A single-molecule magnet （SMM） coupled to two normal metallic electrodes can both switch spin-up and spin- down electronic currents within two different windows of SMM gate voltage. Such spin current switching in the SMM tunnel junction arises from spin-selected single electron resonant tunneling via the lowest unoccupied molecular orbit of the SMM. Since it is not magnetically controlled but all-electrically controlled, the proposed spin current switching effect may have potential applications in future spintronics.
The efficiency of the calculation of Green＇s function （GF） for nano-devices is very important because the calculation is often needed to be repeated countlessly. We present a set of efficient algorithms for the numerical calculation of GF for devices with arbitrary shapes and multi-terminal configurations. These algorithms can be used to calculate the specified blocks related to the transmission, the diagonals needed by the local density of states calculation, and the full matrix of GF, to meet different calculation levels. In addition, the algorithms for the non-equilibrium occupation and current flow are also given. All these algorithms are described using the basic theory of GF, based on a new partition strategy of the computational area. We apply these algorithms to the tight-binding graphene lattice to manifest their stability and efficiency. We also discuss the physics of the calculation results.
The nonequilibrium Kondo effect is studied in a molecule quantum dot coupled asymmetrically to two ferromagnetic electrodes by employing the nonequilibrium Green function technique. The current-induced deformation of the molecule is taken into account, modeled as interactions with a phonon system, and phonon-assisted Kondo satellites arise on both sides of the usual main Kondo peak. In the antiparallel electrode configuration, the Kondo satellites can be split only for the asymmetric dot-lead couplings, distinguished from the parallel configuration where splitting also exists, even though it is for symmetric case. We also analyze how to compensate the splitting and restore the suppressed zero-bias Kondo resonance. It is shown that one can change the TMR ratio significantly from a negative dip to a positive peak only by slightly modulating a local external magnetic field, whose value is greatly dependent on the electron-phonon coupling strength.
Transport properties are theoretically studied through an anisotropy single-molecule magnet symmetrically connected to two identical ferromagnetic leads. It is found that even though in parallel configuration of leads’ magnetizations, the total current still greatly depends on the spin polarization of leads at certain particular bias region, and thus for large polarization a prominent negative differential conductance （NDC） emerges. This originates from the joint effect of single-direction transitions and spin polarization, which removes the symmetry between spin-up and spin-down transitions. The present mechanism of NDC is remarkably different from the previously reported mechanisms. To clarify the physics of the NDC, we further monitored the shot noise spectroscopy and found that the appearance of the NDC is accompanied by the rapid decrease of Fano factor.
