Research On Electronic Properties Of Low-dimensional Materials Based On Green's Function Method

Thermoelectric transport and collection excitations of electronic systems in low-dimensional materials are studied using Green's function method.We focus mainly on the electron transport of two-quantum-dot systems in the Coulomb blockade regime,the charge and spin thermopower of graphene-like zigzag nanoribbons modified by two on-site disorders,and Coulomb screening and plasmon spectra of multilayer graphene in the equilibrium and population inversion conditions.The main contents of the thesis are as follows:(1)The spin-dependent transport through a side-gated double quantum dot system in the Coulomb blockade regime is investigated under a magnetic field B and an electric or thermal bias.The functions of the charge current,the spin current,and the Seebeck coefficient versus the gate voltage VG,the magnetic field B,and the inter-dot coupling tc are calculated systematically.In the case of weak inter-dot coupling,the system behaves as two independent dots.With the increase of tc,the degenerate energy levels in the two dots split and this leads to the strong oscillations of currents versus VG at small positive and large negtive VG.Under a proper VG,a thermal bias can introduce pure spin current and the current sign can be controlled by the magnetic field.In addition,the occupation rates of the states vary greatly with the parameters due to the Coulomb interaction and this makes the prediction of current difficult.Our model can illustrate clearly the physical mechanism of occupation variation when the inter-dot coupling is not too strong.(2)The charge and spin thermopowers are studied systematically in a ferromagnetic junction of graphene-like zigzag nanoribbon modified by two on-site disorders.We emphasize that the symmetries of the transmission spectra and the geometry configurations of the two disorders are important factors in determining the thermoelectric properties of the system.Conditions to achieve pure charge and pure spin thermopower are discussed from the perspective of symmetry.Symmetry breaking is required sometimes to obtain large figure of merit.(3)The plasmon spectra in rhombohedral multilayer graphene under a perpendicular electric field are studied systematically in the random phase approximation.Weakly-damped plasmon modes with low group velocity exist in both nondegenerate and degenerate conditions.In the former case,interband excitations of electrons near the band edges contribute significantly to the Coulomb screening.Similar to bilayer graphene,the energy of the weakly-damped plasmon modes ??2ud is mainly determined by the band gap 2ud which can be well-controlled by the external electric bias.The spectrum region of low group velocity can be enlarged by adjusting the number of graphene layers.In the degenerate case when the temperature is low or the Fermi energy is high,the plasmon modes are dominated by the Fermi energy EF due to the interband excitation near kF.Their spectrum slope minimizes at ud=EF/2 and the region of low group velocity extends with the decrease(increase)of the layer number at high(low)Fermi level(4)In the random phase approximation,the plasmon spectra of graphene multilayers under a perpendicular electric field with population inversion are studied systematically.We calculate the plasmon spectra versus the system parameters such as the graphene layer number,the band gap,the temperature,and the doping level.Different from the spectra in equilibrium,there might appear two extra plasmon modes with positive gain.We observe the 'bimodal peak' in the dielectric function corresponding to four possible plasmon modes in a narrow range of energy.This characteristic phenomenon is relevant to the Berry phase of the system and depends on the parity of graphene layer number.