Tunable Kondo Effect in a Double Quantum Dot Coupled to Ferromagnetic Contacts

We investigate the effects induced by ferromagnetic contacts attached to a serial double quantum dot. Spin polarization generates effective magnetic fields and suppresses the Kondo effect in each dot. The superexchange interaction $J_{\mathrm{AFM}}$, tuned by the interdot tunneling rate $t$, can be used to compensate the effective fields and restore the Kondo resonance when the contact polarizations are aligned. As a consequence, the direction of the spin conductance can be controlled and even reversed using electrostatic gates alone. Our results demonstrate a new approach for controlling spin-dependent transport in carbon nanotube double dot devices.

Figure 1

Upper panel: schematic diagram of a serial double dot for the parallel (P) configuration where the majority spins of both reservoirs are aligned. $t$ is the interdot tunneling amplitude, $\Gamma$ is the lead-dot hybridization strength. The size of the arrow designates the minority or majority character for the spins, while the color describes the spin orientation: light gray (green online) for spin up and dark grey (purple online) for spin down. (a)–(f) Spectral densities $A_{1,\sigma }(\omega )$ of dot 1 for P configuration; due to symmetry, $A_{2,\sigma }(\omega )=A_{1,\sigma }(\omega )$. (g) Spin-spin correlation function at $t=0.192$ for P configuration. (h) Double dot phase diagram for the P case: the three phases are the Kondo phase, the antiferromagnetic phase (AFM) and the molecular-orbital phase (MO). Lower panel: double dot in the antiparallel (AP) configuration where the majority spins of both reservoirs are antialigned. (i)–(l) Spectral densities of dot 1 for AP configuration; due to symmetry, $A_{2,\sigma }(\omega )=A_{1,-\sigma }(\omega )$. Parameters are $ϵ=-U/2$, $U=7$, and $\Gamma =1$.

Figure 2

Spectral function of dot 1 when the system is away from the particle-hole symmetric point. Parameters are $U=7$, $\Gamma =1$, $ϵ=-U/2+1$.

Figure 3

Conductance through the double dot device. (a), (b), (c) Linear conductance for $p=0$, 0.6,0.9 in the parallel arrangement. (d) Spin-up, (e) spin-down conductance, and (f) spin conductance for $p=0.6$. Parameters are $U=7$, $\Gamma =1$. Shade scheme (color scheme online) is the same in (a)–(e), but different in (f), as indicated by contour labels in (a) and (f), respectively. The darker (red online) contour lines in (f) indicate the reversal of the sign of spin conductance. The conductance is expressed in the units of conductance quantum, $2e^2/h$.