Zener quantum dot spin filter in a carbon nanotube

We predict and analyse a novel spin filter in semiconducting carbon nanotubes. By using local electrostatic gates, the conduction and valence bands can be modulated to form a double-barrier structure. The confined region below the valence band defines a Zener quantum dot, which exhibits resonant tunnelling. The resonances split in a magnetic field to make a bipolar spin filter for applications in spintronics and quantum information processing. We model this using k· p envelope function theory and show that this is in excellent agreement with a corresponding tight-binding calculation. A spin filter based on electrostatic gating of single-wall carbon nanotubes (SWCNTs) may have significant advantages over alternative approaches, based on spin injection from ferromagnetic contacts [1, 2]. Experimental techniques are now well established for placing source drain contacts at each end of a carbon nanotube and using a side gate to control electron transport. Such devices exhibit Coulomb blockade in metallic [3, 4] and semiconducting [5] SWCNTs, and field-effect transistor action [6, 7] at room temperature. Electrical conductance through near ideal Ohmic contacts has been predicted [8] and shown experimentally to exhibit ballistic conductance close to the clean quantum limit of 4e 2 /h [9, 10, 7]. Electron interference was first demonstrated in metallic SWCNTs [9, 10] and more recently within the conduction band of semiconducting SWCNTs [5], where the existence of Schottky barriers posed additional difficulties with contact resistance [7]. The Kondo effect has been exhibited in metallic SWCNTs [11] and optical emission from ambipolar FETs has been demonstrated [12]. All these devices have used global gating along the entire length of the active part of the nanotube. Local gating of carbon nanotubes has been demonstrated with split gates, finger gates and top gates fabricated by lithographic means [13, 14, 15, 16]. Through local gating, the nanotube can be made to function as a pn-junction diode without modulation doping by using two closely spaced finger gates, which are positively and negatively charged with respect to source-drain contacts [17]. Top gates have been used to form and control a double quantum dot [16]. In this Letter we demonstrate that local gating also opens up the possibility of spin filtering via Zener tunnelling. Although Zener tunnelling has yet to be demonstrated in semiconducting SWCNTs, our calculations show that there is no fundamental obstacle