Device for charge- and spin-pumped current generation with temperature-induced enhancement

We have proposed a device, a superconducting-lead/quantum-dot/normal-lead system with an ac voltage applied on the gate of the quantum dot induced by a microwave, based on the one-parameter pump mechanism. It can generate a pure charge- or spin-pumped current. The direction of the charge current can be reversed by pushing the levels across the Fermi energy. A spin current arises when a magnetic field is applied on the quantum dot to split the two degenerate levels, and it can be reversed by reversing the applied magnetic field. The increase of temperature enhances these currents in certain parameter intervals and decreases them in other intervals. We can explain this interesting phenomenon in terms of the shrinkage of the superconducting gap and the concepts of photon-sideband and photon-assisted processes.

Figure 1

A sketch of the pump effect induced by level oscillation in the QD at zero temperature for the (a) $\omega >\Delta$ case and (b) $\omega <\Delta$ case. Only one-PA paths contributing to the pumped current are demonstrated. The level ${ϵ}_{\downarrow }$ is not depicted.

Figure 2

Variation of $J_{\uparrow }$ with ${ϵ}_{\uparrow }$ in the case of ${ϵ}_{\uparrow }={ϵ}_{\downarrow }$. The solid, dashed, and dotted curves correspond to $T=0$, $T=0.4{\Delta }_0$, and $T=0.55{\Delta }_0$, respectively. ${\Delta }_0$ is taken to be the unit of energy scale. Other parameters are (a) $\omega =1.3$, $V=0.5$, and $\gamma =0.1$ and (b) $\omega =0.7$, $V=0.5$, and $\gamma =0.1$.

Figure 3

Variation of $J_{\uparrow }$ with $\omega$ in the case of ${ϵ}_{\uparrow }={ϵ}_{\downarrow }$. The solid, dashed, and dotted curves correspond to $T=0$, $T=0.2{\Delta }_0$, and $T=0.55{\Delta }_0$, respectively. ${\Delta }_0$ is taken to be the unit of energy scale. Other parameters are (a) ${ϵ}_{\uparrow }=-1.3$, $V=0.5$, and $\gamma =0.1$ and (b) ${ϵ}_{\uparrow }=-0.4$, $V=0.5$, and $\gamma =0.1$.

Figure 4

A sketch of the one-PA paths influenced by nonzero temperature in the case of ${ϵ}_{\uparrow }<0$. (a) and (b) show the opening of new one-PA paths, which are not allowed at $T=0$. (c) and (d) show the suppression of one-PA paths, which exist already at $T=0$.