Single-parameter quantum charge and spin pumping in armchair graphene nanoribbons

TL;DR

This paper studies quantum charge and spin pumping in armchair graphene nanoribbons driven by a single ac gate voltage, revealing symmetry-breaking effects and predicting pure spin currents under certain conditions.

Contribution

It introduces a novel analysis of charge and spin pumping in graphene nanoribbons with a single ac gate, highlighting symmetry-breaking mechanisms and the generation of pure spin currents.

Findings

Peaks in charge and spin currents occur at subband edges due to symmetry breaking.

Pure spin current can be achieved with ferromagnetic leads and full nanoribbon ac gating.

Dependence of pumping currents on ac field strength and frequency is characterized.

Abstract

We investigate quantum charge and spin pumping in armchair graphene nanoribbons under a single ac gate voltage connected with nonmagnetic/ferromagnetic leads via the nonequilibrium Green's function method. In the case of nonmagnetic leads, where only part of the nanoribbon is subject to an ac gate voltage to break the left-right spatial symmetry, we discover that peaks of the charge pumping current appear at the Fermi energies around the subband edges in the ac-field-free region of the nanoribbon. In the case of ferromagnetic leads with the lead magnetizations being antiparallel to break the left-right symmetry, similar peaks appear in the spin pumping current when the Fermi energies are around the edges of the the majority-spin subbands in the ferromagnetic leads. All these peaks originate from the pronounced symmetry breaking in the transmissions with energies being around the corresponding subband edges. Moreover, we predict a {\em pure} spin current in the case of ferromagnetic leads with the whole graphene nanoribbon under an ac gate voltage. The ac-field-strength and -frequency dependences of the pumping current are also investigated with the underlying physics revealed.