| In this paper, we gave a summary of the history, the current situation, basic theory andapplication prospect about the spintronics. We introduced the define, research techniqueand transport phenomena in the mesoscopic system. And we deduced the Hamiltonianconsidering the spin-orbit coupling.We adopt the model of AB ring embedding parallel double quantum dots. The externalfield only irradiated the left and right lead, but not the potential barrier and the quantumdots (QDs). The coupling terms between the leads and the QDs are time-independent. Theenergy level in the quantum dots is single level and time-independent. And we ignore thedegeneracy of energy level in the quantum dots. The energy levels can shift by controllingthe external field and the grid voltage of the quantum dots. Under the adiabaticapproximation, we gave the system Hamiltonian considering the spin-orbit coupling andsome Green’s functions and the self energy matrix. Began with the Hamiltonian, wededuced the formulas of instantaneous current, average current and occupation numbers inthe system of AB ring embedding parallel double quantum dots applying thenon-equilibrium Green’s function. We provided the numerical calculation about theaverage current and the intradot occupation number by adopting the method ofself-consistent with the help of MATLAB program. The main results are as follows:(1) With the action of different external fields, the average current presents differentform. The current curves present sideband effect when the external field is symmetricaland time-dependent. And the field amplitude is larger, the smaller the main peak of thecurrent curve, but the larger the sideband peak. With the action of asymmetrictime-dependent external field, the field amplitude is larger, the lager the current peak andthe absolute value of the negative current. And an embossment is appear near the negativecurrent maximum value.(2) When we ignore the spin-orbit coupling, the frequency of the symmetricaltime-dependent external field determines the position of the sideband peak. The sidebandresonance peaks are formed at ε_d=nηω (n=0,±1,±2,…). Peak separation is ηω. Withthe action of the asymmetrical time-dependent external field, when the ω is lesser, thecurve peak is narrow but high, and when the is larger, the curve peak wide but low.(3) The every current curve main peak and sideband peak is divided into two peakswith the action of the symmetrical time-dependent external field when considering the existence of the spin-orbit coupling. The intradot occupation numbers are changed. Thespin up occupation number is not equal to the spin down. So spin polarization appears inthe quantum dots. And the spin accumulations in two dots are opposite. So the spinpolarizations in two dots are opposite too.(4) With the action of symmetrical time-dependent external field, the magnetic fluxand the spin-orbit coupling codetermined the spin polarization current. Both the magneticflux and the spin-orbit coupling can lead to a phase difference between the wave functionsof the two ways in the AB ring. To achieve desired objectives, we can choose appropriatemagnetic flux and spin-orbit coupling parameters based on our requirement to the degreeof spin polarization. For example, in our model, when the magnetic flux and the spin-orbitcoupling are both at π/4, the spin up current is separated from the spin down. And thespin down current curve peak is divided, but spin up current curve does not have thisphenomena. The spin up and spin down occupation number curves are changed as ‘steps’.The width of the ‘steps’ is ηω.(5) With the action of the asymmetrical time-dependent external field, we find that aslong as choosing appropriate field frequency, magnetic flux and spin-orbit coupling, wecan get the polarization current. And we can make spin up and down current flow to thereverse.At last, we summarized our working in this paper, drawn our conclusion, and had aoutlook about the mesoscopic physics field. |
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