Coherent single-spin source based on topological insulators

We report on the injection of quantized pure spin current into quantum conductors. In particular, we propose an on-demand single-spin source generated by periodically varying the gate voltages of two quantum dots that are connected to a two-dimensional topological insulator via tunneling barriers. Due to the nature of the helical states of the topological insulator, one or several spin pairs can be pumped out per cycle giving rise to a pure quantized alternating spin current. Depending on the phase difference between two gate voltages, this device can serve as an on-demand single-spin emitter or single-charge emitter. Again, due to the helicity of the topological insulator, the single-spin emitter or charge emitter is dissipationless and immune to disorder. The proposed single-spin emitter can be an important building block of future spintronic devices.

Figure 1

(a) and (c) Schematic plot of pumped current in the whole period for the spin up and spin down. (b) Band structure of TI. Combining (a) and (c), we can get (d) and (e) that depict the generation of pumped pure spin current in the first and second half-periods, respectively.

Figure 2

Pumped current driven by a general ac signal [gray line in (a) and (c)] and harmonic signal [gray line in (b) and (d)]. The red line is the profile of the gate voltage $V_b\left(t\right)$. The width of the lead $W=40a$. The diameter of the circular QD is $D=21a$. The contact width of QPC $L=3a$.

Figure 3

The number of spin pairs pumped out of QD in the first half-period vs the magnitude of the ac gate voltage. Different curves correspond to different $E_0$. The diameter of the circular QD is $D=21a$ and the contact width $L=3a$ (thick lines) and $L=7a$ (thin gray dotted line). The size of square QD is $20a\times 20a$ and the contact width $L=7a$.

Figure 4

The average number of spin pairs and its fluctuation at the gate voltage marked with a red cross in Fig. 3 vs disorder strengths for different $E_0$. (a) and (b) correspond to the circular and square shaped QDs.