Theory of spin blockade, charge ratchet effect, and thermoelectrical behavior in serially coupled quantum dot system

Abstract

The charge transport of a serially coupled quantum dots (SCQD) connected to the metallic electrodes is theoretically investigated in the Coulomb blockade regime. A closed-form expression for the tunneling current of SCQD in the weak interdot hopping limit is obtained by solving an extended two-site Hubbard model via the Green's function method. We use this expression to investigate spin current rectification, negative differential conductance, and coherent tunneling in the nonlinear response regime. The current rectification arising from the space symmetry breaking of SCQD is suppressed by increasing temperature. The calculation of SCQD is extended to the case of multiple parallel SCQDs for studying the charge ratchet effect and SCQD with multiple levels. In the linear response regime, the functionalities of spin filter and low-temperature current filter are demonstrated to coexist in this system. It is further demonstrated that two-electron spin singlet and triplet states can be readily resolved from the measurement of Seebeck coefficient rather than that of electrical conductance.

Effects of interdot hopping and Coulomb blockade on the thermoelectric properties of serially coupled quantum dots

“…Using the Keldysh-Green's function technique, [13] the charge and heat currents of SCQD connected to metallic electrodes are given by…”

“…T ℓ,j (ǫ) denotes the transmission function, which can be calculated by evaluating the on-site retarded Green's function (GF) and lesser Green's GF. [13] The indices ℓ and j denote the ℓth QD and the jth QD, respectively. Based on the equation of motion method, we can obtain analytical expressions of all GFs in the Coulomb blockade regime.…”

“…Results and discussion Here, we consider the case of identical QDs in the optimization of ZT , although it is understood that the size fluctuation of QDs can suppress ZT . [13] In Fig. 1(a) and 1(b), we plot (ZT ) 0 and ZT as a function of temperature for various electron hopping strengths.…”

“…[10] Although many theoretical investigations of the above phenomena have been reported, most of them did not investigate the thermoelectric properties of SCQDs. [11][12][13] This study investigates the ZT of SCQD embedded in a semiconductor nanowire with small phonon thermal conductance. [4] It is expected that SCQD system has a potential to enhance the ZT of nanowires.…”

“…Thus, only one energy level for each quantum dot needs to be considered. A two-level Anderson model [13] is employed to simulate the SCQD junction system.…”