Carbon-nanotube-based quantum pump in the presence of a superconducting lead

Parametric electron pump through superconductor-carbon-nanotube based molecular devices was investigated. It is found that a dc current, which is assisted by resonant Andreev reflection, can be pumped out from such molecular device by a cyclic variation of two gate voltages near the nanotube. The pumped current can be either positive or negative under different system parameters. Due to the Andreev reflection, the pumped current has the double peak structure around the resonant point. The ratio of pumped current of N-SWNT-S system to that of N-SWNT-N system (I-NS/I-N) is found to approach four in the weak pumping regime near the resonance when there is exactly one resonant level at Fermi energy inside the energy gap. Numerical results confirm that in the weak pumping regime the pumped current is proportional to the square of the pumping amplitude V-p, but in the strong pumping regime the pumped current has the linear relation with V-p. For the pumped current via two resonant levels, we found that the current reversal behavior, i.e., the direction of the current reverses when the Fermi energy is varied. This is different from the one dimensional double barrier structure. Our numerical results also predict that pumped current can be obtained more easily by using zigzag tube than by using armchair tube. We have also derived the formula for the total current in the presence of both pumping potential and external bias and applied our formula to the armchair and zigzag carbon nanotubes.