Multichannel effects in Rashba quantum wires

We investigate intersubband mixing effects in multichannel quantum wires in the presence of Rashba spin-orbit coupling and attached to two terminals. When the contacts are ferromagnetic and their magnetization direction is perpendicular to the Rashba field, the spin-transistor current is expected to depend in an oscillatory way on the Rashba coupling strength due to spin coherent oscillations of the traveling electrons. Nevertheless, we find that the presence of many propagating modes strongly influences the spin precession effect, leading to (i) a quenching of the oscillations and (ii) strongly irregular curves for high values of the Rashba coupling. We also observe that in the case of leads’ magnetization parallel to the Rashba field, the conductance departs from a uniform value as the Rashba strength increases. We also discuss the Rashba interaction-induced current-polarization effects when the contacts are not magnetic and investigate how this mechanism is affected by the presence of several propagating channels.

Figure 1

(Color online) Sketch of the physical system (a) and of the spatial variation of Rashba intensity $\alpha \left(x\right)$ and gate potential $V_g\left(x\right)$ (b).

Figure 2

(Color online) Conductance as a function of Rashba coupling intensity. Black corresponds to the complete Rashba interaction while gray (red color) to the neglect of ${\mathcal{H}}_R^{\left(1\right)}$. The leads are spin-polarized along $x$. Upper, intermediate, and lower panel correspond to $N_p=1$, 5, and 10 propagating modes, respectively. We take the parameters $\ell =8{\ell }_0$, $E=N_p\hslash {\omega }_0$, ${\Delta }_{\ell }={\Delta }_r=10\hslash {\omega }_0$, $d_{\ell }=d_r=10{\ell }_0$, and $a=0.1{\ell }_0$.

Figure 3

(Color online) Same as Fig. 2 for polarized leads along $x$ but in antiparallel orientations, i.e., ${\Delta }_{\ell }=10\hslash {\omega }_0$ and ${\Delta }_r=-10\hslash {\omega }_0$.

Figure 4

(Color online) Same as Fig. 2 for parallel polarized leads along $y$.

Figure 5

(Color online) Same as Fig. 2 for polarized leads along $y$ in antiparallel orientation.

Figure 6

(Color online) Conductance $G$, black symbols with left scale, and polarization of transmitted current, gray symbols (red in color) with right scale, as a function of the Rashba intensity. We have used the same parameters as in Fig. 2, except for the Stoner fields which are here taken to vanish. Upper, intermediate, and lower panel correspond to $N_p=4$, 10, and 20 propagating modes, respectively.

Figure 7

(Color online) Conductance $G$, black symbols with left scale, and polarization of transmitted current, gray symbols (red in color) with right scale, as a function of the diffusivity $a$ in the Fermi functions describing the spatial transitions in Fig. 1. We have used the same parameters as in Fig. 6, and a value of the Rashba intensity ${\alpha }_0=\hslash {\omega }_0{\ell }_0$ and $2\hslash {\omega }_0{\ell }_0$ for the upper and lower panels, respectively.