat current and heat generation. With the aid of these formulas, we study:First, we study the spin effects in thermoelectric transport through a lateral double QDs system with ferromagnetic electrodes. A static magnetic field is applied on the tunneling junction between the two QDs. Then both the interdot coupling and the dot-leads coupling become spin-dependent. It is found that in the parallel configuration, the thermoelectric efficiency can reach a considerable value around the spin-down resonance levels when the effective interdot coupling and the tunnel coupling between the QDs and the leads for the spin-down electrons are small. On the other hand, in the presence of the magnetic field, the spin accumulation in the leads strongly suppresses the thermoelectric efficiency. The thermoelectric and the thermo-spin efficiencies are strongly enhanced by the intradot Coulomb interactions and can reach very high values at appropriate temperatures. Moreover, a pure spin thermopower can be obtained in such a double QDs system.Then, we theoretically study thermoelectric transport through a quantum dot coupled to two side quantum dots, and mainly discuss the influence of the Dicke effect on the thermoelectric transport through this triple QDs system. Our results show that at low temperature, the electrical and thermal conductances exhibit characteristics of the Dicke effect. Small (or very large) level shift and large interdot coupling may enhance thermoelectric efficiency near the subradiant state, where density of states almost exhibits a δ-like shape, due to a strong violation of the Wiedemann-Franz law. With increasing temperature the interference effect is weakened, however, considerable figure of merit is still obtained near the subradiant state for large interdot coupling. Moreover, the influence of the asymmetry parameter and the interdot Coulomb interaction will also be discussed.Finally, we consider a system of a QD coupled to a normal metal and a superconducting lead, and study the heat generation in the QD when an electric current passing through the device. For the case with weak dot-leads coupling, we find that the heat generation is not proportional to the current and can be controlled by the gate voltage, bias and temperature. The regions in which ideal condition for device operation is attained are discussed. At high temperature, the heat generation can become negative in the regions where the phonon-assisted Andreev tunneling or phonon-assisted direct tunneling with absorbing a phonon can take place. This means the heat can flow from the phonon bath to the QD when an electric current is flowing through it. So such a system can be designed as a quantum refrigerator.