Thermoelectric Effects In An Isotropic Single Molecule

The thermoelectric energy conversion is a main topic in the field of the science and technology and also is very significant for understanding the information of electron transport and easing the world energy crisis. However, the conventional thermoelectric material parameters obey the Wiedemann-Franz law and the Mott relation, so the thermoelectric conversion efficiency had been low, which seriously affects the use of thermoelectric materials used in industry. With the fast development of nanotechnology, the higher thermoelectric figure of merit is obtained in the nanostructures. Moreover, the high figure of merit (ZT≥1) can also be obtained in molecular junctions. Recently, due to the large spin and magnetic anisotropy of single-molecule magnets, one is beginning to pay attention to its thermoelectric properties, the thermoelectric effect of single-molecule magnets became a hot topic in quantum transport. In this treatise, based on the background above, we investigate thermoelectric effect of isotropic single-molecule magnet and obtain the higher efficiency of thermoelectric conversion than in the conventional bulk materials.In the first chapter, we introduce the quantum transport and thermoelectric effects of single molecular magnet in experimental and theoretical aspects. In the second chapter, we present the non-equilibrium Green’s function method, Lengreth theorem and equation of motion, and give two kinds of truncated approximation method and general formulas for electric and heat current. In the third chapter, by using the non-equilibrium Green function technique based on Hubbard operator, we study thermoelectric properties of the isotropic single molecular magnet coupled to two metal electrodes. The large spin operators are expressed as Hubbard operators, which is the key of calculating nonequilibrium Green function in large spin system. The results show that in the linear response regime, when the Coulomb interaction is infinite, the singly-occupied states determine the effective transport channels and the thermopower and the figure of merit present the large values near bonding state level at the low temperature, and for the given temperature the thermopower and the figure of merit are significantly enhanced with coupling intensity decreasing. For the finite Coulomb interaction, there are four effective transport channels and the spectrums of thermoelectric quantities are split into two sets. When the coupling intensity is small, the peaks of the thermopower and figure of merit corresponding to doubly-occupied transport channels are much larger than unity. In addition, a region of the parameter optimization is given. These properties are useful for understanding the relation between thermoelectric phenomena and quantum tunneling and designing highly efficient thermoelectric devices based on molecular magnet.