AC-driven double quantum dots as spin pumps and spin filters

The emerging field of spintronics aims at creating devices based on the spin of electrons.One of the most important requirements for any spin-based electronics is the ability to generate a spin current.Recently, quantum dots have been proposed as potential spin filters or spin pumps[1,2,3]. In this work we propose a new scheme of realizing both spin filtering and spin pumping by considering ac-driven double quantum dots in external magnetic fields,in the Coulomb Blockade regime.

The electron dynamics is described by means of the density matrix formalism, within the Markov approximation[4], where up to four extra electrons within the double quantum dot are considered. In the regime of gate voltages studied, one electron is excited by the ac gate voltage from the left to the right dot, either to the double occupied singlet state or to the double occupied triplet state, by tuning the ac frequency. In the first case, and for a certain sample configuration, fully spin down polarized current is obtained. Increasing the ac frequency up to the resonance condition for the left dot singlet and the right dot triplet (spin up) double occupied states, the current polarization is reversed and it becomes spin up polarized. By calculating the current through the double quantum dot system, in the sequential tunnelling regime, we show how the spin polarization of the current can be controlled by tuning the ac amplitude and frequency[5].

Recently, electron spin relaxation times in quantum dots have been measured[6]. I will discuss spin relaxation effects in the tunnelling current through our device and how do they affect the robustness of our spin pump.

[1] E. R. Mucciolo et al., Phys. Rev. Lett., 89, 146802 (2002).
[2] T. Aono et al., Phys. Rev. B, 67, 155303 (2003).
[3] Q. Sun et al., Phys. Rev. Lett., 90, 258301 (2003).
[4] E. Cota et al., Nanotechnology, 14, 152 (2003).
[5] E. Cota et al., Phys. Rev. Lett., 94, 1072002 (2005).
[6] J. M. Elzerman et al., Nature, 430, 431 (2004)